Genomic structure and chromosomal localization of the human hepatocyte growth factor activator inhibitor type 1 and 2 genes

Possible role of combined therapy targeting MET and pro-HGF activation for renal cell carcinoma: analysis by human HGF-producing SCID mice

MET is a high-affinity receptor tyrosine kinase of HGF (hepatocyte growth factor). HGF is secreted as an inactive single-chain precursor (pro-HGF), which requires proteolytic activation for conversion to an active form. HGF activator inhibitor (HAI)-2 is a transmembrane Kunitz-type serine protease inhibitor, which inhibits all pro-HGF-activating enzymes. In RCC, increased expression of MET and decreased expression of HAI-2 were reported to be poor prognostic factors. In the current study, we tried to inhibit the growth of RCC cells by dual inhibition of both MET phosphorylation and pro-HGF-activation using MET inhibitor and HAI-2 overexpression. A transgenic mouse model which expressed human HGF (HGF mouse) was used for in vivo analysis to evaluate the HGF/MET signaling axis accurately. Initially, doxycycline-induced HAI-2 overexpression RCC cells (786-O-HAI2) were prepared. The cells were cultured with pro-HGF, and inhibitory effect of MET inhibitor (SCC244) and HAI-2 was evaluated by phosphorylation of MET and cell proliferation. Next, the cells were subcutaneously implanted to HGF mice and the growth inhibition was determined by SCC244 and HAI-2. Single use of each inhibitor showed significant inhibition in MET phosphorylation, migration and proliferation of 786-O-HAI2 cells; however, the strongest effect was observed by combined use of both inhibitors. Although in vivo analysis also showed apparent downregulation of MET phosphorylation and growth inhibition in combined treatment, statistical significance was not observed compared with single use of MET inhibitor. Combined treatment with MET-TKI and HAI-2 suggested to consider as a candidate for new strong therapy for RCC.

Elevated Circulating and Placental SPINT2 Is Associated with Placental Dysfunction

Biomarkers for placental dysfunction are currently lacking. We recently identified SPINT1 as a novel biomarker; SPINT2 is a functionally related placental protease inhibitor. This study aimed to characterise SPINT2 expression in placental insufficiency. Circulating SPINT2 was assessed in three prospective cohorts, collected at the following: (1) term delivery (n = 227), (2) 36 weeks (n = 364), and (3) 24-34 weeks' (n = 294) gestation. SPINT2 was also measured in the plasma and placentas of women with established placental disease at preterm (<34 weeks) delivery. Using first-trimester human trophoblast stem cells, SPINT2 expression was assessed in hypoxia/normoxia (1% vs. 8% O2), and following inflammatory cytokine treatment (TNFα, IL-6). Placental SPINT2 mRNA was measured in a rat model of late-gestational foetal growth restriction. At 36 weeks, circulating SPINT2 was elevated in patients who later developed preeclampsia (p = 0.028; median = 2233 pg/mL vs. controls, median = 1644 pg/mL), or delivered a small-for-gestational-age infant (p = 0.002; median = 2109 pg/mL vs. controls, median = 1614 pg/mL). SPINT2 was elevated in the placentas of patients who required delivery for preterm preeclampsia (p = 0.025). Though inflammatory cytokines had no effect, hypoxia increased SPINT2 in cytotrophoblast stem cells, and its expression was elevated in the placental labyrinth of growth-restricted rats. These findings suggest elevated SPINT2 is associated with placental insufficiency.

Membrane-bound serine protease inhibitor HAI-1 is required for maintenance of intestinal epithelial integrity

Hepatocyte growth factor activator inhibitor type 1 (HAI-1), encoded by the serine protease inhibitor Kunitz type 1 (SPINT1) gene, is a membrane-bound serine protease inhibitor expressed in epithelial tissues. Mutant mouse models revealed that HAI-1/SPINT1 is essential for placental labyrinth formation and is critically involved in regulating epidermal keratinization through interaction with its cognate cell surface protease, matriptase. HAI-1/SPINT1 is abundantly expressed in both human and mouse intestinal epithelium; therefore, we analyzed its role in intestinal function using mice with intestinal epithelial cell-specific deletion of Spint1 generated by interbreeding mice carrying Spint1(LoxP) homozygous alleles with transgenic mice carrying the Cre recombinase gene controlled by the intestine-specific Villin promoter. Although the resulting mice had normal development and appearance, crypts in the proximal aspect of the colon, including the cecum, exhibited histologic abnormalities and increased apoptosis and epithelial cell turnover accompanied by increased intestinal permeability. Distended endoplasmic reticula were observed ultrastructurally in some crypt epithelial cells, indicative of endoplasmic reticular stress. To study the role of HAI-1/SPINT1 in mucosal injury, we induced colitis by adding dextran sodium sulfate to the drinking water. After dextran sodium sulfate treatment, intestine-specific HAI-1/SPINT1-deficient mice had more severe symptoms and a significantly lower survival rate relative to control mice. These results suggest that HAI-1/SPINT1 plays an important role in maintaining colonic epithelium integrity.

ST14 gene variant and decreased matriptase protein expression predict poor breast cancer survival

BACKGROUND

Matriptase plays a role in carcinogenesis, but the role of its genetic variation or that of the hepatocyte growth factor activator inhibitor-1 (HAI-1) has not been evaluated. This study aimed to examine the genetic variation of matriptase (ST14 gene) and HAI-1 (SPINT1 gene) in breast cancer risk and prognosis, to assess matriptase and HAI-1 gene and protein expression in breast tumors, and to identify their clinicopathologic correlations and prognostic significance.

METHODS

Five single nucleotide polymorphisms in ST14 and three in SPINT1 were genotyped in 470 invasive breast cancer cases and 446 healthy controls. Gene expression analysis was done for 40 breast cancer samples. Protein expression was assessed by immunohistochemical analyses in 377 invasive breast tumors. The statistical significance of the associations among genotypes, clinicopathologic variables, and prognosis was assessed.

RESULTS

The ST14 single nucleotide polymorphism rs704624 independently predicted breast cancer survival, a poor outcome associated with the minor allele (P = 0.001; risk ratio, 2.221; 95% confidence interval, 1.382-3.568). Moreover, ST14 gene expression levels were lower among the minor allele carriers (P = 0.009), and negative/low matriptase protein expression was independently predictive of poorer survival (P = 0.046; risk ratio, 1.554; 95% confidence interval, 1.008-2.396).

CONCLUSIONS

The ST14 variant rs704624 and protein expression of matriptase have prognostic significance in breast cancer. This study adds to the evidence for the role of matriptase in breast cancer and has found new evidence for the genotypes having an impact in breast cancer.

IMPACT

This is the first study showing that genetic variation in matriptase has clinical importance. The results encourage further study on the genetic variation affecting protein levels and function in type II transmembrane serine proteases.

Expression of hepatocyte growth factor activator inhibitor type 1 on the epithelial cell surface is regulated by hypoxic and oxidative stresses

Hepatocyte growth factor activator inhibitor type 1 (HAI-1)/spint-1 is a membrane-bound protease inhibitor that is thought to regulate the activities of hepatocyte growth factor activator, matriptase, hepsin, and prostasin. In this study, we show that the membrane form of HAI-1 was significantly upregulated immunohistochemically in epithelial cells under adverse conditions including tissue injury, necroinflammatory reactions, and invasion of carcinomas. To analyze the mechanism underlying these in vivo observations, we examined the effects of hypoxia and oxidative stress on HAI-1 expression in vitro, using three human cell lines, HLC-1, WiDr, and HeLa. Hypoxic condition significantly enhanced the expression of HAI-1 in these cells. Oxidative stress also enhanced HAI-1 expression. Promoter analyses of the human HAI-1/spint-1 gene revealed overlapping binding site for Egr-1-3 and Sp1 near the transcription start site as the key domain for HAI-1/spint-1 transcription. This site was also critical in both hypoxic- and oxidative stress-induced HAI-1 upregulation. In fact, in vivo immunohistochemical studies indicated that areas with HAI-1 upregulation tended to express markers associated with hypoxia and oxidative stress. These observations suggest that the tissue microenvironment regulates the cell surface expression of HAI-1, and thereby may regulate proteolysis and processing of bioactive molecules on the cellular surface.

Regulation of hepatocyte growth factor activator inhibitor 2 by hypoxia in breast cancer

PURPOSE

To examine the in vitro regulation of hepatocyte growth factor activator inhibitor type 2 (HAI-2) in breast cancer cells and the in vivo predictive role for the efficacy of chemoendocrine primary therapy in patients with breast cancer.

MATERIALS AND METHODS

HAI-2 regulation was studied in a panel of breast cancer cell lines comparing normoxia to hypoxia. The effect of HIF-1alpha RNAi on HAI-2 expression was evaluated in these cells. HAI-2 was examined in breast cancer using in situ hybridization and immunohistochemistry. The HAI-2 predictive role was assessed in T(2-4) N(0-1) breast cancers (n = 177) enrolled in a neoadjuvant randomized trial comparing epirubicin versus epirubicin + tamoxifen.

RESULTS

HAI-2 mRNA and protein were regulated by hypoxia in the c-erbB2-positive cell lines, SKBR3 and BT474, and controlled by HIF-1alpha in these cells. Immunohistochemistry confirmed this profile with high expression of HAI-2 in c-erbB2-positive breast cancer. HAI-2 was correlated with T status (P < 0.004), node involvement (P = 0.01), and c-erbB2 expression (P = 0.05). HAI-2 also correlated with hypoxia markers such as carbonic anhydrase IX expression (P = 0.01) and HIF-1alpha. Additionally, high levels of HAI-2 were a significant predictor for poor clinical complete response to preoperative epirubicin in univariate (P = 0.01) and multivariate analyses (P = 0.016). No correlation with disease-free survival and survival was observed.

CONCLUSION

HAI-2 expression in breast cancer correlated with tumor aggressiveness in vivo. It is a HIF target in c-erbB2-positive cells and it is an independent negative predictive factor of efficacy of anthracycline therapy. The interaction of HAI-2 with the hepatocyte growth factor activation pathway may be a useful site for therapeutic intervention.

Hyposecretion, not hyperabsorption, is the basic defect of cystic fibrosis airway glands

Human airways and glands express the anion channel cystic fibrosis transmembrane conductance regulator, CFTR, and the epithelial Na(+) channel, ENaC. Cystic fibrosis (CF) airway glands fail to secrete mucus in response to vasoactive intestinal peptide or forskolin; the failure was attributed to loss of CFTR-mediated anion and fluid secretion. Alternatively, CF glands might secrete acinar fluid via CFTR-independent pathways, but the exit of mucus from the glands could be blocked by hyperabsorption of fluid in the gland ducts. This could occur because CFTR loss can disinhibit ENaC, and ENaC activity can drive absorption. To test these two hypotheses, we measured single gland mucus secretion optically and applied ENaC inhibitors to determine whether they augmented secretion. Human CF glands were pretreated with benzamil and then stimulated with forskolin in the continued presence of benzamil. Benzamil did not rescue the lack of secretion to forskolin (50 glands, 6 CF subjects) nor did it increase the rate of cholinergically mediated mucus secretion from CF glands. Finally, neither benzamil nor amiloride increased forskolin-stimulated mucus secretion from porcine submucosal glands (75 glands, 7 pigs). One possible explanation for these results is that ENaC within the gland ducts was not active in our experiments. Consistent with that possibility, we discovered that human airway glands express Kunitz-type and non-Kunitz serine protease inhibitors, which might prevent proteolytic activation of ENaC. Our results suggest that CF glands do not display excessive, ENaC-mediated fluid absorption, leaving defective, anion-mediated fluid secretion as the most likely mechanism for defective mucus secretion from CF glands.

Nuclear translocation of H2RSP is impaired in regenerating intestinal epithelial cells of murine colitis model

Hepatocyte growth factor activator inhibitor type 2-related small peptide (H2RSP) is a recently identified nuclear peptide that is abundantly expressed in the gastrointestinal tract. In this study, we analyzed the expression of H2RSP in normal and injured intestinal mucosa in a murine experimental colitis induced by oral administration of 2.5% dextran sodium sulfate. Results of immunohistochemistry and in situ hybridization showed that H2RSP was expressed predominantly in the epithelium of normal intestine. Whereas H2RSP was localized in the cytoplasm of cells in the crypt, it was translocated into the nuclei of the surface epithelial cells. In injured intestine, H2RSP was detected in the cytoplasm of regenerating epithelial cells, and the nuclear translocation was impaired even in the surface epithelium. However, the mRNA level was not significantly altered in these cells by real-time reverse transcription-polymerase chain reaction using total RNAs obtained from the fractionated mucosal tissue samples prepared by laser-captured microdissection technique. On the other hand, H2RSP mRNA was significantly upregulated in the stromal cells of injured intestinal mucosa compared with those in normal mucosa, which shows cytoplasmic localization of H2RSP. These circumstantial evidences suggest that the nuclear translocation of H2RSP may be related to a signaling involved in the transition from cellular proliferation to differentiation.

Regeneration of injured intestinal mucosa is impaired in hepatocyte growth factor activator-deficient mice

BACKGROUND AND AIMS

Hepatocyte growth factor activator (HGFA) is a serum proteinase that specifically converts an inactive single-chain form of hepatocyte growth factor (HGF) into an active 2-chain form. HGFA is produced in its precursor form and then activated in injured tissues. To address the precise role of HGFA and to investigate the mechanisms of HGF activation in injured tissues, we generated mice deficient in HGFA.

METHODS

HGFA-deficient mice were generated using targeted gene disruption. The regenerating process of intestinal mucosa damaged by oral administration of dextran sodium sulfate (DSS) or by rectal administration of acetic acid was examined in both HGFA-deficient and control mice. HGF processing activity was analyzed using Western blotting and an HGF activation assay.

RESULTS

Homozygous mutant mice were viable and fertile without obvious abnormalities. When mice were treated with 3% DSS in drinking water for 6 days followed by distilled water without DSS, 72% of HGFA-deficient mice died through day 12 while 75% of control mice survived injury. Similar results were also observed in the acetic acid-induced intestinal injury; the survival rate was 36.6% in HGFA-deficient mice and 84.2% in control mice. In HGFA-deficient mice, the injured mucosa was not sufficiently covered by regenerated epithelium and the activation of HGF was impaired in the injured colon.

CONCLUSIONS

These results indicate that HGFA is required for repair of injured intestinal mucosa but is not essential for normal development during embryogenesis or after birth.

Tissue expression, protease specificity, and Kunitz domain functions of hepatocyte growth factor activator inhibitor-1B (HAI-1B), a new splice variant of HAI-1

Hepatocyte growth factor activator inhibitor-1 (HAI-1) is an integral membrane protein expressed on epithelial cells and contains two extracellular Kunitz domains (N-terminal KD1 and C-terminal KD2) known to inhibit trypsin-like serine proteases. In tumorigenesis and tissue regeneration, HAI-1 regulates the hepatocyte growth factor (HGF)/c-Met pathway by inhibiting the activity of HGF activator (HGFA) and matriptase, two serine proteases that convert pro-HGF into its biologically active form. By screening a placental cDNA library, we discovered a new splice variant of HAI-1 designated HAI-1B that contains an extra 16 amino acids adjacent to the C terminus of KD1. To investigate possible consequences on Kunitz domain function, a soluble form of HAI-1B (sHAI-1B) comprising the entire extracellular domain was produced. First, we found that sHAI-1B displayed remarkable enzyme specificity by potently inhibiting only HGFA (IC50 = 30.5 nm), matriptase (IC50 = 16.5 nm), and trypsin (IC50 = 2.4 nm) among 16 serine proteases examined, including plasminogen activators (urokinase- and tissue-type plasminogen activators), coagulation enzymes thrombin, factors VIIa, Xa, XIa, and XIIa, and activated protein C. Relatively weak inhibition was found for plasmin (IC50 = 399 nm) and plasma kallikrein (IC50 = 686 nm). Second, the functions of the KD1 and KD2 domains in sHAI-1B were investigated using P1 residue-directed mutagenesis to show that inhibition of HGFA, matriptase, trypsin, and plasmin was due to KD1 and not KD2. Furthermore, analysis by reverse transcription-PCR demonstrated that HAI-1B and HAI-1 were co-expressed in normal tissues and various epithelial-derived cancer cell lines. Both isoforms were up-regulated in eight examined ovarian carcinoma specimens, three of which had higher levels of HAI-1B RNA than of HAI-1 RNA. Therefore, previously demonstrated roles of HAI-1 in various physiological and pathological processes likely involve both HAI-1B and HAI-1.

Characterization of transcripts generated from mouse hepatocyte growth factor activator inhibitor type 2 (HAI-2) and HAI-2-related small peptide (H2RSP) genes: chimeric mRNA transcribed from both HAI-2 and H2RSP genes is detected in human but not in mouse

We previously reported a novel small gene, designated hepatocyte growth factor activator inhibitor type 2 (HAI-2) related small peptide (H2RSP), in the process of the search for splicing variant forms of HAI-2 by 3(')-rapid amplification of cDNA ends method [Biochem. Biophys. Res. Commun. 288 (2001) 390]. Human H2RSP gene consisted of four exons spanning approximately 1kbp and was located in 11kbp downstream of HAI-2 gene. In this study, we cloned and characterized the mouse counterpart of H2RSP gene, which was located in 6.6kbp downstream of mouse HAI-2 gene, and analyzed the transcripts generated from both genes. Similar to human, mouse H2RSP mRNA (0.5kb) was detected abundantly in various tissues including the gastrointestinal tract, and has nuclear localization signal (NLS) in the lysine-rich region (exon 4), which was well-conserved between human and mouse genes. However, chimeric mRNA transcribed from both HAI-2 (exons 1-7) and H2RSP (exons 2-4) genes, which was found in the kidney, prostate, and placenta of human by Northern blot analysis, was not detected in mouse tissue even by a reverse transcription-polymerase chain reaction (RT-PCR). Instead of the chimeric mRNA, a novel splicing variant lacking putative transmembrane domain of HAI-2 was found in mouse but not in human as a putative secrete form of HAI-2. These results suggest that the organization of H2RSP and HAI-2 gene complex is well-conserved, but the usage of these genes was quite different between human and mouse.

Role of cancer cell-stroma interaction in invasive growth of cancer cells

Invasive growth is one of the hallmarks of cancer malignancy. To date, a significant body of evidence is accumulating in favor of the notion that invasive growth results from the cross-talk between cancer cells and the host stromal cells, comprising fibroblasts (myofibroblasts), endothelial cells, and leukocytes, all of which are themselves invasive. In this review we describe cross-talk between invasive cancer cells and host stromal fibroblasts and an impact of pericellular microenvironment on the invasive phenotype of cancer cells, focusing on two molecules, extracellular matrix metalloproteinase inducer (EMMPRIN, also known as tumor cell-derived collagenase stimulatory factor, basigin, CD147) and hepatocyte growth factor (HGF, also known as scatter factor). Both molecules are deeply involved in the regulation of invasion-associated cellular activities, such as pericellular proteolysis, migration and ectopic survival of cancer cells.

Expression of hepatocyte growth factor activator inhibitor type 2 (HAI-2) in human testis: identification of a distinct transcription start site for the HAI-2 gene in testis

Hepatocyte growth factor activator inhibitor type 1 (HAI-1) and type 2 (HAI-2) are recently identified integral membrane Kunitz-type proteinase inhibitors. They have important regulatory roles in pericellular activation of hepatocyte growth factor/scatter factor (HGF/SF) which is critically involved in the development and regeneration of various tissues. Recent reports suggest that HGF/SF is also involved in testicular development and spermatogenesis. In this study, we analyzed the expression of HAIs in the testis. In human testis, HAI-2 was strongly expressed whereas HAI-1 mRNA was hardly detectable. Of interest was the observation that the mRNA size of HAI-2 was shorter in the testis (1.2 kb) than those in the other tissues such as placenta (1.5 kb). Subsequent experiments revealed that there are two major transcription start sites of the HAI-2 gene, which are -30 bp and -360 bp upstream from the translation initiation ATG codon. Although the latter site appeared to be mainly used in the placenta and other non-testicular organs, only the former site is used in testis, resulting in the -300 bp shorter mRNA. An immunohistochemical study using a specific monoclonal antibody raised against human HAI-2 protein indicated that HAI-2 is expressed exclusively in primary spermatocytes. These results suggest a distinct regulation of HAI-2 gene expression in testis and that HAI-2 may play a role in the process of spermatogenesis.

Roles of hepatocyte growth factor activator (HGFA) and its inhibitor HAI-1 in the regeneration of injured gastrointestinal mucosa

Hepatocyte growth factor (HGF)/scatter factor (SF) is thought to play an important role in the regeneration of injured gastrointestinal mucosa by promoting the proliferation and migration of epithelial cells. HGF/SF is secreted by stromal cells as an inactive precursor form, and is specifically activated by HGF activator (HGFA) to the active form. HGFA is also produced as a precursor form and activated by thrombin in injured tissues. The activity of HGFA is regulated by two recently identified Kunitz-type serine proteinase inhibitors, namely HGFA inhibitor type 1 (HAI-1) and type 2 (HAI-2). Although the activation of HGF/SF is a critical limiting step in the HGF/SF-induced signaling pathway, little is known about the regulation of HGF/SF activation in injured gastrointestinal mucosa. Immunohistochemically, all these proteins have been detected in gastrointestinal epithelial cells, and HAI-1 was upregulated in regenerative epithelium relative to normal epithelium. During the course of acetic acid-induced murine experimental colitis, HAI-1, but not HAI-2, was indeed upregulated in the recovery phase. In vitro study revealed that HAI-1 is not only an inhibitor, but also a specific cell-surface binding protein, of active HGFA, and acts as a reservoir of this enzyme on the cell surface. Active HGFA/HAI-1 complexes were quickly released from the cell surface by treatment with IL-1beta accompanying significant recovery of HGFA activity in the culture supernatant. These results suggest that HAI-1 is a cell-surface acceptor of activated HGFA in regenerative epithelial cells, and functions on the cell surface to localize the active HGFA that is going to enter the repair process. This concentrated HGFA activity would ensure the efficient pericellular activation of HGF in the injured gastrointestinal mucosa, and promote the proliferation and migration of gastrointestinal epithelial cells.

Identification of hepatocyte growth factor activator inhibitor type 2 (HAI-2)-related small peptide (H2RSP): its nuclear localization and generation of chimeric mRNA transcribed from both HAI-2 and H2RSP genes

A novel small gene, designated hepatocyte growth factor activator inhibitor type 2 (HAI-2)-related small peptide (H2RSP) was cloned and characterized in the process of the search for splicing variant forms of HAI-2 by 3'-rapid amplification of cDNA ends (RACE). The gene consisted of 4 exons spanning approximately 1 kb and was located in 11 kb downstream of HAI-2 gene (19q.13.11). The novel transcript identified by 3'-RACE was thought to be chimerically transcribed from both HAI-2 (exons 1-7) and H2RSP (exons 2-4) genes. Wild-type H2RSP mRNA (0.5 kb) was detected abundantly in various tissues including the gastrointestinal tract, whereas chimeric mRNA (1.5 kb) was found mainly in the kidney, prostate, and placenta by Northern blot analysis. The predicted amino acid sequence of H2RSP contained two unique domains, namely the serine-rich region (exon 3) and the lysine-rich region (exon 4). Transfection of deleted series of H2RSP cDNAs fused to enhanced green fluorescent protein (EGFP) into HeLa cells revealed that H2RSP has nuclear localization signal in the lysine-rich region. Immunohistochemical study using anti-H2RSP polyclonal antibody indeed revealed the nuclear localization of this peptide in vivo. These results suggest that H2RSP and H2RSP/HAI-2 chimeric peptides might function as a transcriptional regulatory peptide at the nucleus.

Mouse hepatocyte growth factor activator inhibitor type 1 (HAI-1) and type 2 (HAI-2)/placental bikunin genes and their promoters

Hepatocyte growth factor (HGF) activator inhibitor type 1 (HAI-1) and type 2 (HAI-2) were recently discovered as specific inhibitors of HGF activator. Each of them contains two Kunitz-type serine protease inhibitor domains and a transmembrane domain, so that their overall structures are similar to each other. In this study, mouse genes encoding HAI-1 and HAI-2 were cloned by screening of a mouse genomic bacterial artificial chromosome library and by polymerase chain reaction of mouse genomic DNA, respectively. The genes (mHAI-1 and mHAI-2) were defined to consist of 11 and eight exons spanning 11 kbp and 9.5 kbp, respectively. Neither a TATA nor CAAT box was found in 5'-flanking regions of both genes and no apparent homologous portion was observed between mHAI-1 and mHAI-2 promoter regions. Promoter assay of mHAI-1 and human HAI-1 revealed that the potential binding sites of a complex of Egr-1-3 and Sp1, which was well-conserved between human (-42 to -58) and mouse (-44 to -57), might be a key portion of its transcriptional regulation to function as not only house-keeping but also early responsive genes.

Hepatocyte growth factor activator inhibitor type 1 is a specific cell surface binding protein of hepatocyte growth factor activator (HGFA) and regulates HGFA activity in the pericellular microenvironment

Hepatocyte growth factor activator (HGFA) is responsible for proteolytic activation of the precursor form of hepatocyte growth factor in injured tissues. To date, two specific inhibitors of HGFA have been identified, namely HGFA inhibitor type 1 (HAI-1) and type 2 (HAI-2)/placental bikunin (PB). Both inhibitors are first synthesized as integral membrane proteins having two Kunitz domains and a transmembrane domain, and are subsequently released from cell surface by shedding. Here we show that an active form of HGFA is specifically complexed with membrane-form HAI-1, but not with HAI-2/PB, on the surface of epithelial cells expressing both inhibitors. This binding required the enzyme activity of HGFA. The selective binding of HGFA to the cell surface HAI-1 was further confirmed in an engineered system using Chinese hamster ovary cells, in which only the cells expressing HAI-1 retained exogenous HGFA. The binding of HGFA to HAI-1 was reversible, and no irreversible modifications affecting the enzyme activity occurred during the binding. Importantly, HAI-1 and the HGFA.HAI-1 complex were quickly released from the cell surface by treatment with phorbol 12-myristate 13-acetate or interleukin 1beta accompanying the generation of 58-kDa fragments of HAI-1, which are less potent against HGFA, as well as significant recovery of HGFA activity in the culture supernatant. This regulated shedding was completely inhibited by BB3103, a synthetic zinc-metalloproteinase inhibitor. We conclude that HAI-1 is not only an inhibitor but also a specific acceptor of active HGFA, acting as a reservoir of this enzyme on the cell surface. The latter property appears to ensure the concentrated pericellular HGFA activity in certain cellular conditions, such as tissue injury and inflammation, via the up-regulated shedding of HGFA.HAI-1 complex. These findings shed light on a novel function of the integral membrane Kunitz-type inhibitor in the regulation of pericellular proteinase activity.