The phosphatidylinositol/AKT/atypical PKC pathway is involved in the improved insulin sensitivity by DHEA in muscle and liver of rats in vivo.

DHEA improves insulin sensitivity and has anti-obesity effect in animal models and men. However, the molecular mechanisms by which DHEA improves insulin action have not been clearly understood. In the present study, we examined the protein levels and phosphorylation state of insulin receptor (IR), IRS-1 and IRS-2, the association between IRSs and PI3K and SHP2, the insulin-induced IRSs associated PI 3-kinase activities, and the phosphorylation status of AKT and atypical PKCzeta/lambda in the liver and the muscle of 6 month-old Wistar rats treated with DHEA. There was no change in IR, IRS-1 and IRS-2 protein levels in both tissues of treated rats analysed by immunoblotting. On the other hand, insulin-induced IRS-1 tyrosine phosphorylation was increased in both tissues while IRS-2 tyrosyl phosphorylation was increased in liver of DHEA treated group. The PI3-kinase/AKT pathway was increased in the liver and the PI3K/atypical PKCzeta/lambda pathway was increased in the muscle of DHEA treated rats. These data indicate that these regulations of early steps of insulin action may play a role in the intracellular mechanism for the improved insulin sensitivity observed in this animal model.

In vivo activation of insulin receptor tyrosine kinase by melatonin in the rat hypothalamus.

Melatonin is the pineal hormone that acts via a pertussis toxin-sensitive G-protein to inhibit adenylate cyclase. However, the intracellular signalling effects of melatonin are not completely understood. Melatonin receptors are mainly present in the suprachiasmatic nucleus (SCN) and pars tuberalis of both humans and rats. The SCN directly controls, amongst other mechanisms, the circadian rhythm of plasma glucose concentration. In this study, using immunoprecipitation and immunoblotting, we show that melatonin induces rapid tyrosine phosphorylation and activation of the insulin receptor beta-subunit tyrosine kinase (IR) in the rat hypothalamic suprachiasmatic region. Upon IR activation, tyrosine phosphorylation of IRS-1 was detected. In addition, melatonin induced IRS-1/PI3-kinase and IRS-1/SHP-2 associations and downstream AKT serine phosphorylation and MAPK (mitogen-activated protein kinase) phosphorylation, respectively. These results not only indicate a new signal transduction pathway for melatonin, but also a potential cross-talk between melatonin and insulin.

Impact of gender on insulin signaling pathway in lacrimal and salivary glands of rats.

The structure and function of lacrimal and salivary glands present gender differences. Previous works have indicated a synergic action between insulin and androgens over lacrimal gland, and insulin-signaling pathways were recently described in lacrimal gland and salivary gland. Our present study investigates whether gender modulates the early steps of the insulin-signaling system in vivo. Eight-week-old male and female Wistar rats (n = 8/group) were compared to evaluate insulin serum levels and insulin tolerance tests by radioimmunoassay and glucose oxidase method, respectively. To assess insulin receptor (IR), Shc, STAT-1, ERK, and Akt phosphorylation in response to insulin in lacrimal gland and salivary gland, tissues from female and male rats (n = 5-8/group) were submitted to immunoprecipitation and immunoblotting or Western blotting protocol, and phosphorylation level was determined by densitometry. No difference was found in insulin serum levels or insulin tolerance tests comparing both groups. Nevertheless, lacrimal gland and salivary gland of female rats had a significantly lower insulin-induced IR phosphorylation compared with males. IR phosphorylation was not affected by the estrous cycle stage in either tissue. In addition, in females an apparent but not significant lower STAT and Akt phosphorylation in response to insulin was observed in the lacrimal gland, compared with males. Our findings suggest that alterations in insulin signal transduction may play a role in lacrimal gland and salivary gland gender differences.