Identification of two RNA cis-elements that function to regulate the 5' splice site selection of Bcl-x pre-mRNA in response to ceramide

Two splice variants derived from the BCL-x gene, proapoptotic Bcl-x(s) and anti-apoptotic Bcl-x(L), are produced via alternative 5' splice site selection within exon 2 of Bcl-x pre-mRNA. In previous studies, our laboratory demonstrated that ceramide regulated this 5' splice site selection, inducing the production of Bcl-x(s) mRNA with a concomitant decrease in Bcl-x(L) correlating with sensitization to chemotherapy (Chalfant, C. E., Rathman, K., Pinkerman, R. L., Wood, R. E., Obeid, L. M., Ogretmen, B., and Hannun, Y. A. (2002) J. Biol. Chem. 277, 12587-12595). We have now identified several possible RNA cis-elements within exon 2 of Bcl-x pre-mRNA by sequence analysis. To study the possible roles of these RNA cis-elements in regulating the alternative 5' splice site selection of Bcl-x pre-mRNA, we developed a BCL-x minigene construct which conferred the same ratio of Bcl-x(L)/Bcl-x(s) mRNA as the endogenous Bcl-x and was responsive to ceramide treatment. Mutagenesis of either a purine-rich splicing enhancer or a pyrimidine tract element within exon 2 induced a change in the ratio of Bcl-x(L)/Bcl-x(s) mRNA from 7 to 1 and 0.23, thereby diminishing the selection of the Bcl-x(L) 5' splice site with a concomitant increase in Bcl-x(s) 5' splice site selection. Furthermore, mutagenesis of these cis-elements abolished the ability of ceramide to affect the 5' splice site selection. In vitro binding assays coupled with competitor studies demonstrated specific binding of RNA trans-activating proteins to these regions. SDS-PAGE analysis of cross-linked RNA trans-activating factors with these RNA cis-elements revealed the binding of 215-, 120-, and 30-kDa proteins to the purine-rich element and 120- and 76-kDa proteins to the pyrimidine tract element. In addition, exogenous treatment of A549 cells with ceramide increased the formation of protein complexes with these RNA cis-elements. Therefore, we have identified two ceramide-responsive RNA cis-elements within exon 2 of Bcl-x pre-mRNA, and this is the first report of an RNA cis-element responsive to a bioactive lipid.