Estrogen directly and specifically downregulates NaPi-IIa through the activation of both estrogen receptor isoforms (ERα and ERβ) in rat kidney proximal tubule

We have previously demonstrated that estrogen (E-2) downregulates phosphate transporter NaPi-IIa and causes phosphaturia and hypophosphatemia in ovariectomized rats. In the present study, we examined whether E-2 directly targets NaPi-IIa in the proximal tubule (PT) and studied the respective roles of estrogen receptor isoforms (ER alpha and ER beta) in the downregulation of NaPi-IIa using both in vivo and an in vitro expression systems. We found that estrogen specifically downregulates NaPi-IIa but not NaPi-IIc or Pit2 in the kidney cortex. Proximal tubules incubated in a "shake" suspension with E-2 for 24 h exhibited a dose-dependent decrease in NaPi-IIa protein abundance. Results from OVX rats treated with specific agonists for either ER alpha[4,4',4 '';-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, PPT] or ER beta [4,4',4 ''-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, DPN] or both (PPT + DPN), indicated that only the latter caused a sharp downregulation of NaPi-IIa, along with significant phosphaturia and hypophosphatemia. Lastly, heterologous expression studies demonstrated that estrogen downregulated NaPi-IIa only in U20S cells expressing both ER alpha and ER beta, but not in cells expressing either receptor alone. In conclusion, these studies demonstrate that rat PT cells express both ER alpha and ER beta and that E2 induces phosphaturia by directly and specifically targeting NaPi-IIa in the PT cells. This effect is mediated via a mechanism involving coactivation of both ER alpha and ER beta, which likely form a functional heterodimer complex in the rat kidney proximal tubule.