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1 Department of Medicine, 2 Department of Physiology, University of Maryland School of Medicine, and 3 Medical Service, Department of Veterans Affairs Medical Center, Baltimore, Maryland 21201; and 4 Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710
Na+/H+ exchanger regulatory factor
(NHERF), an essential protein cofactor in cAMP-mediated inhibition of
Na+/H+ exchange transporter 3 (NHE3),
facilitates the formation of a signal complex of proteins that includes
NHE3, NHERF, and ezrin. This model for NHE3 regulation was developed in
fibroblasts and its applicability to epithelial cells remains to be
established. Opossum kidney (OK) cells were transfected with
either empty vector (control), full-length mouse (m) NHERF(1-355),
or a truncated mNHERF(1-325) that lacked ezrin binding and had
been demonstrated in fibroblasts to bind NHE3 but not mediate its
cAMP-associated inhibition. 8-Bromoadenosine 3',5'-cyclic
monophosphate (8-BrcAMP) at 10
4 M inhibited
Na+/H+ exchange activity in control and OK
cells expressing wild-type mNHERF(1-355) by >60% but by <10%
in cells expressing mNHERF(1-325). NHE3 coimmunoprecipitated with
mNHERF(1-325), but cAMP phosphorylation of NHE3 was impaired in
cells expressing mNHERF(1-325). The inhibitory effect of
hyperosmolality on NHE3 activity and the uptake of
3-O-methyl-D-glucose was the same in all three
cell lines. Cell surface expression of NHE3 was not changed by cAMP in
any of the cells lines. These data indicate that disruption of the
NHERF-ezrin signal complex attenuates the inhibitory effect of cAMP on
NHE3 activity in OK cells and provides evidence supporting the proposed
model of protein kinase A regulation of NHE3 in epithelial cells.
renal electrolyte transport; PSD-95/Dlg/ZO proteins; protein kinase A; sodium/hydrogen exchange transporter 3; acid-base physiology; opossum kidney cells
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