**2.8. Muscarinic acetylcholine receptors in breast cancer**

Muscarinic receptors have been described to be expressed in several non-neuronal cell types, including endothelial cells, smooth muscle cells, or bladder and gastrointestinal tract [104– 107]. In multiple malignancies, including breast, prostate, lung, ovary, pancreas, prostate, skin, stomach, uterus and colon cancer, muscarinic receptors have been demonstrated to contribute to cell proliferation and cancer progression [108–110].

In particular, muscarinic receptors have also been described in normal murine mammary (NMuMG) cells, normal human-breast-derived MCF-10A cells, and homogenates of surgical samples derived from normal human mammary tissue by Western blot and radioligand binding assays [111–113]. The expression of muscarinic acetylcholine receptors, M<sup>3</sup> and M4, was detected in MCF-7 breast cancer cells [114]. Additionally, the expression of muscarinic acetylcholine receptors was evidenced in LM2, LM3, and LMM3 cell lines derived from spontaneous mammary adenocarcinomas developed in Balb/C mice [115–117]. These studies demonstrated the involvement of muscarinic receptors in the tumor cells proliferation, progression, and angiogenesis [115–117], by a mechanism involving the stimulation of the nitric oxide synthase activity [114].

Muscarinic receptors, M<sup>1</sup> , M<sup>3</sup> and M5, are coupled with Gq/11 proteins that activate phospholipase C (PLC) and determine the release of calcium from intracellular reservoirs. In MCF7, human breast cancer cells evidenced the activation of MAP by muscarinic receptors [118]. Considering that acetylcholine exerts neurocrine, paracrine, and autocrine regulation of cancer cell proliferation, migration, etc., we might consider muscarinic receptors among the masterplayers in breast cancer.
