Down-regulation of lck mRNA by T cell activation involves transcriptional and post-transcriptional mechanisms.

The p56lck tyrosine kinase is most likely to be involved in signal transduction of T lymphocyte activation. After full activation through the TcR/CD3 complex lck mRNA is transiently down-modulated. This down-modulation was due to an early decrease of both transcription and stability of the lck mRNA. To study the involvement of transcriptional and post-transcriptional factors in this regulations, we have analysed the effect of cycloheximide, a protein synthesis inhibitor, on the steady-state of the lck mRNA. Cycloheximide superinduced lck mRNA by increasing its stability, although cycloheximide concomitantly decreased lck transcription. This suggests that the constitutive level of lck mRNA observed prior to activation is controlled by transcriptional activator(s) and post-transcriptional destabilizing factor(s). Second, lck mRNA down-modulation observed after full activation was inhibited by cycloheximide. It increased lck mRNA stability whereas lck transcription remained low. Therefore, full activation might increase the synthesis and/or activity of destabilizing factor(s). Cyclosporin A also inhibited the down-modulation of lck mRNA by increasing its transcription with no effect on its stability. Since, lck mRNA down-modulation was always associated with lymphokine mRNA induction, and since CsA blocks both lymphokine transcription and lck decrease of transcription, this indicates that these genes might share common regulatory pathways leading to their inverse transcriptional regulation.