Hypoxia Modulates the PP2A system in Human Cardiovascular Cell Lines: HIF-1α Dependent and Independent Regulation of PP2A in Aortic Smooth Muscle Cells and Ventricular Cardiomyocytes.

BACKGROUND AND PURPOSE: Although protein phosphatases regulate several aspects of cellular function, their expression pattern and modulation under hypoxia remains poorly understood. We investigated the expression of key components of the protein phosphatase system in human cardiovascular cells under normoxic/hypoxic conditions and the mechanism by which hypoxia alters PP2A activity.

EXPERIMENTAL APPROACH: Cardiovascular cell lines were cultured in cell type specific media under normoxic or hypoxic conditions (1% O2 ). Effects on mRNA expression, phosphatase activity, post-translational modification, and involvement of HIF-1α were assessed using RT-PCR, immunoblotting, an activity assay, ELISA, and siRNA silencing.

KEY RESULTS: All components of the protein phosphatase system studied were expressed in each cell line. Hypoxia attenuated mRNA expression of the transcripts in a cell line and time dependent manner. In HASMC and AC16 cells, hypoxia decreased PP2Ac activity and mRNA expression without altering PP2Ac abundance. Hypoxia increased demethylation of PP2Ac and increased PME-1, but decreased LCMT-1 abundance. HIF-1α siRNA prevented the hypoxia-mediated decrease in phosphatase activity and expression of PPP2CA, independently of altering pPP2Ac, DPP2Ac, LCMT-1 or PME-1 abundance.

CONCLUSION AND IMPLICATIONS: Cardiovascular cells express multiple components of the protein phosphatase system. In HASMC and AC16 cells, hypoxia inhibits PP2A activity through HIF-1α dependent and independent mechanisms, with the latter being consistent with altered PP2A holoenzyme assembly. These data indicate a complex inhibitory effect of hypoxia on the PP2A system and expands our understanding of cardiac pathophysiology which has implications regarding development of therapies to modulate diseases associated with dysregulated protein phosphorylation.