Protein C inhibitor (PCI) and heparin cofactor II (HCII): possible alternative roles of these heparin-binding serpins outside the hemostatic system

Protein C inhibitor regulates both cathepsin L activity and cell-mediated tumor cell migration

BACKGROUND

Protein C inhibitor (PCI) is a plasma serine protease inhibitor (serpin) that regulates several serine proteases in coagulation including thrombin and activated protein C. However, the physiological role of PCI remains under investigation. The cysteine protease, cathepsin L, has a role in many physiological processes including cardiovascular diseases, blood vessel remodeling, and cancer.

METHODS AND RESULTS

We found that PCI inhibits cathepsin L with an inhibition rate (k(2)) of 3.0x10(5)M(-)(1)s(-)(1). Whereas, the PCI P1 mutant (R354A) inhibits cathepsin L at rates similar to wild-type PCI, mutating the P2 residue results in a slight decrease in the rate of inhibition. We then assessed the effect of PCI and cathepsin L on the migration of human breast cancer (MDA-MB-231) cells. Cathepsin L was expressed in both the cell lysates and conditioned media of MDA-MB-231 cells. Wound-induced and transwell migration of MDA-MB-231 cells was inhibited by exogenously administered wtPCI and PCI P1 but not PCI P14 mutant. In addition, migration of MDA-MB-231 cells expressing wtPCI was significantly decreased compared to non-expressing MDA-MB-231 cells or MDA-MB-231 cells expressing the PCI P14 mutant. Downregulation of cathepsin L by either a specific cathepsin L inhibitor or siRNA technology also resulted in a decrease in the migration of MDA-MB-231 cells.

CONCLUSIONS

Overall, our data show that PCI regulates tumor cell migration partly by inhibiting cathepsin L.

GENERAL SIGNIFICANCE

Consequently, inhibiting cathepsin L by serpins like PCI may be a new pathway of regulating hemostasis, cardiovascular and metastatic diseases.

Immunolocalization of protein C inhibitor in differentiation of human epidermal keratinocytes

Keratinocytes propagated in low calcium (0.05 mM) serum-free medium grow as monolayers and exhibit morphological and biosynthetic phenotypes similar to the keratinocytes of the basal layer in normal epidermis. When the calcium in the medium is increased to 1.5 mM, the keratinocytes start to stratify and differentiate. Such differentiation is important in the formation of an epidermal barrier. Proteolysis plays a crucial role in the process. The functions of most of the plasminogen activator cascade components in human skin have been studied, but little was known about the expression and role of protein C inhibitor in the differentiation of human epidermal keratinocytes. In the present study, we used immunohistochemistry and immunocytochemistry to examine the immunolocalization of protein C inhibitor in normal human skin and in cultured keratinocytes in serum-free medium with low and high calcium, respectively. The results indicated that protein C inhibitor is mainly localized in superficial and more differentiated keratinocytes in normal human epidermis. Keratinocytes positive for protein C inhibitor were detected in cultures containing both low and high calcium media, and the level of protein C inhibitor was increased in high calcium medium. This increase was accompanied by an altered intracellular distribution, from the perinuclear cytoplasm in undifferentiated keratinocytes to the whole cytoplasm in differentiated keratinocytes. Further study revealed that protein C inhibitor was incorporated into the cornified envelope in normal skin keratinocytes and cultured differentiated keratinocytes. Our results suggest that protein C inhibitor may be involved in the differentiation of keratinocytes.

Essential thrombin residues for inhibition by protein C inhibitor with the cofactors heparin and thrombomodulin

BACKGROUND

Protein C inhibitor (PCI) and antithrombin (AT) are serine protease inhibitors (serpins) that inhibit a wide array of blood coagulation serine proteases including thrombin.

OBJECTIVE

Fifty-five Ala-scanned recombinant thrombin mutants were used to determine thrombin residues important for inhibition by PCI with and without the cofactors heparin and thrombomodulin (TM) and compared with the prototypical serpin, AT.

RESULTS

Residues around the active site (Tyr50 and Glu202) and the sodium-binding site (Glu229 and Arg233) were required for thrombin inhibition by PCI with and without cofactors. Exosite-2 residues (Arg89, Arg93, Glu94, Arg98, Arg245, Arg248, and Gln251) were critical for heparin-accelerated inhibition of thrombin by PCI. Exosite-1 residues (especially Lys65 and Tyr71) were required for enhanced PCI inhibition of thrombin-TM. Interestingly, we also found that the TM chondroitin sulfate moiety is not required for the approximately 150-fold enhanced rate of thrombin inhibition by PCI. Using the aforementioned thrombin exosite-2 mutants that were essential for heparin-catalyzed PCI-thrombin inhibition reactions we found no change in PCI inhibition rates for thrombin-TM.

CONCLUSIONS

Collectively, these results show that (i) similar thrombin exosite-2 residues are critical for the heparin-catalyzed inhibition by PCI and AT, (ii) PCI and AT are different in their thrombin-TM inhibition properties, and (iii) PCI has a distinct advantage over AT in the regulation of the activity of thrombin-TM.

Structure of native protein C inhibitor provides insight into its multiple functions

Protein C inhibitor (PCI) is a multifunctional serpin with wide ranging protease inhibitory functions, unique cofactor binding activities, and potential non-inhibitory functions akin to the hormone-transporting serpins. To gain insight into the molecular mechanisms utilized by PCI we developed a robust expression system in Escherichia coli and solved the crystal structure of PCI in its native state. The five monomers obtained from our two crystal forms provide an NMR-like ensemble revealing regions of inherent flexibility. The reactive center loop (RCL) of PCI is long and highly flexible with no evidence of hinge region incorporation into beta-sheet A, as seen for other heparin-binding serpins. We adapted an extrinsic fluorescence method for determining dissociation constants for heparin and find that the N-terminal tail of PCI and residues adjacent to helix H are not involved in heparin binding. The minimal heparin length capable of tight binding to PCI was determined to be chains of eight monosaccharide units. A large hydrophobic pocket occupied by hydrophobic crystal contacts was found in an analogous position to the hormone-binding site in thyroxine-binding globulin. In conclusion, the data presented here provide important insights into the mechanisms by which PCI exercises its multiple inhibitory and non-inhibitory functions.

Characterization of recombinant human protein C inhibitor expressed in Escherichia coli

The serine protease inhibitor (serpin) protein C inhibitor (PCI; also named plasminogen activator inhibitor-3) regulates serine proteases in hemostasis, fibrinolysis, and reproduction. The biochemical activity of PCI is not fully defined partly due to the lack of a convenient expression system for active rPCI. Using pET-15b plasmid, Ni(2+)-chelate and heparin-Sepharose affinity chromatography steps, we describe here the expression, purification and characterization of wild-type recombinant (wt-rPCI) and two inactive mutants, R354A (P1 residue) and T341R (P14 residue), expressed in Escherichia coli. Wild-type rPCI, but not the two mutants, formed a stable bimolecular complex with thrombin, activated protein C and urokinase. In the absence of heparin, wt-rPCI-thrombin, -activated protein C, and -urokinase inhibition rates were 56.7, 3.4, and 2.3 x 10(4) M(-1) min(-1), respectively, and the inhibition rates were accelerated 25-, 71-, and 265-fold in the presence of 10 mug/mL heparin for each respective inhibition reaction. The stoichiometry of inhibition (SI) for wt-rPCI-thrombin was 2.0, which is comparable to plasma-derived PCI. The present report describes for the first time the expression and characterization of recombinant PCI in a bacterial expression system and demonstrates the feasibility of using this system to obtain adequate amounts of biologically active rPCI for future structure-function studies.

Expression of protein C inhibitor (PCI) in benign and malignant prostatic tissues

BACKGROUND

Protein C inhibitor (PCI) occurs at high concentration in seminal plasma, and inhibits human glandular kallikrein-2 and, less readily, prostate-specific antigen. Previous studies have localized PCI in the male genital tract. Here we have performed a detailed investigation of PCI expression in the prostatic tissues, metastases, and cell lines.

METHODS

Immunohistochemistry, in situ hybridization, and Western blotting were used to study prostatic tissues, metastases, and PC-3, DU-145, and LNCaP cells.

RESULTS

PCI was immunolocalized in tissue microarray spots with BPH epithelium (detection rate 100%), PIN lesions (100%), tumors (96%), metastases (88%), and in all cell lines. ISH and WB supported the findings.

CONCLUSIONS

PCI is widely expressed in benign prostatic epithelium, and may act as a local regulator of enzymatic activity in seminal fluid, of importance for normal sperm function. Lack of PCI expression in a subpopulation of high-grade tumor cells in combination with maintained protease expression may facilitate invasive growth patterns.

Protein C inhibitor is expressed in keratinocytes of human skin

Protein C inhibitor is a member of the serpin family that inhibits a variety of serine proteases. Protein C inhibitor is present in numerous body fluids and is produced in the liver and by various epithelial cells. To determine if this epithelial serpin is present in skin, immunohistochemical studies were performed that showed strong staining for protein C inhibitor antigen in the epidermis. Protein C inhibitor mRNA was detected in the keratinocyte cell line HaCaT and the epidermoid carcinoma cell line A431 using reverse transcription-polymerase chain reaction suggesting that also in normal skin protein C inhibitor is derived from keratinocytes. Conditioned media from these cell lines were analyzed on immunoblots, which revealed a protein C inhibitor-antigen band that comigrated with protein C inhibitor derived from the hepatoma cell line HepG2. Using an enzyme-linked immunosorbent assay specific for total protein C inhibitor antigen the accumulation of protein C inhibitor in the cell culture supernatants of HaCaT keratinocytes was found to be 0.3 ng per h per 1 million cells. This is similar to the amount of plasminogen activator inhibitor-1 produced by these cells, which also produce tissue plasminogen activator and urokinase. Fluorescence-activated cell sorter analysis revealed similar expression of intracellular protein C inhibitor antigen in proliferating and confluent HaCaT cells. These findings demonstrate that protein C inhibitor antigen is present in the normal epidermis and that protein C inhibitor is constitutively expressed by keratinocytes in culture. Therefore, protein C inhibitor may provide protease inhibitory activity not only to internal, but also to the external surface of the body. Additionally, protein C inhibitor could contribute to the regulation of retinoid supply in the epidermis, as we have shown recently that retinoic acid binds specifically to protein C inhibitor.