Date Published: January 24, 2019
Publisher: Public Library of Science
Author(s): Dong-Hong Lee, Eun-Jeong Yu, Joseph Aldahl, Julie Yang, Yongfeng He, Erika Hooker, Vien Le, Jiaqi Mi, Adam Olson, Huiqing Wu, Joseph Geradts, Guang Q. Xiao, Mark L. Gonzalgo, Robert D. Cardiff, Zijie Sun, MOHAMMAD Saleem.
The tumor suppressor p16Ink4a, encoded by the INK4a gene, is an inhibitor of cyclin D-dependent kinases 4 and 6, CDK4 and CDK6. This inhibition prevents the phosphorylation of the retinoblastoma protein (pRb), resulting in cellular senescence through inhibition of E2F-mediated transcription of S phase genes required for cell proliferation. The p16Ink4a plays an important role in tumor suppression, whereby its deletion, mutation, or epigenetic silencing is a frequently observed genetic alteration in prostate cancer. To assess its roles and related molecular mechanisms in prostate cancer initiation and progression, we generated a mouse model with conditional deletion of p16Ink4a in prostatic luminal epithelium. The mice underwent oncogenic transformation and developed prostatic intraepithelial neoplasia (PIN) from eight months of age, but failed to develop prostatic tumors. Given the prevalence of aberrant androgen signaling pathways in prostate cancer initiation and progression, we then generated R26hARL/wt:p16L/L: PB-Cre4 compound mice, in which conditional expression of the human AR transgene and deletion of p16Ink4a co-occur in prostatic luminal epithelial cells. While R26hARL/wt:PB-Cre4 mice showed no visible pathological changes, R26hARL/wt:p16L/L: PB-Cre4 compound mice displayed an early onset of high-grade PIN (HGPIN), prostatic carcinoma, and metastatic lesions. Strikingly, we observed tumors resembling human sarcomatoid carcinoma with intermixed focal regions of signet ring cell carcinoma (SRCC) in the prostates of the compound mice. Further characterization of these tumors showed they were of luminal epithelial cell origin, and featured characteristics of epithelial to mesenchymal transition (EMT) with enhanced proliferative and invasive capabilities. Our results not only implicate a biological role for AR expression and p16Ink4a deletion in the pathogenesis of prostatic SRCC, but also provide a new and unique genetically engineered mouse (GEM) model for investigating the molecular mechanisms for SRCC development.
Mounting evidence has shown aging to be one of the most important risk factors for human prostate cancer (reviewed in ). Aging leads to decreased regenerative capability and an increased risk of malignant transformation (reviewed in ). The tumor suppressor p16INK4a has been shown to play a critical role in cellular aging and proliferation [3, 4]. An inhibitor of cyclin D-dependent kinases, 4 and 6, p16INK4a prevents phosphorylation of the retinoblastoma protein (pRb), which inhibits the transcription of E2F-regulated genes required for cell cycle entry at the G1/S checkpoint [3, 5]. The suppressive role of p16Ink4a on cell cycle progression is frequently disrupted in tumor cells, either by deletions or inactivating mutations of p16Ink4a  or pRb . The expression of p16Ink4a is generally low in normal tissue . The tumor suppressor role of p16Ink4a can be initiated by a number of distinct stimuli, including oncogenic stress, DNA-damage, and aging . It has been shown that deletion of p16Ink4a occurs in human prostate cancer [10, 11]. Conversely, a suppressive growth effect of the induced expression of p16Ink4a has been observed in a prostate cancer xenograft model . These lines of evidence suggest a pivotal role of p16Ink4a in prostate tumorigenesis.
Loss of the CDK4/6 inhibitor, p16INK4a, due to its deletion or reduced expression through promoter hyper-methylation, has been observed in human prostate cancer samples [32, 45, 46]. Although the oncogenic role of p16Ink4a loss has been investigated in several tumor types [47–49], the loss of the tumor suppressor in prostate tumorigenesis has not been evaluated in a relevant mouse model. In this study, we developed a novel mouse model, p16L/L:PB-Cre4 mice, where the p16Ink4a gene was selectively deleted in prostatic luminal epithelial cells through Cre-LoxP-mediated recombination. We observed the development of mouse prostatic intraepithelial neoplasia in p16L/L:PB-Cre4 mice at eight months of age, providing the first line of evidence demonstrating that p16Ink4a deletion can initiate oncogenic transformation and PIN formation in the mouse prostate. In this study, we did not observe further severe pathologic changes in p16L/L:PB-Cre4 mice, suggesting that deletion of p16Ink4a alone is not sufficient to induce progression to prostate cancer. The precise mechanisms for this observation are currently unclear, potentially requiring additional genetic alterations for p16Ink4a-mediated tumor development in the prostate. The current mouse model, p16L/L:PB-Cre4 mice, is a biologically relevant experimental tool for identification of such alterations in the initiation of malignant transformation.