The calcium-sensing receptor in the specification of normal and malignant hematopoietic cell localization in the bone marrow microenvironment

Hematopoiesis occurs through complex interplay between hematopoietic stem and progenitor cells and the supportive bone marrow (BM) microenvironment. Defined by the unique ability to self-renew and differentiate into all of the necessary blood cells to support lifelong hematopoiesis, hematopoietic stem cells (HSCs) are spatially located in BM stem cell niches. It was first discovered that primitive HSCs are localized at the endosteal region of the BM. In this region, bone remodeling mediated by bone-forming osteoblasts and bone-resorbing osteoclasts dynamically alters the endosteal surface and releases Ca²⁺ ions from the bone. HSCs are able to sense extracellular Ca²⁺ concentrations through the cell surface expression of the calcium-sensing receptor (CaR). Previous studies demonstrated the crucial role of the CaR in the localization of HSCs in the endosteal BM niche, where CaR⁻/⁻ HSCs displayed stem cell autonomous defects including impaired lodgment and engraftment. We further explored the role of the CaR on HSC functions by using a pharmacologic approach to stimulate the receptor ex vivo with the clinically approved allosteric agonist, Cinacalcet. With CaR stimulaton, HSC homing, lodgment, and engraftment in the endosteal BM niche were promoted via mechanisms involving increased CXCR4 signaling and cell adhesion to major ECM molecules, suggesting that HSC-niche interactions were enhanced to support hematopoietic recovery in the recipient. Although the regulatory functions of the CaR in HSC lodgment and localization have been investigated in normal hematopoiesis, the functional role of the CaR in pathologic hematopoiesis is still poorly understood. In this regard, we used a genetic approach to study the functional role of the CaR in regulating the pathophysiology of multiple myeloma (MM), as this specific hematological malignancy involves devastating bone destruction caused by an excessively active bone-resorbing osteoclastic population, leading to heightened levels of Ca²⁺ ions in the BM endosteal region. We demonstrated that the CaR stimulates myeloma cell proliferation, with mechanisms involving adhesive interactions with the BM microenvironment and cell cycle entry. We also provided the first evidence that the CaR plays a chemoprotective role in MM, as genetic manipulations of CaR expression altered the chemosensitivity of myeloma cells to the chemotherapeutic agent, bortezomib. Finally, we explored the clinical relevance of the CaR in the specification of a functionally distinct HSC population in human hematopoietic tissues based on the cell surface expression of the receptor. Interestingly, we discovered that human HSCs displaying cell surface CaR have impaired hematopoietic stem and progenitor activity in vitro, but similar in vivo engraftment potential in xenogeneic recipients compared to human HSCs lacking cell surface CaR. Although the precise regulatory mechanisms remain to be elucidated, our findings nonetheless suggest that intrinsic functional differences exist in these human HSC populations. Collectively, these data have implications in developing novel therapeutic strategies to enhance HSC engraftment in the transplantation setting by stimulating the CaR, and also in targeting the CaR to abrogate the pathophysiology of bone metastatic cancers by disrupting the complex interplay between the cancer cells and the BM microenvironment.