Protein phosphatase 2A A regulates A protein expression and stability

Protein phosphatase 2A (PP2A) represses many oncogenic signaling pathways and is an important tumor suppressor. PP2A comprises three distinct subunits and forms through a highly regulated biogenesis process, with the scaffolding A subunit existing as two highly related isoforms, A and A. PP2A's tumor-suppressive functions have been intensely studied, and PP2A inactivation has been shown to be a prerequisite for tumor formation. Interestingly, although partial loss of the A isoform is growth promoting, complete A loss has no transformative properties. Additionally, in cancer patients, A is found to be inactivated in a haploinsufficient manner. Using both cellular and in vivo systems, colorectal and endometrial cancer cell lines, and biochemical and cellular assays, here we examined why the complete loss of A does not promote tumorigenesis. CRISPR/Cas9-mediated homozygous A deletion resulted in decreased colony formation and tumor growth across multiple cell lines. Protein expression analysis of PP2A family members revealed that the A deletion markedly up-regulates A protein expression by increasing A protein stability. A knockdown in control and A knockout cell lines indicated that A is necessary for cell survival in the A knockout cells. In the setting of A deficiency, co-immunoprecipitation analysis revealed increased binding of specific PP2A regulatory subunits to A, and knockdown of these regulatory subunits restored colony-forming ability. Taken together, our results uncover a mechanism by which PP2A A regulates A protein stability and activity and suggests why homozygous loss of A is rarely seen in cancer patients.