Upregulation of cardioprotective SUR2A by sub-hypoxic drop in oxygen

Abstract

The effects of hypoxia on gene expression have been vigorously studied, but possible effects of small changes in oxygen tension have never been addressed. SUR2A is an atypical ABC protein serving as a regulatory subunit of sarcolemmal ATP-sensitive K(+) (KATP) channels. Up-regulation of SUR2A is associated with cardioprotection and improved physical endurance. Here, we have found that a 24h-long exposure to slightly decreased ambient fractional concentration of oxygen (20% oxygen), which is an equivalent to oxygen tension at 350m above sea level, significantly increased levels of SUR2A in the heart despite that this drop of oxygen did not affect levels of O2, CO2 and hematocrit in the blood or myocardial levels of ATP, lactate and NAD/NADH/NAD(+). Hearts from mice exposed to 20% oxygen were significantly more resistant to ischaemia-reperfusion when compared to control ones. Decrease in fractional oxygen concentration of just 0.9% was associated with phosphorylation of ERK1/2, but not Akt, which was essential for up-regulation of SUR2A. These findings indicate that a small drop in oxygen tension up-regulates SUR2A in the heart by activating ERK signaling pathway. This is the first report to suggest that a minimal change in oxygen tension could have a profound signaling effect.

Keywords: ERK; Heart; Oxygen; SUR2A.

Fig. 1

Western blot signals obtained by anti-SUR2A, anti-Kir6.2, anti-ERK1/2 and anti-Akt antibodies were of appropriate molecular weight and clearly distinguishable from any other signals appearing on blots. Typical examples of original Western blots obtained in this study.

Fig. 2

Exposure to 20% oxygen up-regulates SUR2A in the heart without inducing any measurable sings of hypoxia. Bar graphs depicting PO₂ (O₂), PCO₂ (CO2), hematocrit (HCT), total NAD (NADt), NADH (NADH), NAD + (NAD +), ATP (ATP), lactate (Lactate) SUR2A (SUR2A) and Kir6.2; insets in SUR2A and Kir6.2 bar graphs represent original Western blots for SUR2A and Kir6.2 under labeled conditions. Each bar is a mean ± SEM (n = 3–5). *P < 0.01.

Fig. 3

Exposure to 20% oxygen increases heart resistance to ischaemia-reperfusion. Bar graphs depicting CK and LDH concentration in heart perfusate following ischaemia-reperfusion challenge under labeled conditions. Each bar is a mean ± SEM (n = 5). *P < 0.05.

Fig. 4

Exposure to 20% oxygen does not phosphorylate Akt in the heart. Original Western blots with phospho-S473-Akt, phospho-T308-Akt and total Akt antibodies applied on extracts from hearts under depicted conditions and corresponding graphs. Each bar is a mean ± SEM (n = 4–5).

Fig. 5

Exposure to 20% oxygen phosphorylate ERK1/2 in the heart. Original Western blots with phospho-ERK1, phospho-ERK2 and total ERK1 and ERK2 antibodies applied on extracts from hearts under depicted conditions and corresponding graphs. Each bar is a mean ± SEM (n = 4–5). *P < 0.05.

Fig. 6

U0126 inhibits phosphorylation of ERK1 and ERK2 and increase of SUR2A in response to 20% oxygen. Original Western blots with phospho-ERK1, phospho-ERK2, total ERK1/ERK2 and SUR2A antibodies applied on extracts from hearts under depicted conditions and corresponding graphs. Each bar is a mean ± SEM (n = 3). *P < 0.05.