Functional role of TRPC6 and STIM2 in cytosolic and endoplasmic reticulum Ca²⁺ content in resting estrogen receptor-positive breast cancer cells

TRPC6 forms non-selective cation channels activated by a variety of stimuli that are involved in a wide number of cellular functions. In estrogen receptor-positive (ER+) breast cancer cells, the store-operated Ca²⁺ entry has been reported to be dependent on STIM1, STIM2 and Orai3, with TRPC6 playing a key role in the activation of store-operated Ca²⁺ entry as well as in proliferation, migration and viability of breast cancer cells. We have used a combination of biotinylation, Ca²⁺ imaging as well as protein knockdown and overexpression of a dominant-negative TRPC6 mutant (TRPC6dn) to show that TRPC6 and STIM2 are required for the maintenance of cytosolic and endoplasmic reticulum Ca²⁺ content under resting conditions in ER+ breast cancer MCF7 cells. These cells exhibit a greater plasma membrane expression of TRPC6 under resting conditions than non-tumoral breast epithelial cells. Attenuation of STIM2, TRPC6 and Orai3, alone or in combination, results in impairment of resting cytosolic and endoplasmic reticulum Ca²⁺ homeostasis. Similar results were observed when cells were transfected with expression plasmid for TRPC6dn. TRPC6 co-immunoprecipitates with STIM2 in resting MCF7 cells, a process that is impaired by rises in cytosolic Ca²⁺ concentration. Impairment of TRPC6 function leads to abnormal Ca²⁺ homeostasis and endoplasmic reticulum stress, thus, suggesting that TRPC6 might be a potential target for the development of anti-tumoral therapies.