Calcitonin targets extracellular signal-regulated kinase signaling pathway in human cancers

The mitogen-activated protein kinases (MAPKs) signaling pathway is a potential target in cancer therapy. Constitutive phosphorylated extracellular signal-regulated kinase (ERK1/2), which is one of the MAPKs has been detected in a variety of tumors. Calcitonin (CT) is a polypeptide hormone secreted by the thyroid gland and has been used to treat the osteoporosis and humoral hypercalcemia of malignancy. We report that CT decreases ERK1/2 phosphorylation in cancer cells showing constitutive phosphorylated ERK1/2. In MDA-MB-231 cells, a breast cancer cell line showing constitutive phosphorylated ERK1/2, CT phosphorylated c-Raf at Ser(259) via the protein kinase A pathway, resulting in suppression of ERK1/2 phosphorylation. CT significantly reduced the tumor volume of MDA-MB-231 cells showing constitutive phosphorylated ERK1/2 compared with saline buffer. However, CT did not exert any significant effects on the proliferation of MCF-7 cells, a breast cancer cell line, showing non-constitutive phosphorylated ERK1/2. These novel findings indicate that CT may be used to target ERK in the treatment of cancer.