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Simple Summary: Estrogen Receptor α (ERα) is a key target for hormonal treatment of breast cancer. However, treatment with antihormonal therapies such as tamoxifen will eventually fail, presumably through hormone-independent signaling through estrogen receptors. It has been shown that a component of this resistance to hormonal therapy may be through expression of an alternative form of estrogen receptor, ERα36. Here, we show that targeting the enzyme CK2, either with the clinical stage CK2 inhibitor CX-4945, or by disrupting expression of its regulatory β subunit, can result in decreased expression of both the canonical form of ERα protein as well as the ERα36 isoform in tamoxifen-sensitive and tamoxifen-resistant breast cancer cells. Our studies provide a rationale for the clinical investigation of CX-4945 in the treatment of ERα-expressing breast cancers. Aberrant estrogen receptor (ERα) signaling mediates detrimental effects of tamoxifen including drug resistance and endometrial hyperplasia. ERα36, an alternative isoform of ERα, contributes to these effects. We have demonstrated that CK2 modulates ERα expression and function in breast cancer (BCa). Here, we assess if CX-4945 (CX), a clinical stage CK2 inhibitor, can disrupt ERα66 and ERα36 signaling in BCa. Using live cell imaging, we assessed the antiproliferative effects of CX in tamoxifen-sensitive and tamoxifen-resistant BCa cells in monolayer and/or spheroid cultures. CX-induced alterations in ERα66 and ERα36 mRNA and protein expression were assessed by RT-PCR and immunoblot. Co-immunoprecipitation was performed to determine the differential interaction of ERα isoforms with HSP90 and CK2 upon CX exposure. CX caused concentration-dependent decreases in proliferation in tamoxifen-sensitive MCF-7 and tamoxifen-resistant MCF-7 Tam1 cells and significantly repressed spheroid growth in 3D models. Additionally, CX caused dramatic decreases in endogenous or exogenously expressed ERα66 and ERα36 protein. Silencing of CK2β, the regulatory subunit of CK2, resulted in destabilization and decreased proliferation, similar to CX. Co-immunoprecipitation demonstrated that ERα66/36 show CK2 dependance for interaction with molecular chaperone HSP90. Our findings show that CK2 functions regulate the protein stability of ERα66 and ERα36 through a mechanism that is dependent on CK2β subunit and HSP90 chaperone function. CX may be a component of a novel therapeutic strategy that targets both tamoxifen-sensitive and tamoxifen-resistant BCa, providing an additional tool to treat ERα-positive BCa. [ABSTRACT FROM AUTHOR]

Simple Summary: Endometrial cancer is the most common gynecologic cancer in the developed world and in the recurrent setting has a dismal prognosis. As the endometrium is a hormonally active tissue, hormonal therapy has long been an integral treatment modality. There are many options for hormonal therapy including progesterones, agents that target the estrogen pathway, and combinations with targeted therapies, which are described in detail in this review. High disease control rate, long duration of response, and easy tolerability make hormonal therapy an excellent option for patients with low-grade hormone receptor-positive disease who would like to avoid or cannot tolerate cytotoxic chemotherapy or other targeted therapies. Hormonal therapy has long been a treatment modality for recurrent endometrial cancer. It is appealing for patients with low-grade, slow-growing tumors or in patients for which other treatment types may be too toxic. Hormonal therapy is well tolerated and has response rates ranging from 9 to 33%. Hormonal treatment options take advantage of the estrogen-dependent molecular pathways in endometrial cancers. Current options for hormonal therapies include progesterone therapy (medroxyprogesterone acetate and megestrol acetate) as a single agent or in combination and agents that target the estrogen pathway. Aromatase inhibitors have had modest single-agent activity, but synergistic effects have been found when used in combination with targeted therapy including mTOR inhibitors and cyclin-dependent kinase 4/6 (CDK4/6) inhibitors. Molecular profiling of endometrial cancers has begun to help individualize treatments. This review will report on existing data and ongoing trials investigating novel hormonal therapy agents. [ABSTRACT FROM AUTHOR]

Simple Summary: Estrogen receptor-positive (ER+) breast cancer is a notable cause of global death from breast cancer in premenopausal and postmenopausal women. Lethality is driven, amongst other things, by endocrine resistance, which is intrinsic or acquired in approximately 60% of patients after anti-estrogen therapy. Anti-estrogen therapy resistance, which has become a major clinical problem, results from high mutation rates of therapy targets, multiple subclonal diversity, dysregulation of downstream survival pathways, as well as alterations in the expression of non-coding RNAs (e.g., microRNA) that regulate the activity of signaling molecules. This review focuses on the current status of anti-estrogen therapy resistance and the key contributing mechanisms, as well as potential novel combination therapy strategies with anti-estrogen drugs to improve patient survival. Breast cancer (BC) is the most commonly diagnosed cancer in women, constituting one-third of all cancers in women, and it is the second leading cause of cancer-related deaths in the United States. Anti-estrogen therapies, such as selective estrogen receptor modulators, significantly improve survival in estrogen receptor-positive (ER+) BC patients, which represents about 70% of cases. However, about 60% of patients inevitably experience intrinsic or acquired resistance to anti-estrogen therapies, representing a major clinical problem that leads to relapse, metastasis, and patient deaths. The resistance mechanisms involve mutations of the direct targets of anti-estrogen therapies, compensatory survival pathways, as well as alterations in the expression of non-coding RNAs (e.g., microRNA) that regulate the activity of survival and signaling pathways. Although cyclin-dependent kinase 4/6 and phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) inhibitors have significantly improved survival, the efficacy of these therapies alone and in combination with anti-estrogen therapy for advanced ER+ BC, are not curative in advanced and metastatic disease. Therefore, understanding the molecular mechanisms causing treatment resistance is critical for developing highly effective therapies and improving patient survival. This review focuses on the key mechanisms that contribute to anti-estrogen therapy resistance and potential new treatment strategies alone and in combination with anti-estrogen drugs to improve the survival of BC patients. [ABSTRACT FROM AUTHOR]