HIV protease inhibitors induce metabolic dysfunction in part via increased JNK1/2 pro-inflammatory signaling in L6 cells

Protective effect of triterpenes against diabetes-induced β-cell damage: An overview of in vitro and in vivo studies

Accumulative evidence shows that chronic hyperglycaemia is a major factor implicated in the development of pancreatic β-cell dysfunction in diabetic patients. Furthermore, most of these patients display impaired insulin signalling that is responsible for accelerated pancreatic β-cell damage. Indeed, prominent pathways involved in glucose metabolism such as phosphatidylinositol 3-kinase/ protein kinase B (PI3-K/AKT) and 5' AMP-activated protein kinase (AMPK) are impaired in an insulin resistant state. The impairment of this pathway is associated with over production of reactive oxygen species and pro-inflammatory factors that supersede pancreatic β-cell damage. Although several antidiabetic drugs can improve β-cell function by modulating key regulators such as PI3-K/AKT and AMPK, evidence of their β-cell regenerative and protective effect is scanty. As a result, there has been continued exploration of novel antidiabetic therapeutics with abundant antioxidant and antiinflammatory properties that are essential in protecting against β-cell damage. Such therapies include triterpenes, which have displayed robust effects to improve glycaemic tolerance, insulin secretion, and pancreatic β-cell function. This review summarises most relevant effects of various triterpenes on improving pancreatic β-cell function in both in vitro and in vivo experimental models. A special focus falls on studies reporting on the ameliorative properties of these compounds against insulin resistance, oxidative stress and inflammation, the well-known factors involved in hyperglycaemia associated tissue damage.

Amelioration of palmitate-induced metabolic dysfunction in L6 muscle cells expressing low levels of receptor-interacting protein 140

We have shown that reduced expression of receptor-interacting protein 140 (RIP140) alters the regulation of fatty-acid (FA) oxidation in muscle. To determine whether a high level of FA availability alters the effects of RIP140 on metabolic regulation, L6 myotubes were transfected with or without RNA interference oligonucleotide sequences to reduce RIP140 expression, and then incubated with high levels of palmitic acid, with or without insulin. High levels of palmitate reduced basal (53%-58%) and insulin-treated (24%-44%) FA uptake and oxidation, and increased basal glucose uptake (88%). In cells incubated with high levels of palmitate, low RIP140 increased basal FA uptake and insulin-treated FA oxidation and glucose uptake, and decreased basal glucose uptake and insulin-treated FA uptake. Under basal conditions, low RIP140 increased the mRNA content of FAT/CD36 (159%) and COX4 (61%), as well as the protein content of Nur77 (68%), whereas the mRNA expression of FGF21 (50%) was decreased, as was the protein content of CPT1b (35%) and FGF21 (44%). Under insulin-treated conditions, low RIP140 expression increased the mRNA content of MCAD (84%) and Nur77 (84%), as well as the protein content of Nur77 (23%). Thus, a low level of RIP140 restores the rates of FA uptake in the basal state, in part via a reduction in upstream insulin signaling. Our data also indicate that the protein expression of Nur77 may be modulated by RIP140 when muscle cells are metabolically challenged by high levels of palmitate.

Lopinavir inhibits insulin signaling by promoting protein tyrosine phosphatase 1B expression

Treatment with antiretroviral therapy, including protease inhibitors (PIs), may result in metabolic side-effects, for example insulin resistance. The aim of the present study was to investigate the mechanism of the dysregulation of insulin signaling by two PIs, lopinavir and darunavir, by analyzing changes in the expression or activity of proteins associated with insulin signaling. 3T3-L1 preadipocytes were pretreated with lopinavir or darunavir for 48 h and then stimulated with insulin for 30 min. The cell lysates were subjected to western blotting with anti-phospho-insulin receptor substrate (IRS) 1, anti-IRS1, anti-suppressor of cytokine signaling (SOCS) 1, anti-SOCS3 and anti-protein tyrosine phosphatase (PTP) 1B antibodies and to immunoprecipitation with anti-IRS1 antibody. Translocation of glucose transporter 4 (GLUT4) following treatment with lopinavir or darunavir was observed using immunofluorescence. While GLUT4 was recruited to the cellular membrane in control adipocytes following insulin stimulation, it was diffusely distributed in the cytosol in lopinavir-treated adipocytes. In darunavir-treated adipocytes, GLUT4 was mainly recruited to the cellular membrane, but some GLUT4 remained in the cytosol. After insulin stimulation, IRS1 was tyrosine-phosphorylated to a greater extent in control adipocytes compared with darunavir-treated adipocytes. Tyrosine phosphorylation of IRS1 was inhibited in lopinavir-treated adipocytes. The expression of PTP1B was upregulated in adipocytes pretreated with the PIs, particularly lopinavir, compared with those pretreated with a vehicle control. The degree of regulation in insulin signaling differs between lopinavir and darunavir. One mechanism by which lopinavir regulates insulin signaling is by the promotion of PTP1B expression.