Developmental biology. The scattered jigsaw

Hepatocyte growth factor/scatter factor in the eye

Hepatocyte growth factor, also known as scatter factor (HGF/SF) is a multipotential cytokine which can produce a range of responses in target cells and its influence in the eye in health and disease is just beginning to be appreciated. Usually HGF/SF is synthesised by mesenchymally derived cells and targets and signals epithelial cells in a paracrine manner via their c-Met surface receptor. However, there is growing evidence for the existence of autocrine loops in a number of cell systems prominent among which are ocular cells such as the corneal endothelium, the lens epithelium, the retinal pigment epithelium (RPE) and others. Marked cellular proliferation is stimulated when activated HGF/SF is exposed to hepatocytes, renal epithelium, melanocytes and vascular endothelial cells but it is often a poor mitogen for other cell types. In target cells the cytokine promotes other bioactions such as junctional breakdown, shape change, cell scattering, directional and nondirectional migration, cell survival, invasive behaviour and/or tubule formation. These activities seem to depend on HGF/SF linking with the c-Met receptor and pathways to stimulate the various types of cytokine/receptor response are being unravelled at the present time. In corneal wound healing, HGF/SF is produced by stromal keratocytes and targets the repairing epithelium. HGF/SF is a constituent of tears, aqueous humour and vitreous humour at levels above that found in plasma although it is not clear how much is activated. Aqueous HGF/SF may well influence lens epithelial, corneal endothelial and trabecular meshwork cell survival. Vitreous levels of HGF/SF are elevated in proliferative vitreoretinopathy (PVR), where a target cell is the RPE and in proliferative diabetic retinopathy (PDR) where HGF/SF has been shown to be a major angiogenesis factor. Finally HGF/SF may be involved in the metastatic spread of tumour cells from uveal melanomata and in the formation of vascular channels in these tumours.

Activation mechanisms of the urokinase-type plasminogen activator promoter by hepatocyte growth factor/scatter factor

Hepatocyte growth factor/scatter factor (HGF/SF) is a pleiotropic effector inducing invasion and metastasis of tumor cells that express the Met tyrosine kinase receptor. One of the effectors of HGF/SF is the urokinase-type plasminogen activator, a serine protease that facilitates tumor progression and metastasis by controlling the synthesis of the extracellular matrix degrading plasmin. Stimulation of NIH 3T3 cells that were stably transfected with the human Met receptor (NIH 3T3-Methum) with HGF/SF induced a trans-activation of the urokinase promoter and urokinase secretion. Induction of the urokinase promoter by HGF/SF via the Met receptor was blocked by co-expression of a dominant-negative Grb2 and Sos1 expression construct. Further, the expression of the catalytically inactive mutants of Ha-Ras, RhoA, c-Raf, and Erk2 or addition of the Mek1-specific inhibitor PD 098059 abrogated the stimulation of the urokinase promoter by HGF/SF. A sequence residing between -2109 and -1870 base pairs (bp) was critical for stimulation of the urokinase gene by HGF/SF. Mobility shift assays with oligonucleotides spanning an AP-1 site at -1880 bp or a combined PEA3/AP-1 site at -1967 bp showed binding of nuclear factors from NIH 3T3-Methum cells. Expression of an expression plasmid that inhibits DNA binding of AP-1 proteins (A-Fos) abrogated inducible and basal activation of the urokinase promoter. Nuclear extract from unstimulated NIH 3T3-Methum cells contained more JunD and showed a stronger JunD supershift with the AP-1 oligonucleotides, compared with HGF/SF-stimulated cells. Consistent with the levels of JunD expression being functionally important for basal expression of the urokinase promoter, we found that overexpression of wild type JunD inhibited the induction of the urokinase promoter by HGF/SF. These data suggest that the induction of urokinase by HGF/SF is regulated by a Grb2/Sos1/Ha-Ras/c-Raf/RhoA/Mek1/Erk2/c-++ +Jun-dependent mitogen-activated protein kinase pathway.

Hepatocyte growth factor stimulates fetal gastric epithelial cell growth in vitro

BACKGROUND

The growth and development of the fetal gastrointestinal tract is likely mediated, in part, by peptide growth factors. We compared the mitogenic effects of graded doses of hepatocyte growth factor (HGF) to epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), and insulin-like growth factor-1 (IGF-1) on fetal rabbit gastric epithelial cells.

MATERIALS AND METHODS

Fetal rabbit gastric epithelial cells were purified by mechanical dissociation and selected culture and grown in short-term (24 h) and long-term (12 days) culture. Stimulation of fetal gastric epithelial cell growth in response to individual peptide growth factors was measured by [3H]thymidine incorporation and cell counting.

RESULTS

In short-term culture, HGF stimulated [3H]thymidine incorporation in a dose-dependent manner from a threshold at 10 pM to a maximum at 100 pM. For EGF and TGF-alpha, maximal stimulation occurred at 100 pM. For HGF, maximal [3H]thymidine incorporation was 3.6 +/- 0.7 times basal. For EGF and TGF-alpha, maximal [3H]thymidine incorporation was 4.3 +/- 0.4, and 3.6 +/- 0.4 times basal, respectively. For IGF-1, maximal [3H]thymidine incorporation was only 70% of the maximal effect observed for the other growth factors tested. Rabbit amniotic fluid increased [3H]thymidine uptake in a dose-dependent manner. In long-term culture, purification to greater than 90% epithelial cells was attained after 12 days treatment. For HGF, EGF, TGF-alpha, and 20% rabbit amniotic fluid, significant increases in cell number above control (P < 0.05) were observed at 1 nM concentrations. None of these individual factors, however, increased cell growth as significantly as that of 10% fetal bovine serum.

CONCLUSIONS

Our results suggest that: (1) HGF stimulates [3H]thymidine uptake and cell proliferation in fetal rabbit gastric epithelial cells in vitro, and (2) HGF's mitogenic effect on fetal rabbit gastric epithelial cell growth is comparable to that observed for EGF and TGF-alpha, but superior to the effect observed for IGF-1.

Induction of altered gene expression in early embryos

This review focuses on known genes whose expression may be perturbed by teratogens during early embryogenesis (preorganogenesis). Teratogens may disrupt embryogenesis by modifying positional information. Genes controlling positional information include those specifying the primary body axes: anterior-posterior, dorsal-ventral, or left-right. These genes often encode transcription factors, whose regulation or activation can stimulate aberrant tissue differentiation and morphogenesis. Alternatively, teratogens may directly affect cell differentiation, proliferation, or apoptosis. Hydrophilic signalling molecules such as growth factors and hydrophobic molecules such as retinoids regulate these processes. The signalling pathways activated often induce the coordinate regulation of tissue specific gene expression. In addition to modifying individual signalling pathways, teratogens can synergize with or antagonize the effects of other teratogens through inappropriate interactions between signal transduction pathways. Since teratogens may often directly or indirectly perturb the expression of known or as yet undescribed developmentally critical genes, this review also provides a short description of techniques to identify genes whose expression is altered by teratogens.

Immunoreactivity for hepatocyte growth factor/scatter factor and its receptor, met, in human lung carcinomas and malignant mesotheliomas

Paraffin sections from 29 lung carcinomas (28 primary and 1 metastatic) and 9 pleural malignant mesotheliomas were immunostained with antisera to human hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, met. For HGF/SF, immunoreactivity was demonstrated in all 9 mesotheliomas, 9 of 12 adenocarcinomas, and 7 of 10 squamous cell carcinomas. None of seven cases of small cell anaplastic carcinoma was positive. The adenocarcinomas frequently showed enhanced luminal staining, suggesting possible secretion of HGF/SF, and this pattern of staining was also seen occasionally in bronchial epithelium adjacent to the tumour. Stromal fibroblasts also showed immunoreactivity for HGF/SF in 6/8 cases of mesothelioma but in only 3/12 adenocarcinomas, 1/10 squamous cell carcinomas, and 1/4 small cell anaplastic carcinomas. All tumours stained for met, usually strongly. The staining was mainly cytoplasmic in nature, but some plasma membrane staining was usually evident. Adenocarcinomas showed strong luminal membrane staining, as did adjacent, histologically normal bronchial epithelium. This study demonstrates the presence of HGF/SF and met in most of the tumour types described, particularly mesotheliomas, and suggests that the HGF/SF/met signalling system may play a role in the development of these tumours, either by autocrine or by paracrine mechanisms.

C-Met signalling in an HGF/SF-insensitive variant MDCK cell line with constitutive motile/invasive behaviour

The Met protein is a receptor tyrosine kinase for hepatocyte growth factor/scatter factor (HGF/SF), a multifunctional growth factor with mitogenic, motogenic and morphogenic properties. A morphologically altered variant of the MDCK cell line, MDCK-1, spontaneously exhibits a number of features associated with a partial HGF/SF-Met induced phenotype (less adhesive colonies in culture, enhanced invasion and motility, nascent tubule formation), but paradoxically does not respond to HGF/SF treatment. Although the overall cell surface expression and distribution of Met were found to be similar in parental MDCK cells and the MDCK-1 cell line, p145met autophosphorylation (+/ HGF/SF) was significantly reduced in MDCK-1 cells in vitro and in vivo when compared with parental MDCK cells. In contrast, EGF induced cell proliferation and EGF receptor autophosphorylation to similar levels in both cell lines. The basal levels of protein tyrosine phosphorylation were higher in MDCK-1 cells when compared with parental MDCK cells, including that of two prominent proteins with molecular masses of approximately 185 kDa and 220 kDa. Moreover, both p185 and p220 are present and tyrosine phosphorylated in Met immunoprecipitates from MDCK-1 cells (+/-HGF/SF), but not parental MDCK cells. In addition, Met immunocomplexes from MDCK-1 cells exhibited an approximately 3-fold increased tyrosine kinase activity in vitro when compared with MDCK cells, correlating with the higher basal levels of total phosphotyrosine. Treatment of MDCK-1 cells with the tyrosine kinase inhibitor herbimycin A reverted the cell phenotype to a more MDCK-like morphology in culture, with a concomitant reduction in the tyrosine phosphorylation predominantly of p220. Taken together these data suggest that aberrations in Met activity and associated signalling render MDCK-1 cells insensitive to HGF/SF, and may also mediate alterations in MDCK-1 cell behaviour.