The insulin receptor substrate-1 (IRS-1) plays a central role in insulin sensitivity, and association studies have shown that the IRS-1 G972R variant is a risk factor for insulin resistance. However, how this mutation may lead to impaired insulin sensitivity is still to be determined. Our study aimed to evaluate, after transfection of the IRS-1 G972R variant in 3T3L1 adipocytes, the effect of this mutation on insulin signaling and on cell differentiation. The 3T3L1 cells were transfected with pcDNA3 expression vector containing either the human wild-type IRS-1 or the G972R variant. After induction of differentiation, the 3T3L1 transfected with wild-type IRS-1 differentiated in 6–8 days, while the cells transfected with G972R variant did not differentiate. To determine whether the defect in IRS-1 was responsible for this, we analyzed the expression of several genes involved in the insulin signaling pathway. Results showed that PPARγ expression was significantly reduced in cells transfected with the mutated IRS-1, together with a significant decrease in binding of phosphatidylinositol-3 kinase (PI 3-kinase) to IRS-1 G972R and in PI 3-kinase activity. In addition, we observed that the interaction between the insulin receptor (IR) and the IRS-1 G972R protein was increased and that the autophosphorylation of the IR was significantly inhibited in 3T3L1-G972R cells compared with 3T3L1-WT. Treatment of the 3T3L1-G972R cells with pioglitazone (PIO), a PPARγ agonist, restored differentiation with higher level of PPARγ expression and restoration of PI 3-kinase binding to IRS-1 G972R and PI 3-kinase activity. IR autophosphorylation was also increased. Withdrawal of PIO in fully differentiated 3T3L1-G972R cells determined the reappearance of the insulin signaling defect. Finally, we observed higher levels of IRS-2 expression, suggesting that IRS-2 may play a more important role in adipocyte insulin signaling. In conclusion, IRS-1 G972R variant impairs insulin signaling, and treatment with PPARγ agonist restores the normal phenotype of 3T3L1 cells.
F Sentinelli, E Filippi, M G Cavallo, S Romeo, M Fanelli and M G Baroni
N. Torres, M. Fanelli, A. L. Alvarez, D. Santajuliana, S. Finkielman and C. J. Pirola
Betamethasone was administered on alternate days to rats, and the role of the central cholinergic system in the development of hypertension assessed. After 15 days of treatment the systolic blood pressure of treated rats was significantly higher than that of control rats. Peripheral administration of atropine but not of methyl atropine reduced systolic pressure in glucocorticoid-treated rats and had no effect in controls. Therefore, [3H]quinuclidinyl benzylate binding, sodium-dependent high-affinity choline uptake and choline acetyltransferase studies were performed in the septal area, anteroventrolateral medulla (AVLM), anterior hypothalamic preoptic area (AH/PO) and hypothalamus. The density of muscarinic receptors was increased in the hypothalamus and AVLM of treated rats without significant changes in affinity. Choline acetyltransferase activity significantly decreased in the AVLM and increased in the AH/PO. In addition, a decrease in the hypothalamus and an increase in the AH/PO of sodium-dependent high-affinity choline uptake was observed in glucocorticoid-treated rats. These results suggest the presence of an enhanced muscarinic cholinergic activity in several brain nuclei in rats with glucocorticoid-induced hypertension. This activation could be due to pre- and post-synaptic hypersensitivity.
Journal of Endocrinology (1991) 129, 269–274