Collecting duct water permeability inhibition by EGF is associated with decreased cAMP, PKA activity, and AQP2 phosphorylation at Ser.

Prior studies showed that epidermal growth factor (EGF) inhibits vasopressin-stimulated osmotic water permeability in the renal collecting duct. Here, we investigated the underlying mechanism. Using isolated perfused rat inner medullary collecting ducts (IMCDs), we found that the addition of EGF to the peritubular bath significantly decreased 1-deamino-8-d-arginine vasopressin (dDAVP)-stimulated water permeability, confirming prior observations. The inhibitory effect of EGF on water permeability was associated with a reduction in intracellular cAMP levels and protein kinase A (PKA) activity. Using phospho-specific antibodies and immunoblotting in IMCD suspensions, we showed that EGF significantly reduces phosphorylation of AQP2 at Ser and Ser. This effect was absent when 8-cpt-cAMP was used to induce AQP2 phosphorylation, suggesting that EGF's inhibitory effect was at a pre-cAMP step. Immunofluorescence labeling of microdissected IMCDs showed that EGF significantly reduced apical AQP2 abundance in the presence of dDAVP. To address what protein kinase might be responsible for Ser phosphorylation, we used Bayesian analysis to integrate multiple-omic datasets. Thirteen top-ranked protein kinases were subsequently tested by in vitro phosphorylation experiments for their ability to phosphorylate AQP2 peptides using a mass spectrometry readout. The results show that the PKA catalytic-α subunit increased phosphorylation at Ser, Ser, and Ser. None of the other kinases tested phosphorylated Ser. In addition, H-89 and PKI strongly inhibited dDAVP-stimulated AQP2 phosphorylation at Ser. These results indicate that EGF decreases the water permeability of the IMCD by inhibiting cAMP production, thereby inhibiting PKA and decreasing AQP2 phosphorylation at Ser, a site previously shown to regulate AQP2 endocytosis. The authors used native rat collecting ducts to show that inhibition of vasopressin-stimulated water permeability by epidermal growth factor involves a reduction of aquaporin 2 phosphorylation at Ser, a consequence of reduced cAMP production and PKA activity.