5949

AKT-dependent hypersensitivity to mTOR inhibitors is controlled by the differential expression of cyclin D1 and c-myc (Gera et al., 2004, JBC, 279:2737). Translational control of cyclin D1 and c-myc mRNAs has been demonstrated to be a key regulatory mechanism which governs expression of these determinants in response to rapamycin. However, our previous data suggests that cyclin D1 and c-myc may additionally be coordinately regulated in an AKT-dependent manner at the level of transcription and/or mRNA stability. To further investigate the alterations in cyclin D1 and c-myc expression following rapamycin exposure a detailed analysis of both promoter activity and mRNA turnover rates was performed. We describe here the differential coordinate promoter activity and regulation of mRNA stability of cyclin D1 and c-myc in response to rapamycin which is dependent on the relative degree of AKT activity. Under conditions of relatively quiescent AKT activity, treatment of cells with rapamycin resulted in the upregulation of both cyclin D1 and c-myc nascent transcription as well as a marked increase in mRNA stabilility of both of the transcripts. Treatment of cells which had elevated levels of AKT activity with rapamycin resulted in the inhibition of basal cyclin D1 and c-myc transcription and coordinate reductions in their mRNA stabilities. Serial deletions of reporter constructs containing either the 5’ promoter regions of cyclin D1 or c-myc were used to identify AKT-dependent rapamycin responsive elements. Similarly, heterologous chimeric reporter mRNAs containing the 3’ untranslated regions from either the cyclin D1 or c-myc transcripts were analyzed and found to contain cis-acting elements sufficient to confer the AKT-dependent alterations in mRNA turnover rates observed. The characterization of these short elements should permit the identification of interacting factors and signaling components mediating these coordinate responses.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]