Sensitising leukemic cells by targeting microenvironment

The hematopoietic stem cell niche--home for friend and foe?

The hematopoietic stem cell (HSC) niche is involved in the maintainance and regulation of quiescence, self-renewal and differentiation of hematopoietic stem cells and the fate of their progeny in mammals dealing with the daily stresses to the hematopoietic system. From the discovery that perturbations of the HSC niche can lead to hematopoietic disorders, we have now arrived at the prospect that the HSC niche may play a role in hematological malignancies and that this HSC niche may be a target for therapy. This review attempts to capture the discoveries of the last few years regarding the normal and malignant hematopoietic stem cell niche and possible ways to target this niche.

Targeting myelogenous leukemia stem cells: role of the circulation

Unlike stem cells from solid tumors, the stem cells which initiate myelogenous leukemias arise in marrow, an organ with a unique circulation which allows ready access of leukemia cells, including leukemia stem cells (LSCs), to the vasculature. This poses unique problems in the targeting of LSCs since these cells are found circulating in the majority of leukemia cases at diagnosis and are usually not detectable during remission states. Because most cases of leukemia relapse, it is suggested that LSCs remain quiescent in the marrow until they eventually proliferate and circulate again. This indicates that effective targeting of LSCs must occur not only in peripheral circulation but in the micro-circulation of the marrow. Targeting such interactions may overcome cell adhesion-mediated treatment resistance, other multi-drug resistance mechanisms, and opportunities for clonal evolution in the marrow environment. Targeting selectins and integrins, signal transduction mediators, and chemokine/cytokine networks in the marrow micro-circulation may aid in abrogating leukemia-initiating stem cells which contribute to disease relapse. LSCs possess surface antigen profiles and signal transduction activation profiles which may allow differential targeting as compared with normal hematopoietic stem cells.