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

Abstract

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 . Surprisingly, it does not signi®cantly a ect 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 e ciency. Moreover, protein kinase C (PKC) inhibitors prevented both the induction of USF2 protein synthesis and the increase in USF2 translational e ciency 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 di erentiate 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.

A complex composed of USF1 and USF2 activates the human FcεRI α chain expression via a CAGCTG element in the first intron

“…In addition, IL-3 was reported to induce USF2 protein synthesis in murine mast cells [45]. Moreover, by crosslinking of Fc 4 RI, c-Fos protein was shown to be produced.…”

Nonconventional Involvement of LysRS in the Molecular Mechanism of USF2 Transcriptional Activity in FcεRI-Activated Mast Cells

“…In nontumorigenic mammary epithelial cells, USF2 regulates the transcript levels of insulin-like growth factor 2 receptor, but interestingly it loses its ability to transactivate insulin-like growth factor 2 receptor in breast cancer cells (38). Furthermore, extensive studies were carried out on the regulation of USF2 expression in interleukin 3 activated mast cells, where an induction of USF2 protein synthesis was observed via increased translational efficiency (22,48). We previously utilized cell permeable peptides as inhibitors of the activity of the USF2 proteins, to demonstrate its possible role in mast cell survival (9).…”

Studies of Different Aspects of the Role of Protein Kinase C in Mast Cells

“…Initially we demonstrated decreased expression of USF2 as a result of antisense oligonucleotide inhibition of PKC in FcεRI-stimulated mast cells [16]. We also showed that IL-3 induced USF2 protein synthesis in mast cells [17]. However, it did not significantly affect the level of USF2-mRNA in these cells.…”

“…Using polysomal fractionation techniques and RNA analysis, we demonstrated that this translational regulation is mostly the result of increased USF2 translational efficiency [17]. Additionally, PKC inhibitors (calphostin C and long-term exposure to PMA) prevented both the induction of USF2 protein synthesis and the increase in USF2 translational efficiency in IL-3-activated mast cells, showing that PKC is involved in mediating the translational response of USF2 [17]. This observation is intriguing since only few studies have studied the role of PKC in translational regulation.…”