Hypoxia is an important environmental pressure playing fundamental role in lung pathology. Pulmonary circulation responds uniquely to hypoxia with proliferation of vascular cells and pulmonary hypertension (PH). The mechanisms underlying the vascular responses and regulation of hypoxia-inducible factor 1α (HIF-1α) remain to be elucidated.
We report that Ras association domain family 1A (RASSF1A), functions as a stabilizing factor for HIF-1α by inhibiting its hydroxylation, leading to increased HIF-1α mediated transactivation of target genes (PDK1, LDHA, HK2), and promoting proliferation and glycolytic shift in hypoxia-exposed pulmonary vascular cells. Hypoxia initially induces reactive oxygen species (ROS) and stabilizes RASSF1A by protein kinase C (PKC)-mediated phosphorylation. RASSF1A transcription is later upregulated by HIF-1α, participating in a positive feedback loop. Consistently, RASSF1A knockout mice do not develop PH in response to hypoxia. Notably, we observed high expression levels of RASSF1A; HIF-1α and HIF-1 target genes in the pulmonary vasculature of patients with PH (IPAH or PH associated with COPD) and in rodent models of PH (hypoxia and hypoxia+SU5416). In addition, we observed that siRNA mediated inhibition of RASSF1A expression reverts the hyperproliferative and glycolytic phenotype of ex-vivo cultured human PH-PASMCs.
Conclusion and Future Outlook:
We propose that a novel feed-forward regulatory interaction of RASSF1A and HIF-1 drive hypoxia-induced PH via regulation of hypoxia induced proliferation and metabolic shift. Taking a step forward, high throughput metabolic profiling of hypoxia exposed pulmonary artery smooth muscle cells in presence of RASSF1A knockdown displayed significant changes in various metabolites from citric acid cycle and glutamine metabolism. In line, mass spectrometry further revealed prolyl hydroxylase 2 (PHD2), Iso-citrate dehydrogenase (IDH) among others as RASSF1A interacting partners. In depth analysis in this direction, will help to reveal the complete picture of hypoxia driven metabolic changes regulated by RASSF1A.