Transport regulation by the serum- and glucocorticoid-inducible kinase SGK1

(Patho)physiological significance of the serum- and glucocorticoid-inducible kinase isoforms

The serum- and glucocorticoid-inducible kinase-1 (SGK1) is ubiquitously expressed and under genomic control by cell stress (including cell shrinkage) and hormones (including gluco- and mineralocorticoids). Similar to its isoforms SGK2 and SGK3, SGK1 is activated by insulin and growth factors via phosphatidylinositol 3-kinase and the 3-phosphoinositide-dependent kinase PDK1. SGKs activate ion channels (e.g., ENaC, TRPV5, ROMK, Kv1.3, KCNE1/KCNQ1, GluR1, GluR6), carriers (e.g., NHE3, GLUT1, SGLT1, EAAT1-5), and the Na+-K+-ATPase. They regulate the activity of enzymes (e.g., glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, phosphomannose mutase-2) and transcription factors (e.g., forkhead transcription factor FKHRL1, beta-catenin, nuclear factor kappaB). SGKs participate in the regulation of transport, hormone release, neuroexcitability, cell proliferation, and apoptosis. SGK1 contributes to Na+ retention and K+ elimination of the kidney, mineralocorticoid stimulation of salt appetite, glucocorticoid stimulation of intestinal Na+/H+ exchanger and nutrient transport, insulin-dependent salt sensitivity of blood pressure and salt sensitivity of peripheral glucose uptake, memory consolidation, and cardiac repolarization. A common ( approximately 5% prevalence) SGK1 gene variant is associated with increased blood pressure and body weight. SGK1 may thus contribute to metabolic syndrome. SGK1 may further participate in tumor growth, neurodegeneration, fibrosing disease, and the sequelae of ischemia. SGK3 is required for adequate hair growth and maintenance of intestinal nutrient transport and influences locomotive behavior. In conclusion, the SGKs cover a wide variety of physiological functions and may play an active role in a multitude of pathophysiological conditions. There is little doubt that further targets will be identified that are modulated by the SGK isoforms and that further SGK-dependent in vivo physiological functions and pathophysiological conditions will be defined.

Androgens induce prostate cancer cell proliferation through mammalian target of rapamycin activation and post-transcriptional increases in cyclin D proteins

Androgen receptor (AR) plays a central role in prostate cancer, with most tumors responding to androgen deprivation therapies, but the molecular basis for this androgen dependence has not been determined. Androgen [5alpha-dihydrotestosterone (DHT)] stimulation of LNCaP prostate cancer cells, which have constitutive phosphatidylinositol 3-kinase (PI3K)/Akt pathway activation due to PTEN loss, caused increased expression of cyclin D1, D2, and D3 proteins, retinoblastoma protein hyperphosphorylation, and cell cycle progression. However, cyclin D1 and D2 message levels were unchanged, indicating that the increases in cyclin D proteins were mediated by a post-transcriptional mechanism. This mechanism was identified as mammalian target of rapamycin (mTOR) activation. DHT treatment increased mTOR activity as assessed by phosphorylation of the downstream targets p70 S6 kinase and 4E-BP1, and mTOR inhibition with rapamycin blocked the DHT-stimulated increase in cyclin D proteins. Significantly, DHT stimulation of mTOR was not mediated through activation of the PI3K/Akt or mitogen-activated protein kinase/p90 ribosomal S6 kinase pathways and subsequent tuberous sclerosis complex 2/tuberin inactivation or by suppression of AMP-activated protein kinase. In contrast, mTOR activation by DHT was dependent on AR-stimulated mRNA synthesis. Oligonucleotide microarrays showed that DHT-stimulated rapid increases in multiple genes that regulate nutrient availability, including transporters for amino acids and other organic ions. These results indicate that a critical function of AR in PTEN-deficient prostate cancer cells is to support the pathologic activation of mTOR, possibly by increasing the expression of proteins that enhance nutrient availability and thereby prevent feedback inhibition of mTOR.