Targeting of renal proximal tubule Na,K-ATPase by salt-inducible kinase

Abstract

The renal proximal tubule (RPT) is a central locale for Na+ reabsorption, and blood pressure regulation. Na+ reabsorption in the RPT depends upon the Na,K-ATPase, which is controlled by a complex regulatory network, including Salt-Inducible Protein Kinase (SIK). SIKs are recently discovered members of the AMP-activated Protein Kinase (AMPK) family, which regulate salt homeostasis and metabolism in a number of tissues. In the RPT, SIK interacts with the Na,K-ATPase in the basolateral membrane (BM), regulating both the activity and level of Na,K-ATPase in the BM. Thus, Na,K-ATPase activity can be rapidly adjusted in response to changes in Na+ balance. Long-term changes in Na+ intake affect the state of SIK phosphorylation, and as a consequence the phosphorylation of TORCs, Transducers of Regulated CREB (cAMP Regulatory Element Binding Protein). Once phosphorylated, TORCs enter the nucleus, and activate transcription of the ATP1B1 gene encoding for the Na,K-ATPase beta subunit.

Figure 1

Structure of SIK1 and Na,K-ATPase

Figure 2

Acute and Chronic Regulation of Na,K-ATPase by SIK

Figure 3

Promoter of the Na,K-ATPase β Subunit Gene ATP1B1

Figure 4

Regulation of ATP1B1 Transcription by TORCs. A. Regulation in primary RPT cells. Primary RPT cells were transiently transfected as previously described ([30]) with 1 μg pH β1-1141Luc, 0.2 μg pSβGal (to monitor transfection efficiency), and 0.1 μg of either TORC1, TORC2, TORC3, or the empty vector pcDNA3.1/V5 (Control) [22]. After 24 hrs, luciferase activity was determined in quadruplicate. Values are averages (+/− SEM) in comparison to the control). B. Regulation in MDCK cells. MDCK cells were transfected (as above), and incubated 4 hrs either with 1.4 μM PGE₁, or untreated, followed by luciferase determinations. Results are averaged from 3 experiments. C. Regulation via PGRE3. MDCK cells were transfected (as in A. above), and treated with PGE₁, (or untreated), also as above, with the modification that the promoter/luciferase constructs were either pLuc-MCS β72–167 (PGRE3 vector) or pLuc-MCS β72–167 GC trans (PGE3 vector with GC trans mutation, as described previously [22]). Values are averages (+/− SEM) of quadruplicate determinations, as compared with the Control value (in cultures transfected with PGRE1 + pcDNA3.1/V5).