Abstract 3180: Regulation of stem cells by the tumor suppressor PP2A-B56α
Authors: Janghorban Mahnaz , Zachman Derek , Wang Xiaoyan , Agarwal Anupriya , Fleming William , Sears Rosalie 
Published: August 2015

Protein phosphatase 2 A (PP2A) is one of the major cellular Serine-Threonine phosphatases and regulates numerous signaling pathways, including stem cell self-renewal, proliferation, differentiation, and cell survival. The PP2A holoenzyme has 3 subunits: a catalytic (C) subunit, a structural (A) subunit, and a variable regulatory (B) subunit, which directs PP2A to specific targets. PP2A inhibition is required to completely transform human cells and siRNA screens have shown that loss of B56α, B56γ, or PR72/PR130 B subunits results in cell transformation, which are involved in negatively regulating oncogenic pathways including Wnt, Akt, and Myc. Moreover, PP2A plays an important role in regulating human embryonic stem cells, where both the A and C subunit levels and PP2A activity gradually increase during the course of differentiation. Our preliminary data also demonstrates in breast cancer cell lines that B56α overexpression decreases the number of mammospheres whereas inhibiting B56α increases them. Further research is needed to address the function of specific PP2A complexes in regulating stem cells. Despite the fact that B56α depletion is important for human cell transformation, no one has reported on its role in vivo and no mouse model has ever been generated to our knowledge. Therefore, to study the role of B56α in stem cells and cell transformation in vivo, we have generated B56α knockout mice.

We generated B56α knockout mice using commercially available gene trap ES cells. The gene trap sits in the first intron of the B56α gene and, through a splice acceptor site, splices out the first exon to the gene trap and therefore stops whole gene transcription in the whole body. The primary phenotypic effect we observed with B56α loss was spontaneous skin lesion formation. Thus far 40% of mice have developed skin lesions between 11 to 21 months and showed hyper-proliferation of the epidermis, hair follicles, and dermis. Increased number of hair follicles prompted us to perform stem cell assays using BrDU to label long term label-retaining cells and the colony assays for the fast adhering keratinocyte stem cells. We found an increase in skin stem cell numbers. In addition to skin lesions, we found increased inflammation both in skin lesions, liver, and spleen and some enlarged lymph nodes. Next we asked whether loss of B56α also increased the number of hematopoietic stem cell. Although the number of hematopoietic stem cells in the periphery was unchanged under normal condition, treating mice with granulocyte colony-stimulating factor increased the number of circulating stem cells.

Our model suggests that loss of B56α increases the number of stem cells and that B56α is important for cells to maintain homeostasis; B56α loss can lead to increased activity of important oncogenes, such as cMyc, leading to aberrant cell growth and defects in stem cell maintenance. More experiments need to be done to understand the primary effect of B56α loss in skin and the interplay between skin lesions and inflammation.

Citation Format: Mahnaz Janghorban, Derek Zachman, Xiaoyan Wang, Anupriya Agarwal, William Fleming, Rosalie Sears. Regulation of stem cells by the tumor suppressor PP2A-B56α. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3180. doi:10.1158/1538-7445.AM2015-3180