The ubiquitination of ribosomal S6 kinases is independent from the mitogen-induced phosphorylation/activation of the kinase

The splicing factor SRRM2 modulates two S6K kinases to promote colorectal cancer growth

The mechanistic target of rapamycin (mTOR) pathway plays a critical role in cell growth and metabolic homeostasis. The ribosomal protein S6 kinases S6K1 and S6K2 are the major effectors of the mTOR pathway key to translation efficiency, but the underlying regulatory mechanisms remain largely unclear. In this study, we searched for mTOR regulators and found that the splicing factor SRRM2 modulates the levels of S6K1 and S6K2, thereby activating the mTOR-S6K pathway. Interestingly, SRRM2 facilitates the expression of S6K2 by modulating alternative splicing, and enhances the stability of the S6K1 protein by regulating the E3 ubiquitin ligase WWP2. Moreover, SRRM2 is highly expressed in colorectal cancer (CRC) tissues and is associated with a poor prognosis. SRRM2 promotes CRC growth in vitro and in vivo. Combined, these data reveal an oncogenic role of SRRM2 in CRC through activating the mTOR-S6K pathway by two different approaches, further suggesting SRRM2 as a potential therapeutic target for CRC.

S6K2 in Focus: Signaling Pathways, Post-Translational Modifications, and Computational Analysis

S6 Kinase 2 (S6K2) is a key regulator of cellular signaling and is crucial for cell growth, proliferation, and survival. This review is divided into two parts: the first focuses on the complex network of upstream effectors, downstream modulators, and post-translational modifications (PTMs) that regulate S6K2 activity. We emphasize the dynamic nature of S6K2 regulation, highlighting its critical role in cellular homeostasis and its potential as a therapeutic target in diseases like cancer. The second part utilizes in silico analyses, employing computational tools to model S6K2's three-dimensional structure and predict its interaction networks. Molecular dynamics simulations and docking studies reveal potential binding sites and interactions with novel known inhibitors. We also examine the effects of environmental contaminants that potentially disrupt S6K2 function and provide insights into the role of external factors that could impact its regulatory mechanisms. These computational findings provide a deeper understanding of the conformational dynamics of S6K2 and its interactions with its inhibitors. Together, this integrated biochemical and computational approach enhances our understanding of S6K2 regulation and identifies potential new therapeutic strategies targeting S6K2 in the oncology setting.

Investigating the Regulation of Ribosomal Protein S6 Kinase 1 by CoAlation

Ribosomal protein S6 kinases belong to a family of highly conserved enzymes in eukaryotes that regulate cell growth, proliferation, survival, and the stress response. It is well established that the activation and downstream signalling of p70S6Ks involve multiple phosphorylation events by key regulators of cell growth, survival, and energy metabolism. Here, we report for the first time the covalent modification of p70S6K1 by coenzyme A (CoA) in response to oxidative stress, which regulates its kinase activity. The site of CoA binding (CoAlation) was mapped by mass spectrometry to cysteine 217 (Cys217), located in the kinase activation loop and only one amino acid away from the tripeptide DFG motif, which facilitates ATP-binding. The CoAlation of recombinant p70S6K1 was demonstrated in vitro and was shown to inhibit its kinase activity. Our molecular docking and dynamics analysis revealed the most likely mode for CoA binding to p70S6K1. This mechanism involves the non-covalent binding of the CoA ADP moiety to the p70S6K1 nucleotide-binding pocket, positioning the CoA thiol group in close proximity to form a covalent bond with the surface-exposed Cys217 residue. These findings support a "dual anchor" mechanism for protein kinase inhibition by CoAlation in cellular response to oxidative stress. Furthermore, the inhibition of S6K1 by CoAlation may open new avenues for developing novel inhibitors.

The Role of Ribosomal Protein S6 Kinases in Plant Homeostasis

The p70 ribosomal S6 kinase (S6K) family is a group of highly conserved kinases in eukaryotes that regulates cell growth, cell proliferation, and stress response via modulating protein synthesis and ribosomal biogenesis. S6Ks are downstream effectors of the Target of Rapamycin (TOR) pathway, which connects nutrient and energy signaling to growth and homeostasis, under normal and stress conditions. The plant S6K family includes two isoforms, S6K1 and S6K2, which, despite their high level of sequence similarity, have distinct functions and regulation mechanisms. Significant advances on the characterization of human S6Ks have occurred in the past few years, while studies on plant S6Ks are scarce. In this article, we review expression and activation of the two S6K isoforms in plants and we discuss their roles in mediating responses to stresses and developmental cues.

Distinct Roles of mTOR Targets S6K1 and S6K2 in Breast Cancer

The mechanistic target of rapamycin (mTOR) is a master regulator of protein translation, metabolism, cell growth and proliferation. It forms two complexes, mTOR complex 1 (mTORC1) and 2 (mTORC2). mTORC1 is frequently deregulated in many cancers, including breast cancer, and is an important target for cancer therapy. The immunosuppressant drug rapamycin and its analogs that inhibit mTOR are currently being evaluated for their potential as anti-cancer agents, albeit with limited efficacy. mTORC1 mediates its function via its downstream targets 40S ribosomal S6 kinases (S6K) and eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1). There are two homologs of S6K: S6K1 and S6K2. Most of the earlier studies focused on S6K1 rather than S6K2. Because of their high degree of structural homology, it was generally believed that they behave similarly. Recent studies suggest that while they may share some functions, they may also exhibit distinct or even opposite functions. Both homologs have been implicated in breast cancer, although how they contribute to breast cancer may differ. The purpose of this review article is to compare and contrast the expression, structure, regulation and function of these two S6K homologs in breast cancer.

Deacetylation of S6 kinase promotes high glucose-induced glomerular mesangial cell hypertrophy and matrix protein accumulation

S6 kinase acts as a driver for renal hypertrophy and matrix accumulation, two key pathologic signatures of diabetic nephropathy. As a post-translational modification, S6 kinase undergoes acetylation at the C terminus. The role of this acetylation to regulate kidney glomerular cell hypertrophy and matrix expansion is not known. In mesangial cells, high glucose decreased the acetylation and enhanced phosphorylation of S6 kinase and its substrates rps6 and eEF2 kinase that lead to dephosphorylation of eEF2. To determine the mechanism of S6 kinase deacetylation, we found that trichostatin A, a pan-histone deacetylase (HDAC) inhibitor, blocked all high glucose-induced effects. Furthermore, high glucose increased the expression and association of HDAC1 with S6 kinase. HDAC1 decreased the acetylation of S6 kinase and mimicked the effects of high glucose, resulting in mesangial cell hypertrophy and expression of fibronectin and collagen I (α2). In contrast, siRNA against HDAC1 inhibited these effects by high glucose. A C-terminal acetylation-mimetic mutant of S6 kinase suppressed high glucose-stimulated phosphorylation of S6 kinase, rps6 and eEF2 kinase, and inhibited the dephosphorylation of eEF2. Also, the acetylation mimetic attenuated the mesangial cell hypertrophy and fibronectin and collagen I (α2) expression. Conversely, an S6 kinase acetylation-deficient mutant induced all the above effects of high glucose. Finally, in the renal glomeruli of diabetic rats, the acetylation of S6 kinase was significantly reduced concomitant with increased HDAC1 and S6 kinase activity. In aggregate, our data uncovered a previously unrecognized role of S6 kinase deacetylation in high glucose-induced mesangial cell hypertrophy and matrix protein expression.

Molecular Characterization of Two Mitogen-Activated Protein Kinases: p38 MAP Kinase and Ribosomal S6 Kinase From Bombyx mori (Lepidoptera: Bombycidae), and Insight Into Their Roles in Response to BmNPV Infection

Proteins p38 map kinase and ribosomal S6 kinase (S6K) as members of mitogen-activated protein kinases (MAPKs) play important roles against pathogens. In this study, Bmp38 and BmS6K were identified as differentially expressed proteins from iTRAQ database. Bmp38 and BmS6K were expressed, and recombinant proteins were purified. The bioinformatics analysis showed that both proteins have serine/threonine-protein kinases, catalytic domain (S_TKc) with 360 and 753 amino acids, respectively. The real-time quantitative polymerase chain reaction (RT-qPCR) results suggest that Bmp38 and BmS6K had high expression in the midgut and hemolymph. The comparative expression level of Bmp38 and BmS6K in BC9 was upregulated than in P50 in the midgut after Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Western bolt results showed a positive correlation between RT-qPCR and iTRAQ data for Bmp38, but BmS6K data showed partial correlation with iTRAQ. Injection of anti-Bmp38 and anti-BmS6K serum suggested that Bmp38 may be involved against BmNPV infection, whereas BmS6K may require phosphorylation modification to inhibit BmNPV infection. Taken together, our results suggest that Bmp38 and BmS6k might play an important role in innate immunity of silkworm against BmNPV.

S6K2 promises an important therapeutic potential for cancer

S6K2, the newer member of S6 Kinase family, is a crucial modulator of Akt/mTOR signaling pathway and is a member of AGC kinase family that regulates cellular growth and survival. S6K1 and S6K2 share high sequence similarity; therefore, S6K2 had been underestimated. However, recent studies displayed distinct functions of S6K2. Activated by both Akt/mTOR and Ras/Raf/Mek/Erk signaling pathways, S6K2 regulates cancer cell survival via different routes. Complexation with antiapoptotic proteins BRAF and PKCε avoids non-small-cell lung cancer cells  from apoptosis upon FGF-2 stimulation. Indirect upregulation of the translation of antiapoptotic proteins Bcl-XL and XIAP in HEK293T cells and interference with TNF-induced apoptosis in MCF-7 cells are other routes of cancer cell survival. The aforementioned studies on S6K2 necessitate the development of therapies targeting only on S6K2. Studies targeting S6K2 may help to build important roads for cancer therapy.

mAChR-dependent decrease in proteasome activity in the gustatory cortex is necessary for novel taste learning

Regulation of protein degradation via the ubiquitin proteasome system is crucial for normal learning and synaptic plasticity processes. While some studies reveal that increased proteasome degradation is necessary for different types of learning, others suggest the proteasome to be a negative regulator of plasticity. We aim to understand the molecular and cellular processes taking place in the gustatory cortex (GC), which underlie appetitive and aversive forms of taste learning. Previously, we have shown that N-methyl d-aspartic acid receptor (NMDAR)-dependent upregulation of proteasome activity 4h after novel taste learning is necessary for the association of novel taste with malaise and formation of conditioned taste aversion (CTA). Here, we first identify a correlative increase in proteasome activity in the GC immediately after novel taste learning and study the upstream and downstream effectors of this modulated proteasome activity. Interestingly, proteasome-mediated degradation was reduced in the GC, 20min after novel taste consumption in a muscarinic acetylcholine receptor (mAChR)-dependent and NMDAR-independent manner. This reduction in protein degradation led to an increased amount of p70 S6 kinase (p70S6k), which was abolished in the presence of mAChR antagonist scopolamine. Infusion of lactacystin, a proteasome inhibitor, to the GC precluded the amnestic effect of scopolamine. This study shows for the first time that following novel taste learning there is a cortical, mAChR-dependent reduced proteasome activity that enables the memory of taste familiarity. Moreover, inhibition of degradation in the GC attenuates novel taste learning and of p70 S6 kinase correlative increased expression. These results shed light on the complex regulation of protein synthesis and degradation machineries in the cortex following novel taste experience.

p70 Ribosomal protein S6 kinase (Rps6kb1): an update

The Rps6kb1 gene encodes the 70 kDa ribosomal protein S6 kinase (p70S6K), which is a serine/threonine kinase regulated by phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway. p70S6K plays a crucial role in controlling cell cycle, growth and survival. The PI3K/mTOR signalling pathway is one of the major mechanisms for controlling cell survival, proliferation and metabolism and is the central regulator of translation of some components of protein synthesis system. Upon activation, this kinase phosphorylates S6 protein of ribosomal subunit 40S resulting in selective translation of unique family of mRNAs that contain oligopyrimidine tract on 5' transcriptional site (5'TOP). 5'TOP mRNAs are coding the components of translational apparatus including ribosomal proteins and elongation factors. Due to the role of p70S6K in protein synthesis and also its involvement in a variety of human diseases ranging from diabetes and obesity to cancer, p70S6K is now being considered as a new therapeutic target for drug development. Furthermore, p70S6K acts as a biomarker for response to immunosuppressant as well as anticancer effects of inhibitors of the mTOR. Because of the narrow therapeutic index of mTOR inhibitors, drug monitoring is essential, and this is usually done by measuring blood drug levels, therapeutic response and drug-induced adverse effects. Recent studies have suggested that plasma p70S6K is a reliable index for the monitoring of patient response to mTOR inhibitors. Therefore, a better understanding of p70S6K and its role in various pathological conditions could enable the development of strategies to aid diagnosis, prognosis and treatment schedules.

S6K2: The Neglected S6 Kinase Family Member

S6 kinase 2 (S6K2) is a member of the AGC kinases super-family. Its closest homolog, S6K1, has been extensively studied along the years. However, due to the belief in the community that the high degree of identity between these two isoforms would translate in essentially identical biological functions, S6K2 has been largely neglected. Nevertheless, recent research has clearly highlighted that these two proteins significantly differ in their roles in vitro as well as in vivo. These findings are significant to our understanding of S6 kinase signaling and the development of therapeutic strategies for several diseases including cancer. Here, we will focus on S6K2 and review the protein–protein interactions and specific substrates that determine the selective functions of this kinase.

Phosphorylation and degradation of S6K1 (p70S6K1) in response to persistent JNK1 Activation

S6K (ribosomal S6 kinase p70, p70S6K) activation requires phosphorylation at two stages. The first phosphorylation is independent of insulin stimulation and mediated by an unknown kinase. The second phosphorylation is mediated by mTOR in insulin dependent manner. In this study, we identified JNK1 (c-Jun N-terminal kinase 1) as a kinase in the first phosphorylation. S6K protein was phosphorylated by JNK1 at S411 and S424 in the carboxyl terminal autoinhibitory domain. The phosphorylation was observed in kinase assay with purified S6K as a substrate, and in cells after JNK1 activation by TNF-α or MEKK1 expression. The phosphorylation was detected in JNK2 null cells, but not in JNK1 null cells after TNF-α treatment. When JNK1 activation was inhibited by MKK7 knockdown, the phosphorylation was blocked in cells. The phosphorylation led to S6K protein degradation in NF-κB deficient cells. The degradation was blocked by inhibition of proteasome activity with MG132. In wide type cells, the phosphorylation did not promote S6K degradation when IKK2 (IKKβ, IκB kinase beta) was activated. Instead, the phosphorylation allowed S6K activation by mTOR, which stabilizes S6K protein. In IKK2 null cells or cells treated by IKK2 inhibitor, the phosphorylation led to S6K degradation. These data suggest that S6K is phosphorylated by JNK1 and the phosphorylation makes S6K protein unstable in the absence of IKK2 activation. This study provides a mechanism for regulation of S6K protein stability.

S6 kinase 2 deficiency enhances ketone body production and increases peroxisome proliferator-activated receptor alpha activity in the liver

Nutrient homeostasis is tightly regulated by the balance between energy production and utilization. During fasting, production of ketone bodies as an alternative energy source is critical to maintain nutrient homeostasis. An important component in the nutrient-sensitive signaling pathway is S6 kinase 2 (S6K2), a downstream effector of mammalian target of rapamycin. Here, we show that mice lacking S6K2 exhibit elevated levels of ketone bodies and enhanced peroxisome proliferator-activated receptor alpha (PPARα) activity upon nutrient availability. Consistent with this, knockdown of S6K2 increases the transcriptional activity of PPARα. S6K2 suppresses PPARα by associating with its corepressor, nuclear receptor corepressor 1 (NCoR1), and by inducing the recruitment of NCoR1 to the nucleus. Moreover, ob/ob mice, a genetic model of obesity, have markedly elevated S6K2 activity, and S6K2 was strongly associated with NCoR1 in the nucleus of liver cells.

CONCLUSION

Our findings suggest that S6K2 regulates hepatic energy homeostasis by repressing PPARα activity and point to its potential relevance for therapeutic strategies designed to modulate S6K2 activity as a treatment for deregulated ketone body production.

Exploiting p70 S6 kinase as a target for ovarian cancer

INTRODUCTION

The p70 S6 kinase (p70(S6K)) is frequently active in ovarian and a wide range of cancer types, and it has a crucial role in several processes considered hallmarks of cancer. Therefore, blocking p70(S6K) expression or activity may present a promising strategy for anticancer treatment.

AREAS COVERED

The current understanding of the molecular mechanisms that govern p70(S6K) regulation as well as its tumorigenic effects, which are involved in the initiation and progression in ovarian cancer, in particular the emerging new role of p70(S6K) in cell migration, which is a prerequisite of tumor metastasis. The p70(S6K) cellular substrates and/or interacting proteins. The current state of drugs that target this kinase, either alone or in combination with other targeted agents.

EXPERT OPINION

Targeting p70(S6K) through the use of small-molecule inhibitors, microRNAs and natural compounds may represent a beneficial new avenue for cancer therapy and opens new areas of investigation in p70(S6K) biology.

Regulation and function of ribosomal protein S6 kinase (S6K) within mTOR signalling networks

The ribosomal protein S6K (S6 kinase) represents an extensively studied effector of the TORC1 [TOR (target of rapamycin) complex 1], which possesses important yet incompletely defined roles in cellular and organismal physiology. TORC1 functions as an environmental sensor by integrating signals derived from diverse environmental cues to promote anabolic and inhibit catabolic cellular functions. mTORC1 (mammalian TORC1) phosphorylates and activates S6K1 and S6K2, whose first identified substrate was rpS6 (ribosomal protein S6), a component of the 40S ribosome. Studies over the past decade have uncovered a number of additional S6K1 substrates, revealing multiple levels at which the mTORC1-S6K1 axis regulates cell physiology. The results thus far indicate that the mTORC1-S6K1 axis controls fundamental cellular processes, including transcription, translation, protein and lipid synthesis, cell growth/size and cell metabolism. In the present review we summarize the regulation of S6Ks, their cellular substrates and functions, and their integration within rapidly expanding mTOR (mammalian TOR) signalling networks. Although our understanding of the role of mTORC1-S6K1 signalling in physiology remains in its infancy, evidence indicates that this signalling axis controls, at least in part, glucose homoeostasis, insulin sensitivity, adipocyte metabolism, body mass and energy balance, tissue and organ size, learning, memory and aging. As dysregulation of this signalling axis contributes to diverse disease states, improved understanding of S6K regulation and function within mTOR signalling networks may enable the development of novel therapeutics.

Functions and regulation of the 70kDa ribosomal S6 kinases

The 70kDa ribosomal protein S6 kinases, S6K1 and S6K2 are two highly homologous serine/threonine kinases that are activated in response to growth factors, cytokines and nutrients. The S6 kinases have been linked to diverse cellular processes, including protein synthesis, mRNA processing, glucose homeostasis, cell growth and survival. Studies in model organisms have highlighted the roles that S6K activity plays in a number of pathologies, including obesity, diabetes, ageing and cancer. The importance of S6K function in human diseases has led to the development of S6K-specific inhibitors by a number of companies, offering the promise of improved tools with which to study these enzymes and potentially the effective targeting of deregulated S6K signalling in patients. Here we review the current literature on the role of S6Ks in the regulation of cell growth, survival and proliferation downstream of various signalling pathways and how their dysregulation contributes to the pathogenesis of human diseases.

Histone acetyltransferases interact with and acetylate p70 ribosomal S6 kinases in vitro and in vivo

The 70kDa ribosomal protein S6 kinases (S6K1 and S6K2) play important roles in the regulation of protein synthesis, cell growth and survival. S6Ks are activated in response to mitogen stimulation and nutrient sufficiency by the phosphorylation of conserved serine and threonine residues. Here we show for the first time, that in addition to phosphorylation, S6Ks are also targeted by lysine acetylation. Following mitogen stimulation, S6Ks interact with the p300 and p300/CBP-associated factor (PCAF) acetyltransferases. S6Ks can be acetylated by p300 and PCAF in vitro and S6K acetylation is detected in cells expressing p300. Furthermore, it appears that the acetylation sites targeted by p300 lie within the divergent C-terminal regulatory domains of both S6K1 and S6K2. Acetylation of S6K1 and 2 is increased upon the inhibition of class I/II histone deacetylases (HDACs) by trichostatin-A, while the enhancement of S6K1 acetylation by nicotinamide suggests the additional involvement of sirtuin deacetylases in S6K deacetylation. Both expression of p300 and HDAC inhibition cause increases in S6K protein levels, and we have shown that S6K2 is stabilized in cells treated with HDAC inhibitors. The finding that S6Ks are targeted by histone acetyltransferases uncovers a novel mode of crosstalk between mitogenic signalling pathways and the transcriptional machinery and reveals additional complexity in the regulation of S6K function.