Post-transcriptional regulation of thromboxane A2 synthase in U937 cells

Translation of mRNAs is a process usually tightly coupled to transcription of genes. However, there are examples of mRNA species which accumulate without being translated. Some mRNAs present in oocytes and ferritin mRNA are the most studied models. Studying the biogenesis of thromboxane A2 (TXA2) in the promonocytic line U937, we have noted that in proliferating cells high levels of TXA2 synthase mRNA are detectable by Northern blot, whereas no TXA2 could be recovered in the medium. This has been explained on the basis of Western blot experiments: TXA2 synthase was not detectable in proliferating cells, while a band of about 55 kd appears after treatment with the differentiating agent TPA. Immunofluorescence detection by confocal microscopy was in agreement with Immunoblot results. Thus, in U937 cells, TPA behaves as a regulator of translation of TXA2 synthase mRNA. We have further observed that the induced enzyme in U937 cells has many characteristics in common with the human monocytic enzyme: a long half life (> 24 hrs), a marked stability during catalysis and similar Km and Vmax values. Thus, U937 cells are a good model to study the mechanism by which a mRNA is efficiently translated only after differentiation has been triggered.