Combined Targeting of Estrogen Receptor Alpha and XPO1 Prevent Akt Activation, Remodel Metabolic Pathways and Induce Autophagy to Overcome Tamoxifen Resistance

Abstract

A majority of breast cancer specific deaths in women with ERα (+) tumors occur due to metastases that are resistant to endocrine therapy. There is a critical need for novel therapeutic approaches to resensitize recurrent ERα (+) tumors to endocrine therapies. The objective of this study was to elucidate mechanisms of improved effectiveness of combined targeting of ERα and the nuclear transport protein XPO1 in overcoming endocrine resistance. Selinexor (SEL), an XPO1 antagonist, has been evaluated in multiple late stage clinical trials in patients with relapsed and /or refractory hematological and solid tumor malignancies. Our transcriptomics analysis showed that 4-Hydroxytamoxifen (4-OHT), SEL alone or their combination induced differential Akt signaling- and metabolism-associated gene expression profiles. Western blot analysis in endocrine resistant cell lines and xenograft models validated differential Akt phosphorylation. Using the Seahorse metabolic profiler, we showed that ERα-XPO1 targeting changed the metabolic phenotype of TAM-resistant breast cancer cells from an energetic to a quiescent profile. This finding demonstrated that combined targeting of XPO1 and ERα rewired the metabolic pathways and shut down both glycolytic and mitochondrial pathways that would eventually lead to autophagy. Remodeling metabolic pathways to regenerate new vulnerabilities in endocrine resistant breast tumors is novel, and given the need for better strategies to improve therapy response in relapsed ERα (+) tumors, our findings show great promise for uncovering the role that ERα-XPO1 crosstalk plays in reducing cancer recurrences.

Combined Targeting of Estrogen Receptor Alpha and Exportin 1 in Metastatic Breast Cancers

“…Our RNA-Seq analysis showed that PPARGC1A (PGC1α) mRNA [ 13 ], ( Figure 5 A) was induced by SEL treatment in endocrine resistant cell lines and protein expression of PGC1α was also increased ( Figure 5 B). PGC-1α is a major regulator of mitochondrial biogenesis, and overexpression of this protein was implicated in regulation of glutamine metabolism [ 18 ] and resistance to metabolic drugs [ 19 ].…”

“…Briefly, we used 6-week-old BALB/c athymic (RRID: IMSR_TAC:balb), ovariectomized, nude female mice from Taconic Biosciences. Power calculations were done based on our previously published studies and five animals per group were used [ 11 , 13 ]. After one week of acclimatization, we injected 1 × 10 6 cells resuspended in 50% PBS and 50% Matrigel subcutaneously into both the right and left flank of each mouse.…”

“…XPO1 up-regulation caused nuclear export of factors that are required in the nucleus for ERα-dependent transcriptional responses [ 11 , 12 ]. We previously showed that XPO1 mRNA and protein expression are higher in Luminal B subtype of tumors, which are more refractory to endocrine treatments [ 11 , 13 ]. High XPO1 mRNA expression is associated with a poor outcome in women who are treated with tamoxifen (TAM) [ 11 ].…”

“…High XPO1 mRNA expression is associated with a poor outcome in women who are treated with tamoxifen (TAM) [ 11 ]. In cells that are resistant to the active form of TAM, 4-hydroxy-tamoxifen (4-OHT), combined targeting of ERα and XPO1 prevented 4-OHT induced cell proliferation and anchorage independent growth [ 11 ], and induced autophagy [ 13 ]. In BT474 tumor xenografts, TAM stimulated tumor growth whereas SEL inhibited growth.…”

“…In our follow-up studies, we showed that XPO1 induced endocrine resistance by modulating PI3K/Akt signaling and increased survival of BCa cells by increasing mitochondrial and glycolytic respiration. Combined ERα and XPO1 targeting inhibited activation of cell survival mechanisms to provide sustained tumor regression [ 13 ]. However, we do not know if we can combine XPO1 inhibitors with other therapies currently used in the clinics for treatment of metastatic ER + BCas.…”