DISTINCT MECHANISMS REGULATE TIMP-1 EXPRESSION AT DIFFERENT STAGES OFPHORBOL ESTER-MEDIATED DIFFERENTIATION OF U937 CELLS

Upon exposure to 12-O-tetradecanoylphorbol 13-acetate (PMA), promonocy te-like U937 cells differentiate into macrophage-like cells and begin to express certain metalloproteinases and TIMP-1. We report here that distinct mechanisms regulate TIMP-1 production in PMA-treated U937 cel ls. TIMP-1 protein and steady-state mRNA levels increased about 10-fol d in PMA-differentiated cells compared to undifferentiated cells. TIMP -1 transcription increased about 2.5-fold, but this stimulation was no t detected until at least 48 h post-PMA. In contrast, the half-life fo r TIMP-1 mRNA increased about 3-fold and was detected at 8 h post-PMA. Using in vitro translation assays, we found that TIMP-1 mRNA from PMA -differentiated cells translated about 5-fold less efficiently than th at from basal cells, suggesting structural differences in TIMP-1 mRNA in basal and differentiated U937 cells. Although primer extension and RNase protection analyses showed 5' heterogeneity of TIMP-1 transcript s, all forms were equally stimulated in response to PMA-mediated diffe rentiation. The poly(A) tail length of TIMP-1 mRNA, however, was longe r in PMA-treated cells. Our findings suggested that up-regulation of T IMP-1 expression in PMA-treated U937 cells is mediated early by enhanc ed TIMP-1 mRNA stability, possibly related to increased poly(A) tail l ength, and later by an increase in transcription rate.