Abstract 5080: CD24 expression by prostate and bladder cancer cells is induced by hypoxia and promotes tumor growth and metastasis

Abstract

CD24 is a cell surface glycoprotein similar to mucin. Our lab has previously shown that CD24 expression is necessary for in vitro survival, proliferation and anchorage independent growth of a variety of human cancer cell lines, including ones from prostate and bladder cancers (Smith et al. Cancer Res. 2006 Feb 15;66(4):1917-22). Elevated CD24 expression in human bladder cancer tissue was also found to be an independent, negative indicator for patient disease-free survival. Despite these studies, and numerous other correlations of CD24 expression to tumorigenicity or tumor aggressiveness, the role of CD24 in cancer remains elusive. The purpose of the study reported here was to determine the necessity of CD24 expression for in vivo growth and metastatic competence of prostate and bladder cancer xenografts. Additionally, we sought to determine if elevated CD24 expression found in these types of tumors could be a response to stimuli from the tumor microenvironment, namely hypoxia. Stable knock-down of CD24 expression in the prostate cancer cell line PC-3 and the bladder cancer cell line UMUC-3 was achieved by introduction of shRNA. Tumor growth after subcutaneous inoculation in immune-compromised murine hosts was evaluated for cells with CD24 knock-down and for control cells harboring a non-targeted shRNA. CD24 knock-down reduced the tumorigenicity of both cell lines. Next, we evaluated the requirement of CD24 expression for metastatic competence in these cell lines. After intracardic injection, PC-3 cells with abrogated CD24 expression produced significantly fewer metastases at all sites than control cells. UMUC-3 bladder cancer cells were injected via the tail vein and metastasis was evaluated in the mouse lungs. Lung metastasis by UMUC-3 cells with CD24 knock-down was reduced significantly compared to controls. Regional hypoxia is a common feature of solid tumors, and hypoxia induces a number of genes responsible for increased tumor growth, invasion, metastasis and resistance to therapy. We therefore examined the effects of hypoxia on CD24 expression. Hypoxia increased CD24 protein and mRNA expression in PC-3, LNCaP and DU-145 prostate cancer cells, as well as in UMUC-3 bladder cancer cells. In order to study the mechanism of CD24 induction by hypoxia, we have constructed a series of deletion mutants from the CD24 promoter region controlling the luciferase reporter gene. When transfected into UMUC-3 cells, the reporters show that hypoxia induction of CD24 is dependent upon a cis-acting element within a 123 b.p. interval of the CD24 promoter. Our study shows that CD24 gene expression is induced by hypoxia in prostate and bladder cancer cells, and this regulation may account for its elevated expression in tumors. CD24 expression under hypoxic conditions could also play a role in the demonstrated requirement for CD24 in tumorigenicity and metastasis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5080.