Growth-dependent and PKC-mediated translational regulation of the upstream stimulating factor-2 (USF2) mRNA in hematopoietic cells

Upstream stimulating factor (USF2) is a basic helix-loop-helix leucine zipper transcription factor, which is found in most tissues. A critical role for USF2 in cellular proliferation has been proposed based on its importance in the regulation of various cyclins and P53 and its capability to antagonize c-myc. In this paper we report that IL-3, which is a major growth factor for mast cells, induces USF2 protein synthesis in murine mast cells (MC-9). Surprisingly, it does not significantly affect the level of USF2 mRNA in these cells at any of the time points tested. Using polysomal fractionation and RNA analysis we then demonstrated that this translational regulation is mostly the result of increased USF2 translational efficiency. Moreover, protein kinase C (PKC) inhibitors prevented both the induction of USF2 protein synthesis and the increase in USF2 translational efficiency in IL-3-activated mast cells. Two other hematopoietic cell lines were used to determine whether the translational regulation of USF2 is of a more general nature: mouse lymphosarcoma cells whose proliferation is inhibited by dexamethasone; and mouse erythroleukemia cells that differentiate upon exposure to hexamethylen bisacetamide. In both cell types, USF2 translation was repressed in the non-dividing cells. This strongly implies that USF2 is translationally repressed in quiescent hematopoietic cells. Considering the proposed role of USF in proliferation it seems that translational regulation of USF2 might have an important role in cellular growth.