Abstract 11991: Peroxisome Proliferator-activated Receptor Gamma, Coactivator-1 beta is Downregulated by a Hypoxia-inducible Factor-1 alpha-dependent Mechanism Involving MYC Suppression in Hypoxic Endothelial Cells
As hypoxia-induced endothelial cell (EC) dysfunction plays an integral role in pulmonary hypertension, we examined the effects of hypoxia on mitochondrial content and energy metabolism in human pulmonary artery ECs. In these cells, gene expression of peroxisome proliferator-activated receptor gamma, coactivator-1 beta (PGC-1β), a regulator of mitochondrial biogenesis, was reduced by 64 ± 2% (p<0.001) following exposure to 24 hours of hypoxia. As expected, hypoxia stabilized hypoxia-inducible factor-1α (HIF-1α) protein. Targeted knockdown of HIF-1α blunted the suppressive effect of hypoxia on PGC-1β expression by 59 ± 1% (p<0.01). MYC is known to promote the expression of PGC-1β. Hypoxia caused an increase in gene (4.5 ± 0.3-fold, p<0.001) and protein (16 ± 0.1-fold, p<0.05) expression of MXI1, an inhibitor of MYC activity, and significantly decreased expression of MYC target genes (p<0.05). Knockdown of MYC decreased PGC-1β gene expression (67 ± 4%, p<0.05), confirming a role for MYC in modulating PGC-1β expression in ECs. Hypoxia decreased cellular ATP levels (63 ± 6%, p<0.01) and mitochondrial DNA content (22 ± 2%, p<0.05). PGC-1β knockdown under normoxic conditions, however, had no effect on cellular ATP or on mitochondrial DNA content. Hypoxia and targeted knockdown of PGC-1β lessened the expression of MFN2, a regulator of mitochondrial networks, by 22 ± 1% (p<0.05) and 52 ± 11% (p<0.05), respectively, although neither hypoxia nor knockdown of PGC-1β produced observable changes in mitochondrial network formation, as assessed qualitatively by Mitotracker Green staining. MYC knockdown, however, caused fragmented mitochondrial networks. Thus, exposure to hypoxia decreases ATP and mitochondrial DNA content, and reduces expression of PGC-1β by stabilizing HIF-1α and inhibiting MYC pathways. Targeted knockdown of PGC-1β alone, however, is insufficient to alter EC mitochondrial DNA content or cellular ATP. In addition, MYC, but not PGC-1β, appears to influence mitochondrial architecture in ECs. Taken together, these data suggest that hypoxia-mediated changes in mitochondrial content and bioenergetics are regulated in part by independent as well as interrelated effects of HIF-1α stabilization, PGC-1β expression, and the MYC pathway.
- © 2012 by American Heart Association, Inc.