Regulation of rat Na+/P-i cotransporter-1 gene expression: The roles of glucose and insulin

Cytosolic inorganic phosphate (P-i) is important for glucose metabolism. It plays a role in homeostatic regulation of glucose by insulin and glucagon. Recently, we isolated two cDNA clones for rat Na+/P-i cotransporter-1 (rNaPi-1) and demonstrated that they are expressed primarily in the rat liver and kidney. We now report that the expression of rNaPi-1 in these tissues is regulated by fasting and streptozotocin-induced diabetes. Using rat hepatocytes in primary culture, we also demonstrate that glucose and insulin upregulate rNaPi-1 expression, whereas glucagon and elevated intracellular adenosine 3',5'-cyclic monophosphate levels downregulate its expression. Because 2-deoxyglucose exhibits no effect on rNaPi-1 gene expression, we suggest that some metabolite accumulated during glucose metabolism may be responsible for the effects of glucose and insulin on rNaPi-1 gene expression. Our data also reveal that other known Na+/P-i cotransporter genes, NaPi-2 and Ram-1 (a receptor for amphotropic murine retrovirus), are not regulated by insulin and glucose. It is therefore proposed that various subtypes of Na+/P-i cotransporters are differentially regulated and that each subtype may be involved in a specific cellular function. rNaPi-1 may be responsible for P-i uptake by liver and kidney for glucose metabolism, whereas NaPi-2 may play a key role in P-i reabsorption in the kidney.