Intracellular calcium and NF-B-k regulate hypoxia-induced leptin, VEGF, IL-6 and adiponectin secretion in human adipocytes

Aims: Hypoxia-induced adipokine release has been attributed mainly to HIF-1 alpha. Here we investigate the role of intracellular calcium and NF-kB in the hypoxia-dependent release of leptin, VEGF, IL-6 and the hypoxia-induced inhibition of adiponectin release in human adipocytes. Main methods: We used intracellular calcium imaging to compare calcium status in preadipocytes and in adipocytes. We subjected both cell types to hypoxic conditions and measured the release of adipokines induced by hypoxia in the presence and absence of HIF-1 alpha inhibitor YC-1, NF-B-kappa inhibitor SN50 and intracellular calcium chelator BAPTA-AM. Key findings: We demonstrate reduced intracellular calcium oscillations and increased oxidative stress as the cells transitioned from preadipocytes to adipocytes. We show that differentiation of preadipocytes to adipocytes is associated with distinct morphological changes in the mitochondria. We also show that hypoxia-induced secretion of leptin, VEGF, IL-6 and hypoxia-induced inhibition of adiponectin secretion are independent of HIF-1 alpha expression. The hypoxia-induced leptin, VEGF and IL-6 release are [Ca++](i) dependent whereas adiponectin is NF-B-k dependent. Significance: Our work suggests a major role for [Ca++](i) in preadipocyte differentiation to adipocytes and that changes in mitochondrial morphology in the adipocytes might underlie the reduced calcium oscillations observed in the adipocytes. It also demonstrates that multiple signaling pathways are associated with the hypoxia-induced adipokine secretion.