Examination of the potential structure of human salivary cystatins based on computer modelling

Elevated expression of CST1 promotes breast cancer progression and predicts a poor prognosis

Cystatin SN (CST1) belongs to the type 2 cystatin (CST) superfamily, which restricts the proteolytic activities of cysteine proteases. CST1 has been recently considered to be involved in the development of several human cancers. However, the prognostic significance and function of CST1 in breast cancer remains unknown. In the current study, we found that CST1 was generally upregulated in breast cancer at both mRNA and protein level. Furthermore, overall survival (OS) and disease-free survival (DFS) in the low CST1 expression subgroup were significantly superior to the high CST1 expression subgroup (OS, p < 0.001; DFS, p < 0.001), which indicated that CST1 expression level was closely correlated to the survival risk of these patients. Univariate and multivariate analyses demonstrated that CST1 expression was an independent prognostic factor, the same as ER status and nodal status. Next, CST1 overexpression promoted breast cancer cell proliferation, clonogenicity, migration, and invasion abilities. By contrast, knockdown of CST1 attenuated these malignant characteristics in breast cancer cells. Collectively, our study indicates that CST1 cannot only serve as a significant prognostic indicator but also as a potential therapeutic target for breast cancer.

KEY MESSAGES

High CST1 expression is negatively correlated with survival of breast cancer patients. CST1 promotes cell proliferation, clone formation, and metastasis in breast cancer cells. CST1 is a novel potential prognostic biomarker and therapeutic target for breast cancer.

Influence of substratum hydrophobicity on salivary pellicles: organization or composition?

Different physico-chemical properties (eg adsorption kinetics, thickness, viscoelasticity, and mechanical stability) of adsorbed salivary pellicles depend on different factors, including the properties (eg charge, roughness, wettability, and surface chemistry) of the substratum. Whether these differences in the physico-chemical properties are a result of differences in the composition or in the organization of the pellicles is not known. In this work, the influence of substratum wettability on the composition of the pellicle was studied. For this purpose, pellicles eluted from substrata of different but well-characterized wettabilities were examined by means of sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). The results showed that substratum hydrophobicity did not have a major impact on pellicle composition. In all substrata, the major pellicle components were found to be cystatins, amylases and large glycoproteins, presumably mucins. In turn, interpretation of previously reported data based on the present results suggests that variations in substratum wettability mostly affect the organization of the pellicle components.

Cystatin SN neutralizes the inhibitory effect of cystatin C on cathepsin B activity

Cystatin SN (CST1) is one of the several salivary cystatins that form tight equimolar complexes with cysteine proteases, such as the cathepsins. High expression of CST1 is correlated with advanced pTNM stage in gastric cancer. However, the functional role of CST1 in tumorigenesis has not been elucidated. In this study, we showed that CST1 was highly expressed in colon tumor tissues, compared with nontumor regions. Increased cell proliferation and invasiveness were observed in HCT116 cell lines stably transfected with CST1 cDNA (HCT116-CST1) but not in CST3-transfected cells. We also demonstrated that CST1-overexpressing cell lines exhibited increased tumor growth as well as metastasis in a xenograft nude mouse model. Interestingly, CST1 interacted with cystatin C (CST3), a potent cathepsin B (CTSB) inhibitor, with a higher affinity than the interaction between CST3 and CTSB in the extracellular space of HCT116 cells. CTSB-mediated cellular invasiveness and proteolytic activities were strongly inhibited by CST3, but in the presence of CST1 CTSB activities recovered significantly. Furthermore, domain mapping of CST1 showed that the disulfide-bonded conformation, or conserved folding, of CST1 is important for its secretion and for the neutralization of CST3 activity. These results suggest that CST1 upregulation might be involved in colorectal tumorigenesis and acts by neutralizing the inhibition of CTSB proteolytic activity by CST3.

Homology Model and Molecular Dynamics Simulation of Carp Ovum Cystatin

In this study, a homology model of carp ovum cystatin was constructed based on the crystal structure of chicken egg white cystatin. The results of amino acid sequence alignment indicate that these two proteins exhibit 36.11% of sequence identity. The resultant homology model reveals that carp ovum cystatin shares similar folds as chicken egg white cystatin, particularly in the conserved regions of Q48-V49-G52 and P98-W99 and the locations of two disulfide bonds, C67-C76 and C90-C110. However, the results of 1 ns molecular dynamics simulations show that carp ovum cystatin exhibits less structural integrity than chicken egg white cystatin in explicit water at 300 K. The relatively hydrophilic Met62 of carp ovum cystatin, corresponding to the hydrophobic Leu68 of human cystatin C and Ile66 of chicken egg white cystatin, may destabilize the hydrophobic core and form a dimeric structure more easily through domain swapping. A total of 16 positively charged residues are equally distributed on the surface of carp ovum cystatin, resulting in agglutination with the negatively charged spermatozoa via electrostatic interaction. Thus, carp ovum cystatin is considered to be important in preventing carp eggs from polyspermy.

Extracellular production of human cystatin S and cystatin SA by Bacillus subtilis

We herein describe the development of a Bacillus subtilis system that can be used to produce large quantities of recombinant (r-) human salivary cystatins, a cysteine protease inhibitor of family 2 in the cystatin superfamily. The B. subtilis that lacked the alkaline protease E gene (DeltaaprE type mutant strain) was prepared by homologous recombination. The cDNA fragments coding for mature cystatins (S and SA) were ligated in frame to the DNA segment for the signal peptide of endoglucanase in the pHSP-US plasmid vector that was then use to transform the DeltaaprE type mutant strain of B. subtilis. The transformants carrying the expression vectors were cultivated in 5-L jar fermenters for 3 days at 30 degrees C. Both r-cystatin S and r-cystatin SA were successfully expressed and secreted into the culture broth, and were purified using a fast performance liquid chromatography system. The first use of DeltaaprE type mutant strain of B. subtilis made it possible to obtain a high yield of secreted protein, which makes this system an improvement over expression in Escherichia coli. We conclude that this system has high utility for expression of commercial quantities of secreted proteins.

Towards novel anti-cancer strategies based on cystatin function

Cystatins have recently emerged as important players in a multitude of physiological and patho-physiological settings that range from cell survival and proliferation, to differentiation, cell signaling and immunomodulation. This group of cysteine protease inhibitors forms a large super-family of proteins composed of one, two, three, and, in some species, more than three cystatin domains. Over the last 20 years or so, members of the cystatin super-family have been primarily explored with respect to their capacity to inhibit intracellular cysteine proteases. Yet, this classical mode of action does not fully explain their remarkably diverse biological functions. Due to the space limitations, the author will discuss here the most recent findings that suggest that some of the single-domain, cytoplasmic and cell-secreted cystatins may play important roles in the promotion or suppression of tumor growth, invasion and metastasis. Based on the present understanding of cystatin function, novel avenues for anti-cancer strategies are proposed.

Salivary (SD-type) cystatins: over one billion years in the making--but to what purpose?

Human saliva contains relatively abundant proteins that are related ancestrally in sequence to the cystatin superfamily. Most, although not all, members of this superfamily are potent inhibitors of cysteine peptidases. Four related genes have been identified, CST1, 2, 4 and 5, encoding cystatins SN, SA, S, and D, respectively. CST1, 4, and probably CST5 are now known to be expressed in a limited number of other tissues in the body, primarily in exocrine epithelia, and the term SD-type cystatin is more appropriate than 'salivary cystatin'. These genes are co-ordinately regulated in the submandibular gland during post-natal development. The organization of these tissue-specifically-expressed genes in the genome, and their phylogeny, indicate that they evolved from an ancestral housekeeping gene encoding the ubiquitously expressed cystatin C, and are members of a larger protein family. Their relationship to rat cystatin S, a developmentally regulated rodent submandibular gland protein, remains to be established. In this review, the evolution of the SD-type cystatins in the cystatin superfamily, their genomics, expression, and structure-function relationships are examined and compared with known cystatin functions, with the goal of providing clues to their biological roles.

Purification of large quantities of human salivary cystatins S, SA and SN: their interactions with the model cysteine protease papain in a non-inhibitory mode

OBJECTIVES

Our aim was to purify large quantities of human salivary cystatins S, SA and SN in order to determine whether these salivary cystatins have a stable interaction with cysteine proteases at a second binding site, other than the protease active site. This property may affect their availability to act as cysteine protease inhibitors within the oral environment.

METHODS

Salivary cystatins S, SA and SN were purified from human submandibular sublingual saliva to homo- geneity by column chromatography. Formation of stable complexes between the model cysteine protease papain in the absence of reductant was assessed by SDS-PAGE and probing Western blots with antibody to human salivary cystatin SN. Proteolytic activity of the complex was determined in the gel after electrophoresis.

RESULTS AND CONCLUSIONS

Only cystatin SN (14.3 kD) was found to form a stable complex with papain (22 kD) that could be separated by SDS-PAGE producing a Coomassie stained band at (37 kD). After western transfer this same band (37 kD) cross-reacted with antibody to SN. In the presence of E64, an active site inhibitor of cysteine proteases, the same complex was formed, suggesting that SN is able to bind to papain at a site other than the active site. Activity staining of the gel confirmed that this complex (-E64) retained proteolytic activity. Such complex formation between cystatin SN and cysteine proteases in a non-inhibitory mode may reduce its availability to act as an effective cysteine protease inhibitor in the oral environment.