Taxol- and okadaic acid-induced destabilization of bcl-2 mRNA is associated with decreased binding of proteins to a bcl-2 instability element.

The observation that overexpression of the anti-apoptotic protein Bcl-2 is associated with both cancer development and anti-cancer drug resistance suggests that factors which regulate bcl-2 expression may be important therapeutic targets. We report here that taxol or okadaic acid (OA) treatment of HL-60 cells reduced bcl-2 mRNA steady state levels to 50% of control cell levels in 20-24hr of treatment. The 3'-untranslated region of bcl-2 mRNA contains four potential A+U-rich elements (AREs), which are associated with mRNA destabilization. RNA gel mobility shift assays revealed that HL-60 cell extracts contain proteins that bind to RNA transcripts containing the first bcl-2 ARE (ARE 1). ARE 1 binding activity was substantially reduced in extracts of cells treated for 20 hr with taxol or OA and was abolished after 32 hr of treatment. UV-induced RNA cross-linking assays revealed that untreated HL-60 cell extracts contain approximately eight proteins, ranging in size from 32 to 100 kDa, that bind to ARE 1 RNA. Following 20 hr of taxol or OA treatment, RNA cross-linking to approximately 70 and approximately 38 kDa proteins was greatly reduced, and cross-linking to four proteins of 45-60 kDa sizes was progressively reduced with 10-34 hr of OA or taxol treatment. Collectively, these results suggest a novel action of taxol and OA on bcl-2 expression, which involves bcl-2 mRNA downregulation through inactivation of bcl-2 mRNA stabilizing factors.