The tissue inhibitor of metalloproteinases 1 increases the clonogenic efficiency of human hematopoietic progenitor cells through CD63/PI3K/Akt signaling

Initially described as an endogenous inhibitor of proteases, the tissue inhibitor of metalloproteinases 1 (TIMP-1) also displays cytokine-like functions. TIMP-1 is a soluble protein whose levels are increased under inflammatory conditions. We recently found that TIMP-1-/- mice have decreased bone marrow (BM) cellularity and that the engraftment capability of TIMP-1-/- hematopoietic stem cells (HSCs) is impaired, owing to proliferation defects. Here, we investigated the role of recombinant human TIMP-1 (rhTIMP-1) in human hematopoietic stem/progenitor cells (HSPCs) and elucidated the downstream pathway ignited by rhTIMP-1. We found that rhTIMP-1 affects in vitro cell survival, proliferation, and particularly clonogenic expansion of CD34+ HSPCs without compromising their short-term engraftment potential after transplantation into immunodeficient mice. These effects are independent on matrix metalloproteinase (MMP) inhibition and rely on TIMP-1's binding to the tetraspanin membrane receptor CD63. Further investigation indicated that rhTIMP-1 stimulation induces phosphatidylinositol 3-kinase (PI3K) recruitment and Akt phosphorylation, both presiding over survival/proliferation pathways in HSPCs. Downstream targets of phosphorylated Akt (pAkt) are also modulated, including the proliferation marker cyclin D1 (CycD1), whose levels are increased upon exposure to rhTIMP-1. These findings indicate that rhTIMP-1 promotes clonogenic expansion and survival in human progenitors via the activation of the CD63/PI3K/pAkt signaling pathway, suggesting that TIMP-1 might be a key player in the network of proinflammatory factors modulating HSPC functions.