Autocrine and paracrine regulatory loops involved in PC survival within bone marrow microenvironment. PPCs express high levels of GAS6, but none of its receptors, and CXCR4 and CCR2, but not their ligands. On the contrary, BMPCs express GAS6, CXCR4, and CCR2 but also MER, SDF1, and MCP2, suggesting the existence of autocrine survival loops. In addition, BMPCs express MET, the receptor for HGF produced by bone marrow stromal cells. CD138 is a coreceptor for HGF, but also for other growth factors, in particular in association with the tetraspanin CD9. The combination of all these autocrine and paracrine signals could participate in the up-regulation of antiapoptotic proteins that is a hallmark of BPMCs.