p60-S6K1 represents a novel kinase active isoform with the mode of regulation distinct from p70/p85-S6K1 isoforms

摘要

The phosphatidylinositol-3-kinase (PI3K)/mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway controls plenty of cellular functions regulating phosphorylation one of its mediators ribosomal protein S6 kinase 1 (S6K1). Alternative translation of the common S6K1 transcript can generate three protein kinase isoforms, including p85-S6K1, p70-S6K1 and p60-S6K1. The catalytic activity of S6K1 is modulated by mitogens and growth factors via phosphorylation at three critical sites such as the activation loop (T-loop site), turn motif (TM site), and hydrophobic motif (HM site). Both members of the PI3K/mTORC1 pathway, PDK1 and mTORC1, directly phosphorylate the T-loop site and HM site, respectively. Indeed, most studies aimed at elucidating S6K1 regulation have focused on p70- and p85-S6K1. Meanwhile, however, the activity of p60-S6K1 and its regulation have not been elucidated so far. To test whether p60-S6K1 was an active kinase isoform that was regulated similar to p70/p85-S6K1, we employed previously generated p85sup–/sup/p70sup–/sup/p60sup+/supHEK-293 cells. First, an in vitro kinase assay confirmed the ability of p60-S6K1 to phosphorylate ribosomal protein S6 (rpS6), a well-known S6K1 substrate. Next, analysis of p60-S6K1 phosphorylation under different cell growth conditions showed that p60-S6K1 does not have detectable levels of phosphorylation at PDK1- and mTORC1-regulated sites, yet this isoform undergoes phosphorylation at the TM site. Finally, we found that activity of p60-S6K1 was not sensitive to mitogenic stimulation and cell treatment by potent inhibitor of the PI3K1/mTORC1-dependent signaling pathway rapamycin suggesting the existence of a PI3K/mTORC1-independent mechanism of p60-S6K1 regulation in HEK-293. The data of the current study suggest that the p60-S6K1 isoform possesses intrinsic kinase activity that is independent of PI3K/mTORC1 signaling regulation in HEK-293 cells. What is more, modulation of p60-S6K1 activity via the PI3K/mTORC1 signaling pathway seems to be cell-type specific, since the p60-S6K1 isoform undergoes PDK1- and mTORC1-mediated phosphorylation in breast cancer cell line MCF-7.