Paracrine regulation of liver growth by IGF and HGF

Abstract

Organ growth and its manifestations at cellular level, i.e. proliferation and differentiation, are under strict regulation including in the liver. The regulation of liver growth involves endocrine, paracrine and autocrine factors. In the present study, we investigated the importance of a transcription factor family - CCAAT/enhancer binding protein (C/EBP), and paracrine interactions involving the insulin-like growth factor (IGF) and hepatocyte growth factor (HGF) system in hepatocytes and hepatic stellate cells.The levels of the transcription factors C/EBP· and C/EBP were unchanged during induction of liver growth by treatment with cyproterone acetate, although these factor have been associated with hepatocyte proliferation as well as expression of differentiated hepatocyte functions. On the other hand, an increase in C/EBP preceded a decrease in C/EBP· during liver regeneration after 2/3 partial hepatectomy in rats. Studies on cultured rat hepatic stellate cells (HSC) showed that IGF-1 time- and dose-dependently increased HGF mRNA and HGF immunoreactivity. IGF-1 also enhanced DNA synthesis in the cultures. The IGF-1 effect on HGF production was dependent on both PI-3 kinase and MAP kinase signal transduction pathways. Inhibition of p70S6K activation blocked IGF-1 induced DNA synthesis but not the increase in HGF. Hepatocyte-conditioned medium (PCcM) also time- and dose-dependently increased HGF mRNA and immunoreactivity in cultured hepatic stellate cells. PCcM also enhanced the DNA synthesis in hepatic stellate cell cultures. The PCcM was prepared under serum- and hormone free conditions. It contained low levels of immunoreactive IGF-1 (3.6-5.0 ng/ml) and had no IGF-1 bioactivity measured as phosphorylation of type 1 IGF receptor subunit and a protein with a size consistent with that of insulin receptor substrate-1. A partial characterization of the factor(s) responsible for the mitogenic activity in hepatocyte-conditioned medium suggested that it was a heat, acid and protease sensitive protein(s) with an apparent molecular size between 30 and 100 kD. The mitogenic effect was sensitive to inhibition by transforming growth factor -1 (TGF*1), but only in very high doses. The effect of PCcM seemed to be more dependent on PI3-K than on MAPK dependent signaling pathways in HSC. The in vivo results presented suggest that an increase in C/EB and a suppression of C/EBP* are not prerequisites for all types of liver cell growth. The in vitro results indicate that both IGF-1and IGF-1 independent factor(s) from hepatocytes can stimulate HGF production by hepatic stellate cells. It remains to be investigated whether hepatocyte-derived IGF-1 and other unknown factors could stimulate hepatocyte proliferation indirectly via HSC derived HGF during any in vivo conditions.