Abstract
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, Aα and Aβ. 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 Aα isoform is growth promoting, complete Aα loss has no transformative properties. Additionally, in cancer patients, Aα 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 Aα does not promote tumorigenesis. CRISPR/Cas9-mediated homozygous Aα deletion resulted in decreased colony formation and tumor growth across multiple cell lines. Protein expression analysis of PP2A family members revealed that the Aα deletion markedly up-regulates Aβ protein expression by increasing Aβ protein stability. Aβ knockdown in control and Aα knockout cell lines indicated that Aβ is necessary for cell survival in the Aα knockout cells. In the setting of Aα deficiency, co-immunoprecipitation analysis revealed increased binding of specific PP2A regulatory subunits to Aβ, and knockdown of these regulatory subunits restored colony-forming ability. Taken together, our results uncover a mechanism by which PP2A Aα regulates Aβ protein stability and activity and suggests why homozygous loss of Aα is rarely seen in cancer patients.
Highlights
Protein phosphatase 2A (PP2A) represses many oncogenic signaling pathways and is an important tumor suppressor
To define the molecular basis for why homozygous A␣ deletion appears to be unfavorable for cancer cell growth, we used a combination of biochemical and cellular assays to examine the functional ramifications of complete loss of the A␣ subunit
Colorectal cancer and endometrial cancer cell lines were chosen for these analyses because both heterozygous mutations as well as heterozygous loss of the A␣ subunit are recurrent in these contexts, indicating that haploinsufficiency of A␣ in these cancers may be a mechanism of PP2A inactivation (The Cancer Genome Atlas) [16, 17]
Summary
Protein phosphatase 2A (PP2A) represses many oncogenic signaling pathways and is an important tumor suppressor. 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. Whereas complete loss of A results in transformation, A␣ functions as a tumor suppressor in a haploinsufficient manner These data are reflective of what has been seen in large sequencing cohorts, including The Cancer Genome Atlas (TCGA). These studies have revealed that the A␣ isoform is altered in 35% of human cancers, homozygous deletions of A␣ are exceedingly rare, occurring in only 0.3% of patient tumors (Fig. S2, A and B). Modulating specific A holoenzymes by knockdown of B56␥ restored colony growth, indicating that B56␥-A holoenzymes are at least partially responsible for the growth-suppressive effects of
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
