The role of cAMP-MAPK signalling in the regulation of human hepatocellular carcinoma growth in vitro.

We have previously identified that primary human hepatocellular carcinoma (HCC) is associated with altered guanine nucleotide regulatory protein (G-protein) expression concomitant with decreased adenylyl cyclase (AC) and increased mitogen activated protein kinase (MAPK) activity in vivo. This study aims to address the potential link between Gs protein regulation of AC activity/ cyclic adenosine monophosphate (cAMP) production and the subsequent downstream regulation of MAPK activity and mitogenesis. Pharmacological agents which selectively interact with specific target proteins involved in signal transduction via the Gs-AC-cAMP-MAPK signalling pathway were employed in cultured human HCC cell lines in these studies. These agents allow us to address the role of individual components of these pathways in the regulation of mitogenesis in HCC. These studies utilized three distinct human HCC cell lines (HepG2, Hep3B and SKHep) in the absence and presence of agents that alter AC-cAMP dependent signalling. De novo DNA synthesis was determined as a marker of altered cellular proliferation, and MAPK activity was determined as the ability to catalyse myelin basic protein (MBP) phosphorylation. 8-Bromo-cAMP (8-Br-cAMP; a cell-permeable cAMP analogue) and forskolin (AC activator) dose-dependently decreased thymidine incorporation in all three cell lines. In addition, serum-stimulated [3H] thymidine incorporation was significantly decreased in HepG2, Hep3B and SKHep cell lines following treatment with either 8-Br-cAMP or forskolin. By contrast, MDL12330A (MDL; irreversible AC inhibitor) enhanced thymidine incorporation in all three cell lines. Treatment with either 8-Br-cAMP or forskolin significantly decreased serum-stimulated MAPK activity. These data suggest that cAMP acts as an anti-mitogenic agent in these hepatic tumorigenic cell lines in vitro such that inhibition of AC activity promotes MAPK activity and cellular mitogenesis in HCC.