Hepatocyte growth factor remains as an inactive single chain after partial hepatectomy or unilateral nephrectomy

Hepatocyte Growth Factor Activator: A Proteinase Linking Tissue Injury with Repair

Hepatocyte growth factor (HGF) promotes pleiotropic signaling through its specific receptor tyrosine kinase, MET. As such, it has important roles in the regeneration of injured tissues. Since HGF is produced mainly by mesenchymal cells and MET is expressed in most epithelial, endothelial and somatic stem cells, HGF functions as a typical paracrine growth factor. HGF is secreted as an inactive precursor (proHGF) and requires proteolytic activation to initiate HGF-induced MET signaling. HGF activator (HGFAC) is a serum activator of proHGF and produces robust HGF activities in injured tissues. HGFAC is a coagulation factor XII-like serine endopeptidase that circulates in the plasma as a zymogen (proHGFAC). Thrombin, kallikrein-related peptidase (KLK)-4 or KLK-5 efficiently activates proHGFAC. The activated HGFAC cleaves proHGF at Arg494-Val495, resulting in the formation of the active disulfide-linked heterodimer HGF. Macrophage stimulating protein, a ligand of RON, is also activated by HGFAC in vivo. Although HGFAC functions primarily at the site of damaged tissue, a recent report has suggested that activated HGFAC relays a signal to stem cells in non-injured tissues via proHGF activation in the stem cell niche. This review focuses on current knowledge regarding HGFAC-mediated proHGF activation and its roles in tissue regeneration and repair.

Matrix metalloproteinase-13 promotes recovery from experimental liver cirrhosis in rats

OBJECTIVE

To evaluate the role of matrix metalloproteinase (MMP)-13 gene expression in the early phase of recovery from liver fibrosis/cirrhosis.

METHODS

Liver fibrosis was induced in male Wistar rats by administration of carbon tetrachloride (CCl(4)) for 10 weeks. Recombinant adenovirus-mediated human MMP-13 gene transfer (RAdMMP-13) was performed via the femoral vein on day 3 after the last CCl(4) injection. The role of MMP-13 in stably expressing cell lines was also analyzed.

RESULTS

Fibrous deposition in the liver was decreased in RAdMMP-13-injected rats by day 3 after gene transfer compared with empty vector RAd66-injected rats. Furthermore, MMP-2 and MMP-9 enzymatic activity was markedly enhanced in the liver of RAdMMP-13 injected rats. Hepatocyte growth factor (HGF) induction was also increased in RAdMMP-13 injected rats. In established stable HT-1080 cells transfected with MMP-13, HGF-α expression and MMP-2 and MMP-9 enzymatic activity were increased. The conversion of precursor HGF into mature HGF was also increased in the MMP-13 expressing cell lines.

CONCLUSION

Forced MMP-13 expression effectively accelerated recovery from liver cirrhosis via the effects of MMP-13-mediated HGF, MMP-2, and MMP-9 expression, which induced the degradation of collagen fibers and promoted hepatic regeneration.

Hepatocyte growth factor activator (HGFA): pathophysiological functions in vivo

Hepatocyte growth factor activator (HGFA) is a serine protease initially identified as a potent activator of hepatocyte growth factor/scatter factor. Hepatocyte growth factor/scatter factor is known to be critically involved in tissue morphogenesis, regeneration, and tumor progression, via its receptor, MET. In vivo, HGFA also activates macrophage-stimulating protein, which has roles in macrophage recruitment and inflammatory processes, cellular survival and wound healing through its receptor, RON. Therefore, the pericellular activity of HGFA might be an important factor regulating the activities of these multifunctional cytokines in vivo. HGFA is secreted mainly by the liver, circulates in the plasma as a zymogen (pro-HGFA), and is activated in response to tissue injury, including tumor growth. In addition, local production of pro-HGFA by epithelial, stromal or tumor cells has been reported. Although the generation of HGFA-knockout mice revealed that the role played by HGFA in normal development and physiological settings can be compensated for by other protease systems, HGFA has important roles in regeneration and initial macrophage recruitment in injured tissue in vivo. Insufficient activity of HGFA results in impaired regeneration of severely damaged mucosal epithelium, and may contribute to the progression of fibrotic lung diseases. On the other hand, deregulated excess activity of HGFA may be involved in the progression of some types of cancer.

Translational research to identify clinical applications of hepatocyte growth factor

Hepatocyte growth factor (HGF), originally purified from the plasma of patients with fulminant hepatic failure, has been shown to carry out various physiological functions. HGF not only stimulates liver regeneration, but also acts as an antiapoptotic factor in in vivo experimental models. Therefore, HGF is a promising therapeutic agent for the treatment of fatal liver diseases, including fulminant hepatic failure. After performing a number of preclinical tests, our group began an investigator-initiated registered phase I/II clinical trial of patients with fulminant hepatic failure to examine the safety and clinical efficacy of recombinant human HGF. In this article, we will discuss the basic research results as well as the translational research that underpins current attempts to use HGF in various clinical settings.

Impairment of activation of hepatocyte growth factor precursor into its mature form in rats with liver cirrhosis

BACKGROUND

Hepatocyte growth factor (HGF) has a crucial role in liver regeneration following injury. The conversion of an inactive precursor form of HGF (proHGF) into a biologically active form (mature HGF) is essential, as HGF is involved in the recovery of liver damage. Liver regeneration is markedly poor in patients with liver cirrhosis after resection. We hypothesized that impairment of liver regeneration in cirrhosis is in part because of the absence of activation of proHGF to mature HGF. Studies were performed to clarify the molecular form of HGF in the liver of rats with fibrosis/cirrhosis before and after liver resection.

METHODS

Rat models of liver fibrosis/cirrhosis were induced by intraperitoneal administration of dimethylnitrosamine, followed by 45% partial hepatectomy or sham operation. HGF was purified from the liver and plasma on a SP-Sepharose column and was analyzed by Western blotting.

RESULTS

Production of proHGF in the liver increased in the following order: rats with normal liver, rats with fibrosis, and rats with cirrhosis. However, the levels of proHGF were similar after liver resection in the liver of these groups. A small but significant level of mature HGF was detected before resection in the fibrosis group, but not in the normal and cirrhosis groups. Liver resection increased the levels of mature HGF in the normal and fibrosis groups, but marginally in the cirrhosis group.

CONCLUSIONS

These results demonstrate that the conversion of proHGF into mature HGF is impaired after liver resection in liver cirrhosis, while proHGF production is similar in the livers of normal, fibrosis, and cirrhosis groups. Acceleration of the processing of the HGF molecule may contribute to the improvement of liver dysfunction in cirrhosis.

Exogenously administered HGF activator augments liver regeneration through the production of biologically active HGF

Hepatocyte growth factor (HGF) plays a crucial role in the recovery of injured liver. Liver functions are mostly impaired in patients with liver diseases including cirrhosis. However, a significant amount of inactive HGF precursor (proHGF) is reported in the plasma of these patients. proHGF is proteolytically converted to an active form (mature HGF) by HGF-activator. Thus conversion of proHGF into mature HGF presumably contributes to the recovery of liver functions. In this study, rats with a partial hepatectomy were used, as proHGF is accumulated in the remnant liver. Recombinant human HGF-activator was administered via the portal vein to investigate the effect on molecular forms of HGF and its biological signaling. rhHGF-activator promptly converted proHGF into mature HGF, reaching maximal levels at 5-10 min after the injection, while the decreased proHGF was quickly recovered to the initial levels in the liver. The HGF receptor/c-Met was found to be autophosphorylated in the liver treated with rhHGF-activator. Further, the proliferating cell nuclear antigen labeling index and the liver regeneration rate were significantly higher in rhHGF-activator group than in control animals. These results indicate that exogenously administered HGF-activator produces a biologically active HGF from its precursor form and increases the potential for liver regeneration in vivo.

Effects of pHGF on renal cellular DNA synthesis after partial hepatectomy in rats

The effects of pro-hepatocyte growth factor (pHGF) on the changes of renal cellular proliferative cycle of partial hepatectomized rats were observed by flow cytometry (FCM). S phase fraction (SPF) of control rats (group A) accounted for 7.58% and increased gradually within 6 h, following a peak at 12th or 36th h after operation, but in pHGF-treated rats (group B) the peak appeared at 24th h after operation. Proliferation index (PI) of group A was 13.2% before partial hepatectomy, increased within 6 h and reached a peak at 12th or 36th h after operation, and in group B the peak appeared at 48th h after operation. There were significant differences in SPF and PI between two groups (P < 0.01). These findings suggest that pHGF may nonspecifically promote the DNA synthesis of renal cells.

Transforming growth factor alpha levels in liver and blood correlate better than hepatocyte growth factor with hepatocyte proliferation during liver regeneration

Transforming growth factor alpha (TGFalpha) and hepatocyte growth factor (HGF) are mitogens for hepatocytes in vitro and in vivo, produced by hepatocytes or nonparenchymal cells such as stellate cells in the liver. It is still uncertain whether TGFalpha and HGF are essential for liver regeneration. To assess the role of these growth factors in liver regeneration, their circulating and hepatic levels were studied in various rat models of liver regeneration. Hepatic and plasma HGF levels were increased with increased number of mitotic hepatocytes in rats after partial hepatectomy or carbon tetrachloride intoxication. However, hepatic HGF levels were decreased despite an increased number of mitotic hepatocytes and increased or unchanged plasma HGF levels in rats given phenobarbital and in rats after dimethylnitrosamine intoxication, which can induce hepatic necrosis after apoptosis of hepatic stellate cells. In contrast, hepatic and serum TGFalpha levels were increased in all of the models. In sham-operated rats with no increased number of mitotic hepatocytes, hepatic and circulating levels of HGF were increased, whereas those levels of TGFalpha were unchanged. The results indicate that TGFalpha levels in liver and blood more closely correlate with hepatocyte mitogenesis than HGF levels.

Hepatocyte growth factor induces hepatocyte proliferation in vivo and allows for efficient retroviral-mediated gene transfer in mice

Recombinant retroviral vectors are an attractive means of transferring genes into the liver because they integrate into the host cell genome and result in permanent gene expression. However, efficient in vivo gene transfer is limited by the requirement of active cell division for integration. Traditional approaches to induce liver proliferation have the disadvantage of inducing hepatocellular injury by delivery of toxins or by surgical partial hepatectomy. As a nontraumatic alternative, we show that exogenous hepatocyte growth factor (HGF) is a powerful and safe mitogen for the mature intact murine liver when delivered continuously into the portal vein. A 5-day infusion of human HGF (5 mg/kg/d) resulted in > 140% increase in relative liver mass, which returned to normal in 4 to 5 weeks. This clearly shows that an exogenous growth factor can induce robust liver proliferation in vivo. In addition, we show that the HGF-induced proliferation was independent of interleukin-6, an essential cytokine involved in liver regeneration after partial hepatectomy. When recombinant retroviral vectors were infused in combination with HGF, 30% of hepatocytes were stably transduced with no indication of hepatic injury or histopathology. These results show the ability to obtain a clinically relevant transduction efficiency with retroviral vectors in vivo without the prior induction of liver injury. The level of hepatic gene transfer achieved has the potential to be curative for a large number of genetic liver diseases.

Plasma heparin-binding EGF-like growth factor levels in patients after partial hepatectomy as determined with an enzyme-linked immunosorbent assay

We recently showed that heparin-binding EGF-like growth factor (HB-EGF) has hepatotrophic effects. In this study, we developed an ELISA system with high specificity and sensitivity for human plasma HB-EGF. In 14 patients who underwent partial hepatectomy, plasma HB-EGF levels were measured serially after surgery. In patients who underwent gross hepatectomy (lobectomy and segmentectomy), plasma HB-EGF levels increased, reaching maximal levels approximately 5 to 7 days after surgery. In patients who underwent minor hepatectomy (subsegmentectomy), plasma HB-EGF levels did not increase. Maximal plasma HB-EGF levels were significantly higher in patients who had a percent increased volume of the remaining liver (%ILV) above 20% than those who had a %ILV below 20% (32.4 +/- 19.6 pg/ml vs 7.4 +/- 2.7, P < 0.05). The plasma HB-EGF values did not correlate with WBC counts, C-reactive protein, or alanine aminotransferase. Plasma HB-EGF may be a marker for liver regeneration after hepatectomy in humans.

Effects of pHGF on hepatocyte DNA synthesis after partial hepatectomy in rats

The effects of pHGF on the changes of hepatocyte proliferative cycle and liver regenerative capacity after partial hepatectomized rats were observed by flow cytometry (FCM). The results were as follows: 1) S phase fraction (SPF) in group of normal rats (group A) accounted for 9.89% and increased gradually within 6 h, following a peak at 12th h or 36th h after operation, but in the group of pHGF-treated rats (group B) the peak appeared at 24th h after operation; 2) Proliferation index (PI) of group A was 19.6% before partial hepatectomy, increased to 34.91% within 6 h and reached a peaks at 12th or 36th h after operation, and in group B the peak appeared at 48th h after operation. There were significant differences between two groups in SPF and PI (P < 0.01). The weight of liver began to increase 12 h after operation, and almost reached the preoperative weight 5 days after operation. These findings suggest that pHGF can promote the DNA synthesis and segmentation of hepatocyte.

Overexpression and activation of hepatocyte growth factor/scatter factor in human non-small-cell lung carcinomas

Hepatocyte growth factor/scatter factor (HGF/SF) stimulates the invasive growth of epithelial cells via the c-MET oncogene-encoded receptor. In normal lung, both the receptor and the ligand are detected, and the latter is known to be a mitogenic and a motogenic factor for both cultured bronchial epithelial cells and non-small-cell carcinoma lines. Here, ligand and receptor expression was examined in 42 samples of primary human non-small-cell lung carcinoma of different histotype. Each carcinoma sample was compared with adjacent normal lung tissue. The Met/HGF receptor was found to be 2 to 10-fold increased in 25% of carcinoma samples (P = 0.0113). The ligand, HGF/SF, was found to be 10 to 100-fold overexpressed in carcinoma samples (P < 0.0001). Notably, while HGF/SF was occasionally detectable and found exclusively as a single-chain inactive precursor in normal tissues, it was constantly in the biologically-active heterodimeric form in carcinomas. Immunohistochemical staining showed homogeneous expression of both the receptor and the ligand in carcinoma samples, whereas staining was barely detectable in their normal counterparts. These data show that HGF/SF is overexpressed and consistently activated in non-small-cell lung carcinomas and may contribute to the invasive growth of lung cancer.