David Pearce, M.D.
Work in my laboratory focuses on mechanisms of hormone signaling, particularly as it relates to the regulation of epithelial ion transport. This process, we have found, is central to the interconnections between diabetes and hypertension (high blood pressure). Abnormalities in glucose metabolism and blood pressure, together with abnormal lipid/cholesterol metabolism, are the leading causes of heart disease and stroke world-wide. Interestingly, diabetes and hypertension frequently track together clinically, and our work has uncovered mechanistic links at the molecular level. We are particularly interested in understanding how hormones alter sodium and potassium transport in kidney epithelia, since this is a key determinant of blood pressure, as well as of blood ion concentrations. We use a combination of animal-based, cell culture, and in vitro methods to explore the mechanistic basis of these processes.
1) SGK1 control of ion channel trafficking.
SGK1 is a serine-threonine kinase whose gene transcription is stimulated by the principal blood pressure regulatory hormone, aldosterone. We identified SGK1 as an aldosterone-regulated gene in kidney epithelial cells. Interestingly, SGK1 activity is also controlled by PI3-kinase, a key mediator of insulin action. Hence, SGK1 sits at the intersection of blood pressure and glucose regulation. We have shown that SGK1 stimulates sodium transport by regulating membrane trafficking of the epithelial sodium channel (ENaC). Current efforts focus on the underlying mechanisms. In particular, we have shown that Sgk1 acts to inhibit a ubiquitin ligase, Nedd4-2, by triggering recruitment of 14-3-3 chaperone proteins. Sgk1 is targeted to its appropriate subcellular location by a combination of protein-protein and protein-lipid interactions. Its interactions with phosphoinositides control its subcellular localization. These latter observations provide potential targets for identification of novel treatments for hypertension.
2) GILZ and the control of Raf-MEK-ERK signaling.
GILZ is another aldosterone-regulated gene product, and our recent data has uncovered surprising connections between this mediator and the Raf-MEK-ERK signaling module. Notably, recent data has suggested that signaling through Raf-MEK-ERK inhibits Na+ transport in epithelia, and our data demonstrate that GILZ stimulates Na+ transport by inhibiting Raf, i.e. through a disinhibitory mechanism. Our data support the idea that GILZ binds to and inhibits c-Raf. Interestingly, the Raf-MEK-ERK pathway also mediates important effects of insulin on cell proliferation and glucose metabolism. Hence, here again we see the intertwining of ion transport and energy metabolism.
The figure below shows the integrated effects of GILZ and SGK1 in a kidney epithelial cell.
Schematic depiction of ENaC regulation by GILZ and SGK1 in a kidney epithelial cell, showing potential convergence of these regulatory pathways at the ubiquitin ligase, Nedd4-2. ERK inhibits ENaC by stimulating its interaction with Nedd4-2, while GILZ inhibits ERK by binding to its upstream regulator, Raf, and displacing Ras. SGK1 inhibits Nedd4-2 by stimulating its interaction with 14-3-3 chaperone proteins. Thus, GILZ and SGK1 work together to disinhibit (and thus stimulate) ENaC. For simplicity, only some of the proposed complexes and effects are shown. Note that in this cell, the net flow of Na+ ions is from the lumen (top) across the cell to the extracellular fluid (bottom); from here, Na+ reenters the bloodstream. ENaC mediates the initial entry, and constitutes the rate-limiting step for transit. Note that for simplicity, activation of the “PI3K pathway” by insulin, and the Raf-MEK-ERK pathway by EGF are shown, however, this is an over-simplification; there is crosstalk. Small circled “P”: phosphate; Ins: insulin. Aldo: aldosterone; MR: mineralocorticoid receptor (steroid receptor for aldosterone); ins (insulin); EGF: epidermal growth factor; EGFR: EGF receptor. For additional details, see: Am J Physiol: Renal Phys, 291:F714-21, 2006.