Pharmacological Targeting of SERCA in Breast Cancer
Authors: Colomer-Saucedo Jesika B. , Loulousis Melanie M. , Copello Valeria A. , Krager Stacey L. , Tischkau Shelley L. , Copello Julio A. 
Published: April 2020

About 12% of U.S. woman develop breast cancer at some time during their lifetime. In addition to surgery and radiation therapy, chemotherapy is a key part of treatment following diagnosis. However, current drugs have variable success due to challenging disease subtypes that exhibit drug resistance. As a result, over 40,000 women die every year from intractable metastatic breast cancer. Thus, various new targets against breast cancer cells are currently under study. Aberrant intracellular Ca2+ homeostasis is observed in most cancer cells, hence, proteins that modulate Ca2+ levels may serve as a potential therapeutic target. Thapsigargin (TG) and cyclopiazonic acid (CPA) are agents known to potently inhibit the endoplasmic/sarcoplasmic reticulum (ER/SR) Ca2+‐ATPase (SERCA) and have shown high efficacy for impairing cancer cell growth. However, TG and CPZ are highly cytotoxic for normal cells. In recent years, our lab used spectrometry UV‐visible Ca2+ loading and ATPase activity assays in SR microsomes to identify novel drugs that target SERCA in striated muscle via an inhibitory, calcium‐dependent mechanism. These include drugs of the benzothiazepine (BZT) family (K201, CGP, PH00095) and antiepileptic drugs (sipatrigin, pimozide). The action of these new agents would cause partial SR depletion in cells by affecting Ca2+ sequestration at low cytosolic Ca2+. Interestingly, some of these drugs have been found to be cytoprotective under various stressful conditions. It is possible that these drugs prevent damaging SR Ca2+ overload under pathological conditions, including ischemia, via its action on SERCA. These findings contrast with the cytotoxicity associated with TG and CPZ. We compared the efficacy of these new SERCA inhibitors versus TG/CPZ on cell viability of breast cancer cell lines MCF‐7 and MDA‐MB‐231. MTS assay revealed TG and CPZ were more efficacious than our new compounds to decrease MCF‐7 viability. However, none of the agents tested significantly affected MDA‐MB‐231, a more metastatic breast cancer cell line. When paired with chemotherapeutics, K201, TG and CPZ synergistically reduce MCF‐7 viability. Still, the effects of chemotherapeutics on MDA‐MD‐231 was not significantly changed. Based on our studies, targeting intracellular Ca2+ homeostasis may serve as a potential therapeutic target for some breast cancer cell types but not all. Here, we find that drugs of the BZT family, as well as some antiepileptics, reduce MCF‐7 cell viability and potentially synergize with the action of chemotherapeutics. However, MDA‐MB‐231 remain largely unaffected by these agents. In principle, we thought that MDA‐MB‐231 may not have significant intracellular Ca2+ stores; yet immunoblot analysis reveal comparable levels of SERCA1/2 protein expression in both cell lines. Still, MDA‐MB‐231 seems to preferentially utilize Ca2+ entry from the extracellular space for cell survival. Further studies of plasma membrane Ca2+ transporters are required to elucidate the dependence of intracellular Ca2+ signaling among these breast cancer cell lines.

Support or Funding Information

CT Moy Endowed Fund, Eskridge Foundation Grant, SIUSOM ‐ Team Development Grant