The protein-tyrosine phosphatases PTP1B and Syp have both been implicated as modulators of the mitogenic actions of insulin. However, the roles of these protein-tyrosine phosphatases in the metabolic actions of insulin are not well characterized. In this study, we directly assessed the ability of PTP1B and Syp to modulate insulin-stimulated translocation of the insulin-responsive glucose transporter GLUT4 in a physiologically relevant insulin target cell. Primary cultures of rat adipose cells were transiently transfected with either wild-type PTP1B (PTP1B-WT), wild-type Syp (Syp-WT), or the catalytically inactive mutants PTP1B-C/S or Syp-C/S. The effects of overexpression of these constructs on insulin-stimulated translocation of a co-transfected epitope-tagged GLUT4 were studied. Cells overexpressing either PTP1B-C/S or Syp-WT had insulin dose-response curves similar to those obtained with control cells expressing only epitope-tagged GLUT4. In contrast, for cells overexpressing PTP1B-WT the level of GLUT4 on the cell surface at each insulin dose (ranging from 0 to 60 nM) was significantly lower than that observed in the control cells. Interestingly, cells overexpressing the dominant inhibitory mutant Syp-C/S also had a small but statistically significant impairment in insulin responsiveness. At a maximally stimulating concentration of insulin (60 nM), cell surface epitope-tagged GLUT4 was approximately 20% less than that of the control cells. It is possible that effects from high level overexpression of Syp and PTP1B constructs may not reflect what occurs under physiological conditions. Nevertheless, our data raise the possibility that PTP1B may be a negative regulator of insulin-stimulated glucose transport, while Syp may have a small role as a positive mediator of the metabolic actions of insulin.

Sarcopenia is an age-related decline in skeletal muscle mass and function that is multifactorial in etiology. Age-related changes in the renin-angiotensin system (RAS), increased oxidative stress, and chronic inflammation likely all contribute to its development. Losartan, an angiotensin II type I receptor blocker (ARB) decreases RAS activity and likely influences oxidative stress and inflammation. Given this, we hypothesized that losartan would improve activity levels and parameters related to inflammation and oxidative stress in older mice. We sought to test this hypothesis by comparing functional and molecular parameters between 18-month-old C57BL/6 mice treated with 50-70 mg/kg/day of losartan over a 4 month-period and age- and gender-matched mice receiving placebo. Losartan treatment significantly improved several activity measurements during treatment period compared to placebo controlled group, including increased time on treadmill, traveling activity, standing activity, and decreased grid contacts (p-values < 0.05, 0.001, 0.01; and 0.04 respectively). Grip strength did not improve in treatment group relative to control group over time. Serum IL-6 level in the treated group was significantly lower than that in the control group at the end of treatment, (30.3±12.9 vs. 173.0±59.5 pg/ml, p< 0.04), and mRNA expression of antioxidant enzymes catalase (3.9±0.9 vs. 1.0±0.4) and glutathione peroxidase (4.7±1.1 vs. 1.0±0.4) was significantly higher, P-value: 0.02, and 0.03 respectively) in quadriceps muscle after 4 months of treatment in treated and control groups. These results support the hypothesis that chronic losartan treatment improves skeletal muscle related activity measures in older mice, and that it is associated with more favorable relevant biological profiles in the treatment group. Additional studies are needed to 1) further quantify this functional improvement, 2) further identify mechanisms that influence this improvement, and 3) provide additional rationale for translating these findings into older adults.