Novel wiring of the AKT-RSK2 signaling pathway plays an essential role in cancer cell proliferation via a G₁/S cell cycle transition

p90 Ribosomal S6 kinase 2 (RSK2), a member of mitogen-activated protein kinase regulating cell proliferation and transformation induced by tumor promoters, such as epidermal growth factor, plays a vital role as a signaling hub to modulate cell proliferation, transformation, cell cycle transition, and chromatin remodeling by tumor promoter stimulation such as epidermal growth factor. On the other hand, the RSK2-mediated signaling networks that regulate cancer cell proliferation are unclear. In this study, SKOV3, an ovarian cancer cell that exhibits chemoresistant properties, and TOV-112D cells showed different sensitivities to colony growth in soft agar. Based on the protein profile shown in a previous report, RSK2 knockdown preferentially and significantly suppressed cell proliferation and colony growth. Moreover, RSK2 interacted with AKTs (AKT 1-3) via the N-terminal kinase domain (NTKD) of RSK2, resulting in the phosphorylation of RSK2. The AKT-mediated phosphorylation consensus sequence, RxRxxS/T, on RSK2 NTKD (Thr115) was well conserved in different species. In particular, an in vitro kinase assay showed that NTKD deleted and Thr115Ala mutants of RSK2 abolished AKT1-mediated phosphorylation. In the physiological assay of RSK2 phosphorylation at Thr115 on cell proliferation, AKT1-mediated RSK2 phosphorylation at Thr115 played an essential role in cell proliferation. The re-introduction of RSK2-T115A to RSK2^−/− MEF attenuated the EGF-induced G₁/S cell cycle transition compared to RSK2-wt introducing RSK2^−/− MEFs. This attenuation was observed by EGF stimulations and insulin-like growth factor-1. Overall, these results show that novel wiring of the AKT/RSKs signaling axis plays an important role in cancer cell proliferation by modulating the G₁/S cell cycle transition.