Cancer Biology and Signal Transduction PIK3R3 Induces Epithelial-to-Mesenchymal Transition and Promotes Metastasis in Colorectal Cancer

Class IAPI3Kplays an essential role in the invasion andmetastasis of cancer. However, themechanisms and specific functions of PI3K isoforms in tumor invasion andmetastasis arenot fullyunderstood.Weevaluated the role of PIK3R3, a PI3K regulatory subunit encoded by the PIK3R3 gene, in colorectal cancer invasion and metastasis. Clinical specimens and cell lines data show that the expression level of PIK3R3 is associated with colorectal cancer metastasis. Overexpression of PIK3R3 increases tumor migration and invasion in vitro and promotes metastasis of colorectal cancers in vivo. Furthermore, we investigated that the overexpression of PIK3R3depends on SNAI2, inducing significant epithelial-to-mesenchymal transition (EMT).Downregulation of PIK3R3 reverses this process, which possibly contributes to the enhanced invasive and metastasizing abilities of colorectal cancer cells. In this study, we found that PIK3R3 plays an important role in colorectal cancermetastasis andmight be a potential and specific target for therapies againstmetastatic colorectal cancer. Mol Cancer Ther; 13(7); 1837–47. 2014 AACR. Introduction Metastasis is one of the biggest challenges in cancer therapy and is responsible for 90% of cancer patient deaths (1, 2). Targeting tumor metastasis process will improve the tumor therapy and give patients a longer overall survival (1). Activation of class IA PI3K/AKT signaling is necessary for many events involved in the metastatic pathway including escape of cells from the tumor environment, into and out of the circulation, blockageof apoptosis, activation of proliferation, andactivation of angiogenesis (3–5). Targeting the PI3K/AKT pathway byPI3K inhibitors can improve the outcomeof cancer, and many PI3K pathway inhibitors have been developed and are being evaluated in preclinical studies and in early clinical trials (3). Class IA PI3Ks consist of a regulatory subunit and a catalytic subunit (6). Three mammalian genes, PIK3R1, PIK3R2, and PIK3R3, encode p85a (p85a, p55a, and p50a isoforms), p85b, andp55g (also namedp55PIK) regulatory subunits (7). The catalytic isoforms, p110a, p110b, and p110d, are the products of 3 genes, PIK3CA, PIK3CB, and PIK3CD (8). PI3K signaling is activated in human cancers via several different mechanisms, including direct mutational activation or amplification of genes encoding key components of the PI3K pathway such as PIK3CA and AKT1 or loss of PTEN (3, 9). The role of several PI3K subunits in tumor development and metastasis has been well-investigated, including p110a, p110b, p85a, and p85b (10–12). But even now, the role and function of PIK3R3 in tumormetastasis still remain to be little known. The PIK3R3, encoded by the PIR3R3 gene, can bind to the p110 catalytic subunit through the iSH2 domain (13). Compared with other regulatory subunits, PIK3R3 contains a unique NH2 terminal. We previously study found that the NH2 terminal of PIK3R3 mediated PIK3R3-specific functions different from other regulatory subunits, via binding to some cell growth keyproteins including the retinoblastoma (RB1) protein and proliferating cell nuclear antigen (PCNA; refs. 14, 15). We have reported that the PIK3R3 regulatory subunit is important for cell proliferation and tumor growth and is overexpressed in some cancers (15–17). Zhang and colleagues also reported that the mRNA and protein levels of PIK3R3 in ovarian cancer were elevated compared with those in normal ovarian epithelia and that PIK3R3 knockdown induces ovarian cell apoptosis in vitro (18). Moreover, in our previous studies, we investigated that blocking PIK3R3 inhibits cell-cycle progression, induces cell differentiation, and inhibits tumor angiogenesis (15, 16, 19). Although PIK3R3 is especially important in the tumorigenesis, cell-cycle regulation, cell differentiation, and angiogenesis, in our previous study, blocking PIK3R3 by PIK3R3-specific inhibitor Ad-N24 can prevent colorectal cancer liver metastasis in animal model (17), which showed that PIK3R3 may play an important role in cancer metastasis, but the function and the mechanisms of PIK3R3 in tumor metastasis remain unknown. Authors'Affiliation: CancerResearch Institute, Tongji Hospital, Huazhong University of Science and Technology, Wuhan, China Note: G. Wang and X. Yang contributed equally to this work. Corresponding Author: Junbo Hu, Cancer Research Institute, Tongji Hospital, Huazhong University of Science and Technology, Wuhan 430030, China; Phone: 86-027-83663301; Fax: 86-027-83663301; E-mail: [email protected] doi: 10.1158/1535-7163.MCT-14-0049 2014 American Association for Cancer Research. Molecular Cancer Therapeutics www.aacrjournals.org 1837 on April 20, 2017. © 2014 American Association for Cancer Research. mct.aacrjournals.org Downloaded from Published OnlineFirst May 16, 2014; DOI: 10.1158/1535-7163.MCT-14-0049