Essential role for NHERF in cAMP-mediated inhibition of the Na+-HCO3- Co-transporter in BSC-1 cells

Abstract

Prior studies have indicated a requirement for the PDZ domain-containing protein, Na+/H+ Exchanger Regulatory Factor (NHERF), for protein kinase A (PKA)-mediated inhibition of the renal basolateral Na+-HCO3- co-transporter (NBC). The present studies explore the potential mechanisms by which NHERF transduces cAMP signals to inhibit NBC. In BSC-1 cells, cells that express NBC but lack NHERF, 8-bromo·cAMP (100 μM for 15 min) failed to inhibit transport until wild-type mNHERF-(l-355) was expressed. mNHERF-(116-355) containing PDZ II and C-terminal ezrin-binding sequences or a mutant unphosphorylated form of rabbit NHERF effectively transduced the cAMP signals that inhibited NBC. By contrast, mNHERF-(l-126) encompassing N-terminal PDZ I and mNHERF-(l-325), which lacks ezrin-binding, failed to support cAMP inhibition of NBC activity. NBC and NHERF did not associate with each other in yeast twohybrid or co-immunoprecipitation assays, and confocal microscopy indicated distinct subcellular localization of the two proteins. NBC was phosphorylated in BSC-1 cells, but its phosphorylation was not increased by cAMP nor was immunoprecipitated NBC phosphorylated by PKA in vitro. Acute exposure of mNHERF-(l-355)-expressing BSC-1 cells to cAMP did not change cell surface expression of NBC. Although these results established an essential role for NHERF in cAMP-mediated inhibition of NBC in BSC-1 cells, they also suggest a novel mechanism for NHERF-mediated signal transduction distinct from that previously characterized from studies of other NHERF targets.