Mechanism of lipid induced insulin resistance: Activated PKCε is a key regulator

Abstract

Fatty acids (FAs) are known to impair insulin signaling in target cells. Accumulating evidences suggest that one of the major sites of FAs adverse effect is insulin receptor (IR). However, the underlying mechanism is yet unclear. An important clue was indicated in leptin receptor deficient (db/db) diabetic mice where increased circulatory FAs was coincided with phosphorylated PKCε and reduced IR expression. We report here that central to this mechanism is the phosphorylation of PKCε by FAs. Kinase dead mutant of PKCε did not augment FA induced IRβ downregulation indicating phosphorylation of PKCε is crucial for FA induced IRβ reduction. Investigation with insulin target cells showed that kinase independent phosphorylation of PKCε by FA occurred through palmitoylation. Mutation at cysteine 276 and 474 residues in PKCε suppressed this process indicating participation of these two residues in palmitoylation. Phosphorylation of PKCε endowed it the ability to migrate to the nuclear region of insulin target cells. It was intriguing to search about how translocation of phosphorylated PKCε occurred without having canonical nuclear localization signal (NLS). We found that F-actin recognized phospho-form of PKCε and chaperoned it to the nuclear region where it interact with HMGA1 and Sp1, the transcription regulator of IR and HMGA1 gene respectively and impaired HMGA1 function. This resulted in the attenuation of HMGA1 driven IR transcription that compromised insulin signaling and sensitivity.

Saturated Fatty Acids Induce Post-transcriptional Regulation of HAMP mRNA via AU-rich Element-binding Protein, Human Antigen R (HuR)

“…Both classical (PKC␣) and novel (PKC␦) PKC isoforms have been shown to phosphorylate HuR at serine residues within or adjacent to the HuR-nucleo-cytoplasmic shuttling sequence and thereby activate its translocation to the cytoplasm (46,47). Numerous studies have clearly shown that PA treatment activates both the classical and novel class isoforms of PKC (22)(23)(24)48), but the effect of PA on HuR activation is unknown. We have established a connection between PA and HuR and validated the importance of HuR in PA-induced HAMP mRNA stabilization.…”

“…Independently, PA has also been shown to induce the activation of multiple isoforms of PKC (21)(22)(23)(24)(25). We therefore examined the role of PKC in post-transcriptional regulation of HAMP by PA. For these studies, HepG2 cells were first treated with staurosporine, a potent broad-spectrum protein kinase C inhibitor (39,40).…”

“…The phosphorylation-mediated nucleo-cytoplasmic shuttling of HuR is essential for its mRNA stabilization function (20). Saturated fatty acids have been shown to activate various signaling pathways in hepatoma cells, including PKCs (21)(22)(23)(24)(25). However, the role of HuR or PKCs in HAMP regulation has not been investigated.…”

Mechanism of Lipid Induced Insulin Resistance: An Overview

“…we found it is F-actin, which has 13 NLS recognizes pPKCε and chaperoned it to nuclear region which coincided with reduced IR expression in skeletal muscle cells [47]. Question is how pPKCε is involved in IR downregulation.…”

“…We have reported that FA initiates disruption of insulin signal by phosphorylating PKCε in a PDK1 independent manner [21,22,47]. Palmitate and myristate phosphorylated PKCε (pPKCε) which then moves to nuclear region.…”