Stimulation of rat type iia sodium dependent phosphate cotransporter (NAPI-2)‎ by serum and glucocorticoid-induced kinase sgk1 and na+H+ exchange regulating factor nherf2

الملخص EN

Inorganic phosphate (Pi) is reabsorbed in the renal proximal convoluted tubules mainly via the electrogenic sodium dependent phosphate cotransporter NaPi Type IIa (NaPi-IIa).

The isoforms of NaPi-IIa have been cloned from different species such as rat (NaPi-2).

Serum and glucocorticoid-induced kinase 1 (SGK1) and rat sodium dependent phosphate cotransporter (NaPi-2) are highly expressed in the brush border membrane (BBM) of proximal tubule cells.

The significance of the kinase in regulation of sodium dependent phosphate cotransporter (NaPi-2) has, however, remained elusive.

On the other hand, the carboxyl-terminal tail of NaPi-2 contains information for apical expression, and interacts by means of its three terminal amino acids with several PSD95/DglA/ZO-1-like domains (PDZ)-containing proteins such as Na+/H+ exchanger 3 regulatory factors NHERF1 or NHERF2.

Both, NHERF1 and NHERF2 modulate the targeting and trafficking of several proteins into the plasma membrane.

Trafficking of the Na+/H+ exchanger NHE3 is controlled by NHE regulating factor NHERF2 and serum and glucocorticoid-inducible kinase SGK1.

To test for a possible involvement in NaPi-2 regulation, cRNA encoding NaPi-2 was injected into Xenopuslaevis oocytes with or without additional injection of cRNA encoding SGK1 and/or NHERF2.

Using two-electrode voltage-clamp, the transport activity was quantified as the substrate-induced current.

Exposure to 1 mM phosphate induced an inward current (IP) in NaPi-2 expressing oocytes but not in water injected oocytes.

Coexpression of SGK1 in NaPi-2 expressing oocytes significantly stimulated the phosphate-induced inward current.

Moreover, coexpression of NHERF2 also significantly stimulated the phosphate-induced inward current in NaPi-2 expressing oocytes.

The effect of SGK1 on NaPi-2 is mimicked by additional coexpression of NHERF2.

The observations suggest that SGK1 and NHERF2 regulate NaPi-2 activity and are thus likely to participate in the stimulatory effect of some hormones, such as growth hormone and insulin, on renal phosphate transport.

The present results thus disclose novel signaling mechanisms regulating NaPi-2 activity and renal phosphate transport, which may be important for regulation of phosphate homeostasis.