Bcl-x is alternatively spliced into two mRNAs. The protein product of the larger Bcl-x mRNA (Bcl-xL) functions as a repressor of programmed cell death (Kroemer, selleck products 1997), whereas the smaller splicing product Bcl-xS, encodes a protein capable of accelerating cell death (Antonsson and Martinou, 2000; Tsujimoto and Shimizu, 2000). While it becomes increasingly clear that the two close relatives Bcl-2 and Bcl-xL show different cellular expression patterns and may complement each other’s antiapoptotic function, the exact mechanisms of action remain unclear (Kroemer and Reed, 2000; Robertson and Orrenius, 2000). The antiapoptotic effects of Bcl-xL against IR- and chemotherapy-induced apoptosis have been demonstrated in various human cancer cell lines (Huang et al, 1997; Amarante-Mendes et al, 1998; Nagane et al, 1998; Srinivasan et al, 1998).

The most pronounced effects were observed in cells containing the highest levels of Bcl-xL expression. Antisense (AS) oligonucleotides are modified single-strand stretches of nucleotides capable of inhibiting protein expression by complexing with the complementary target mRNA preventing translation. Antisense oligonucleotides hold great promise as agents for specific manipulation of gene expression and have been used to inhibit gene expression both in vitro and in vivo (Kitada et al, 1994; Keith et al, 1995). Bcl-xL downregulation by AS oligonucleotides has been observed in different types of cancer cells leading to an increase in susceptibility to apoptotic stimuli (Amarante-Mendes et al, 1998; Lebedeva et al, 2000).

Recently, it was shown that Bcl-xL AS oligonucleotides are capable of sensitising colon cancer cells in vitro to 5-fluorouracil (Nita et al, 2000). Furthermore, bcl-2/bcl-xL bispecific oligonucleotides significantly reduced Bcl-xL expression that leads to increased apoptosis and delayed tumour growth in a xenotransplantation model for colon cancer (Gautschi et al, 2001). Taylor et al (1999) demonstrated specific downregulation of Bcl-xL by AS oligonucleotides (ISIS 16009) in keratinocytes and epithelial cells and sensitisation to UV-B radiation- and cisplatin-induced apoptosis. However, the effect of Bcl-xL AS oligonucleotides on radiosensitivity of colon cancer has not yet been explored.

Given the overexpression of Bcl-xL protein in more than 60% of human colon cancers (Krajewska et al, 1996; Maurer et al, 1998) and its positive correlation with poor prognosis (Biroccio et al, 2001), we hypothesised that downregulation Anacetrapib by Bcl-xL by AS oligonucleotides may sensitise colon cancer cells to IR or cisplatin. MATERIALS AND METHODS Cell culture The human colorectal carcinoma cell line Caco-2 was obtained from American Type Culture Collection (ATCC, Manassas, VA, USA) and maintained in basal tissue culture medium (DMEM) supplemented with 8% foetal calf serum, 1% penicillin, and 1% streptomycin (all Gibco BRL, Paisley, UK) in a humidified 5% CO2, 95% ambient air atmosphere at 37��C.