NOVEL CDK2 MECHANISM CONTROLS CELL PROLIFERATION

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Nontransformed T cells require IL-2 for proliferation, and IL-2Ralpha is an obligatory component of the high affinity biologically relevant IL-2R. Numerous studies have characterized the transcriptional up-regulation of IL-2Ralpha in mitogenically stimulated T cells. On the other hand, and of potential biological importance, post transcriptional control of IL-2Ralpha expression has yet to be adequately explored or definitively demonstrated. Data presented in this proposal clearly show that IL-2Ralpha expression is regulated at the translational level in primary T cells exposed to mitogenic stimuli. We show that increased translation of the IL-2Ralpha transcript is paralleled by the induction of IL-2 signaling pathways, and we suggest that translational up-regulation of IL-2Ralpha requires the activity of the cell cycle regulatory kinase, cdk2. Interestingly, we found that cdk2 expression was also controlled at a post- transcriptional level and, most likely, by a cdk2-dependent mechanism. These studies identify novel actions of cdk2 and suggest that previously unrecognized mechanisms contribute to the expression of IL-2Ralpha and cdk2. We also present data showing that cdk2 activation and IL-2 signaling are interdependent processes in splenic T cells. We suggest that cdk2 activity is required for the efficient translation of IL-2R and that signals generated by the IL-2R facilitate cdk2 activation by maintaining the down-regulation of the cdk2 inhibitor, P27kip1. These interactions and the involvement of cdk2 in the translation of two important cell cycle regulatory molecules - IL-2Ralpha and cdk2 itself - form the focus of our proposal. Specifically, we will examine potential mechanisms by which cdk2 activity might control the translation of IL-2Ralpha in primary splenocytes. We will identify the region of the 5' untranslated region of the IL-2Ralpha mRNA that is responsible for translational regulation and will determine if proteins bind to this region in a manner dependent on cdk2 activity. We will also determine if cdk2 activity modulates the synthesis or the stability of cdk2. Additional experiments will assess the contribution of IL-2 signaling to the continued down-regulation of p27kip1 in TCR-activated splenocytes. Lastly, the mechanism by which TCR activation promotes the expression of cyclin A will be delineated.