Motility, proliferation, bone marrow (BM) retention and egress to the circulation of hematopoietic stem and progenitor cell (HSPCs) are key elements in normal hematopoiesis and also in the pathogenesis of acute myeloid leukemia (AML). HSPCs are tightly regulated in the BM niche by various molecules. Among them, CXCL12 and its major receptor CXCR4, expressed by both HSPCs and BM niche components, govern HSPC retention in the BM. Administration of CXCR4 antagonists lead to mobilization of both normal HSPCs and leukemia-inducing stem cells (LICs) from the BM to the circulation. We have previously found that normal human HSPCs functionally express b2-adrenergic receptors (b2-AR) that are up-regulated by myeloid cytokines such as G-CSF. The catecholaminergic neurotransmitters epinephrine and norepinephrine (NE) activate both Wnt and GSK3β signaling pathways via b2-AR, leading to enhanced HSPC proliferation, motility and BM repopulation (Spiegel et al, Nat Immunol 2007; Lapid et al, JCI 2013). These findings indicate HSPC regulation by dynamic interactions of the sympathetic nervous and hematopoietic systems. However, the role of catecholamines in regulation of AML remains elusive.
We found that several human AML cell lines from different FAB subtypes express b2-AR. NE, a b2-AR activating ligand, increased b2-AR expression and the CXCL12-induced migration of AML primary patients’ cells and cell lines. In addition, NE treatment significantly enhanced CXCL12induced actin polymerization, which drives most of the cellular movements. Looking for a downstream effector of b2-AR, we focused on RGS16, a G-protein signaling regulator, which negatively regulates CXCL12/CXCR4 axis (Berthebaud et al., Blood 2005). Concurrently with the increase in cell migration, NE decreased RGS16 expression (both in protein and mRNA level) in monocytic AML cells. However, no effect on either cell migration or RGS16 expression was observed in non-monocytic AML cells. These data provided evidence for the involvement of catecholamines in the regulation of AML cell migration and showed a correlation between AML FAB subtypes, cell motility and RGS16 expression. One of the regulators of RGS16 levels is miR126, which is highly expressed in AML and normal HSPCs and plays an important role in mobilization and proliferation of normal HSPCs. Indeed, in search for the mechanisms underlying the above observed differences, we found that the enhancing effect of NE on CXCL12-induced migration of monocytic AML cells was accompanied by up-regulation of miR126 expression concurrently with down-regulation of RGS16 expression, whereas in non-monocytic AML cells we observed the opposite effects, suggesting that NE differently regulates AML cells belonging to different FAB subtypes. Upon studying human normal HSPCs, we found that in steady state, normal cells express low levels of β2-AR and NE did not affect either RGS16 expression or CXCL12-induced migration of both mononuclear and CD34+ cells derived from human cord blood and BM.
Our results demonstrate that while normal and AML cells share common mechanisms that govern their motility, there are unrevealed yet mechanisms, apparently cell-type dependent, which uniquely lead to opposite effects in normal HSPCs, monocytic and non-monocytic AML cells. Altogether, these findings suggest that targeting of miR126 and RGS16 pathways by specific agonists and antagonists may serve as a new approach for selective eradication of LICs.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.