MAPK3 and neoplasm: A putative molecular mechanism by which serotonin increases cell viabilty is shown in Fig. 4a: briefly, serotonin, acting both in a paracrine and autocrine fashion, activates 5HT receptors on tumour cells; this activation leads to an increased production of cAMP, and subsequent activation of PKA37, 38; the subsequent signaling cascade ultimately activates the pro-mitogenic ERK1/2 proteins through phosphorylation39.