S6K1 and S6K2 belong to the AGC kinase family. AGC kinases are serine–threonine kinases that share some structural features and are implicated to regulate several cellular processes, such as growth, survival and metabolism. Besides S6Ks, proteins PKA, PKG, PKC and Akt, amongst others, are members of the AGC kinase family.
Phosphorylation and activation of S6Ks
Several studies have investigated the S6K1 activation process, but few of them describe the S6K2 activation. The S6K1 C-terminal region has an auto-inhibitory function and interacts with the N-terminal region, shutting the kinase domain for its targets. The S6K1 activation mechanism begins with the phosphorylation of four serine residues in the C-terminal domain, S411, S418, S421 and S424 (Figure 1). These initial phosphorylations are responsible for exposing the internal region of the protein, allowing mTOR to phosphorylate the T389 residue, followed by phosphorylation of the T229 residue in the kinase domain by PDK1 (3-phosphoinositide dependent protein kinase 1), culminating in S6K1 activation. S371 and T404 have also been reported to be phosphorylated, although it is not known which kinases act in these residues. Moreover, the S6K1 C-terminal acetylation blocks the T389 phosphorylation mediated by mTOR in vitro, preventing the protein activation. On the other hand, C-terminal acetylation of the S6K1 is inhibited by SIRT1 and SIRT2, members of the deacetylase sirtuin family, allowing the T389 phosphorylation by mTOR and S6K1 activation. These results indicate a cross-talk between mTOR and sirtuin pathways, which regulates S6K1 activity.
Figure 1. S6K isoforms are produced from two different genes and may have alternative start codon usage and alternative splicing variants. A) Domain architecture of S6K proteins. NTD: Nterminal domain; CTD: C-terminal domain; NLS: nuclear localization signal; Pro: proline-rich domain; P: phosphorylated residue. (Tavares M R; et al. 2015)
S6K1 and inflammation
The effects of the mTOR/S6K1 pathway in immunity and inflammation are controversial. Although a recent study has demonstrated a positive effect of rapamycin administration in immune system of elderly, another study has shown impairment of the adaptive immunity in an old mice model by rapamycin administration. The involvement of S6K1 in inflammatory response has been studied. Phosphatidic acid (PA) induces the production of proinflammatory cytokines, such as TNF-α, IL-1β and IL-6, by stimulating Akt/mTOR/S6K1 pathway. Indeed, PA, produced by phospholipases D1 (PLD1) and D2 (PLD2), is able to directly bind and stimulate mTOR. LPS treatment of macrophage cell line Raw 264.7 activated S6K1 by PLD2 activation. Conversely, pretreatment of these cells with rapamycin blocked LPS-induced nitric oxide synthase (iNOS) expression and nitric oxide (NO) synthesis, by suppression of ERK-1/2 activity. Another study demonstrated that inhibition of S6K1 blocked TNF-α expression induced by LPS in Raw 264.7 cells. Moreover, PLD1 inhibition blocked S6K1 activation, leading to reduction of TNF-α expression driven by JNK. On the other hand, S6K1 may negatively regulate inflammatory signaling. S6K1 overexpression in vitro impaired NF-κB activity and production of proinflammatory cytokines, such as TNF-α, IL-1β and IL-6,. This was mediated by the S6K1 inhibition of TAK1 (TGFβ-Activated Kinase 1), a key regulator of inflammatory signaling transduction. S6K1 interacts with TAK1 N-terminal and impairs its connection with TAB1 (TAK1-Binding Protein 1), responsible for regulating TAK1 activity, thus blocking proinflammatory signaling. The discrepancy found by those studies concerning the role of S6K1 in inflammation may reflect the different methods used by the authors and also by the complexity of the signaling pathways involved in the inflammation process. More studies are needed to fully understand the role of S6K1 in inflammation, although it may be possible that S6K1 inhibition of inflammation, mediated by TAK1 interaction, represents a negative feedback loop.
S6Ks as therapeutic targets
Due to their important role in the development and progression of several diseases, S6K isoforms are frequently indicated as potential therapeutic targets and a specific S6K1 inhibitor, PF-4708671, has been recently developed. Before, S6K inhibition was attained by upstream inhibitors such as rapamycin, which could cause misleading results since they interfere in the activity of several other proteins. PF-4708671 high specificity provides two major advantages against the other inhibitors used so far: it does not significantly inhibit other AGC kinases (with the exception of MSK1) and it may be useful to distinguish different roles of S6K1 and S6K2, since it only inhibits the S6K1 isoform. Hence, PF-4708671 might provide substantial support to elucidate S6K roles in human diseases and to establish effective therapeutic approaches.
1. Tavares M R, et al.; The S6K protein family in health and disease. Life Sciences 2015, 131:1-10.