OBJECTIVETo investigate the expression changes and effects of hypoxia inducible factor-1α (HIF-1α) on non-lethal high temperature induced thermotolerance and its role on thermotolerance protection.

METHODSH9c2 cardiomyocytes were cultured and pretreated with the HIF-1α inhibitor YC-1, the cells were then subjected to normal temperature (37 °C), thermotolerance induction (40 °C, 3 h), or hyperthermia (43 °C, 2 h). The cells were divided into 8 groups (n = 3 each): normal temperature control group; thermotolerance group; thermotolerance/hyperthermia group; hyperthermia group; DMSO+normal temperature group; YC-1+thermotolerance group; YC-1+thermotolerance/hyperthermia group; YC-1+hyperthermia group. Cell apoptotic rate was assessed by flow cytometry. Western blot was used to detect the expression of HIF-1α and caspase-3.

RESULTSFlow cytometry results showed that apoptosis rate was similar between control group and thermotolerance group, between DMSO+normal temperature group and YC-1+thermotolerance group, between YC-1+thermotolerance/hyperthermia group and YC-1+hyperthermia group, but was significantly higher in hyperthermia group [(17.35 ± 1.07)%] than in control group [(7.52 ± 1.55)%, P < 0.01] which was partly reduced in thermotolerance/hyperthermia group [(12.58 ± 1.97)%, P < 0.01 vs. thermotolerance group]. Cell apoptosis rate of YC-1+thermotolerance/hyperthermia group (23.75 ± 1.92)% was significantly higher than that of thermotolerance/hyperthermia group [(12.58 ± 1.97)%, P < 0.01], and in YC-1+hyperthermia group [(24.89 ± 1.83)%] than in hyperthermia group [(17.35 ± 1.07)%, P < 0.01]. HIF-1α expression was obviously upregulated in thermotolerance cells compared with control cells, in thermotolerance/hyperthermia cells than in hyperthermia cells, in YC-1+thermotolerance group, YC-1+thermotolerance/hyperthermia group and YC-1+hyperthermia group than in DMSO group (all P < 0.05). Caspase-3 expression was similar between control group and thermotolerance group, but was significantly lower in thermotolerance/hyperthermia group than in hyperthermia group (P < 0.05), significantly higher in YC-1+thermotolerance group, YC-1+thermotolerance/hyperthermia group and YC-1+hyperthermia group than in DMSO group (all P < 0.05) and significantly higher in YC-1+thermotolerance/hyperthermia group than in thermotolerance/hyperthermia group (P < 0.01) and in YC-1+hyperthermia group than in hyperthermia group (P < 0.01).

CONCLUSIONNon-lethal high temperature induced thermotolerance can reduce heat stress-induced cardiomyocytes apoptosis rate via upregulating the expression of HIF-1α and inhibiting caspase-3 signalling pathways.