Regulation of the energy sensor AMP-activated protein kinase in the kidney by dietary salt intake and osmolality

Abstract

The AMP-activated protein kinase (AMPK) is a key controller of cellular energy metabolism. We studied its expression and regulation by salt handling in the kidney. Immunoprecipitation and Western blots of protein lysates from whole rat kidney using subunit-specific antibodies showed that the alpha1-catalytic subunit is expressed in the kidney, associated with the beta2- and either gamma1- or gamma2-subunits. Activated AMPK, detected by immunohistochemical staining for phospho-Thr172 AMPK (pThr172), was expressed on the apical surface of the cortical thick ascending limb of the loop of Henle, including the macula densa, and some parts of the distal convoluted tubule. Activated AMPK was also expressed on the basolateral surface of the cortical and medullary collecting ducts as well as some portions of the distal convoluted tubules. AMPK activity was increased by 25% in animals receiving a high-salt diet, and this was confirmed by Western blotting for pThr172. Low-salt diets were associated with reduced levels of the alpha-subunit of AMPK, which was highly phosphorylated on Thr172. Surprisingly, both low- and high-salt media transiently activated AMPK in the macula densa cell line MMDD1, an effect due to changes in osmolality, rather than Na+ or Cl- concentration. This study, therefore, demonstrates regulation of AMPK by both a high- and a low-salt intake in vivo and suggests a role for the kinase in the response to changes in osmolality within the kidney.

AMPK Potentiates Hypertonicity-induced Apoptosis by Suppressing NFκB/COX-2 in Medullary Interstitial Cells

“…14,15 Surprisingly, we found that AMPK phosphorylation in RMICs was transiently increased at 5 minutes, then rapidly decreased by hypertonicity within 12 hours, and then gradually returned to baseline levels, which is in sharp contrast to a previous report suggesting that hyperosmolality activated AMPK in MMDD1 cells, a clonally derived macula densa cell line, within 30 minutes. 15 This inhibitory effect on AMPK activity appears to be specific for RMICs, because hypertonicity significantly increased AMPK activity in IMCD-3 cells, which is consistent with a previous report in MDCK cells. 32 The mechanism by which hypertonicity suppresses AMPK phosphorylation in RMICs is currently unknown.…”

“…In fact, almost all subunits of AMPK can be found in the kidney, and activated AMPK was found strongly expressed at the apical surface of cortical thick ascending limbs and the macula densa. 15 However, the detailed expression profile of AMPK and its activity in each cell type of the kidney is still under investigation. This study demonstrates constitutive expression of AMPK in RMICs, where its activity is essential for determining the survival of RMICs under the hypertonic condition both in vitro and in vivo.…”

“…Although the role of AMPK activation in the viability of these cells was not the focus of this study, the presence of relatively high levels of phosphorylated AMPK␣ suggests that AMPK may play an important role in regulating collecting duct function by sensing the energy stress. 15 Therefore, examining the effect of AMPK activation on collecting duct cell viability under the hypertonic condition would be equally important.…”

“…A high AMP or low ATP level activates AMPK, which inhibits energyconsuming processes and enhances energy-producing processes to restore the energy homeostasis. 11 AMPK is regulated by various stimuli including energy depletion, exercise, hypoxia, and hypertonicity in many cell types, [12][13][14][15] where it inhibits cell viability and induces apoptosis by up-regulating p53 expression, activating the caspase cascade and reducing the activation of the cytoprotector cyclooxygenase 2 (COX-2). 16 -18 Because renal medullary interstitial cells (RMICs) are normally subjected to hypoxia and hypertonic challenges and NFB-driven COX-2 expression is critical to the capability of RMICs to survive under hyperosmotic stress, 19 we aimed to investigate whether AMPK plays an important role in regulating cell survival of RMICs and whether its activation results in massive apoptosis under hypertonic conditions.…”

Mechanistic Insight into Control of CFTR by AMPK

“…The actions of AMPK are complex as this kinase regulates a number of transport proteins involved in secretion or absorption of electrolytes in epithelia. Thus epithelial Na ϩ channels (30 -32), the renal NKCC2 (33,34), CFTR (5-7), and probably the secretory Na (34) can all be controlled by AMPK. Regulation occurs either indirectly, as in the case of epithelial Na ϩ channels (ENaC), or directly through AMPK phosphorylation as for CFTR, NKCC2, and probably NKCC1.…”