GNA13, a Novel Component of the GPR84/Beta-Catenin Signaling Axis in AML Stem Cells, Regulates the Maintenance of MLLAF9 Leukemogenesis

Beta-catenin is required for the establishment of leukemic stem cells (LSC) in mixed lineage leukemia (MLL)-rearranged acute myeloid leukemia (AML). Targeted inhibition of beta-catenin signaling has been hampered by the lack of pathway components amenable to pharmacological inhibition. Our recent study has identified a new beta-catenin regulator, GPR84, a member of the G protein-coupled receptor family that represents a highly tractable class of drug targets. Since no GPR84-specific anti-cancer compound has been developed to date, we aim to further delineate the GPR84/beta-catenin axis in order to inhibit other components of this signaling pathway pharmacologically.

Preliminary data from our lab has implicated GNA13 as a potential downstream target gene of GPR84. In this study we sought to further examine the involvement of GNA13 in the GPR84/beta-catenin signaling axis. Functional studies showed that shRNA-mediated ablation of GNA13 significantly decreased the capacity of MLLAF9-LSC to form colonies in vitro (P=0.0184). We further demonstrated that GNA13 inhibition in LSC induced a significant G1 cell cycle arrest in vitro (P=0.0192). Subsequent in vivo studies showed that GNA13 downregulation in MLLAF9-LSC significantly delayed the onset of AML (P=0.0016) and significantly decreased LSC proliferation (P=0.0066). Hence GNA13 inhibition mimics the previously described GPR84-deficient phenotype in MLLAF9-LSC (Blood 2013;122:3781). In contrast, the GPR84-deficient phenotype could be rescued in MLLAF9-LSC by GNA13 overexpression in vitro (P=0.0130) and in vivo (P=0.0012). Further analysis showed that there was a significant increase in the mitotic index of GPR84-deficient LSC transduced with GNA13 cDNA in vitro, evidenced by an increase in G2/M-phase accumulation (P=0.0030) and p-histone H3 (P=0.0040). In addition, our data demonstrated a significant increase in BrdU incorporation of GPR84-deficient LSC overexpressing GNA13 in vivo (P<0.0001).

In conclusion, our studies have identified GNA13, a novel component of the GPR84/beta-catenin axis. Since this pathway is pivotal for MLLAF9 AML maintenance by sustaining LSC self-renewal, targeting the GPR84/GNA13/beta-catenin signaling axis may provide a novel and promising strategy for improving the therapy and outcome of AML patients.