Pseudohypoxic stabilization of HIF1α via suppression of cyclophilin D promotes metastasis of melanoma

Stabilization of hypoxia-inducible factor 1 alpha (HIF1α), which is pivotal for regulating cellular responses to insufficient oxygen, is implicated in cancer progression, particularly epithelial-mesenchymal transition and metastasis. Despite its crucial role in tumorigenesis, the precise mechanisms governing HIF1α stabilization are not fully understood. Here, we show that stabilization of HIF1α in metastasizing melanoma under mild hypoxia is regulated primarily by mitochondrial reactive oxygen species (ROS) rather than by reduced oxygen levels. Activated HIF1α suppresses expression of cyclophilin D (CypD), a regulator of the mitochondrial permeability transition pore (mPTP), as a reciprocal regulatory mechanism to sustain HIF1 signaling via upregulation of microRNAs miR-23a/27a. Reduced expression of CypD closes the mPTP to increase mitochondrial calcium accumulation and oxidative phosphorylation, thereby boosting ROS production to inactivate a prolyl hydroxylase and establish a psuedohypoxic state. This HIF1-reinforced and mitochondria-driven pseudohypoxic induction is essential for maintaining HIF1 signaling under conditions of mild hypoxia or temporary elevation of oxygen levels during melanoma metastasis. Overexpression of CypD reversed the pseudohypoxic state, and potently inhibited melanoma metastasis. Thus, mitochondria-driven pseudohypoxic induction is critical for sustaining HIF1 signaling in metastasizing cancer cells, and can be exploited to develop anti-metastatic therapies