AMP-Activated Protein Kinase alpha1 Dependent Signaling in Renal Tissue Fibrosis

Abstract

Tubulointerstitial fibrosis is a common hallmark of chronic kidney disease caused by diabetes, hypertension, ischemia, renal injury and obstructive uropathy. The hetereotrimeric AMP-activated protein kinase (AMPK) consists of three subunits (α, β, γ) and is a master sensor of cellular energy status. Activation of AMPK contributes to monocyte-fibroblast transition and production of matrix protein even though accumulating evidence suggests that activated AMPK inhibits tissue fibrosis, which may be due to isoform specific effects of AMPK. Therefore, the isoform specific function of AMPKα1 in renal fibrosis was investigated. To this end, gene-targeted mice lacking functional AMPKα1 (AMPKα1-/-) and corresponding wild-type mice (AMPK α1+/+) were subjected to unilateral ureteral obstruction (UUO) for 3 days, 7 days and 3 weeks or angiotensin II infusion (ANG II) for 2 weeks. Quantitative RT-PCR, western blotting and immunostaining were used to determine transcript levels and protein abundance respectively. Both, UUO and ANG II treatment increased the protein abundance of AMPKα1 in kidney tissues of AMPKα1+/+ mice. In contrast, AMPKα2 protein in UUO treated renal tissue of AMPKα1+/+ mice was down-regulated, but not in ANG II infused mice. Importantly, α-smooth muscle actin (α-SMA) was upregulated in renal tissue by both UUO and ANG II treatment, an effect significantly blunted by AMPKα1 deficiency. TGF-β activated kinase 1 (TAK1) phosphorylation was increased by UUO and ANG II treatment in renal tissue of AMPKα1+/+ mice, an effect significantly blunted by AMPKα1 deficiency. The UUO induced elevated transcript levels of Col1a1 and Col3a1 was reduced in AMPKα1-/- mice, although collagen deposition remained unchanged following UUO. The increased mRNA expression of pro-inflammatory and pro-fibrotic cytokines was attenuated in AMPKα1-/- mice following both, UUO or ANG II treatment. Furthermore, infiltration of inflammatory cells was impaired in AMPKα1-/- mice indicated by lower mRNA expression of Cd206 and Cxcl16 after UUO. Nonetheless, AMPKα1-/- mice displayed aggravated tubular injury and apoptosis induced by UUO. In Xenopus oocytes, co-expression of wild-type and a mutated constitutively active AMPK, but not a mutated inactive AMPK reduced Kv1.5-mediated currents. Constitutively active AMPK and Nedd4-2 reduced Kv1.5 K+ channel protein abundance in the cell membrane. In conclusion, the isoform shift from AMPKα2 towards AMPKα1 potentiates myofibroblast formation and contributes to signaling involved in renal fibrosis. AMPK affects TAK1 phosphorylation and regulates Kv1.5 abundance, which may mediate the effects of AMPK during renal fibrosis.