| Apolipoprotein B100 (apoB), the structural protein component of triglyceride-rich very low density lipoprotein (VLDL) and atherogenic low density lipoprotein, is considered an important risk indicator of atherosclerosis. In insulin resistant states, hepatic overproduction of apoB leads to metabolic dyslipidemia, characterized by high circulating VLDL and hypertriglyceridemia. Since the mitogen-activated protein kinases (MAPKs) and the inhibitor of NFκB kinase (IKK)-NFκB cascades are perturbed in insulin resistance, we hypothesized that the MAPKs (ERK, p38 and JNK) and the IKK-NFκB pathways regulate hepatic apoB output. We modulated these pathways in HepG2, a human hepatoma cell line, and primary hamster hepatocytes using chemical inhibitors and protein overexpression. ApoB synthesis and secretion were examined by metabolic pulse labeling. HepG2 is typically defective in secreting apoB as large VLDL particles and secretes smaller triglyceride-poor apoB-particles. Under continuous pulse labeling, ERK inhibition not only increased apoB secretion, it enabled HepG2 to secrete VLDL-sized particles in the presence of exogenous fatty acid (oleate). Concomitant with the increased apoB-particle size, ERK inhibition raised intracellular triglyceride level and diacylglycerol acyltransferase (DGAT) 1 and DGAT2 mRNA levels. Conversely, ERK activation decreased VLDL-apoB secretion from primary hepatocytes. In contrast to ERK, p38 or JNK inhibition decreased apoB secretion without affecting apoB-particle size from oleate-treated HepG2 cells. JNK inhibition also modulated apoB levels in primary hamster hepatocytes. Interestingly, the development of diet-induced hepatic insulin resistance was associated with decreased ERK, and enhanced p38 and NFκB activities. Thus we investigated the role of the NFκB pathway in regulating hepatic apoB production. IKK inhibition decreased and IKK overexpression increased apoB levels by modulating apoB mRNA translation and protein stability. IKK inhibition also suppressed hepatic apoB overproduction in an insulin resistance model, the fructose-fed hamster. Altogether, our results suggest that among the MAPK cascades, the MEK-ERK pathway is crucial in regulating apoB-lipoprotein assembly, possibly by modulating lipid availability to newly-synthesized apoB. The inflammatory IKK-NFκB cascade is also involved in regulating apoB synthesis and secretion. We postulate that dysregulation in the MAPK or NFκB cascades in insulin resistant and inflammatory states may contribute to hepatic apoB overproduction, and the common phenotype of hypertriglyceridemia and dyslipidemia. |
