GROWTH-PROMOTING EFFECTS OF CA2-MOBILIZING AGENTS IN HEPATOCYTES - LACK OF CORRELATION BETWEEN THE ACUTE ACTIVATION OF PHOSPHOINOSITIDE-SPECIFIC PHOSPHOLIPASE-C AND THE STIMULATION OF DNA-SYNTHESIS BY ANGIOTENSIN-II, VASOPRESSIN, NOREPINEPHRINE, AND PROSTAGLANDIN-F2-ALPHA()

Although several hormones that promote hepatocyte proliferation also a ctivate phosphoinositide-specific phospholipase C (PI-PLC) and mobiliz e Ca2+, the role of PI-PLC in the growth-stimulating effect of these a gents is not clear. We have investigated this issue further, by exposi ng freshly isolated adult rat hepatocytes to vasopressin, angiotensin II, norepinephrine (in the presence of the beta-adrenoceptor blocker t imolol) or PGF(2 alpha), and examined both acute responses and the sub sequent DNA synthesis when the cells were grown in monolayer culture. All the agonists elevated the level of inositol 1,4,5-trisphosphate (I nsP(3)) and enhanced the DNA synthesis, amplifying the response to epi dermal growth factor (EGF), and this comitogenic effect could be exert ed by a single exposure of the cells 24 h prior to the addition of EGF . The acute activation of PI-PLC, measured as the early rise (peak 15- 60 s) in InsP(3), was 8-10-fold with vasopressin or angiotensin II, 3- 4-fold with norepinephrine, and similar to 2-fold with PGF(2 alpha). F or all the agonists, a rise in cytosolic free Ca2+ in 100% of the cell s and a maximal increase in glycogen phosphorylase activity were evoke d at concentrations that approximately doubled the level of InsP(3). H owever, the growth-stimulatory effects of these agonists showed a diff erent order of efficacy as compared to the activation of PI-PLC; in te rms of the maximal stimulation of DNA synthesis, the effects were: nor epinephrine approximate to PGF(2 alpha) > angiotensin II > vasopressin . Also, norepinephrine, PGF(2 alpha), and angiotensin II, but not vaso pressin, further enhanced the DNA synthesis when their concentrations were increased above those yielding maximal elevation of InsP(3). In e xperiments where vasopressin and angiotensin II were combined, their e ffects on the DNA synthesis were additive while the InsP(3) responses were not. The results show that the extent of the initial activation o f PI-PLC is not the determinant for the magnitude of the growth effect s of Ca2+-mobilizing hormones in hepatocytes. This suggests either (a) that the proliferative response to these agents is determined by the activity of PI-PLC at a later time, or its integral over an extended p art of the prereplicative period, rather than by the acute activation, or (b) that additional, PI-PLC-independent, mechanisms are required. (C) 1996 Wiley-Liss, Inc.