Bone Abstracts

Recent evidence suggests that osteoblastic bone marrow niche is vital for the maintenance and self-renewal of hematopoietic stem cells (HSC). It has been recently proposed that cholesterol efflux pathways participate in HSC mobilization and that cholesterol-sensing pathways control the proliferation of HSC progenitors. Moreover, we have recently documented that HDL perturbations result in impaired osteoblastic function in mice. In the present study, we aimed at investigating the role of ApoA-1, the cardinal regulator of HDL biosynthesis in the regulation of HSC quiescence-mobilization and consequently in hematologic malignancies.

Materials and methods: Whole bone marrow cells (WBMCs) were isolated, from the femora of ApoA-I^-/- (n=6) and WT (n=6) C57BL/6 mice and assessed for the expression of factors that are differentially expressed in the BM microenvironment and affect HSC fate. More specifically, we tested the expression of the chemoattractant cytokine CLCX12, its receptor CXCR4, the Jagged-1/Notch (1,2) signaling cascade elements as well as N-cadherin and osteopontin, factors that promote HSC quiescence and self-renewal with qRT-PCR. Additionally, we assessed the expression of CLCX12 and CXCR4 (the most significant regulators of HSC microenvironment) with flow cytometry.

Results: The expression of CLCX12 was significantly reduced, while the expression CXCR4 was greatly augmented (possibly via feedback cell reaction-mechanism) in the WBMC of the ApoA-I^-/- compared to the WT mice. WBMCs from ApoA-I^-/- mice displayed strongly decreased mRNA levels of Jagged-1, Notch-1 and -2 and ostepontin. The levels of N-cadherin were greatly elevated in the knock-out compared to the WT animals. In symphony with the RT-PCR results, flow cytometry confirmed the RT-PCR results as regards CLCX12 and CXCR4 expression.

Discussion: The present study suggests for the first time that ApoA-I deficiency halts HSC maintenance and quiescence, whereas it promotes HSC differentiation facilitating the development of hematologic malignancies and possibly bone metastases.