Insulin resistance is mechanistically linked to hepatic mitochondrial remodeling in non-alcoholic fatty liver disease

OBJECTIVE

Insulin resistance and altered hepatic mitochondrial function are central features of type 2 diabetes (T2D) and non-alcoholic fatty liver disease (NAFLD), but the etiological role of these processes in disease progression remains unclear. Here we investigated the molecular links between insulin resistance, mitochondrial remodeling, and hepatic lipid accumulation.

METHODS

Hepatic insulin sensitivity, endogenous glucose production, and mitochondrial metabolic fluxes were determined in wild-type, obese (ob/ob) and pioglitazone-treatment obese mice using a combination of radiolabeled tracer and stable isotope NMR approaches. Mechanistic studies of pioglitazone action were performed in isolated primary hepatocytes, whilst molecular hepatic lipid species were profiled using shotgun lipidomics.

RESULTS

Livers from obese, insulin-resistant mice displayed augmented mitochondrial content and increased tricarboxylic acid cycle (TCA) cycle and pyruvate dehydrogenase (PDH) activities. Insulin sensitization with pioglitazone mitigated pyruvate-driven TCA cycle activity and PDH activation via both allosteric (intracellular pyruvate availability) and covalent (PDK4 and PDP2) mechanisms that were dependent on PPARγ activity in isolated primary hepatocytes. Improved mitochondrial function following pioglitazone treatment was entirely dissociated from changes in hepatic triglycerides, diacylglycerides, or fatty acids. Instead, we highlight a role for the mitochondrial phospholipid cardiolipin, which underwent pathological remodeling in livers from obese mice that was reversed by insulin sensitization.

CONCLUSION

Our findings identify targetable mitochondrial features of T2D and NAFLD and highlight the benefit of insulin sensitization in managing the clinical burden of obesity-associated disease.

Dynamic proteome profiling of human pluripotent stem cell-derived pancreatic progenitors

A comprehensive characterization of the molecular processes controlling cell fate decisions is essential to derive stable progenitors and terminally differentiated cells that are functional from human pluripotent stem cells (hPSCs). Here, we report the use of quantitative proteomics to describe early proteome adaptations during hPSC differentiation toward pancreatic progenitors. We report that the use of unbiased quantitative proteomics allows the simultaneous profiling of numerous proteins at multiple time points, and is a valuable tool to guide the discovery of signaling events and molecular signatures underlying cellular differentiation. We also monitored the activity level of pathways whose roles are pivotal in the early pancreas differentiation, including the Hippo signaling pathway. The quantitative proteomics data set provides insights into the dynamics of the global proteome during the transition of hPSCs from a pluripotent state toward pancreatic differentiation.

Insulin receptor-mediated signaling regulates pluripotency markers and lineage differentiation

Objectives

Insulin receptor (IR)-mediated signaling is involved in the regulation of pluripotent stem cells; however, its direct effects on regulating the maintenance of pluripotency and lineage development are not fully understood. The main objective of this study is to understand the role of IR signaling in pluripotency and lineage development.

Methods

To explore the role of IR signaling, we generated IR knock-out (IRKO) mouse induced pluripotent stem cells (miPSCs) from E14.5 mouse embryonic fibroblasts (MEFs) of global IRKO mice using a cocktail of four reprogramming factors: Oct4, Sox2, Klf4, cMyc. We performed pluripotency characterization and directed the differentiation of control and IRKO iPSCs into neural progenitors (ectoderm), adipocyte progenitors (mesoderm), and pancreatic beta-like cells (endoderm). We mechanistically confirmed these findings via phosphoproteomics analyses of control and IRKO iPSCs.

Results

Interestingly, expression of pluripotency markers including Klf4, Lin28a, Tbx3, and cMyc were upregulated, while abundance of Oct4 and Nanog were enhanced by 4-fold and 3-fold, respectively, in IRKO iPSCs. Analyses of signaling pathways demonstrated downregulation of phospho-STAT3, p-mTor and p-Erk and an increase in the total mTor and Erk proteins in IRKO iPSCs in the basal unstimulated state. Stimulation with leukemia inhibitory factor (LIF) showed a ∼33% decrease of phospho-ERK in IRKO iPSCs. On the contrary, Erk phosphorylation was increased during in vitro spontaneous differentiation of iPSCs lacking IRs. Lineage-specific directed differentiation of the iPSCs revealed that cells lacking IR showed enhanced expression of neuronal lineage markers (Pax6, Tubb3, Ascl1 and Oligo2) while exhibiting a decrease in adipocyte (Fas, Acc, Pparγ, Fabp4, C/ebpα, and Fsp27) and pancreatic beta cell markers (Ngn3, Isl1, and Sox9). Further molecular characterization by phosphoproteomics confirmed the novel IR-mediated regulation of the global pluripotency network including several key proteins involved in diverse aspects of growth and embryonic development.

Conclusion

We report, for the first time to our knowledge, the phosphoproteome of insulin, IGF1, and LIF stimulation in mouse iPSCs to reveal the importance of insulin receptor signaling for the maintenance of pluripotency and lineage determination.

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Insulin receptor signaling regulates expression of key pluripotency genes including Oct4 and Nanog.

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IRKO iPSCs show upregulation of neuronal markers during differentiation.

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Adipocyte and pancreatic beta cell differentiation are perturbed in IRKO iPSCs.

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Phosphoproteomics analyses confirmed the role of IR in regulation of pluripotency and developmental proteins.

Cellular stress drives pancreatic plasticity

Controversy has long surrounded research on pancreatic beta cell regeneration. Some groups have used nonphysiological experimental methodologies to build support for the existence of pancreatic progenitor cells within the adult pancreas that constantly replenish the beta cell pool; others argue strongly against this mode of regeneration. Recent research has reinvigorated enthusiasm for the harnessing of pancreatic plasticity for therapeutic application--for example, the transdifferentiation of human pancreatic exocrine cells into insulin-secreting beta-like cells in vitro; the conversion of mouse pancreatic acinar cells to beta-like cells in vivo via cytokine treatment; and the potential redifferentiation of dedifferentiated mouse beta cells in vivo. Here, we highlight key findings in this provocative field and provide a perspective on possible exploitation of human pancreatic plasticity for therapeutic beta cell regeneration.