Liver X receptor agonists ameliorate TNFα-induced insulin resistance in murine brown adipocytes by downregulating protein tyrosine phosphatase-1B gene expression

Abstract

Aims/hypothesis The nuclear receptors, including nuclear receptor subfamily 1, group H, member 3 (NR1HR, also known as liver X receptor [LXR]), are sensors of cholesterol metabolism and lipid biosynthesis that have recently been proposed as insulin sensitisers. TNFα has been described as a link between obesity and the development of insulin resistance, an important contributor to the pathogenesis of type 2 diabetes. Therefore, we decided to investigate the ability of NR1HR agonists to ameliorate TNFα-induced insulin resistance in brown adipocytes. Methods Primary brown adipocytes from rat fetuses, and from wild-type neonate mice and neonate mice deficient in the gene encoding protein tyrosine phosphatase-1B (Ptpn1, also known as Ptp1b) were cultured in the absence or presence of TNFα and different nuclear receptor agonists. Among them, the unrelated NR1HR ligands T0901317, GW3965 and (22R)-hydroxycholesterol were tested. After insulin stimulation, glucose uptake and solute carrier family 2 (facilitated glucose transporter), member 4 (SLC2A4, formerly known as GLUT4) translocation were measured. Next the insulin signalling cascade was determined by submitting cells to lysis, immunoprecipitation and immunoblotting. Results NR1HR agonists ameliorate TNFα-induced insulin resistance restoring completely insulin-stimulated glucose uptake and SLC2A4 translocation to plasma membrane. This effect is parallel to the recovery of the insulin cascade insulin receptor/IRS-2/phosphatidylinositol 3-kinase/protein kinase B, and could be due to the fact that T0901317 prevents the increase of PTPN1 production and phosphatase activity produced by TNFα. In this regard, Ptpn1-deficient brown adipocytes showed protection against insulin resistance by TNFα. Moreover, we observed that T0901317 produced in itself a significant increase over basal glucose uptake consistent with an increase of SLC2A4 protein content in plasma membrane, attributable to the activation of protein kinase ζ and/or the increase of Slc2a4 expression. Conclusions/interpretation Nuclear receptors NR1HR are interesting potential targets for drug treatment of insulin resistance. Keywords

Dual Role of Interleukin-6 in Regulating Insulin Sensitivity in Murine Skeletal Muscle

“…Among these molecules, tumor necrosis factor (TNF)-␣ and interleukin (IL)-6 have been proposed as a link between obesity and insulin resistance because 1) the majority of type 2 diabetic patients are obese, 2) TNF-␣ and IL-6 are overexpressed in adipose tissues of obese animals and humans, and 3) elevated plasma concentrations of IL-6 are detected in obese and insulin-resistant patients (4,5). We previously investigated how TNF-␣ treatment induces a state of insulin resistance in vivo and in vitro at the level of insulin receptor substrate (IRS) (6,7). Accordingly, we identified the Ser307 residue in IRS-1 as a site for TNF-␣-impaired insulin signaling in myotubes, and p38 mitogen-activated protein kinase (MAPK) and inhibitor B (IB) kinase are involved in the phosphorylation of this residue (8).…”

“…Furthermore, mice lacking PTP1B also exhibit increased insulin sensitivity under both dietary or polygenic insulin resistance (31,32). We recently found upregulation of PTP1B by TNF-␣ and protection against insulin resistance by this cytokine in mice and cells lacking PTP1B (6,7). Accordingly, our final goal was to investigate whether PTP1B deficiency confers protection against insulin resistance by IL-6.…”

New emerging role of protein-tyrosine phosphatase 1B in the regulation of glycogen metabolism in basal and TNF-α-induced insulin-resistant conditions in an immortalised muscle cell line isolated from mice

“…Insulin was able to abolish this TNF-α effect in cells lacking PTP1B but not in wild-type cells. Thus, our data revealed a dual role for TNF-α, enhancing the basal GS activity ratio and blocking insulin action, not only in terms of glucose uptake [27,28] but also in several steps within the insulin signalling cascade and glycogen synthesis.…”

“…One of the best-characterised phosphatases, proteintyrosine phosphatase 1B (PTP1B), dephosphorylates tyrosine residues of IR and IRS-1 with the subsequent attenuation of the insulin signalling cascade [22,23]. Interestingly, the activity of PTP1B and mRNA level of PTP1B/Ptp1b (also known as PTPN1/Ptpn1) are elevated in muscle and adipose tissue of obese and diabetic humans and rodents [24,25] and, as our group has observed, TNF-α and IL-6 action enhance PTP1B levels [26][27][28]. Moreover, PTP1B overproduction in L6 muscle cells impairs glucose uptake and insulin-stimulated IRS-1 phosphorylation [29][30][31].…”

“…In addition, insulin action is enhanced in PTP1B-deficient myocytes. Several studies have proposed TNF-α as a link between insulin resistance and obesity [48] and, as our group has previously described, both adipocytes and myocytes showed defects at different levels in insulin action after chronic exposure to the cytokine [16,27]. Nevertheless, few data on the role of TNF-α on glycogen metabolism have emerged in recent years [49].…”