Abstract 1140: CD24 exists in the nucleus and drives cancer growth

CD24 is an oncoprotein whose high level of expression in patient tumors yields a poor prognosis. This observation has been shown in breast, prostate, pancreas, ovary, colorectal, and bladder cancers. In vitro, elevated expression of CD24 has been shown to drive cancer cell proliferation while in vivo, it promotes tumor growth and metastasis. Cancer stem cells (CSC), in many different tissue types, are denoted by the presence or absence of CD24 cell surface expression. Most prominent among these tissue types is the finding that sorting breast cancer cells for a CD24neg population yields a CSC cohort. Using bladder cancer cells we discovered that a CD24neg sorted population exhibits a 2 fold increase in anchorage independent growth relative to parental cells and a 5 fold increase relative to cells depleted of CD24 by RNAi (shCD24). In vivo we determined that CD24neg cells were dramatically more successful than shCD24 cells at establishing metastases (6/10 vs 0/10 mice). Strikingly, we found that treatment of CD24neg cells with CD24 siRNA eliminated their increased growth potential. These results strongly suggest a significant biological role for non-surface CD24. Therefore, we set out to determine the precise locations of CD24 within cells and assess any possible biological relevance. Cellular fractionation experiments revealed the existence of CD24 in the nuclear fraction in 3 different bladder lines as well as lines from breast, lung, colon and prostate cancer cells. Furthermore, this nuclear signal was the same in both CD24neg cells and parental cells. Using immunofluorescence we observed CD24 signal colocalizing with DAPI signal. Again, this nuclear signal remained relatively unchanged in CD24neg cells compared to parental cells. We also found that blocking nuclear import (Ivermectin) lead to a decrease in nuclear CD24 signal and blocking nuclear export (Leptomycin B) lead to an increase in CD24 signal in the nucleus, suggesting cells actively regulate nuclear CD24 levels. To assess the potential functional significance of nuclear CD24 we expressed a mutant which localizes exclusively to the nucleus (NLS-CD24). Comparing cells expressing NLS-CD24 to those expressing either wild-type CD24 (Wt-CD24) or a control construct with a scrambled amino acid sequence in place of NLS (Scr-CD24) revealed that NLS-CD24 expressing cells exhibit a 2-3 fold increase in anchorage independent growth over both Wt-CD24 and Scr-CD24. Similarly, growth rates of subcutaneous tumors in mice expressing NLS-CD24 were 3 times higher than those expressing Wt-CD24. Taken together, we discovered and validated a nuclear cohort of CD24 and found that nuclear specific expression of CD24 promotes greater growth phenotypes both in vitro and in vivo. Thus, it is important that cancer models and therapeutic approaches involving CD24 expression consider this nuclear cohort. Citation Format: Jason Duex, Changho Lee, Charles Owens, Ana Chauca-Diaz, Dan Theodorescu. CD24 exists in the nucleus and drives cancer growth. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1140.