Hepatic Autophagy Is Suppressed in the Presence of Insulin Resistance and Hyperinsulinemia

Abstract

Autophagy is essential for maintaining both survival and health of cells. Autophagy is normally suppressed by amino acids and insulin. It is unclear what happens to the autophagy activity in the presence of insulin resistance and hyperinsulinemia. In this study, we examined the autophagy activity in the presence of insulin resistance and hyperinsulinemia and the associated mechanism. Insulin resistance and hyperinsulinemia were induced in mice by a high fat diet, followed by measurements of autophagy markers. Our results show that autophagy was suppressed in the livers of mice with insulin resistance and hyperinsulinemia. Transcript levels of some key autophagy genes were also suppressed in the presence of insulin resistance and hyperinsulinemia. Conversely, autophagy activity was increased in the livers of mice with streptozotocin-induced insulin deficiency. Levels of vps34, atg12, and gabarapl1 transcripts were elevated in the livers of mice with insulin deficiency. To study the mechanism, autophagy was induced by nutrient deprivation or glucagon in cultured hepatocytes in the presence or absence of insulin. Autophagy activity and transcript levels of vps34, atg12, and gabarapl1 genes were reduced by insulin. The effect of insulin was largely prevented by overexpression of the constitutive nuclear form of FoxO1. Importantly, autophagy of mitochondria (mitophagy) in cultured cells was suppressed by insulin in the presence of insulin resistance. Together, our results show that autophagy activity and expression of some key autophagy genes were suppressed in the presence of insulin resistance and hyperinsulinemia. Insulin suppression of autophagy involves FoxO1-mediated transcription of key autophagy genes.Macroautophagy (autophagy) is a catabolic process whereby long lived large molecules and cellular organelles, such as mitochondria and endoplasmic reticulum (ER), 3 are degraded by lysosomes for an alternative energy source during starvation (1, 2). Autophagy is normally activated by glucagon or deprivation of amino acids during starvation (1) but inhibited by amino acids and/or insulin through the mTOR-or/and Akt-dependent pathways after food intake (3, 4). Thus, autophagy activity fluctuates with food intakes and fasts. Importantly, autophagy is also essential for maintaining cellular health by removing misfolded large molecules and aged/dysfunctional cellular organelles, such as mitochondria and ER (1, 5). In other words, decreased autophagy will inevitably slow the removal of misfolded large molecules and aged/dysfunctional cellular organelles. Accumulation of these molecules and dysfunctional cellular organelles may not only contribute to the development of cancers (1) but also contribute to the development of metabolic diseases, such as insulin resistance. For example, the accumulation of dysfunctional mitochondria will most likely cause increased mitochondrion-derived oxidative stress, which is known to contribute to the development of insulin resistance (6 -10).Insulin resistance is either a precursor or...

High-fat diet increases autophagic flux in pancreatic beta cells in vivo and ex vivo in mice

“…3c,d). This finding suggests that the highfat diet reduced autophagic flux in the liver as previously suggested [10,11], which was confirmed with another marker, p62/SQSTM1 (sequestosome 1), which is cleared in lysosomes and thus correlates inversely with autophagy. p62/SQSTM1 staining in the liver was increased (consistent with reduced autophagic flux) by both chloroquine treatment and a high-fat diet (ESM Fig.…”

“…Although LC3 can also associate with lipid droplets [10] these were not detected in beta cells under conditions in which GFP-LC3 puncta were readily observable. The specificity of our approach is further underscored by the contrast with liver, where autophagic flux was inhibited by high-fat feeding, consistent with previous work [10,11].…”

Human liver epigenetic alterations in non-alcoholic steatohepatitis are related to insulin action

“…35,36 SQSTM1 encodes a multifunctional protein that besides binding ubiquitin and regulating activation of the nuclear factor kappa-B (NF-kB) signaling pathway, has also effects related to impairment of autophagic flux, 37 thereby contributing to hepatic insulin sensitivity. [38][39][40] Moreover, correlations of NASH-associated CpG sites with mRNA expression of CEACAM1, SH3BP4 (encoding transferrin receptor-trafficking protein), and NAT2 also support a role for our findings relating differential DNA methylation in NASH with impaired insulin signaling/action. [41][42][43][44][45][46][47] The mechanisms that could alter DNA methylation in the insulin-resistant liver remain to be elucidated.…”

Overview of macroautophagy regulation in mammalian cells

“…Expression of a constitutive form of FoxO3 induces autophagy in adult mouse skeletal muscle. Recently, another member of the FoxO protein family, FoxO1, has been shown to regulate the expression of key autophagy genes Pik3c3, Atg12, and Gabarapl1 in hepatocytes in an insulin-dependent manner [151]. p53 p53 is a pivotal factor involved in regulating cell death and survival, and in regulating metabolism [152].…”