Mitochondrial Autophagy Is an HIF-1-dependent Adaptive Metabolic Response to Hypoxia

Abstract

Autophagy is a process by which cytoplasmic organelles can be catabolized either to remove defective structures or as a means of providing macromolecules for energy generation under conditions of nutrient starvation. In this study we demonstrate that mitochondrial autophagy is induced by hypoxia, that this process requires the hypoxia-dependent factor-1-dependent expression of BNIP3 and the constitutive expression of Beclin-1 and Atg5, and that in cells subjected to prolonged hypoxia, mitochondrial autophagy is an adaptive metabolic response which is necessary to prevent increased levels of reactive oxygen species and cell death.The survival of metazoan organisms is dependent upon their ability to efficiently generate energy through the process of mitochondrial oxidative phosphorylation in which reducing equivalents, derived from the oxidation of acetyl CoA in the tricarboxylic acid cycle, are transferred from NADH and FADH 2 to the electron transport chain and ultimately to O 2 , a process which produces an electrochemical gradient that is used to synthesize ATP (1). Although oxidative phosphorylation is more efficient than glycolysis in generating ATP, it carries the inherent risk of generating reactive oxygen species (ROS) 2 as a result of electrons prematurely reacting with O 2 at respiratory complex I or complex III. Transient, low level ROS production is utilized for signal transduction in metazoan cells, but prolonged elevations of ROS result in the oxidation of protein, lipid, and nucleic acid leading to cell dysfunction or death.O 2 delivery and utilization must, therefore, be precisely regulated to maintain energy and redox homeostasis.Hypoxia-inducible factor 1 (HIF-1) plays a key role in the regulation of oxygen homeostasis (2, 3). HIF-1 is a heterodimer composed of a constitutively expressed HIF-1␤ subunit and an O 2 -regulated HIF-1␣ subunit (4). Under aerobic conditions, HIF-1␣ is hydroxylated on proline residue 402 and/or 564 by prolyl hydroxylase 2 a dioxygenase that utilizes O 2 and ␣-ketoglutarate as co-substrates with ascorbate as co-factor in a reaction that generates succinate and CO 2 as side products (5-8). Under hypoxic conditions the rate of hydroxylation declines, either as a result of inadequate substrate (O 2 ) or as a result of hypoxia-induced mitochondrial ROS production, which may oxidize Fe(II) in the catalytic center of the hydroxylase (9, 10). Hydroxylated HIF-1␣ is bound by the von HippelLindau protein, which recruits a ubiquitin protein ligase complex that targets HIF-1␣ for proteasomal degradation (11)(12)(13)(14).HIF-1 regulates the transcription of hundreds of genes in response to hypoxia (15, 16), including the EPO (17) and VEGF (18) genes that encode proteins required for erythropoiesis and angiogenesis, respectively, which serve to increase O 2 delivery. In addition, HIF-1 controls a series of molecular mechanisms designed to maintain energy and redox homeostasis. First, HIF-1 coordinates a switch in the composition of cytochrome c oxidase (mitochondrial electron-transp...

Autophagy at the crossroads of catabolism and anabolism

“…For example, damaged and superfluous mitochondria are targeted to autophagosomes in a process termed mitophagy 24,25 . Numerous studies have led to the identification of mitophagy receptors such as Atg32 in yeast, as well as BCL2/adenovirus E1B 19 kDa proteininteracting protein 3 (BNIP3), NIP3-like protein X (NIX) and FUN14 domain-containing protein 1 (FUNDC1) in mammals [26][27][28][29][30][31][32][33] . These receptors all possess an LIR motif to directly target mitochondria to autophagosomes.…”

BNIP3 protects HepG2 cells against etoposide-induced cell death under hypoxia by an autophagy-independent pathway

“…BNIP3 has been associated with cell death by necrosis, apoptosis and autophagy. Moreover, a protective role has been conferred to BNIP3 in part through the upregulation of autophagy [10]. To determine the putative role of BNIP3 on autophagy and apoptosis in the protection conferred by hypoxia, the expression of BNIP3 was silenced using specific siRNA.…”

“…In addition, hypoxia selects cancer cells with reduced apoptotic potential [9]. Autophagy is first a cellular response in which proteins, organelles and portions of cytoplasm are engulfed, digested and recycled to sustain cellular metabolism during stress such as nutrient starvation and hypoxia [10].…”

“…However, the precise mechanisms involved are still poorly understood. It seems that BNIP3 could be involved in autophagosome-lysosome fusion or in earlier stages such as Beclin-1 activation and mTOR inhibition [10,16,17].…”

Cardiac reanimation: targeting cardiomyocyte death by BNIP3 and NIX/BNIP3L

“…The exact function of autophagy in programmed cell death is not fully understood (Kroemer and Levine, 2008). It first and foremost represents an evolutionarily conserved survival pathway that includes a mechanism to selectively discard damaged or irreversibly injured organelles, particularly mitochondria (mitophagy) (Zhang et al, 2008). Early removal of damaged mitochondria after injury could limit cell death by curtailing the mitochondrial regulated intrinsic death pathway.…”