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

Abstract

Insulin resistance with resultant hyperinsulinemia refl ects an impairment in insulin-regulated glucose homeostasis, and is a key pathogenic element in obesity, metabolic syndrome features, and type 2 diabetes mellitus (T2DM). In this regard, primary impairments in insulin-stimulated glucose transport in muscle owing to muscle-specifi c knockout of the insulin receptor, the Glut4 glucose transporter, or atypical protein kinase C (aPKC), PKC-can induce insulin resistance and development of obesity, metabolic syndrome, and T2DM ( 1, 2 ). However, dietary excesses appear to be particularly important contributors to the high prevalence of insulin-resistant disorders in Western/Westernized populations. Unfortunately, defects in glucose metabolism and underlying mechanisms that underlie insulin resistance in diet-induced obesity, and are only partly understood. Indeed, both the causes and required forms of dietary indiscretion that lead to insulin resistance are uncertain. In this regard, however, excessive activity of hepatic aPKC seems to be a commonly observed and critically important contributor to insulin resistance in high-fat-fed (HFF) mice ( 3-5 ), muscle-specifi c PKC-knockout mice ( 6 ), obese overtly diabetic ob/ob mice ( 3, 7 ), and type 2 diabetic rats ( 3, 7 ) and humans ( 8 ). Abstract This work was supported by funds from the Department of Veterans Affairs Merit Review Program and the National Institutes of Health Grants (7RO1DK 065969-09) to R.V.F. The authors declare no fi nancial confl icts of interest.Manuscript received 17 July 2014 and in revised form 5 November 2014. Published, JLR Papers in Press, November 13, 2014 DOI 10.1194 Hepatic insulin resistance in ob/ob mice involves increases in ceramide, aPKC activity, and selective impairment of Akt-dependent FoxO1 phosphorylation Abbreviations: ACC, acetyl-coA carboxylase; ACPD, 2-acetyl-1,3-cyclopentanedione; Akt, protein kinase B; aPKC, atypical protein kinase C; FoxO1, forkhead box O1 protein; G6Pase, glucose-6-phosphatase; GSK3 ␤ , glycogen synthase-3 ␤ ; GTT, glucose tolerance test; HFF, high-fatfed; NF B, nuclear factor B; mTOR, mammalian target of rapamycin; PEPCK, phosphoenolpyruvate carboxykinase; PI3K, phosphatidylinositol 3-kinase; SREBP-1c, sterol receptor binding protein-1c; T2DM, type 2 diabetes mellitus; WD40/ProF, 40 kDa WD(tryp-x-x-asp)-repeat propeller/FYVE protein .

Targeted Induction of Ceramide Degradation Leads to Improved Systemic Metabolism and Reduced Hepatic Steatosis

“…We subsequently injected both the transgenic and WT mice with either PBS or 2-acetyl-1,3-cyclopentanedione (ACPD), a known PKCζ inhibitor demonstrated in previous studies to effectively reduce PKCζ activity (Sajan et al, 2014b), and compared the results with WT and transgenic mice fed a doxycycline diet. For the Alb-AC cohort, we found that injection of ACPD for one month in WT and Alb-AC mice resulted in the same improvements in insulin sensitivity and reduction in hepatic lipid accumulation as Alb-AC mice fed a HFD-dox diet for the same time (Fig.…”

“…This phosphorylation site alters the affinity for Akt to 3,4,5-trisphosphate, thus interfering with Akt translocation to plasma membrane microdomains where it is activated (Fox et al, 2007). A recent report by the Farese group suggests that aberrant activation of atypical PKC, is evident in livers of Lep ob/ob mice and is essential for impairments in Akt mediated signaling to FoxO1; this is rescued in vivo by pharmacological inhibition of atypical PKCs (Sajan et al, 2014b). PKCζ has also attracted attention for its role in lipid homeostasis, particularly as a mediator of SREBP-1c driven lipogenesis (Sajan et al, 2004; Sajan et al, 2009; Taniguchi et al, 2006).…”

BMI-related progression of atypical PKC-dependent aberrations in insulin signaling through IRS-1, Akt, FoxO1 and PGC-1α in livers of obese and type 2 diabetic humans

“…Similarly, in mouse diet-induced obesity (DIO), caloric excesses provoke initial increases in hepatic gluconeogenic enzymes that lead to systemic insulin resistance and liverdependent decreases in insulin signaling in muscle (3, 4). Assuming that dietary excesses are also important in human obesity, the startup of insulin resistance therein is likely to involve alterations in hepatic factors that promote gluconeogenesis.…”

“…Thus, in liver, dietary excesses of fat (3) or carbohydrate (4) in mouse DIO lead to increases in ceramide, which, perhaps with other lipids [e.g., phosphatidic acid (PA)], directly activates aPKC. Combined activation of aPKC by ceramide and hyperinsulinemia-derived PIP 3 causes excessive aPKC accumulation on scaffolding protein, WD40/ProF, and displacement of activated Akt from this platform, where Akt otherwise phosphorylates/inhibits FoxO1 (3, 4, 21–23).…”

Roux-en-Y Gastric Bypass Improves Hepatic Glucose Metabolism Involving Down-Regulation of Protein Tyrosine Phosphatase 1B in Obese Rats

“…Hepatic gluconeogenic and glycolytic pathways are predominantly regulated by the transcription factor forkhead box O1 (FoxO1), one of the major intracellular targets of insulin, through inhibition of transcriptional activation of relevant key enzymes [16,17]. However, Akt-dependent FoxO1 phosphorylation is selectively impaired in ob/ob mice with hepatic insulin resistance [18]. Furthermore, overexpression of FoxO1 facilitates hepatic glucose production, indicating a critical role of FoxO1 in the regulation of glucose homeostasis [19].…”