Loss of SIMPL compromises TNF-α-dependent survival of hematopoietic progenitors

Objective: Emerging work has revealed an integral role of the tumor necrosis factor-α (TNF-α) nuclear factor (NF)-κB pathway in the regulation of hematopoiesis. TNF-α inhibition of hematopoietic stem/progenitor cell growth involves type I TNF-α receptor (TNF-RI) and type II TNF-α receptor (TNF-RII). However, the role of TNF-RI vs TNF-RII in mediating this response is less clear. Full induction of NF-κB-dependent gene expression through TNF-RI requires the transcriptional coactivator SIMPL (substrate that interacts with mouse pelle-like kinase). To address the role of SIMPL in TNF-α-dependent signaling in hematopoiesis, endothelial cells and hematopoietic progenitors expressing SIMPL short hairpin RNA were characterized. Material and Methods: In vitro gene expression and progenitor assays employing SIMPL short hairpin RNA were used to examine the requirement for SIMPL in TNF-α-dependent effects upon cytokine gene expression and hematopoietic progenitor cell growth. Competitive repopulation studies were used to extend these studies in vivo. Results: SIMPL is required for full TNF-RI-dependent expression of NF-κB-controlled cytokines in endothelial cells. Hematopoietic progenitor cell expansion is not affected if progenitors lacked SIMPL or if progenitors are treated with human TNF-α, which signals through TNF-RI. In the absence of SIMPL, human TNF-α leads to a dramatic decrease in progenitor cell expansion that is not due to apoptosis. Loss of SIMPL does not affect the activity of transforming growth factor-β1 and interferon-γ, other known suppressors of hematopoiesis. Conclusions: Suppression of myeloid progenitor cell expansion requires signaling through TNF-RI and TNF-RII. Signals transduced through the TNF-α-TNF-RI-SIMPL pathway support hematopoietic progenitor cell survival, growth and differentiation.