Glucose phosphorylation is essential for the turnover of neutral lipid andthe second stage assembly of triacylglycerol-rich ApoB-containing lipoproteins in primary hepatocyte cultures

Primary hepatocytes cultured in a medium supplemented with amino acids and lipogenic substrates responded to increased extracellular glucose by increa sing the secretion of VLDL apoB. This effect was accompanied by an increase d secretion of VLDL triacylglycerol (TAG) derived from endogenous stores. G lucose also stimulated intracellular TAG mobilization via the TAG lipolysis /esterification cycle. All these effects were abolished in the presence of mannoheptulose (MH), an inhibitor of glucose phosphorylation. Glucose also gave rise to a modest (50% to 60%) increase in the incorporation of S-35 me thionine into newly synthesized apoB (P<0.05) and to a doubling of newly-sy nthesized apoB secretion as VLDL (P<0.05). The magnitude of these effects w as similar for apoB-48 and for apoB-100. NIH inhibited apoB-48 and apoB-100 synthesis and VLDL secretion at all glucose concentrations. The effects of glucose and MH on the secretion of newly-synthesized apoB-48 or apoB-100 a s small dense particles were less pronounced. Glucose had no effects on the posttranslational degradation of newly-synthesized apoB-100 or apoB-48. Ho wever, this process was significantly enhanced by MH. The results suggest t hat glucose stimulates TAG synthesis, turnover, and output as VLDL. These e ffects are associated with an increased VLDL output of apoB mediated mainly by an increase in the net synthesis of both apoB-48 and apoB-100. All thes e changes are prevented by interference with glucose phosphorylation. Outpu t of small, dense, apoB-containing particles is relatively unaffected by th e glucose and MH-induced changes in TAG synthesis and lipolysis, an observa tion which suggests that only the bulk lipid addition step of VLDL assembly is affected by changes in glucose metabolism.