Ukr.Biochem.J. 2019; Volume 91, Issue 4, Jul-Aug, pp. 17-25

doi: https://doi.org/10.15407/ubj91.04.017

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

12.06.2019   Uncategorized   No comments

I. V. Zaiets, V. V. Holiar, A. S. Sivchenko,
V. V. Smialkovska, V. V. Filonenko

Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine, Kyiv;
e-mail: filonenko@imbg.org.ua

Received: 13 March 2019; Accepted: 17 May 2019

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 p85–/p70–/p60+HEK-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.

Keywords: , , ,

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