Protein kinase D as a modulator of aldosterone action in the kidney
Warren Thomas, Ruth Dooley & Brian Harvey
The distal nephron is the principal site for salt conservation in the body. Through its action on basolateral and apical membrane transporters, aldosterone regulates ion fluxes across the cells of the distal nephron. Aldosterone elicits physiological responses by modulating the expression of the transporter subunits; for example in the case of the epithelial Na+ channel (ENaC) and Na+/K+-ATPase, and also through the expression of the regulatory proteins which affect the activity of these transporters such as the serum and glucocorticoid regulated kinase-1. The interaction of aldosterone with its specific receptor the mineralocorticoid receptor (MR) also stimulates the rapid activation of protein kinase signal transduction cascades and the trans-activation of the epidermal growth factor receptor (EGFR). The Protein kinase D (PKD) family of serine/ threonine kinases are important regulators of key cellular processes including proliferation and sub-cellular trafficking. Aldosterone treatment of renal cortical collecting duct (CCD) cells resulted in the EGFR-dependent activation of PKD1. The activation of this signalling cascade was coupled to c-Src-dependent phosphorylation of EGFR. Silencing of PKD1 expression inhibited aldosterone-induced membrane insertion of ENaC and the stimulation of an amiloride-sensitive trans-epithelial current. We found that PKD1 also modulated the activation of ERK1/2 in response to aldosterone in CCD cells and that PKD1 suppression attenuated the growth response to aldosterone in these cells. Aldosterone promoted the sub-cellular redistribution of ERK1/2 to the nuclei of M1-CCD cells after 2 min treatment and to sites, proximal to the nuclei after 30 min. The sub-cellular redistribution of ERK1/2 was inhibited in PKD1 knock-down cells. Stabilization of ERK1/2 activation by PKD1 and ERK1/2 sub-cellular redistribution are a pre-requisite for aldosterone-induced CCD cell growth. Since aldosterone stimulates the growth of renal stem cells in culture, so PKD1 activation by aldosterone may be an important factor in both renal differentiation and proliferative disease.