Hypoxia-Induced Autophagy Is Mediated through Hypoxia-Inducible Factor Induction of BNIP3 and BNIP3L via Their BH3 Domains

Abstract

While hypoxia-inducible factor (HIF) is a major actor in the cell survival response to hypoxia, HIF also is associated with cell death. Several studies implicate the HIF-induced putative BH3-only proapoptotic genes bnip3 and bnip3l in hypoxia-mediated cell death. We, like others, do not support this assertion. Here, we clearly demonstrate that the hypoxic microenvironment contributes to survival rather than cell death by inducing autophagy. The ablation of Beclin1, a major actor of autophagy, enhances cell death under hypoxic conditions. In addition, the ablation of BNIP3 and/or BNIP3L triggers cell death, and BNIP3 and BNIP3L are crucial for hypoxia-induced autophagy. First, while the small interfering RNA-mediated ablation of either BNIP3 or BNIP3L has little effect on autophagy, the combined silencing of these two HIF targets suppresses hypoxia-mediated autophagy. Second, the ectopic expression of both BNIP3 and BNIP3L in normoxia activates autophagy. Third, 20-mer BH3 peptides of BNIP3 or BNIP3L are sufficient in initiating autophagy in normoxia. Herein, we propose a model in which the atypical BH3 domains of hypoxia-induced BNIP3/BNIP3L have been designed to induce autophagy by disrupting the Bcl-2-Beclin1 complex without inducing cell death. Hypoxia-induced autophagy via BNIP3 and BNIP3L is clearly a survival mechanism that promotes tumor progression.The evolutionarily conserved hypoxia-inducible factor (HIF) transcriptional complex is rapidly activated when the O 2 tension decreases (26,33). HIF orchestrates the expression of a myriad of genes, the function of which primarily is to ensure cell survival under a short-and long-term hypoxic stress, thereby attempting to restore O 2 homeostasis (32). By exploring the functionality of HIF-1␣ and, in particular, the role of its two transactivation domains (N-TAD and C-TAD), we recently brought to light the bifunctional activity of HIF-1␣ (8). This duality of action is discriminated by FIH (factor inhibiting HIF-1), an inhibitor of the C-TAD. Among the genes expressed only by the N-TAD is the putative proapoptotic gene bnip3 (Bcl-2/adenovirus E1B 19-kDa interacting protein 3). This was an intriguing finding. Why would a death-promoting protein be induced under conditions of moderate hypoxia, where HIF-1 would be expected to promote cell survival? However, a closely related gene, bnip3l (Bcl-2/adenovirus E1B 19-kDa interacting protein 3 like, also known as BNIP3␣ and Nix), was classified as an FIH-inhibited gene, and it is induced by both the N-TAD and C-TAD domains. Its expression reached its maximum in severe hypoxia, which is encountered close to necrotic areas of tumors.Therefore, we specifically focused our interest on understanding HIF-1-mediated cell death by studying the role of the HIF-dependent gene products BNIP3 and BNIP3L (7, 12) in both normal and cancer cells. BNIP3 and BNIP3L are members of the so-called BH3-only subfamily of Bcl-2 family proteins (40) that heterodimerize and antagonize the activity of the prosurvival proteins (Bcl-2 and Bcl-X L )...

NDPK-D (NM23-H4)-mediated externalization of cardiolipin enables elimination of depolarized mitochondria by mitophagy

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Association and dissociation of autophagy, apoptosis and necrosis by systematic chemical study

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The autophagy conundrum in cancer: influence of tumorigenic metabolic reprogramming

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