Adipose tissue is an active endocrine organ that secretes various humoral factors (adipokines), and its shift to production of proinflammatory cytokines in obesity likely contributes to the low-level systemic inflammation that may be present in metabolic syndrome-associated chronic pathologies such as atherosclerosis. Leptin is one of the most important hormones secreted by adipocytes, with a variety of physiological roles related to the control of metabolism and energy homeostasis. One of these functions is the connection between nutritional status and immune competence. The adipocyte-derived hormone leptin has been shown to regulate the immune response, innate and adaptive response, both in normal and pathological conditions. The role of leptin in regulating immune response has been assessed in vitro as well as in clinical studies. It has been shown that conditions of reduced leptin production are associated with increased infection susceptibility. Conversely, immune-mediated disorders such as autoimmune diseases are associated with increased secretion of leptin and production of proinflammatory pathogenic cytokines. Thus, leptin is a mediator of the inflammatory response.

Hyperhomocysteinemia and insulin resistance are independent factors for cardiovascular disease. Most of the angiotoxic effects of homocysteine are related to the formation of homocysteine thiolactone and the consequent increase in oxidative stress. The oxidative stress has also been shown to impair insulin action, therefore leading to insulin resistance. In order to study a putative direct effect of homocysteine on insulin signaling, we have characterized the molecular counter-regulation of the early events in the signal transduction of the insulin receptor, and the metabolic end-point of glycogen synthesis. We employed HTC rat hepatoma cells transfected with the human insulin receptor. A 10 min exposure to homocysteine thiolactone (50 microM) resulted in a significant inhibition of insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit and its substrates IRS-1 and p60-70, as well as their association with the p85 regulatory subunit of phosphatidylinositol 3-kinase. These effects led to impairment of the insulin-stimulated phosphatidylinositol 3-kinase activity, which plays a central role in regulating insulin action. Thus, insulin-stimulated glycogen synthesis was also inhibited by homocysteine thiolactone. To investigate whether oxidative stress was mediating the counter-regulatory effect of homocysteine thiolactone on insulin signaling, we preincubated the cells (5 min) with 250 microM glutathione prior to the incubation with homocysteine (10 min) and subsequent insulin challenge. Glutathione completely abolished the effects of homocysteine thiolactone on insulin-receptor signaling and restored the insulin-stimulated glycogen synthesis. In conclusion, these data suggest that homocysteine thiolactone impairs insulin signaling by a mechanism involving oxidative stress, leading to a defect in insulin action.

Hyperhomocysteinemia and insulin resistance are independent factors for cardiovascular disease. Most of the angiotoxic effects of homocysteine are related to the formation of homocysteine thiolactone and the consequent increase in oxidative stress. We have recently found that homocysteine thiolactone inhibits insulin receptor tyrosine kinase activity, which results in decreased phosphatidylinositol 3-kinase (PI3K) activity and inhibition of glycogen synthesis. Oxidative stress seemed to be the mechanism underlying these effects, since glutathione was able to restore the insulin signaling as well as the insulin-mediated glycogen synthesis. In the present work we have further investigated insulin receptor signaling studying mitogen-activated protein kinase (MAPK), glycogen synthase kinase-3 (GSK-3) and p70 S6K phosphorylation. Again, homocysteine thiolactone (50 μM) prevented insulin-mediated MAPK, GSK-3 and p70 S6K phosphorylation and these effects were blocked by glutathione (250 μM). Since MAPK and PI3K pathways, including GSK3 and S6K, seem to mediate insulin-mediated growth and proliferation, we measured DNA and protein synthesis. We have found that homocysteine thiolactone (50 μM) inhibits insulin-mediated growth and proliferation, as previously shown for glycogen synthesis. Again, these effects seem to be mediated by oxidative stress, since 250 μM glutathione completely abolished the effects of homocysteine thiolactone on insulin-stimulated DNA and protein synthesis. In conclusion, these data suggest that homocysteine thiolactone impairs insulin signaling by a mechanism involving oxidative stress, leading to a defect in the action of insulin on growth and proliferation.