Impaired insulin secretion and decreased expression of the nutritionally responsive ribosomal kinase protein S6K-1 in pancreatic islets from malnourished rats

Low protein diet has been shown to affect the levels and activities of several enzymes from pancreatic islets. To further extend the knowledge on how malnutrition affects insulin secretion pathway, we investigated in this work the insulin release induced by glucose or leucine, an insulin secretagogue, and the expression of insulin receptor (IR), insulin receptor substrate 1 (IRS1), phosphatidylinositol 3-kinase (PI3K), and p70S6K1 (S6K-1) proteins from pancreatic islets of rats fed a normal (17%; NP) or a low (6%; LP) protein diet for 8 weeks. Isolated islets were incubated for 1 h in Krebs–bicarbonate solution containing 16.7 mmol/L of glucose, or 2.8 mmol/L of glucose in the presence or absence of 20 mmol/L of leucine. Glucose- and leucine-induced insulin secretions were higher in NP than in LP islets. Western blotting analysis showed an increase in the expression of IR and PI3K protein levels whereas IRS1 and S6K-1 protein expression were lower in LP compared to NP islets. In addition, S6K-1 mRNA expression was also reduced in islets from LP rats. Our data indicate that a low protein diet modulates the levels of several proteins involved in the insulin secretion pathway. Particularly, the decrease in S6K-1 expression might be an important factor affecting either glucose- or leucine-induced insulin secretion.