MAPK3 and neoplasm: Erk1 and Erk2 regulate transcription indirectly by phosphorylating the 90 kDa ribosomal protein S6 kinases (RSKs), a family of broadly expressed Ser/Thr kinases activated in response to mitogenic stimuli, including growth factors and tumor-promoting phorbol esters [35,36], Therefore, it is not unexpected that GH would not increase phosphorylation of RSK1 in normal cells but increases phosphorylation of RSK1 in cells pre-treated with the AKT and eNOS inhibitors.