Molecular Stress Responses to Short Cycle Hypoxia in the Intact Heart: Evidence for Grp94 and HSP70 Oxygen Sensitivity

Abstract

Glucose related protein 94 (Grp 94) and heat shock protein 70 (HSP70) mRNA induction occur as part of a generalized stress response and Grp94 upregulation occurs via hypoxia inducible factor-1 (HIF-1) transactivation. This study tested the hypothesis that these genes respond to mild hypoxia, which does not alter energy state and injury in intact heart. We developed a short-cycle hypoxia protocol in isolated perfused rabbit heart. By altering cycling conditions we identified a specific cycle with O2 content and duration, which operated near a hypoxic threshold for causing functional injury. Profound cyclic hypoxia caused marked ATP depletion also with induction of these genes, and depletion of β-F1-ATPase, while mild short cycle hypoxia caused no ATP or total nucleoside depletion, or degradation of mRNA for the constitutive mitochondrial protein β-F1-ATPase, yet induced Grp94 and HSP70. In conclusions, these stress genes respond rapidly to mild short cycle hypoxia indicating oxygen sensitivity in the intact heart. Induction occurs without metabolic stress in the form of ATP depletion. Grp94 in these experiments may respond through enhanced HIF-1 activation, as the Grp94 promoter contains HIF-1 binding sequences, though the mechanism for HSP70 oxygen sensitivity remains unknown.