Cancer/Testis Antigen PASD1 Positively Regulate STAT3 Activation To Promote Tumorigenesis

Recent studies have suggested a strong association between chronic infection, inflammation and cancer, and the transcription factors NF-κB and STAT3 have been demonstrated to be crucial in linking inflammation to cancer development. The inflammatory immune cells in tumor microenvironment secrets various kinds of cytokines, such as TNF-a, IL-6, to activate NF-κB and STAT3. The activated NF-κB and STAT3 independently or cooperately induce a series of pro-oncogenic mediators to promote tumor initiation, promotion and progression, and NF-κB-IL-6-STAT3 signal axis has been proved to play pivotal roles in inflammation-associated cancer development.Activation of STAT3 is tightly regulated under physiological conditions by various mechanisms at multiple layers. To identify additional molecules that regulate IL-6-induced STAT3 activation, we screened about 13000 human and mouse cDNA expression clones by STAT3 reporter assay, and ultimately identified PASD1 as a positive regulator of STAT3 activation. PASD1 is a newly identified cancer/testis antigen and its function has not been clarified. In this study, overexpression of PASD1 specifically activated STAT3 and potentiated IL-6-induced activation of STAT3, whereas knockdown of PASD1 had opposite effects. Then we found that PASD1 interacted with STAT3 and regulated its phosphorylation. Overexpression of PASD1 enhanced both basal and IL-6-induced STAT3 phosphorylation, whereas knockdown of PASD1 had the opposite effects. Further investigations indicated that PASD1 could disrupt the interaction between TC45 and STAT3 by competing with TC45 to associate with STAT3. Next, we investigated the role of PASD1 in tumorigenesis. We found that knockdown of PASD1 inhibited the expression of many STAT3 target genes, such as FOS, BCLXL etc, which played important roles in tumorigenesis. Consistently, knockdown of PASD1 inhibited cell proliferation, anchor-independent growth and migration in vitro and tumor growth in vivo. Taken all together, our results demonstrate that PASD1 serves as a critical positive regulator of STAT3 activity by competing with TC45 to associate with STAT3, thus inhibits TC45-mediated dephosphorylation of STAT3, and consequently promotes tumorigenesis.However, the physiological functions of PASD1 remain enigmatic. The restricted expression of PASD1 in spermatogonia implies PASD1 may be involved in spermatogenesis. As there is no homologue of PASD1 in mice, it is impossible to utilize mouse model to investigate the functions of PASD1 in inflammation-mediated tumorigenesis. Since PASD1 can significantly potentiate IL-6-induced STAT3 activation, it is possible that PASD1 is involved in tumorigenesis in inflammation condition. Whether PASD1 plays roles in other biological processes requires further investigation, nevertheless, our study provides the first evidence for a role of PASD1 in tumorigenesis by activating IL-6-STAT3 signal pathway. This study helps to understand the complicated regulatory process of STAT3 activation and molecular mechanisms of inflammation-meditated tumorigenesis, and provides novel strategy for cancer therapy.