Role of Proinflammatory Cytokines in the Regulation of Hematopoiesis

The differentiation of CD34+ progenitor cells into erythroid and myeloid elements require a balance between the growth factors such as erythropoietin, granulocyte-macrophage colony stimulating, stem cell factor and interleukin 3 and the inhibitory cytokines such as tumor necrosis factor alpha (TNF-&agr;), interleukin (IL) 1beta and Interferon (IFN) &agr;,beta,gamma. The same cytokines that are essential for normal hematopoiesis, when over-produced lead to hematopoietic suppression. Over the years a number of studies have been performed to help better understand the underlying molecular mechanism behind the hematopoietic suppression caused by these proinflammatory cytokines. We have used two independent studies to demonstrate the role of two distinct proteins that get activated by proinflammatory cytokines but they both play very different roles in the signaling pathways.;In Part A of the thesis we demonstrate the role of a pro-apoptotic protein in inducing the myelosuppressive effects of TNF&agr; and IFNgamma on hematopoiesis. Interferon gamma (IFNgamma) and tumor necrosis factor &agr; (TNF&agr;) are potent inhibitors of hematopoiesis and have been implicated in the pathophysiology of bone marrow failure and myelodysplastic syndromes (MDS). We examined the role of protein kinase R (PKR) a pro-apoptotic kinase, in the generation of the inhibitory effects of these myelosuppressive cytokines on hematopoiesis. Our data demonstrate that PKR is rapidly phosphorylated/activated in response to engagement of IFNgamma or TNF&agr; receptors in normal human hematopoietic progenitors. Such engagement of PKR is important for the suppressive effects of these cytokines on normal hematopoiesis. Altogether, our data establish that PKR plays an important roles in signaling for IFNgamma and other myelosuppressive cytokine receptors as a common mediator of signals for hematopoietic suppression.;Part B of thesis we focus on a protein that act as negative regulator of IFN signaling. Interferons (IFNs) have important antiviral and antineoplastic properties, but the precise mechanisms required for generation of these responses remain to be defined. We provide evidence that during engagement of the Type I IFN receptor (IFNR), there is upregulation of expression of Sprouty (Spry) proteins 1, 2 and 4. Our studies demonstrate that IFN-inducible upregulation of Spry proteins act as a negative regulator of interferon-stimulated genes (ISGs). We establish that ISG15 mRNA expression and IFN-dependent antiviral responses are enhanced in Spry 1/2/4 triple knockout mouse embryonic fibroblasts. In other studies, we found that siRNA-mediated knockdown of Spry1, Spry2 or Spry4 promotes IFN-inducible antileukemic responses. Altogether, our findings demonstrate that Spry proteins are potent regulators of Type I IFN signaling and negatively control induction of Type IFN mediated biological responses.