We have previously established that rat and human hepatic HMG-CoA reductase activity is modulated in vitro and in vivo in a bicyclic cascade system involving reversible phosphorylation of both HMG-CoA reductase and reductase kinase. Recently we have identified two additional kinase systems for the regulation of HMG-CoA reductase activity by short-term covalent modification, involving a Ca2+/calmodulin-dependent kinase and protein kinase C. In order to understand the coordinate regulation of HMG-CoA reductase, cholesterol synthesis, and the role of apolipoproteins such as apolipoprotein A-I (apoA-I) and apolipoprotein B (apoB-100) in the transport and regulation of cellular cholesterol, a systematic investigation of their role in plasma lipid and lipoprotein transport and metabolism has been undertaken. Recently we have shown the post-transnational modification of human plasma apoA-l involving reversible phosphorylation. Plasma LDL (apoB-100) have been correlated directly with the development of premature cardiovascular disease. During the last year we have established that both secreted and cellular apoB-100 from Hep G-2 cells were phosphorylated. We have also demonstrated the phosphorylation of human plasma apoB-100 (LDL) using protein kinase C, a cAMP dependent protein kinase and a Ca 2+/calmodulin-dependent kinase. We have also shown that both secreted and cellular phospho-apoB-100 do respond to increases in the levels of the above three intracellular protein kinases when cells were challenged with phorbol ester, glucagon and a Ca 2+ ionophore, respectively. The phosphorylation of apoB-100 may play an important role in the intracellular transport of hepatic VLDL during lipid assembly and secretion.
