Abstract C63: Understanding prostate cancer initiation and development through differentiation: Role of ING4 and Myc in prostate differentiation and cancer development

Abstract Introduction: Genes involved in differentiation and cell fate determination are frequently altered in cancers. Understanding how normal differentiation is controlled, and how its deregulation leads to cancer development is paramount to developing better diagnostic and therapeutic strategies for prostate cancer. Our laboratory recently developed an in vitro differentiation model in which human AR-negative basal prostate epithelial cells can be differentiated in to AR-positive secretory cells. Loss of integrin expression and cell-matrix adhesion is crucial to generating stable AR-expressing cells. This is consistent with the observation that integrins are only expressed in basal cells and AR is only expressed in secretory cells. However, in prostate cancer basal cells are lost and AR and integrin α6β1 are co-expressed in the tumor cells. The mechanism by which this occurs is unknown. Our hypothesis is that oncogenic conversion disrupts an intermediate step in normal prostate epithelial differentiation, causing retention of integrin α6β1 and AR expression in the same cell. The earliest alterations in cell adhesion during differentiation are the loss of the extracellular matrix laminin 5 and its respective ligand-specific integrin subunits, α3 and β4. This results in the pairing of the α6 and β1 subunits to generate α6β1, which is not lost until several days later. Several lines of evidence suggest Myc is required to suppress α6β1 expression. Misregulated expression of Myc may contribute to prostate cancer development through up-regulation of integrin α6β1. ING4, a PHD finger containing nuclear protein, is a tumor suppressor that blocks Myc-induced hyperplasia. We hypothesize that ING4 controls Myc, is vital for the normal prostate epithelial differentiation, and loss of ING4 is required for tumorigenesis. Methods: We manipulated Myc and ING4 expression in differentiating human prostate epithelial cells and measured the effects on differentiation. We generated tumorigenic variants by over expressing Myc and Erg, and suppressing Pten. We measured ING4 expression in prostate cancer tissues. Results: Myc over expression resulted in faster differentiation, i.e. loss of integrin expression and induction of AR-responsive genes. However, the differentiated Myc over expressing cells eventually died while control cells did not. ING4 expression increased transiently during differentiation. Loss of ING4 expression did not affect early differentiation; however, cells could not complete differentiation. Myc over expression resulted in an increase in ING4 expression, and ING4 over expression, like Myc, initially augmented differentiation, but induced death of the differentiated cells. Knock-down of ING4 reversed the effect of Myc over expression. Epithelial cells over expressing Myc, Erg, and shPten were tumorigenic in mice, but failed to properly differentiate in vitro. They initiated the AR program, but failed to switch off integrin expression. Over-expression of ING4 in the tumorigenic cells restored normal differentiation. In a cohort of 50 patients, ING4 expression was lost in 67% of primary tumors. Conclusions: These data indicate Myc stimulates ING4 expression in mid differentiation and ING4 is required transiently for proper differentiation and loss of integrin expression. Sustained over expression of ING4, directly or indirectly by Myc, results in death of the differentiated cells. In tumorigenic cells, the differentiation program is altered such that Myc over expression no longer promotes full differentiation, ING4 is not turned on, and cells survive. We hypothesize that ING4 loss is required for prostate cancer development to overcome the death-inducing effects of Myc over expression. Funding: Association for International Cancer Research and VARI. Citation Format: Penny Berger, Suwon Kim, Cindy K. Miranti. Understanding prostate cancer initiation and development through differentiation: Role of ING4 and Myc in prostate differentiation and cancer development [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr C63.