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EDITORIAL FOCUS
Department of Physiology, University of Texas Health Science Center, San Antonio, Texas
IN THIS ISSUE of the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, the most recent recipient of the New Investigator Award in Regulatory and Integrative Physiology, Dr. V. Vallon of the Departments of Medicine and Pharmacology at the University of California San Diego and Veterans Affairs Medical Center San Diego, reviews evidence supporting a central role for serum- and glucocorticoid-regulated kinase 1 (Sgk1) in regulating systemic Na+ and K+ homeostasis (19). This important and timely review focuses on new insight gleaned from molecular genetic studies linking polymorphisms in the gene encoding Sgk1 with blood pressure (3). Moreover, this review highlights recent findings from in vivo studies on Sgk1-deficient mice showing that Sgk1 is necessary for normal upregulation of Na+ reabsorption in the kidney in response to stress, such as restricted dietary NaCl intake, as well as for normal increases in renal K+ secretion in response to enhanced K+ intake (9, 25).
Sgk1 is a member of the AGC family of serine/threonine protein kinases (reviewed in Refs. 7, 10, 13). This kinase was originally cloned by the Firestone laboratory as a glucocorticoid-responsive gene from mammary tumor cells (23, 24). Many diverse extracellular and intracellular factors are now recognized to regulate Sgk1 mRNA levels. Commensurate with its dynamic regulation, Sgk1 is a central player in a number of essential cellular responses, including increased protein synthesis, proliferation, protection against apoptosis, electrical activity, cellular electrolyte metabolism, and volume regulation. Likely because of its ability to serve as a central cell signaling integrator, Sgk1 is widely expressed in many tissues and is an ancient protein with homologues identified in all metazoans examined to date. Remarkably, Sgk1 orthologues have even been identified in yeast. Three isoforms of Sgk, encoded by distinct genes, have been identified, Sgk1, Sgk2 and Sgk3; however, only Sgk1 is responsive to glucocorticoids at the level of transcription. Similar to glucocorticoids, another adrenal corticosteroid hormone, aldosterone, regulates Sgk1 at the level of transcription. It is regulation by this latter mineralocorticoid, as discussed by Vallon and colleagues, which positions Sgk1 as a central cellular integrator affecting systemic salt and K+ balance.
The need for systemic salt conservation is a relatively new concept in the broader scheme of evolution. This need is intimately associated with terrestrial life where water availability becomes limiting. Water moves only passively in physiological systems following osmotic gradients established by active transport of osmolytes/electrolytes, such as Na+. The most abundant extracellular osmolytes/electrolytes are Na+ and its conjugated anions, Cl– and HCO3–. In contrast, the most abundant intracellular electrolytes are K+ and its associated bases. Thus plasma and interstitial volumes are ultimately established by regulated Na+ movement with plasma and intracellular K+ levels playing critical roles in the electrical activity of many cell types, including muscle and neurons. In terrestrial vertebrates, particularly mammals, the ability to conserve salt protects plasma and interstitial fluid volumes. Disruption of proper Na+ balance leads to disease states, including hypertension and renal salt wasting.
The primary effector organs setting systemic Na+ balance are the colon and kidneys. In the colon, Na+ is (re)absorbed from ingested foods and pancreatic secretions. In the kidney, Na+ filtered from the plasma at the glomerulus is reabsorbed from the ultrafiltrate within the renal tubule and collecting duct system. Most Na+ (re)absorption is constitutive and necessary for life. However, (re)absorption at the aldosterone-sensitive distal renal nephron and colon is regulated allowing fine tuning of plasma Na+ levels and related volumes.
At the distal nephron and colon, activity of the epithelial Na+ channel (ENaC) is limiting for Na+ (re)absorption. This channel resides in the luminal plasma membrane of the epithelial cells that comprise the physical barrier separating the internal environment from the external environments of the lumen of the alimentary canal and nephron. ENaC is one end effector of the renin-angiotensin-aldosterone system (RAAS) with aldosterone acting on distal nephron and colonic epithelia to increase Na+ (re)absorption via activating ENaC. The RAAS is sensitive to changes in blood pressure and plasma volume and acts in a classic negative-feedback manner to maintain systemic Na+ and fluid balance. Thus aldosterone control of ENaC is directly linked to control of blood pressure. Indeed, gain of function mutations in ENaC and its upstream regulatory RAAS pathway lead to inappropriate salt conservation and hypertension (reviewed in Ref. 11).
Aldosterone, through its actions on ENaC, as well as independent of these actions, also impinges upon distal nephron K+ secretion and thus systemic K+ balance. Loss of function mutations in ENaC and decreases in aldosterone signaling lead to renal salt wasting associated with hyperkalemia and decreases in blood pressure (4, 8).
While the relationship between aldosterone, Na+ reabsorption, K+ secretion, and blood pressure have been established for over 50 years, little was actually known about the cellular mechanisms of aldosterone action on distal nephron epithelia until recently. It was, however, realized that aldosterone influenced Na+ and K+ transport through a genomic mechanism requiring changes in gene expression (reviewed by Refs. 17, 20). The expression levels of the apical channels (e.g., ENaC and renal outer medullary K+ channel, ROMK) and basolateral pumps mediating electrolyte transport, however, do not increase until well after transport increases. This led to the hypothesis that aldosterone impacts transport by controlling the expression of cell signaling molecules that transduce the aldosterone response from the nucleus to luminal plasma membrane Na+ and K+ channels and serosal Na+-K+-ATPases. Sgk1 has been identified as an aldosterone-responsive protein controlled at the level of transcription, and as discussed by Vallon and colleagues, central to increases in Na+ and K+ transport.
The laboratories of Pearce (5) and Naray-Fejes-Toth (12) showed first that corticosteroids, such as aldosterone, control Sgk1 expression in distal nephron cells at the level of transcription. Trans-activation of Sgk1 by aldosterone is through a classic canonical pentadecamer, cis-acting steroid response element, found in the promoter region of the gene encoding this kinase (23, 24). In addition to trans-activation, aldosterone also influences Sgk1 activity by regulating the phosphorylation (active) state of this kinase (18, 22). Sgk1, similar to closely related AGC kinases, such as Akt, is activated on phosphorylation by phospholipid-dependent kinases, which themselves are controlled by production of phosphatidylinositol 3,4,5-trisphosphate by phosphoinositide 3-OH kinase (PI3-K). It currently is unclear how aldosterone activates PI3-K; however, it is known that aldosterone does increase activity of this kinase in renal epithelia (2). One possible mechanism, although it remains controversial, is that aldosterone activates PI3-K through trans-activation of the small G protein K-Ras (16, 18). PI3-K is a known first effector of this GTPase. Similar to aldosterone, insulin, which also increases Na+ reabsorption at the distal nephron, exerts this effect by stimulating PI3-K with subsequent activation of Sgk and ENaC (1, 22). Thus Sgk1 is well positioned to be a critical integrator for many extracellular and intracellular singling inputs that ultimately impinge upon the activity of the luminal ion channels that are limiting for Na+ and K+ transport. Indeed, it is well established that Sgk1 increases that activity of both ENaC and ROMK (5, 12, 26). The latter channel is thought to play a pivotal role in K+ secretion at the distal nephron.
Sgk1 increases ENaC activity, at least in part, by phosphorylating and inhibiting the ubiquitin ligase Nedd4–2 (and possibly the related Nedd4; Refs. 6, 14). Ubiquitin ligases associate with the COOH-terminal tails of ENaC subunits to promote channel internalization and decreases in channel activity with disruption of the Nedd4–2/Nedd binding sites in ENaC, leading to hypertension associated with improper salt conservation (15). Similarly, Sgk1 increases activity of ROMK channels by increasing the plasma membrane levels of this channel (26). In addition, Sgk1 increases the membrane number and thus activity of serosal Na+-K+ pumps to maintain the proper electrochemical gradients needed for Na+ and K+ transport (21). Vallon and colleagues in the current review suggest that it actually may be this latter action of Sgk1 on serosal pumps and ENaC, rather then direct effects on ROMK, that predominate with respect to regulating K+ secretion. Nonetheless, not only is Sgk1 an integrator for signaling inputs affecting salt and K+ balance, but it also serves as a divergence point affecting a number of the critical transport proteins involved in these processes.
The primary importance of the current paper by Vallon and colleagues (19) lies in reviewing recent evidence linking Sgk1 to blood pressure control and control of Na+ and K+ balance in mammals. This is a crucial connection for it establishes Sgk1 as a critical cellular mediator of the physiological actions of aldosterone. A significant point noted by Vallon and colleagues is that Sgk1, in contrast to ENaC and the mineralocorticoid receptor, is not absolutely required for Na+ and K+ transport with it being absolutely necessary only in extreme cases of Na+ depravation and elevated K+ intake. This positions Sgk1 as an integrator that finetunes transport of these electrolytes and suggests that other currently unrealized factors, also possibly responsive to aldosterone, influence epithelial handling of Na+ and K+. As concluded by Vallon and colleagues, only through further investigation will these other factors and the complete role of Sgk1 in Na+ and K+ homeostasis be determined.
FOOTNOTES
Address for reprint requests and other correspondence: J. D. Stockand, Dept. of Physiology, Univ. of Texas Health Science Center, San Antonio, TX 78229 (stockand{at}uthscsa.edu)
REFERENCES
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  F. Lang, C. Bohmer, M. Palmada, G. Seebohm, N. Strutz-Seebohm, and V. Vallon (Patho)physiological Significance of the Serum- and Glucocorticoid-Inducible Kinase Isoforms. Physiol Rev, October 1, 2006; 86(4): 1151 - 1178. [Abstract] [Full Text] [PDF]
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