Feeding State, Vitamin A Status and Atypical Protein Kinase C Modulate the Insulin-Regulated Gene Expression in Rat Hepatocytes

Hormonal and nutritional stimuli coordinately regulate the glucose and lipid metabolism in the liver. Dysregulated hormonal balance and nutrient metabolism not only disrupt the energy homeostasis, but also predispose individuals to the development of obesity and its related metabolic diseases. Research presented in this dissertation investigated the effects of a nutritional factor, vitamin A (VA), and a potential component of insulin signaling cascade, atypical protein kinase C (aPKC), on the glucose and lipid metabolic genes transcription in primary hepatocytes from Zucker lean (ZL) and fatty (ZF) rats.

To examine VA effects, we put ZL and ZF rats with different VA statuses on various feeding conditions. After the isolation of their primary hepatocytes, we analyzed the expressions of hepatic model genes in response to the treatments of insulin and retinoic acid (RA). We report that the insulin- and RA-regulated glucokinase (Gck), sterol regulatory element-binding protein-1c (Srebp-1c) and cytosolic form of phosphoenolpyruvate carboxykinase (Pck1) expressions are impaired in hepatocytes of ZF rats fed chow or a VA sufficient (VAS) diet ad libitum. The impairment is partially corrected when ZF rats are fed a VA deficient (VAD) diet ad libitum or pair-fed a VAS diet to the intake of their VAD counterparts in non-fasting conditions. Interestingly, in the pair-fed ZL and ZF rats, transient overeating on the last day of pair-feeding regimen changes the expression levels of some VA catabolic genes, and impairs the insulin- and RA-regulated gene expression in hepatocytes.

To investigate the roles of aPKC, PKCζ(zeta) and PKCι/λ(iota/lambda), they were overexpressed in primary hepatocytes from ZL rats using recombinant adenoviruses. aPKC overexpression in primary hepatocytes significantly attenuated the insulin- regulated expressions of Gck, Srebp-1c and Pck1. Additionally, aPKC overexpression increased their basal transcript levels. The impairment is associated with reduced protein level of insulin receptor substrate 1, and the phosphorylation of protein kinase B in response to insulin treatment.

Taken together, the data demonstrate that feeding state, VA status, and aPKC expression regulate the hepatic glucose and lipid metabolism at the gene expression level, and provide interesting insights into the development of metabolic diseases.