Human kallikrein-related peptidases 3, 4, 11, and KLK2, the activator of KLK3/PSA, belong to the prostatic group of the KLKs, whose major physiological function is semen liquefaction during the fertilization process. Notably, these KLKs are upregulated in prostate cancer and are used as clinical biomarkers or have been proposed as therapeutic targets. However, this potential awaits a detailed characterization of these proteases. In order to study glycosylated prostatic KLKs resembling the natural proteases, we used Leishmania (LEXSY) and HEK293 cells for secretory expression. Both systems allowed the subsequent purification of soluble pro-KLK zymogens with correct propeptides and of the mature forms. Periodic acid-Schiff reaction, enzymatic deglycosylation assays, and mass spectrometry confirmed the glycosylation of these KLKs. Activation of glycosylated pro-KLKs 4 and 11 turned out to be most efficient by glycosylated KLK2 and KLK4, respectively. By comparing the glycosylated prostatic KLKs with their non-glycosylated counterparts from Escherichia coli, it was observed that the N-glycans stabilize the KLK proteases and change their activation profiles and their enzymatic activity to some extent. The functional role of glycosylation in prostate-specific KLKs could pave the way to a deeper understanding of their biology and to medical applications.
Among the 15 human kallikrein-related peptidases two subgroups in male and female reproductive organs and fluid secretions exhibit a rather complementary expression pattern on the RNA and protein level: KLKs 2, 3, 4 and 11 are mostly detected in testis, prostate and semen, while KLKs 6, 7, 8, 10, 12 and 13 are mainly present in uterus, vagina and cervico-vaginal fluid (Harvey et al., 2000; Shaw and Diamandis, 2007; Uhlén et al., 2015). Prostatic KLKs fulfill important tasks in impregnation, in particular KLK2 and KLK3/PSA digest the gel-forming proteins semenogelin I and II, which liquefies the seminal clot and increases the motility of spermatozoa (Sotiropoulou et al., 2009; Emami and Diamandis, 2012). Although KLK11 does not cleave semenogelins, it may activate KLKs 2 and 3, whereby KLK2 seems to be the major activator of KLK3 (Lovgren et al., 1997; Luo et al., 2006). KLKs 3 and 11 could be activators of KLK4 by cleaving the unusual propeptide SCSQ-I16ING (Yoon et al., 2007). Similarly, the role of KLK4 in semen liquefaction is not entirely clear and might be the degradation of extracellular matrix (ECM) proteins, such as fibronectin (Matsumura et al., 2005). Under pathological conditions, KLK4 appears to adopt a more prominent role by degradation of ECM proteins and, possibly, by activation of matrix metalloproteinase-1 (MMP-1), which is believed to be critical for tumor cell invasion and metastasis (Fuhrman-Luck et al., 2016). In addition, the prostatic KLKs 2, 3, 4 and 11 can cleave insulin-like growth factor binding protein IGFBP3 and activate urokinase-type plasminogen activator uPA, as well as proteinase-activated receptors PAR1 and PAR2, which are expressed at much higher levels in prostate cancer tissues than in benign and normal prostate tissues (Avgeris et al., 2012; Fuhrman-Luck et al., 2014). Apart from uPA, KLK4 can also directly cleave uPAR and thereby modulate the activity of the uPA/uPAR system (Beaufort et al., 2006).
Despite a less crucial role in prostate cancer, the enhanced expression and concomitantly enhanced serum levels rendered KLK3 as prostate specific antigen the most relevant prognostic biomarker based on an immunoassay (Papsidero et al., 1980; Kuriyama et al., 1981). In order to achieve a better distinction between malignant and benign forms of prostate cancer, various KLK3 forms, including pro-KLK3 plus KLK2, KLK4 and KLK11 have been tentatively used as prognostic markers, which improved the quality of prognosis significantly in some approaches (Stephan et al., 2007; Gupta et al., 2010; Hong, 2014). Recent studies corroborated that KLK4 levels in seminal plasma and blood serum have no prognostic value for prostate cancer patients, whereas KLK4 participates in the protease network, which is responsible for the dissemination of metastasizing prostate cancer cells (Karakosta et al., 2016; Reid et al., 2017).
Approximately 50% of human proteins are N- and/or O-glycosylated, mostly representing secreted soluble proteins or integrated membrane proteins with their extracellular regions (Apweiler et al., 1999). All KLKs possess potential N-glycosylation sites at asparagine in so-called sequons, such as Asn-Xaa-Ser/Thr (NXS/T), with Xaa being usually not Pro, whereas O-glycosylation takes place at Ser or Thr in Pro-rich regions without a distinct consensus sequence (Guo et al., 2014). In addition, non-consensus sequons, such as NXC, NGX and NXV can be N-glycosylated, but the degree of glycosylation varies as for standard sequons, depending on the three-dimensional (3D) protein architecture, whereby surface loops are strongly favored as glycosylation sites (Zielinska et al., 2010; Aebi, 2013). Structure-function analyses of glycan-free and glycosylated KLK2 demonstrated that the single core-glycan at Asn95 in the kallikrein/99-loop near the active site suffices to regulate the activation, enzymatic activity and deactivation (Figure 1A) (Skala et al., 2014; Guo et al., 2016). Similarly, KLK3 possesses a single N-glycan at Asn61, whose role has not been revealed yet, while additional O-glycosylation has been reported for KLK3 at Thr125 (Figure 1B) (Stura et al., 2011). The KLK4 sequence contains a potential N-glycosylation site at Asn159, which is also present in the mouse and pig homologs mKlk4 and pKLK4, linked to variable multiantennary N-glycans, besides additional N-glycans attached to Asn120 and Asn202 (Figure 1C) (Yamakoshi et al., 2011). The sequence of KLK11 contains two sequons corresponding to the glycosylation sites at Asn159 and Asn202, while it possesses two additional sequons at Asn95 and Asn175 (Guo et al., 2014). Glycosylation in vivo has been confirmed for several KLKs and has attracted growing attention, as the effects of glycosylation are widespread and critical in the life cycle of many proteases from expression to regulation of activity and their degradation (Goettig, 2016).
Aberrant glycosylation has been observed in a variety of diseases, especially in various cancer types, such as in prostate cancer for KLK3/PSA (Tabares et al., 2006; Sarrats et al., 2010). Glycosylated KLK3 expressed by the prostate tumor cell line LNCaP was remarkably different from KLK3 present in seminal plasma (Peracaula et al., 2003). The carbohydrate component of glyco-KLK3 from healthy individuals exclusively exhibits α-2,6 sialic acid, whereas that from PCa patients was mainly α-2,3 sialic acid (Yoneyama et al., 2014). The altered glycosylation pattern appears to be a predictive marker for discrimination between aggressive and non-aggressive cancers and may allow for a personalized medicine approach (Gilgunn et al., 2013; Padler-Karavani, 2014; Drake et al., 2015). However, full use of such an approach awaits detailed in vitro studies accompanied by information on biological and pathological functions of KLKs. Since protein glycosylation has a remarkable influence on folding, stability, molecular recognition, activity, specificity and regulation, it is worth investigating the effects of glycosylation on KLKs 3, 4 and 11, in order to obtain a more complete picture, which was outlined before with glycosylated KLK2 (Guo et al., 2016).
Eukaryotic expression of proforms and mature glyco-KLKs 3, 4, 11
Secretory expression of prostatic kallikrein-related peptidases in eukaryotic cells was chosen, in order to obtain folded and glycosylated proteins, resembling the natural human KLKs. The variants will be designated according to their expression origin as KLKlx (LEXSY), KLKS or KLKT (HEK293S/T) and KLKe (Escherichia coli). Expression of native-like pro-KLK3 constructs in Leishmania (LEXSY) and HEK293T cells was not sufficient. Both HEK293T and S cells produced larger amounts of the construct for mature KLK3 comprising an artificial propeptide with an N-terminal His-tag and an enterokinase (EK) cleavage site, exhibiting a molecular mass of about 30 kDa on sodium dodecyl sulfate (SDS) gels after EK digest (data not shown).
By contrast, pro-KLK4 constructs were successfully expressed in LEXSY and HEK293T with N-terminal His-tags. After TEV protease digest, the native-like pro-KLK4lx form was obtained as a stable zymogen. N-terminally His-tagged KLK4 with EK cleavages sites were expressed in LEXSY and HEK293T cells with higher yields. According to SDS gels, activated mature KLK4T had a molecular weight around 28 kDa, while KLK4lx displayed a molecular weight slightly above 26 kDa (data not shown).
Expression of pro-KLK11 in HEK293T and LEXSY cells with the legumain (AEP) cleavage site His6-GSN-ETR-I16 was feasible and purified samples of pro-KLK11lx were rather stable. LEXSY and HE293 cells expressed sufficient amounts of mature KLK11 with an artificial His-tag propeptide. After EK digest, mature KLK11lx showed a double band around 34 kDa in SDS gels, indicating slightly inhomogeneous glycosylation (data not shown).
Characterization of glycosylated KLKs 3, 4, 11
Edman degradation confirmed the correct N-terminal sequences IINGDCYP and IIKGFECK of the LEXSY expressed KLK4lx and KLK11lx, respectively. In addition, the IVGGWECEKHS N-terminal sequences of KLK3 from HE293S/T cells were confirmed, as the corresponding sequences of the KLK4T construct. Glycan staining with the periodic acid-Schiff reaction (PAS) indicated that all investigated samples of KLKs 3, 4, and 11 were glycosylated to some extent (data not shown).
Also, KLK3T with an artificial propeptide and His6-tag, exhibited a molecular weight of about 32 kDa on SDS gels, which shifted roughly to 28 kDa upon treatment with PNGase F, while Endo Hf treatment resulted in weaker bands around 30 kDa, indicating the presence of a complex, multiantennary N-glycan (Figure 2A). The sequon around Asn61 is the only predicted N-glycosylation site of KLK3, which was confirmed by a crystal structure (Stura et al., 2011). However, mass spectrometric analysis of KLK3 glycosylation was inconclusive, probably due to inhomogeneous N-glycosylation or loss of glycosylated peptides in the experiment.
Enzymatic deglycosylation for LEXSY and HEK293T cell expressed KLK4 was only successful with PNGase F, whereby the shift for KLK4lx was about 1 kDa and more than 2 kDa for KLK4T to about 26 kDa. The predicted N-glycosylation site at Asn159 was glycosylated according to mass spectrometry, as peptides in tryptic and chymotryptic digests carried a small core glycan of 892.32 Da in case of KLK4lx and a larger one of 1216.42 Da in case of KLK4T (Figure 2B). Most likely, the first one is a GlcNAc2Man3 core known for Leishmania glycosylation, while the second one comprises two more hexoses, e.g. GlcNAc and galactose, or mannose of a hybrid glycan. Peptides from tryptic digest showed smaller fractions of unpredicted additional N-glycans, which were not linked to standard sequons (N95RP, N101DL), with N202GY exhibiting the highest degree of glycosylation (Figure 2B).
The apparent molecular weight of KLK11lx was roughly 36 kDa, which is significantly higher than the expected 26 kDa for the unmodified protein. PNGase F treatment resulted only in a minor shift on SDS gels to about 34 kDa. Mass spectrometry indicates only partial glycosylation of Asn95, Asn159 and Asn175, in contrast to Asn202, which carried a core glycan of 892.32 Da, corresponding to GlcNAc2Man3. In particular, peptides containing Asn95 exhibited a significant glycan-free fraction. Moreover, these glycosylation sites could be glycosylated with more variations than the one at Asn202, for which an additional acetylation seems possible. Thr161 and Ser204, belonging to the sequons at Asn159 and Asn202, respectively, might be linked to O-glycans, which was not clearly resolved, whereas a comparable O-glycosylation pattern at sequons has been reported for human KLK1 (Kellermann et al., 1988).
Pro-KLKs 3, 4 and 11 activation
As prostate-specific KLKs have a three to seven residue propeptide, the additional residues in the N-terminal His-tag and protease recognition sequence may allow detecting molecular weight shifts upon analysis by SDS-polyacrylamide gel electrophoresis (PAGE). The two available fusion proteins pro-KLK4lx with His6-GS-ENLYFQ-SCSQ-↓-IINGEDC22 and pro-KLK11lx with His6-GS-N-ETR-↓-IIKGFEC22 were incubated with mature prostatic KLKs at 37°C for 24 h. The activating cleavage at the scissile peptide bonds of pro-KLK4 and pro-KLK11 at Gln-Ile16 and Arg-Ile16 require a modified chymotryptic and a regular tryptic activator protease, respectively. The properly activated zymogens of KLK4 and KLK11 will result in mature proteases with trypsin-like activity. Beside SDS-PAGE detection, the tryptic activity of these samples was determined to quantify the amount of activated species. From the activity assay, KLK2lx and KLK2e showed the strongest activation of pro-KLK4, followed by KLK11lx. This finding seems inconsistent with previous reports, indicating that KLK11 and KLK3 are the only members of the KLK family, which can process propeptides with Gln in the P1 position (Yoon et al., 2007). Surprisingly, the best activator for KLK4 seemed to be KLK2, which usually has a strong preference for P1-Arg residues (Cloutier et al., 2002; Skala et al., 2014). Apparently, the N-glycosylation of KLK2lx nearly doubles the activation efficiency (Figure 3A).
Activation of pro-KLK11 with Arg15 at the P1 position requires tryptic proteases such as KLK2, 4 and 11, rather than the chymotryptic KLK3 (Figure 3B). Glycosylated KLK2lx was more active in pro-KLK11 activation than the corresponding non-glycosylated KLK2e and comparable to KLK4T. A further 30% increase in activation efficiency was observed for the KLKs with glycosylation of the LEXSY type compared with the one of the HEK293 cell type. KLK4lx proved to be the best activator of pro-KLK11 (Figure 3B). Due to the low zymogen activity of pro-KLK11, auto-activation was not observed at all over 24 h.
Enzymatic activity and inhibition of glycosylated KLKs 3, 4 and 11
Several chromogenic and fluorogenic substrates were tested with the various glycosylated prostatic KLKs and non-glycosylated counterparts from E. coli expression, in order to assess the influence of the glycans on the enzymatic activity. A comparison for glycosylated KLK2lx with KLK2e revealed significant differences in enzyme kinetic parameters and processing of large proteins (Guo et al., 2016). KLK3S and KLK3T preferred chymotryptic substrates, e.g. Suc-AAF-AMC, and showed activity stimulation with 20 mm citrate and inhibition by 20 μm Zn2+, as it was reported for KLK3 from E. coli and insect cell expression (Hsieh and Cooperman, 2000; Andrade et al., 2010). In contrast to the chymotryptic KLK3, glycosylated KLK4lx, KLK4T and KLK11lx strongly preferred tryptic substrates, such as Bz-PFR-pNA, while the chymotryptic H-AAF-AMC was not cleaved (data not shown).
A systematic investigation of the three glycosylated prostatic KLKs was carried out and whenever possible compared to the activity of the non-glycosylated KLKe variant. The same substrates were used for the respective KLKs, in order to obtain comparable Michaelis constants (KM values). Proper determination of kcat values and the catalytic efficiency was based on active site titration for each sample. Overall, the glycosylated KLK variants exhibited higher fractions of active proteases, as it was already observed for KLK2e and KLK2lx (Guo et al., 2016). For example, KLK4e and KLK11e were only 7% and 14% active, whereas their glycosylated counterparts KLK4lx and KLK11lx were 48.5 and 73% active, indicating a stabilizing effect of the various glycans. Interestingly, KLK4e and KLK4lx have nearly equal KM (536 μm and 635 μm) for the substrate Suc-AAPR-pNA, but the kcat of KLK4e (0.89 s−1) is significantly lower than the one of KLK4lx (1.99 s−1), which resembles the situation for KLK11 (Table 1). In case of KLK11e and KLK11lx with Bz-IEGR-pNA only minor differences were observed in KM (660 μm and 820 μm), however, the kcat values (1.52 and 2.78 s−1) differed by more than 50% as the resulting catalytic efficiencies (kcat/KM) (Figure 4A, Table 1). No comparison was possible for KLK3S with a KLK3e sample, since only the glycosylated variant exhibited a little turnover of the chymotryptic substrate Suc-AAPF-pNA with a high KM (1.5 mm) and a low kcat (0.033 s−1), which is comparable to enzyme kinetic data from the literature (Watt et al., 1986). Apparently, Zn2+ does not inhibit both KLK11e and KLK11lx significantly, whereas the KLK3S activity was reduced by Zn2+ with an inhibition constant (IC50) of 17 μm (Figure 4B, Table 1). This value is close to the reported 20 μm for natural seminal KLK3, as well as the 15% residual activity beyond 100 μm (Malm et al., 2000). Even around 500 μm Zn2+ KLK4lx has a residual activity of about 60%, with an apparent IC50 of 15 μm. Glycan-free KLK4e is more efficiently inhibited by Zn2+, albeit with an elevated IC50 of 34 μm, showing a residual activity around 15% for 500 μm Zn.
|KLK||Substrate||KM (μm)||kcat (s−1)||kcat/KM (m−1·s−1)||IC50 Zn2+|
For Michaelis constants KM and turnover number kcat, the weighted standard errors of the non-linear regression fitting were used, while the error of the catalytic efficiency was calculated according to the formula given by Fenner (1931). Active site titrations were performed for tryptic KLKs with PPACK in which KLK11e was 14% active, whereas the KLK11lx protein showed 73% activity. No significant inhibition was observed in the range 0–500 μm Zn2+ for both KLK11lx and KLK11e. For KLK3S, the substrate Bz-AEPF-pNA was used. Errors are derived from experimentally measured and weighted data and given as standard deviations values.
Both glycosylated KLK3S and KLK3T processed the natural substrate fibronectin in a comparable manner, indicating that the different glycans may not affect their substrate specificity and turnover (Figure 5A). KLK4T and KLK4lx degraded fibronectin very rapidly, whereas KLK11lx cleaved it to a much lower extent (Figure 5A). Additionally, KLK3S and KLK3T formed SDS-stable complexes with natural inhibitors, such as the serpin α1-antichymotrypsin (ACT), which is the primary molecular form of serum KLK3, reaching up to 95% of total KLK3/PSA (Figure 5B) (Stenman et al., 1991). Seemingly, the different glycosylation patterns of KLK3 generated by HEK293S and T cells resemble the glycosylation variations of KLK3 isoforms A and B, which possesses more terminal sialic acids (Zhang et al., 1995). The reported slow KLK3-ACT complex formation in vitro was equally observed, whereby long incubation times resulted in liberation of KLK3 and increasing degradation of ACT as for natural KLK3 (Christensson et al., 1990). Similarly, the unspecific protease inhibitor α2-macroglobulin (A2M), which is present in human serum, formed a complex with our recombinant KLK3T (Figure 5B). Moreover, the physiologically relevant complex formation with the protein C inhibitor (PCI) from the serpin family was confirmed for KLK3S and KLK3T (Figure 5C) (Christensson and Lilja, 1994). Whereas KLK4T degraded the PCI completely, an SDS-stable complex with PCI and KLK11lx was formed, exhibiting an approximate molecular mass of 80 kDa (Figure 5C). This complex would be novel and might be relevant under physiological conditions in reproductive processes, as PCI complexes with an unspecified tissue kallikrein have been described in the literature (España et al., 2007).
Although the prostatic KLKs 2, 3, 4 and 11 are highly homologous, their expression as zymogens and mature proteases differs significantly in the three expression systems of E. coli, Leishmania and HEK293 cells according to our results (Skala et al., 2014). Expression of KLK3 in bacteria in bacteria is possible, but refolding from inclusion bodies is problematic, which can be explained by the requirement of the N-glycan at Asn61 for easy folding (Debela et al., 2006b). Similarly, expression of KLK5 failed completely in E. coli, whereas expression was successful in Sf9 insect cells that generate small N-glycans (Debela et al., 2007). Otherwise, it is not clear, why KLK3 expression was negligible in LEXSY cells, while KLK2 expression was preferred in this system, despite 80% identical sequences (Guo et al., 2016).
Since the type and extent of glycosylation, in particular the molecular weight differences, cannot be easily assessed by shifts on SDS gels upon deglycosylation, a thorough interpretation of the available mass spectrometry data allows to judge which glycosylation is reliable (Figures 2 and 6). Unfortunately, peptides of KLK3S/T from both tryptic and chymotryptic digest were not resolved in these measurements, most likely due to the presence of inhomogeneous N-glycosylation at Asn61. This single glycosylation site may carry mannose-rich hybrid glycans and a mixture of bi- and triantennary N-glycans with varying numbers of fucose and terminal sialic acids that were observed for KLK3, similar to mammalian KLK4 (Figure 1C) (Sarrats et al., 2010; Stura et al., 2011; Yamakoshi et al., 2011; Ishikawa et al., 2017). Nevertheless, the high molecular weight on SDS gels and downshifts upon enzymatic deglycosylation confirm the N-glycosylation of KLK3S/T. Regarding KLK4, the expected N-glycosylation at Asn159 appears to be much more reliable (Figure 2B). The lack of deglycosylation of KLK4lx by Endo Hf might depend on slightly extended core N-glycans, which could carry a fucose and two Gal-Sia antennae (Breitling et al., 2002). In general, Leishmania are capable of the unusual posphoserine-O-glycosylation, while common O-glycosylation of human proteins in L. tarentolae was recently confirmed (Ilg et al., 1994; Klatt et al., 2013). Among the four sequons of KLK11lx, the one Asn202 is most likely linked to a GlcNAc2Man3 core glycan, however, Asn159, Asn175 and in particular Asn95 might be incompletely occupied. Moreover, these N-glycans seem to be extended by up to two sugars. The partial O-glycosylation of Ser95b, Thr161, Thr177, and Ser204 is less reliable according to the MS analysis, while the O-glycan masses could be comparable to core glycans, which would explain the high apparent molecular weight of 36 kDa for KLK11lx on SDS polyacrylamide gels (Figure 2A).
Although the activation assays for pro-KLK4 and pro-KLK11 were not as systematic as in previous studies, they focus on the physiological set of prostatic KLKs with native-like glycosylation (Yoon et al., 2007, 2009; Beaufort et al., 2010). Remarkably, glycosylated KLK2lx was most efficient in cleaving the propeptide SCSQ from pro-KLK4, which does not match the known specificity profiles of KLK2e and KLK2lx with P1-Gln as disfavored residue (Skala et al., 2013; Guo et al., 2016). Otherwise, the results for pro-KLK11 activation with the propetide ETR are similar to the one reported for propeptide fusion proteins (Yoon et al., 2007). The native-like pro-KLK4 activation experiment clearly demonstrates that not only the three-dimensional protein structures of enzyme and substrate have to be considered, but also their distinct post-translational modifications, such as glycosylation (Figure 3A). Additional molecular properties of KLK4 could play role in the interaction with KLK2, such as the negatively charged patch in the region of the usually positively charged patch of thrombin and several KLKs, termed ‘anion binding exosite I’ (Debela et al., 2008). KLK2 may bind this region in a preferential manner, resulting in activation of pro-KLK4 despite the unfavorable P1-Gln. Thus, the 3D-structure derived tertiary specificity of a protease is an additional level of information, which is required for a full understanding of physiological and pathological processes.
The conformation of a protein structure, as a whole or in parts of it, can be significantly influenced by glycosylation. Thus, it is not surprising that the glycosylated and glycan-free KLKs exhibit slightly different enzyme kinetic parameters, such as a roughly doubled kcat of glycosylated KLK4lx and KLK11lx compared with the values of the glycan-free counterparts (Figure 4A and Table 1). Depending on the higher kcat, the catalytic efficiency was enhanced, probably based on a conformational optimization of the active site, as it was observed for the cleavage of large protein substrates by KLK2lx compared with KLK2e (Guo et al., 2016). Even if a glycan does not directly influence the active site as in KLK2lx, more distant, functionally connected amino acid positions could explain the enzyme kinetic differences according to the concept of protein sectors (Halabi et al., 2009). The N-glycosylated Asn159 of KLK4lx and KLK11lx would be positioned right between the structural core sector, Cys157 and the S1 pocket surrounding sector at residue 161, if the analogy to rat trypsin is valid. Seemingly, the strong difference of Zn2+ inhibition with respect to the residual activity depends on the single glycan linked to Asn159 KLK4lx. Interestingly, a residual activity of 25% was observed for KLK4e even at 1 mm Zn2+, whereby the Zn2+-binding site was unambiguously identified at His25 and Glu77 (Debela et al., 2006a). However, these samples were pre-treated with mm Ca2+, in order to maintain a monomeric state of KLK4, which tends to oligomerize and lose activity. Glycosylation at Asn159 could prevent the formation of higher KLK4 oligomers, but it is compatible with tetramers and octamers (PDB code 2BDI).
The degradation of the natural substrate fibronectin by glycosylated KLK3S/T and KLK4T/lx shows no significant differences (Figure 5A). Only a comparison with glycan-free KLKs 3 and 4 might demonstrate effects of glycosylation on activity, which seems rather unlikely, as the glycans of both KLKs are located distant to the active site cleft (Figures 1 and 2B). The tryptic KLK11lx hardly cleaved fibronectin, which confirmed an earlier study on KLK11 from seminal plasma (Luo et al., 2006). This finding can be explained by the specific three-dimensional architecture of substrate and protease, e.g. by altered exosite interactions or the influence of glycans. Remarkably, the ternary complex of the serpin antithrombin with thrombin and a bridging heparin corroborates that N-glycans of both protein molecules can serve as additional determinants of the heparin binding mode (Li et al., 2004). Noteworthy, KLK3 binds heparin as well and Asn61 is located similar to Asn60G of thrombin, which agrees with a similar role in the heparin enhanced complex formation of KLK3 with PCI (Christensson and Lilja, 1994).
Regardless of potential functional roles, a major structural and concomitantly regulatory role of the glycans is long-term stabilization, most likely by protection of sensitive cleavage sites (Figure 6A and B). This concept was confirmed for glycosylated KLK2lx and is further supported by the higher fractions of active glycosylated KLK3S, KLK4lx and KLK11lx, which are around 50% according to active site titrations, compared to their glycan-free counterparts, which are only active around 10% on average (Guo et al., 2016). An additional role in regulation of enzyme activity for the N-linked Glycan at Asn95 of KLK2 was described by Guo et al. (2016). Apart from the physiological roles of glycosylation, aberrant glycosylation of prostatic KLKs in cancer deserves future investigations, which might improve the detection of aggressive forms by glycan analysis of KLK3 and could be supported by corresponding analyses of the three other prostatic KLKs. In addition, the glycosylation patterns of all KLKs expressed under pathological conditions could help to target them in diagnostic and therapeutic approaches.
Materials and methods
Cloning and expression of glycosylated prostatic KLKs
Similar cloning and expression procedures for glycosylated KLK2lx have been published previously (Guo et al., 2016). All constructs for native-like proforms and mature KLKs 3, 4 and 11 require artificial propeptides for purification with His-tags and prevention of autodegradation. For mature KLKs, the EK recognition sequence His6-GS-DDDDK- was introduced, preceding Ile16 of the mature proteases. For pro KLKs 3 and 4 TEV protease cleavage sites His6-GS-ENLYFQG-A/S were used, while the AEP cleavage site was His6-GS-N- for KLK11 (Dando et al., 1999; Kapust et al., 2002). Encoding DNA fragments were amplified by polymerase chain reaction (PCR) using appropriate primers with XbaI and NotI for N-terminal His-tags or KpnI restriction sites for C-terminal His-tags, respectively. PCR products were inserted into the pLEXSY-sat2 vector (Jena Bioscience, Jena, Germany) following the established protocols (Breitling et al., 2002; Guo et al., 2016).
For transient secretory expression in HEK293 cells, encoding DNA fragments of KLKs were amplified by PCR and inserted into the mammalian expression vector pHLsec (Aricescu et al., 2006) at AgeI and XhoI restriction sites for N-terminal His-tags or AgeI and KpnI for C-terminal His-tag fusion proteins. HEK293 cells were grown in a humidified 37°C incubator with 5% CO2 in Dulbecco’s Modified Eagle’s Medium (DMEM) containing 2 mm L-glutamine, non-essential amino-acids (NEAA) and 10% fetal bovine serum (FBS, Invitrogen, Karlsruhe, Germany) in T-175 tissue culture flasks (Greiner, Frickenhausen, Germany). Adherent HEK293 cells were detached with 80% confluent using 1× trypsin/EDTA followed by PBS wash, and then passaged at a ratio of 1:10–1:20 for general maintenance. For expression, the HEK293 cells were cultivated to confluence 80% and transfected with cDNA encoding prostatic KLKs with polyethylenimine (PEI, Sigma, Munich, Germany) at a weight ratio for DNA:PEI of 1:1.5. Transfected cells were cultivated in DMEM medium containing 2% FBS, L-glutamine and NEAA. Secreted protein was detected by immunoblots of conditioned media with a penta-His antibody. Large-scale cultures for protein expression were performed in an expanded-surface polystyrene roller bottle (2280 cm2, Greiner Bio-One, Frickenhausen, Germany). When the media color change from pink-red to yellow, the supernatants were collected and refilled with new condition media, repeated twice to maximize the production of secreted prostatic KLKs.
Protein purification including generation of correct N-termini
Secreted His-tagged KLKs were purified from the supernatant medium after centrifugation of the cells (4500 rpm, 30 min) with incubation on Ni-NTA resin (Qiagen Inc., Valencia, CA, USA) at 4°C for 4 h or overnight in case of LEXSY cells. In the case of HEK293 cells, this was preceded by filtration through 0.45 μm polyether sulfone (PES) membranes and dialysis against 1000 times excess of 100 mm NaCl, 5% glycerol, 20 mm Tris/HCl, pH 8.0, in order to remove the 66 mm Ca2+ from the culture media, which would reduce the IMAC efficiency. Proforms of KLKs were eluted from Ni-NTA resin with 50 mm imidazole in the same buffer.
Mature KLK constructs with the artificial propeptides DDDDK were treated with C-terminally His-tagged EK, generating the correct N-terminus for activation pocket of trypsin-like serine proteases (Skala et al., 2013). Recombinant KLKs were activated by EK at molar ratios of 50:1–1000:1 at room temperature overnight. Cleavage of the pro-KLK11 construct at the AEP recognition site His6-GS-N-ETR-I16 was performed at pH 4.5–5.5 at a ratio of 50:1. The more specific TEV protease cleaves at small, unbranched P1′ residues as in the KLK3 and KLK4 constructs His6-GS-ENLYFQG-APLILSR-I16 or -SCSQ-I16 (Kapust et al., 2002). Instead of DTT, which could reduce disulfides, a glutathione redox buffer was used, which maintains reducing conditions for TEV protease. The reactions were performed with a 5:1 ratio of KLK fusion proteins to TEV at room temperature in 50 mm Tris/HCl (pH 8.0), 0.5 mm EDTA, 3 mm reduced glutathione and 0.3 mm oxidized glutathione.
Uncleaved fusion proteins with N-terminal His-tags, as well as the artificial propeptides with His-tags and C-terminally His-tagged EK were removed by chromatography on Ni-NTA columns (GE Healthcare, Munich, Germany). Imidazole was removed by dialysis against 50 mm Tris/HCl (pH 8.0), 500 mm NaCl in 6–8 kDa cutoff membranes (Spectra/Por, Spectrum Labs, Rancho Dominguez, CA, USA). The His-tagged molecules were removed by Ni-NTA columns from target proteins. Additional purification of KLKs 4 and 11 with trypsin-like specificity, was achieved by benzamidine sepharose columns in 50 mm Tris/HCl (pH 8.0), 500 mm NaCl as loading and wash buffer. KLKs were eluted in this buffer plus 100 mm benzamidine or 20 mmp-amino-benzamidine or 0.5 m arginine (pH 3.0). For size exclusion chromatography (SEC), samples were concentrated to 500 μl with 10 kDa cutoff centrifugal filters (Amicon, Millipore, Billerica, MA, USA). Usually, SEC on Superdex 75 10/300 GL columns (GE Healthcare, Munich, Germany) in 100 mm NaCl, 5% glycerol, 20 mm Tris/HCl (pH 8.0), resulted in >95% pure and homogeneous samples.
The periodic acid-Schiff (PAS) reaction was used for the detection of glycosylated proteins, which was successful for KLK2lx (Guo et al., 2016). The sensitivity is 25~100 ng glycoprotein, while the reagent GlycoProfile III and PVDF membranes increased the detection limits. Recombinant KLKs were separated by 15% SDS-PAGE and transferred to nitrocellulose membranes in semidry Western blots, followed by 5 min wash and incubation in 1% periodic acid/3% acetic acid 15 min in the dark. After washing with H2O (5 min, three times), the membrane was developed with the Schiff reagent, resulting in magenta staining after 3–10 min. Rinsing with water (5 min) and air-drying fixed the membranes permanently.
Enzymatic deglycosylation under denaturing conditions was performed following the instructions of the enzyme producer New England Biolabs (Frankfurt a. M., Germany). Ten micrograms of prostatic KLK sample were treated with 500 units of PNGase F in 50 mm sodium phosphate, pH 7.5 or with 1000 units of Endo Hf in 50 mm sodium acetate pH 6.0 at 37°C for 1 h. Reactions were terminated with sample buffer and products were monitored by SDS-PAGE.
For mass spectrometry, 50 μg purified pro-KLK3/4/11 from LEXSY or HEK293S/T cells stored in a freezer before the test. The thawed solution was digested either by trypsin or by chymotrypsin and the proteolytic fragments were sequenced by means of tandem mass spectrometry with a Q-Exactive LC-MS/MS system (Thermo Scientific, Dreieich, Germany). Data analysis was performed with PEAKS Studio version 7.0 (Bioinformatics Solutions, Waterloo, ON, Canada), a de novo sequencing program that also considers mass shifts caused by amino acid modifications. This makes it possible to deduce modification sites from peptide fragment spectra in a reliable manner.
Pro-KLK fusion protein hydrolysis and enzymatic assays
The ability of prostate-specific KLKs to convert individual pro-KLKs into enzymatic active species was assessed by incubation of 10 μg pro-KLK fusion protein with 0.1 μg KLK protease in 150 mm NaCl, 10 mm Tris/HCl (pH 8.0) at 37°C for 24 h. The reactions were stopped on ice and samples prepared for SDS-PAGE and activity assays. The activation of pro-KLKs by KLK proteases was evaluated with 1 mm fluorogenic H-PFR-AMC, which was measured in an Infinite M200 microplate reader (Tecan, Männedorf, Switzerland) with wavelengths of 380 nm (excitation) and 460 nm (emission). The fluorescence intensity generated by activated pro-KLKs was compared to control samples of pro-KLKs without activating protease, protease alone and buffer. Relative turnover velocities were calculated with the highest velocity defined as 100%, considering 10% as suitable cutoff for activation.
All para-nitroanilide (pNA) and 7-amino-4-methylcoumarin (AMC) substrates and the inhibitors D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone (PPACK) and carboxy-benzyl-L-leucyl-L-tyrosine chloromethyl ketone (Z-LY-cmk) were purchased from Bachem (Weil am Rhein, Germany) and stock solutions were prepared in DMSO. For enzymatic assays in 100 μl samples with 50 mm Tris/HCl (pH 7.5), 100 mm NaCl at 37°C with 0.1% (w/v) BSA were used, which was replaced by 0.005% Tween-20 for enzyme kinetic measurements in 50 μl samples. Protein concentrations were measured with a NanoDrop 2000c spectrophotometer (Thermo Scientific, Dreieich, Germany) at 280 nm, using computed extinction coefficients. Released pNA was recorded photometrically at 405 nm on the Infinite M200 microplate reader. Standard curves of pNA and AMC (Sigma, Munich, Germany) were measured under identical conditions as the assays and used in calculating the rate of product formation, i.e. the turnover velocity.
The initial velocity of turnover was measured in triplicates in the range of 20 μm–5 mm substrate with an enzyme concentration of 500 nm. Data were fitted to the Michaelis-Menten equation by non-linear regression analysis to obtain Vmax and KM, performed with the ORIGIN software (Origin Lab, Northampton, MA, USA). The fraction of active protease was determined with suitable chromogenic substrates by active site titration after 30 min incubation with the irreversible inhibitor PPACK for tryptic KLKs, while KLK3 was titrated with Z-LY-cmk (Myles et al., 2001; Skala et al., 2014; Guo et al., 2016). After calculation of kcat values, the standard deviation for kcat/KM was calculated according to Fenner (Fenner, 1931). Zn2+ inhibition curves were measured in assay buffer (pH 7.0) with 0.005% Tween-20.
Interaction with protein substrates and inhibitors
For fibronectin degradation, 50 μg human serum fibronectin (Innovative research, Novi, MI, USA) was incubated with 0.5 μg tryptic KLKs or 2.5 μg KLK3 for 0, 1, 2, 4 and 24 h at 37°C in 50 mm Tris/HCI, pH 8.0, containing 0.1% NP40, 150 mm NaCl. An equal amount of fibronectin or protease alone was monitored under identical conditions as a control. The covalent bond formation of KLKs with serpins or A2M was evaluated by SDS-gels. Six micrograms A2M were incubated with 1.3 μg KLK at 37°C for 1 h in 150 mm NaCl, 20 mm Tris/HCl (pH 8.0). Similarly, the interaction of protease with 5.7 μg PCI or 4.5 μg anti-chymotrypsin was tested under the same condition, with a molar ratio of inhibitor to protease of 2:1. An equal amount of inhibitor or protease alone was monitored as control.
Funding source: Austrian Science Fund
Award Identifier / Grant number: I631-B11
Award Identifier / Grant number: W_01213
Funding statement: We are thankful to Sabine Creutzburg for molecular biological work and Reinhard Mentele for N-terminal sequencing. In addition, we would like to thank Wolfgang Skala for providing KLKs from E. coli expression. This study was supported by the Austrian Science Fund (FWF) with D-A-CH project, Funder Id: 10.13039/501100002428, I631-B11 and W_01213 (S.G.) as well as with project P25003-B21 (P.G.). The German Research Foundation (DFG) contributed with financial support from Funder Id: 10.13039/501100001659, grant MA 1236-10-1 (V.M.).
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