Activation of the Non-Canonical Estrogen Receptor Gper As a Novel Therapeutic Strategy Against Waldenström Macroglobulinemia

Although estrogens have been shown to regulate normal B cell proliferation and differentiation, the impact of estrogen receptor (ER) signaling on B cell malignancies, including Waldenström Macroglobulinemia (WM), remains unexplored. To address this issue, we have analyzed, through preclinical WM models, the druggability either of the classical estrogen receptor ERβ, an emerging target for acute leukemias and certain types of lymphomas, and of the novel, more recently deorphanized G-protein coupled ER GPER, whose therapeutic potential in solid tumors has recently emerged.

Both ERβ and GPER were found expressed at mRNA and protein level in a panel of WM and IgM-secreting lymphoma cell lines. However, while treatment with ERβ agonists did not elicit any effect, selective GPER activation significantly reduced WM cell viability. On this basis, we investigated GPER expression pattern and therapeutic potential in WM.

By interrogating public microarray datasets, we found GPER transcript significantly upregulated in WM as compared to normal B cells; moreover, immunohistochemical analysis showed elevated GPER expression in lymph node biopsies from newly-diagnosed WM patients as compared to healthy lymph nodes. Treatment with the selective GPER agonist G-1 ( (±) -1-[(3aR*,4S*,9bS*)-4-(6-Bromo-1,3-benzodioxol-5-yl) -3a,4,5,9b-tetrahydro-3H cyclopenta [c]quinolin-8-yl] ethanone) led to reduced viability, clonogenicity and migration of WM and IgM-secreting lymphoma cell lines (IC50 at 48h ranging between 2.5 and 5.0 mM), including MYD88 and CXCR4 mutated cell lines, and even in the presence of bone marrow-derived stromal cells, while it did not affect healthy CD19⁺ B cell viability; conversely, GPER antagonists G-36 and G-15 slightly enhanced cell proliferation. The growth inhibitory activity of G-1 was associated with accumulation in G2/M cell cycle phase and induction of apoptosis, the latter phenomenon assessed by Annexin V/7AAD staining, analysis of mitochondrial membrane depolarization and cleavage of caspase 3, 7 and 9; hybridization of an antibody-array also highlighted G-1-induced down-regulation of anti-apoptotic proteins, including survivin and Bcl2. A significant effect of G-1 was also observed in primary tumor cells from refractory WM patients. Moreover, G-1 synergistically enhanced bortezomib and ibrutinib cytotoxicity in vitro. Importantly, intraperitoneal injection of G-1 (2mg/kg) significantly reduced the growth of BCWM-1 xenografts in NOD/SCID mice.

To gain further insight into the consequences of selective GPER activation, we performed gene expression profile and GSEA analysis of WM cell lines treated with G-1. Of note, the p53 signaling pathway was strongly induced upon GPER activation, and upregulation of p53 and of its target genes or microRNAs (p21^CIP1, Bax, Bad, PUMA and miR-34a) was confirmed both in G-1 treated WM cell lines and primary WM cells; moreover, G-1 combination with bortezomib or ibrutinib led to stronger increase in p53 and p21^CIP1 levels as compared to single agent treatment. Finally, p53 knock-down partially reversed G-1-dependent anti-WM effects, thus suggesting p53 as downstream mediator of GPER.

In summary, we report a significant anti-tumor activity upon selective GPER activation in WM cells, with apoptosis induction and a potent synergistic anti-WM activity in combination with ibrutinib and bortezomib. Altogether, these preclinical data suggest GPER agonists as a potential therapeutic option in WM.