Therefore THP 1 cells show a phenotype similar to the one observed in siRhoH BaF3 cells, with low RhoH levels and upregulated IRF 1 and CD123 e pression. Discussion Previous work has shown that RhoH is a negative regu lator for growth, survival and cytoskeletal modifications. We show here that the e pression level of RhoH modulates the activity of STAT transcription factors STAT5 and STAT1. In the IL3 dependent cell line BaF3, RhoH acts as a specific negative regulator of IL3, but not Epo induced proliferation and silencing of RhoH gene e pression allows the cells to proliferate faster in response to IL3. The JAK STAT pathway is a major signalling pathway of haematopoietic cells that links proliferative signals to the cell cycle machinery.

In IL3 mediated signalling, STAT5 plays a major role in the regulation of proliferation, differentiation and anti apoptotic signalling. We demonstrate that overe pression of RhoH leads to a decrease in the activity of STAT5, whereas silencing of RhoH e pression causes an increased activ ity of STAT5 compared to control cells. No changes in the e pression level of total STAT5 protein were detect able and we therefore conclude that RhoH does not modulate STAT5 activity through regulation of STAT5 e pression levels. Most interestingly, we also could show a link between RhoH e pression levels and changes in the surface e pression of the ILR3 a chain CD123. It had previously been suggested that an elevated CD123 e pression, as it can be found in patients with acute myeloid leukaemia, may contribute to the increased proliferation of leukaemic blasts, hyperactiva tion of STAT5 and poor prognosis.

Low e pression levels of RhoH were recently described as yet another factor linked to poor patient prognosis. Our data now show that these two findings indeed might be con nected. Because low RhoH e pression leads to an increased STAT5 activity, STAT5 might then induce e pression of the IRF 1 gene, which in turn allows an IRF 1 dependent upregulation of the CD123 gene, even tually leading to an increase in the surface levels of the protein. Although the regulatory influence of RhoH on STAT5 activity would be sufficient to account for the differ ences GSK-3 in proliferation, we observed an additional mechanism by which RhoH negatively regulates IL3 induced growth, namely the activation of STAT1 in RhoH overe pressing cells. STAT1 is the key factor that transduces the antiproliferative effects of interferons and activation of STAT1 coincides with cell cycle arrest or apoptosis. As a consequence, STAT1 knock out mice develop tumours more rapidly. When we screened control cells and RhoH overe pressing cells for differences in their sensitivity towards apoptotic sti muli, we were not able to find any.