Phorbol esther-induced differentiation in human macrophages reveals the existence of factors destabilizing MHC class II transactivator (CIITA) mRNA

The human promyelocytic U937 and THP1 cell lines express detectable levels of MHC class II (MHC-II) molecules. Treatment with 12-O-tetradecanoyl phorbol 13-acetate (TPA), inducing macrophage-like differentiation, produces a dramatic decrease of MHC-II expression as result of down-modulation of the AIR-1-encoded MHC-II transcriptional activator CIITA. This event is specific, as MHC-I remains unaffected. Similar results are observed with U937 cells expressing an exogenous full length CIITA. Molecular studies demonstrate that TPA treatment affects the stability of CIITA mRNA rather than CIITA transcription. In fact the destabilizing function of CIITA 30 untranslated region (UTR) may be transferred also on EGFP reporter cDNA. By this analysis, it is shown that cis–acting elements, within the distal 650 bp of the 1035 bp 30 UTR, are associated to transcript instability. Transcription inhibitors actinomycin-D and 5,6-dichlororibofuranosyl benzimidazole, and the translation inhibitor cycloheximide significantly rescue the accumulation of CIITA mRNA in TPA-treated cells. A similar effect is also observed after treatment with staurosporine and the PKC-specific inhibitor GF109203X. The instability of CIITA mRNA produced by TPA in U937 is not seen in B cells. These results demonstrate the presence of an additional level of control of MHC-II expression in the macrophage cell lineage depending upon the control of CIITA mRNA stability most likely mediated by differentiation-induced, 30UTR-interacting factors which require kinase activity for their destabilizing function. Instability of CIITA mRNA during differentiation toward antigen presenting (APC) function should be a novel, intriguing negative feedback to avoid redundancy in the triggering of T helper responses.