The survival effect of mitochondrial Higd-1a is associated with suppression of cytochrome C release and prevention of caspase activation

Abstract

Higd-1a (hypoxia induced gene domain family-1a) is a mitochondrial inner membrane protein with a conformation of N-terminal outside-C-terminal outside and loop inside. There are four Higd genes, Higd-1a, -1b, -1c and -2a, in the mouse. Higd-1a and -2a are expressed primarily in the brain, heart, kidney and leukocytes. HIF (hypoxia-inducible factor) overexpression induced the endogenous expression and promoter activity of Higd-1a. Mutation of the HRE (hypoxia-response element) site at -32bp in the Higd-1a promoter reduced the promoter activity, suggesting that transcription of Higd-1a is regulated by binding of the transcription factor HIF to the HRE. Higd-1a promoted cell survival under hypoxia. RAW264.7 cells stably transfected with Higd-1a underwent less apoptosis than control cells in a hypoxic condition, and hypoxia-induced apoptosis was strongly enhanced when endogenous Higd-1a was silenced by siRNA. The survival effect of Higd-1a was completely abolished by deletion of the 26 N-terminal amino acids, and we showed that Higd-1a increased survival by inhibiting cytochrome C release and reducing the activities of caspases. However, expression of Bcl-2, Bax, Bad, and BNIP3 and translocation of AIF were unaffected under the same conditions. Higd-2a also enhanced cell survival under hypoxia. Cells transfected with Higd-2a underwent less apoptosis than control cells in hypoxic conditions, and hypoxia-induced apoptosis increased when endogenous Higd-2a was depleted. Together these observations indicate that Higd-1a is induced by hypoxia in a HIF-dependent manner and its anti-apoptotic effect results from inhibiting cytochrome C release and reducing caspase activities.

Higd-1a interacts with Opa1 and is required for the morphological and functional integrity of mitochondria

“…Although it is induced by hypoxia and promotes survival, its modes of action remain to be clarified. We showed that Higd-1a-transfected cells underwent significantly less apoptosis, as the result of inhibition of the release of cytochrome c and reduction of caspase activities (22). In the present study, we found that Higd-1a protects the Opa1 L isoform from induced cleavage.…”

“…Wang and coworkers showed that such apoptosis was inhibited by ectopic expression of Higd-1a in pancreatic β cells (18,36). We have shown that hypoxia-induced apoptosis can be prevented by overexpression of Higd-1a in RAW264.7 macrophages (22).…”

“…We constructed a mutant form of Higd-1a, designated Higd-1a-ΔNT (27-95 amino acids), with 26 N-terminal amino acids deleted. We showed that the missing region was necessary for the survival effect of Higd-1a (22). We used this mutant in the present study to examine whether the same region was required for its interaction with Opa1.…”

“…The human and mouse forms of Higd-1a are highly expressed in the brain, heart, and kidney (18,22). By regulating mitochondrial homeostasis, Higd-1a may permit these major organs to resist external stimuli such as hypoxia.…”

“…Although Higd-1a is ubiquitous, it is induced by hypoxia in a HIF-1-dependent manner (22). The human and mouse forms of Higd-1a are highly expressed in the brain, heart, and kidney (18,22).…”