Aggressive phenotype for prostate cancer cells that survive high-dose ionizing radiation.

Abstract

236 Background: Radiation therapy (RT) is an important primary treatment for localized prostate cancer (PCa). However, certain patients can relapse and become resistant to RT. To identify the molecular mechanisms underlying RT resistance, we studied human PCa cells that survive fractionated high dose IR in vitro. Methods: DU145 (DU), a human cell line from PCa metastasized to brain, was exposed to fractionated ionizing radiation (IR) up to 40 Gy. The cancer biology of the surviving cells (DUIR) after 4 month in culture was studied. Cell growth and survival were assessed using cell counting, SRB and clonogenic assay. Invasiveness was measured using Boyden chambers. Caspase activity was determined for apoptosis. Activation of signaling molecules and expression of biomarkers was assessed using Immunoblot. Hormone binding to receptors was assayed using iodinated ligands. Results: Whereas DU was squamous in shape, DUIR displayed fibroblast-like morphology with long protrusions resembling cells undergoing epithelial-mesenchymal transition (EMT) confirmed by reduced E-cadherin and enhanced vimentin expression. DUIR cells showed higher level of neurotensin receptor 1 (NTR1) and chromogranin B resembling a neuroendocrine (NE) phenotype. DUIR has significantly higher survival than DU when exposed to IR (6–10 Gy) or zinc-directed toxins and grows significantly faster in response to serum or NT at a dose dependent manner. Addition of NTR1 antagonist SR486923 abolished the NT enhanced cell growth. DUIR had increased invasiveness through the matrigel-coated membrane. ERK and PKC, but not AKT, were constitutively active in DUIR. BCL2 level was upregulated in DUIR. NT stimulation reduced apoptosis while SR486923 enhanced apoptosis in DUIR cells upon IR. Conclusions: PCa cells that survive high dose IR display an aggressive phenotype characterized by NE and EMT features including invasion and resistance to IR/drug induced cell death. These changes could be due to constitutive activation of growth signaling pathways and upregulation of anti-apoptosis/necrosis molecules. Further study may provide insight into the mechanism of cellular resistance to therapy and establish strategies for novel therapeutic applications.