Regulation of human Hepatocytes by P2Y receptors:: Control of glycogen phosphorylase, Ca2+, and mitogen-activated protein kinases

In the rat both short-term liver function, such as glycogen metabolism, and long-term events such as proliferation after partial hepatectomy, are in part controlled by release of nucleotides such as ATP acting on hepatocyte P2Y(1) and P2Y(2) receptors ( members of a family of P2Y receptors for extracellular nucleotides such as ATP and UTP). Here, we have studied P2Y receptor regulation of signaling pathways involved in glycogen phosphorylase activation and proliferation of primary human hepatocytes. Stimulation of cultured hepatocytes with either ATP and UTP, but not UDP or 2-methylthio ADP, led to concentration-dependent increases in cytosolic free Ca2+ concentration ([Ca2+](c); EC50 for ATP = 3.3 mu M, for UTP = 2.3 mu M) and [H-3] inositol (poly) phosphates (EC50 for ATP = 9.4 mu M, for UTP = 15.4 mu M). ATP and UTP also stimulated glycogen phosphorylase in human hepatocytes, each with a threshold for activation of less than 1 mu M. Application of 2-methylthio ADP up to 100 mu M was ineffective. Phosphorylation of both extracellular signal- related kinase and c-Jun N-terminal kinase was stimulated by ATP and UTP, but not by 2-methylthio ADP or UDP, either alone or when costimulated with epidermal growth factor. In conclusion, in human hepatocytes P2Y receptors control both glycogen metabolism and proliferation-associated responses such as increased [Ca2+](c) and mitogen-activated protein kinase cascades. Regulation seems to be primarily through P2Y(2) receptors. In contrast with previous studies on rat hepatocytes, there is an absence of responses mediated by P2Y(1) receptors.