Disruption of sarcolemmal ATP-sensitive potassium channel activity impairs the cardiac response to systolic overload

Abstract

Sarcolemmal ATP-sensitive potassium channels (K(ATP)) act as metabolic sensors that facilitate adaptation of the left ventricle to changes in energy requirements. This study examined the mechanism by which K(ATP) dysfunction impairs the left ventricular response to stress using transgenic mouse strains with cardiac-specific disruption of K(ATP) activity (SUR1-tg mice) or Kir6.2 gene deficiency (Kir6.2 KO). Both SUR1-tg and Kir6.2 KO mice had normal left ventricular mass and function under unstressed conditions. Following chronic transverse aortic constriction, both SUR1-tg and Kir6.2 KO mice developed more severe left ventricular hypertrophy and dysfunction as compared with their corresponding WT controls. Both SUR1-tg and Kir6.2 KO mice had significantly decreased expression of peroxisome proliferator-activated receptor gamma coactivator (PGC)-1alpha and a group of energy metabolism related genes at both protein and mRNA levels. Furthermore, disruption of K(ATP) repressed expression and promoter activity of PGC-1alpha in cultured rat neonatal cardiac myocytes in response to hypoxia, indicating that K(ATP) activity is required to maintain PGC-1alpha expression under stress conditions. PGC-1alpha gene deficiency also exacerbated chronic transverse aortic constriction-induced ventricular hypertrophy and dysfunction, suggesting that depletion of PGC-1alpha can worsen systolic overload induced ventricular dysfunction. Both SUR1-tg and Kir6.2 KO mice had decreased FOXO1 after transverse aortic constriction, in agreement with the reports that a decrease of FOXO1 can repress PGC-1alpha expression. Furthermore, inhibition of K(ATP) caused a decrease of FOXO1 associated with PGC-1alpha promoter. These data indicate that K(ATP) channels facilitate the cardiac response to stress by regulating PGC-1alpha and its target genes, at least partially through the FOXO1 pathway.

Figure 1

After 4 weeks of TAC, SUR1-tg mice had significantly more ventricular hypertrophy (A) and pulmonary congestion (B), a greater increase of LV end-systolic diameter (C), a marked decrease of LV ejection fraction (D), and significantly higher mRNA and protein levels of ANP (F and G) as compared with WT mice. E, representative echocardiograms. *p<0.05 as compared with sham; #, p<0.05 as compared with WT.

Figure 2

TAC induced more severe cardiac hypertrophy (A) and pulmonary congestion (B) in the Kir6.2 KO mice. There was a trend toward deterioration of cardiac function (C and D) and a significant increase of ANP mRNA level (E) in the Kir6.2 null mice as compared with their wild type littermates.

Figure 3

Following TAC the expression of myocardial energy metabolism related enzymes was significantly decreased at both protein (A–C) and mRNA levels (D) in SUR1-tg mice as compared with WT. The mRNA levels of these enzymes were also significantly decreased in the Kir6.2 KO mice (E). *p<0.05 as compared to the corresponding sham group; # p<0.05 as compared to WT.

Figure 4

TAC caused significant increases in PGC-1α and PPARα mRNA in WT mice, which were absent in the SUR1-tg mice (A). The PGC-1α protein level was decreased in SUR1-tg mice at basal conditions, and further decreased following TAC (B and C). The mRNA level of PGC-1α was also significantly decreased after TAC in the Kir6.2 KO mice (D). * p<0.05 as compared to sham; #, p<0.05 as compared to wild type.

Figure 5

The mRNA levels of PGC-1α and its target gene CPT-1b were significantly when K_ATP channels were pharmacologically blocked with glibenclamide (A) or genetically inhibited by SUR2 specific siRNA (B). Glibenclamide treatment and SUR2 gene silencing reduced the luciferase activity of the reporter gene driven by the PGC-1α promoter (C).

Figure 6

TAC induced more severe hypertrophy (A), pulmonary congestion (B), LV dilation (C) and dysfunction (D), and ANP expression (E) in the PGC-1α null mice. * p<0.05 as compared to sham; #, p<0.05 as compared to wild type.

Figure 7

Following TAC both total and phos-Akt levels were increased in SUR1-tg mice as compared with WT (A and B). Total FOXO1 levels were significantly less in both SUR1-tg mice (A and B) and Kir6.2 KO mice (D) as compared with their corresponding controls. Nuclear FOXO1 was significantly decreased in the banded SUR1-tg hearts (C). *p<0.05 as compared to sham; #, p<0.05 as compared to WT. In rat neonatal cardiomyocytes, blocking K_ATP activity enhanced Akt phosphorylation and reduced the nuclear fraction of FOXO1 (E). * p<0.05 as compared with cells transfected with non-specific siRNA and treated with vehicle (DMSO).

Figure 7

Following TAC both total and phos-Akt levels were increased in SUR1-tg mice as compared with WT (A and B). Total FOXO1 levels were significantly less in both SUR1-tg mice (A and B) and Kir6.2 KO mice (D) as compared with their corresponding controls. Nuclear FOXO1 was significantly decreased in the banded SUR1-tg hearts (C). *p<0.05 as compared to sham; #, p<0.05 as compared to WT. In rat neonatal cardiomyocytes, blocking K_ATP activity enhanced Akt phosphorylation and reduced the nuclear fraction of FOXO1 (E). * p<0.05 as compared with cells transfected with non-specific siRNA and treated with vehicle (DMSO).

Figure 8

Three IRS were identified by sequence alignment and underlined bases (in blue) were mutated (in red) (A). Reporter activity was reduced when IRSs were mutated (B). Disruption of K_ATP activity disassociated FOXO1 from PGC1-1α promoter (C). A diagram of regulation of PGC-1α expression by K_ATP channels (D). RLU, relative luciferase unit.