Human Cancer-Associated Mutations in the Aα Subunit of Protein Phosphatase 2A Increase Lung Cancer Incidence in Aα Knock-In and Knockout Mice

Abstract

Strong evidence has indicated that protein phosphatase 2A (PP2A) is a tumor suppressor, but a mouse model for testing the tumor suppressor activity was missing. The most abundant forms of trimeric PP2A holoenzyme consist of the scaffolding A␣ subunit, one of several regulatory B subunits, and the catalytic C␣ subunit. A␣ mutations were discovered in a variety of human carcinomas. All carcinoma-associated mutant A␣ subunits are defective in binding the B or B and C subunits. Here we describe two knock-in mice expressing cancer-associated A␣ point mutants defective in binding B subunits, one knockout mouse expressing truncated A␣ defective in B and C subunit binding, and a floxed mouse for generating conditional A␣ knockouts. We found that the cancer-associated A␣ mutations increased the incidence of cancer by 50 to 60% in lungs of FVB mice treated with benzopyrene, demonstrating that PP2A acts as a tumor suppressor. We show that the effect of A␣ mutation on cancer incidence is dependent on the tumor suppressor p53. The finding that the A␣ mutation E64D, which was detected in a human lung carcinoma, increases the lung cancer incidence in mice suggests that this mutation also played a role in the development of the carcinoma in which it was discovered.Protein phosphatase 2A (PP2A) plays a role in many fundamental cellular processes, such as signal transduction, DNA repair, transcription, translation, and growth control (15,24,26,43,77). The basis for this multifunctionality of PP2A rests on the large number of subunits that control its phosphatase activity, substrate specificity, and subcellular localization. The trimeric holoenzyme is composed of a catalytic C subunit, a scaffolding A subunit, and one of many regulatory B subunits. The dimeric core enzyme consists of one A and one C subunit. Both forms coexist in cultured cells and in tissue (5,30,42). The A subunit exists as two isoforms, A␣ and A␤, which are 87% identical. The catalytic C subunit also exists as two isoforms, C␣ and C␤, which are 96% identical. The B subunits fall into four families designated B, BЈ, BЉ, and Bٞ. The B or PR55 family has four members, the BЈ family (also designated B56 or PR61) consists of five isoforms and additional splice variants, and the BЉ or PR72 family has four members, including splice variants. B, BЈ, and BЉ are largely unrelated by sequence except for two common regions involved in binding to the A subunit (34). The Bٞ family has two members, striatin and S/G 2 nuclear autoantigen (40). The combination of all subunits could give rise to over 70 different holoenzymes. In addition, the ability of PP2A to associate with approximately 150 other proteins further increases its regulatory potential (15,24,26,42). Figure 1 shows a schematic diagram of the BЈ-containing holoenzyme whose structure has been revealed by biochemical analysis (53, 56) and X-ray crystallography (9,19,44,78,79).The first indication that PP2A might be a tumor suppressor came from the finding that okadaic acid is both a tumor promoter and an inhibitor of...

A Protein Array Screen for Kaposi's Sarcoma-Associated Herpesvirus LANA Interactors Links LANA to TIP60, PP2A Activity, and Telomere Shortening

“…SV40 small tumor antigen and polyomavirus small and middle tumor antigens each associate with PP2A and displace B subunits, and this activity is essential for their cell transformation function (18). Cancer-associated mutations in the PP2A A subunit render the A subunit defective for binding B or C subunits, and expression of these mutant PP2A proteins in mice increases tumor formation, providing additional evidence that dysregulation of PP2A activity is cancer promoting (87).…”

Ppp2ca knockout in mice spermatogenesis

“…Several studies have demonstrated a link between PP2A and a number of diseases (Janssens et al 2005, Rivera & Losada 2006, Krapf et al 2010, Brestovitsky et al 2011, Ruediger et al 2011. Its catalytic subunit PP2AC plays a critical role in embryonic development and human diseases, such as Alzheimer's disease, cancer, and diabetes (Goedert et al 1995, Zolnierowicz 2000, Kowluru 2005, Eichhorn et al 2009, Liu & Wang 2009).…”

Loss of protein phosphatase 6 in oocytes causes failure of meiosis II exit and impaired female fertility

“…Within the PPP family, the catalytic subunits of PP2A, PP4 and PP6 are most closely related, and the three proteins form a subfamily called PP2A-like protein phosphatases that account for the majority of cellular serine/ threonine phosphatase activity (Janssens and Goris, 2001;Moorhead et al, 2007). PP2A is involved in regulating chromosome condensation, DNA damage repair, the G2/M transition and sister chromatid cohesion (Ruediger et al, 2011). We have recently shown that PP2A is essential for female meiosis and fertility because oocyte-specific depletion of PP2A facilitates GVBD, causes elongated MII spindles and precocious separation of sister chromatids, resulting in defective early embryonic development, and thus subfertility .…”