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Atypical PKC Knockout Models: Effect on Glucose and Lipid Homeostasis


Insulin stimulates glucose transport in muscle and adipocytes, and lipid synthesis in liver by activating atypical protein kinase C (aPKC) and Akt (PKB). Muscle-specific knockout of one PKC-lambda allele (Het-Mlambda KO) diminishes glucose transport, thus causing impaired glucose tolerance, insulin resistance, abdominal obesity, hepatosteatosis, and hyperlipidemia, i.e., a metabolic syndrome (MS), and ultimately type 2 diabetes mellitus (T2DM). Hyperinsulinemia in Het-M KO mice inordinately activates hepatic sterol receptor element receptor binding protein-1c (SREBP-1c) and NF B, and resultant increases in lipid and cytokine production presumably contribute to insulin resistance, and this is probably abetted by recently observed increases in hepatic activities of conventional and novel PKCs (alpha,B2,epsilon,theta ) in older Het-Mlambda KO mice, We have also generated adipocyte (A)-specific PKC-lambda KO (Alambda KO) mice and glucose transport is diminished in isolated adipocytes. However, unlike Mlambda KO mice, Alambda -KO mice are lean and have normal glucose tolerance, normal serum levels of insulin and lipids, and clamp studies reveal increases in hepatic insulin sensitivity and suppression of hepatic glucose output, owing to decreased expression of phosphoenolpyruvate carboxykinase (PEPCK). We postulate that diminished glucose transport in Alambda -KO adipocytes limits glycerol-PO4 availability for fat synthesis, and resulting leanness, possibly via hypoleptinemia, downregulates hepatic PEPCK expression and thereby confers metabolic protection. Liver-specific PKC-lambda KO (Llambda KO) also protects lipid and glucose homeostasis by diminishing activation of hepatic SREBP-1c and NF B, and by diminishing fasting-dependent expression of PEPCK and glucose-6-phosphatase (G6Pase). Thus, specific inhibition of hepatic aPKC by adenoviral-mediated expression of kinase-inactive aPKC, or our newly developed PKC-lambda inhibitors, diminishes insulin activation of SREBP-1c/NF B, and fasting expression of PEPCK/G6Pase, thereby markedly improving obesity, hyperlipidemia, hyperglycemia, insulin signaling in muscle, and insulin resistance in obese/T2DM mice. We have also generated mice with total body loss of one PKC-lambda allele (Het-TBlambda KO) and as expected glucose transport in muscle and adipose tissues is diminished; however, these mice unexpectedly have poor activation of the insulin receptor and Akt, as well as aPKC, in muscle, adipose and liver tissues. Despite these signaling defects, aside from mild glucose intolerance and hyperinsulinemia, Het-TBlambda KO mice do not develop MS or T2DM. We postulate that partial loss of hepatic PKC-lambda in Het-TB KO confers metabolic protection by diminishing activity/expression of SREBP-1c, NF B, PEPCK and G6Pase. In Specific Objective 1, to define mechanisms underlying leanness and metabolic protection in AlambdaKO mice, we will examine: lipogenic and lipolytic processes in adipocytes; factors that contribute to leanness; alterations in hepatic lipogenic, inflammatory and gluconeogenic pathways during high-fat feeding; and, the role of hypoleptinemia in diminishing hepatic gluconeogenesis. In Specific Objective 2, we will test the idea that the partial loss of hepatic PKC-lambda confers metabolic protection in Het-TB KO mice by replenishing hepatic aPKC. In Specific Objective 3, we will examine age-dependent alterations in activities of conventional and novel PKCs in tissues of Het-Mlambda KO mice and correlate these alterations with impairments in insulin signaling as these mice become progressively diabetic.

Funded by the NIH National Center for Advancing Translational Sciences through its Clinical and Translational Science Awards Program, grant number UL1TR002541.