Role of atypical protein kinase C in activation of sterol regulatory element binding protein-1c and nuclear factor kappa B (NFκB) in liver of rodents used as a model of diabetes, and relationships to hyperlipidaemia and insulin resistance

Abstract

Previous findings in diabetic rodents suggest that insulin activation of atypical protein kinase C (aPKC) is impaired in muscle, but surprisingly conserved in liver, despite impaired hepatic PKB/Akt activation. Moreover, aPKC at least partly regulates two major trans-activators, viz.,hepatic sterol receptor binding protein-1c (SREBP-1c), which controls lipid synthesis, and NFκB, which promotes inflammation and systemic insulin resistance. Presently, in type 2 diabetic Goto-Kakizaki (GK) rats, we examined whether: (a) differences in hepatic aPKC and PKB activation reflect differences in activation of insulin receptor substrate (IRS)-1- and IRS-2-dependent phosphatidylinositol 3-kinase (PI3K); (b) hepatic SREBP-1c and NFκB are excessively activated by aPKC; and (c) metabolic consequences of excessive hepatic aPKC/SREBP-1c/NFκB activation. We found that, in liver as well as in muscle, whereas IRS-1/PI3K activation by insulin was impaired, IRS-2/PI3K was intact. Moreover, selective inhibition of hepatic aPKC by adenoviral-mediated expression of either kinase-inactive aPKC, or shRNA that targets and partially depletes hepatic IRS-2, which controls hepatic aPKC during insulin activation, diminished hepatic SREBP-1c expression and NFκB activities, and concomitantly improved serum lipids and insulin signalling in muscle and liver. Similar improvements in SREBP-1c, NFκB and insulin signaling were seen in ob/ob mice following inhibition of hepatic aPKC. Our findings suggest that, in diabetic rodents: (a) in liver, diminished PKB activation largely reflects impaired IRS-1/PI3K activation, and conserved aPKC activation reflects retained IRS-2/PI3K activity; (b) hepatic aPKC contributes importantly to excessive SREPB-1c and NFκB activities; and (c) excessive hepatic aPKC-dependent activation of SREBP-1c and NFκB contributes importantly to hyperlipidaemia and systemic insulin resistance.

Bifurcation of insulin signaling pathway in rat liver: mTORC1 required for stimulation of lipogenesis, but not inhibition of gluconeogenesis

“…Previous reports raised the possibility that PKCλ, an atypical protein kinase C, may play a role in mediating the insulinmediated increase in SREBP-1c transcription (32)(33)(34), although these results have been controversial (17). Inasmuch as a specific inhibitor for PKCλ is not available, we were not able to directly examine its role in mediating the insulin stimulation of SREBP-1c expression in rat hepatocytes.…”

PKCδ regulates hepatic insulin sensitivity and hepatosteatosis in mice and humans

“…Deletion of PKCε improves glucose-stimulated insulin secretion, reduces insulin clearance, and protects against hepatic insulin resistance, whereas muscle-specific inactivation of PKCι/λ has been shown to impair glucose transport (28). Excessive activation of the atypical PKCζ has been shown to activate SREBP1c and NF-κB and contribute to hyperlipidemia and systemic insulin resistance (29), and we have also demonstrated the existence of divergent regulation of hepatic glucose and lipid metabolism dependent on PI3K and PKCι/ζ (30). Studies on PKCθ inactivation have reported conflicting effects on HFD-induced insulin resistance (31,32).…”

Hepatic insulin resistance in ob/ob mice involves increases in ceramide, aPKC activity, and selective impairment of Akt-dependent FoxO1 phosphorylation

“…In keeping with these ideas, the inhibition of hepatic aPKC in ob/ob mice diminished the association of aPKC with the WD40/ProF platform, and this was attended by increased association of Akt with the WD40/ProF platform, increased FoxO1 phosphorylation, and subsequent decreases in expression and abundance of gluconeogenic enzymes, PEPCK and G6Pase. were not altered by ACPD, these alterations in lipogenic and proinfl ammatory gene function most likely refl ect diminished activity of hepatic aPKC, which is required for insulin-induced activation of SREBP-1c and NF B, and subsequent increases in expression of lipogenic and proinfl ammatory factors (4)(5)(6)(7)(8)(27)(28)(29). In this regard, note that the phosphorylation of mTOR, which mediates the stimulatory effects of Akt on lipogenesis during insulin action ( 14 ), was elevated basally and during insulin treatment in parallel with increases in hepatic Akt activity.…”

“…Liver and muscle samples were homogenized as described (3)(4)(5)(6)(7)(8)24 ). Nuclei were isolated as described ( 4-7 ).…”

“…For this purpose, we used ob/ob mice, as these mice genetically lack the CNS appetite-suppressant, leptin, and develop obesity, metabolic syndrome, and T2DM while eating excessive quantities of standard low-fat mouse chow. On the other hand, we previously evaluated 8-12-week-old overtly diabetic ob/ob mice and found that they have markedly impaired activation of hepatic Akt, but elevated activation of hepatic aPKC ( 3,7 ). However, here, we took advantage of the fact that ob/ob mice generated on a C57Bl/6 background are at fi rst markedly diabetic, but, after several months, their diabetes is largely ameliorated by a surge in pancreatic insulin production.…”