[en] Functional loss of the Ataxia Telangiectasia-Mutated (ATM) gene product leads to a pleiotropic phenotype, with radiation hypersensitivity as one of the most widely studied events. We have recently reported that treatment of LNCaP human prostate cancer cells with the Protein Kinase C activator 12-O-tetradecanoylphorbol 13-acetate (TPA) with radiation initiated an apoptotic response via activation of the enzyme ceramide synthase (CS) and de novo synthesis of the sphingolipid ceramide (Garzotto M et. al. 1999), while radiation alone failed to induce apoptosis in these cells. Recent studies in our laboratory showed that the CS pathway is activated in response to DNA damage and that this activation is regulated by the ATM gene product (Liao W-C et. al. 1999). Here, we show that TPA reduces the ATM protein level in both LNCaP and CWR22-Rv1 cells. Radiation alone had no significant effect on ATM levels in these prostate cancer cell lines, nor when applied together with TPA. TPA-induced reduction of ATM protein correlated with increased apoptosis in these cell lines. Quantitative RT-PCR showed a 50% reduction of ATM mRNA between 8-16 h of TPA treatment. Gel-shift analysis showed a significant reduction in the amount of Sp-1 transcription factor binding to the ATM promoter, as early as 4 h post TPA-treatment for LNCaP, but with slower kinetics for CWR22-Rv1. The use of morpholino-antisense ATM replaced the TPA effects by significantly reducing the levels of ATM protein present, and conferred intense XRT-induced activation of CS, ceramide and apoptosis. These data demonstrate that in some human prostate cell lines, transcription of ATM can be attenuated by TPA treatment, enhancing sensitivity to ionizing radiation through a CS-mediated pathway. This investigation defines a new approach to overcome radiation resistance in human prostate cancer cells in a cell-type specific manner