Fibrinogen Enhances Homing-Related Responses of CD34⁺ by Incorporating Membrane Type1-Matrix Metalloproteinase into Membrane Lipid Rafts.

Clinical studies have shown that transplanted mobilized peripheral blood (mPB) hematopoietic stem/progenitor cells (HSPC) home faster to the bone marrow (BM) than harvested BM or cord blood cells. We previously showed that fibrinogen has a priming effect on HSPC homing by enhancing the chemotactic responses of CD34⁺ cells towards a low SDF-1 gradient by incorporating CXCR4 into membrane lipid rafts (

We found that fibrinogen

• highly upregulated MT1-MMP protein expression (Western blot) in CD34⁺ cells as well as proMMP-2 and proMMP-9 secretion (zymography);

• primed trans-Matrigel chemoinvasion of CD34⁺ cells towards a low SDF-1 gradient (20 ng/mL), which was inhibited by epigallocatechin-3-gallate, a potent inhibitor of MT1-MMP; and

• stimulated MMP-2 activation in co-cultures of stromal cells (BM fibroblasts and HUVEC) with CD34⁺ cells.

Moreover, we demonstrate by confocal microscopy, for the first time, that in CD34⁺ cells MT1-MMP is localized in the GM1-fraction of lipid rafts where it co-localizes with CXCR4; this co-localization is enhanced when CD34⁺ cells are stimulated with fibrinogen. Furthermore, disruption of lipid raft formation by the cholesterol-depleting agent methyl-b-cyclodextrin inhibited MT1-MMP incorporation into membrane lipid rafts and also trans-Matrigel chemoinvasion of CD34⁺ cells towards an SDF-1 gradient. Thus we conclude that fibrinogen enhances homing-related responses of CD34⁺ cells towards SDF-1 by increased incorporation and co-localization of CXCR4 and MT1-MMP in membrane lipid rafts. Further, we postulate that while the presence of CXCR4 in lipid rafts allows the cells to better sense the SDF-1 chemokine gradient, the upregulated MT1-MMP in the lipid rafts facilitates their migration through the ECM and possibly towards the BM niches.