The p85 isoform of the kinase S6K1 functions as a secreted oncoprotein to facilitate cell migration and tumor growth

p85S6K sustains synaptic GluA1 to ameliorate cognitive deficits in Alzheimer's disease

BACKGROUND

Ribosomal protein S6 kinase 1 (S6K1) is a serine-threonine kinase that has two main isoforms: p70S6K (70-kDa isoform) and p85S6K (85-kDa isoform). p70S6K, with its upstream mammalian target of rapamycin (mTOR), has been shown to be involved in learning and memory and participate in the pathophysiology of Alzheimer's disease (AD). However, the function of p85S6K has long been neglected due to its high similarity to p70S6k. The role of p85S6K in learning and memory is still largely unknown.

METHODS

We fractionated the postsynaptic densities to illustrate the differential distribution of p85S6K and p70S6K. Coimmunoprecipitation was performed to unveil interactions between p85S6K and the GluA1 subunit of AMPA receptor. The roles of p85S6K in synaptic targeting of GluA1 and learning and memory were evaluated by specific knockdown or overexpression of p85S6K followed by a broad range of methodologies including immunofluorescence, Western blot, in situ proximity ligation assay, morphological staining and behavioral examination. Further, the expression level of p85S6K was measured in brains from AD patients and AD model mice.

RESULTS

p85S6K, but not p70S6K, was enriched in the postsynaptic densities. Moreover, knockdown of p85S6K resulted in defective spatial and recognition memory. In addition, p85S6K could interact with the GluA1 subunit of AMPA receptor through synapse-associated protein 97 and A-kinase anchoring protein 79/150. Mechanistic studies demonstrated that p85S6K could directly phosphorylate GluA1 at Ser845 and increase the amount of GluA1 in synapses, thus sustaining synaptic function and spine densities. Moreover, p85S6K was found to be specifically decreased in the synaptosomal compartment in the brains of AD patients and AD mice. Overexpression of p85S6K ameliorated the synaptic deficits and cognitive impairment in transgenic AD model mice.

CONCLUSIONS

These results strongly imply a significant role for p85S6K in maintaining synaptic and cognitive function by interacting with GluA1. The findings provide an insight into the rational targeting of p85S6K as a therapeutic potential for AD.

The Impact of Oxidative Stress and AKT Pathway on Cancer Cell Functions and Its Application to Natural Products

Oxidative stress and AKT serine-threonine kinase (AKT) are responsible for regulating several cell functions of cancer cells. Several natural products modulate both oxidative stress and AKT for anticancer effects. However, the impact of natural product-modulating oxidative stress and AKT on cell functions lacks systemic understanding. Notably, the contribution of regulating cell functions by AKT downstream effectors is not yet well integrated. This review explores the role of oxidative stress and AKT pathway (AKT/AKT effectors) on ten cell functions, including apoptosis, autophagy, endoplasmic reticulum stress, mitochondrial morphogenesis, ferroptosis, necroptosis, DNA damage response, senescence, migration, and cell-cycle progression. The impact of oxidative stress and AKT are connected to these cell functions through cell function mediators. Moreover, the AKT effectors related to cell functions are integrated. Based on this rationale, natural products with the modulating abilities for oxidative stress and AKT pathway exhibit the potential to regulate these cell functions, but some were rarely reported, particularly for AKT effectors. This review sheds light on understanding the roles of oxidative stress and AKT pathway in regulating cell functions, providing future directions for natural products in cancer treatment.

IKBKE phosphorylates and stabilizes Snail to promote breast cancer invasion and metastasis

IKBKE, a non-canonical inflammatory kinase, is frequently amplified or activated, and plays predominantly oncogenic roles in human cancers, especially in breast cancer. However, the potential function and underlying mechanism of IKBKE contributing to breast cancer metastasis remain largely elusive. Here, we report that depletion of Ikbke markedly decreases polyoma virus middle T antigen (PyVMT)-induced mouse mammary tumorigenesis and subsequent lung metastasis. Biologically, ectopic expression of IKBKE accelerates, whereas depletion of IKBKE attenuates breast cancer invasiveness and migration in vitro and tumor metastasis in vivo. Mechanistically, IKBKE tightly controls the stability of transcriptional factor Snail in different layers, in particular by directly phosphorylating Snail, which markedly blocks the E3 ligase β-TRCP1-mediated Snail degradation, resulting in breast cancer epithelial-mesenchymal transition (EMT) and metastasis. These findings together reveal a novel oncogenic function of IKBKE in promoting breast cancer metastasis by governing Snail abundance, and highlight the potential of targeting IKBKE for metastatic breast cancer therapies.

Beyond controlling cell size: functional analyses of S6K in tumorigenesis

As a substrate and major effector of the mammalian target of rapamycin complex 1 (mTORC1), the biological functions of ribosomal protein S6 kinase (S6K) have been canonically assigned for cell size control by facilitating mRNA transcription, splicing, and protein synthesis. However, accumulating evidence implies that diverse stimuli and upstream regulators modulate S6K kinase activity, leading to the activation of a plethora of downstream substrates for distinct pathobiological functions. Beyond controlling cell size, S6K simultaneously plays crucial roles in directing cell apoptosis, metabolism, and feedback regulation of its upstream signals. Thus, we comprehensively summarize the emerging upstream regulators, downstream substrates, mouse models, clinical relevance, and candidate inhibitors for S6K and shed light on S6K as a potential therapeutic target for cancers.

p70 S6 kinase as a therapeutic target in cancers: More than just an mTOR effector

p70 S6 kinase (p70^S6K) is best-known for its regulatory roles in protein synthesis and cell growth by phosphorylating its primary substrate, ribosomal protein S6, upon mitogen stimulation. The enhanced expression/activation of p70^S6K has been correlated with poor prognosis in some cancer types, suggesting that it may serve as a biomarker for disease monitoring. p70^S6K is a critical downstream effector of the oncogenic PI3K/Akt/mTOR pathway and its activation is tightly regulated by an ordered cascade of Ser/Thr phosphorylation events. Nonetheless, it should be noted that other upstream mechanisms regulating p70^S6K at both the post-translational and post-transcriptional levels also exist. Activated p70^S6K could promote various aspects of cancer progression such as epithelial-mesenchymal transition, cancer stemness and drug resistance. Importantly, novel evidence showing that p70^S6K may also regulate different cellular components in the tumor microenvironment will be discussed. Therapeutic targeting of p70^S6K alone or in combination with traditional chemotherapies or other microenvironmental-based drugs such as immunotherapy may represent promising approaches against cancers with aberrant p70^S6K signaling. Currently, the only clinically available p70^S6K inhibitors are rapamycin analogs (rapalogs) which target mTOR. However, there are emerging p70^S6K-selective drugs which are going through active preclinical or clinical trial phases. Moreover, various screening strategies have been used for the discovery of novel p70^S6K inhibitors, hence bringing new insights for p70^S6K-targeted therapy.

Attacking the PI3K/Akt/mTOR signaling pathway for targeted therapeutic treatment in human cancer

Cancer is the second leading cause of human death globally. PI3K/Akt/mTOR signaling is one of the most frequently dysregulated signaling pathways observed in cancer patients that plays crucial roles in promoting tumor initiation, progression and therapy responses. This is largely due to that PI3K/Akt/mTOR signaling is indispensable for many cellular biological processes, including cell growth, metastasis, survival, metabolism, and others. As such, small molecule inhibitors targeting major kinase components of the PI3K/Akt/mTOR signaling pathway have drawn extensive attention and been developed and evaluated in preclinical models and clinical trials. Targeting a single kinase component within this signaling usually causes growth arrest rather than apoptosis associated with toxicity-induced adverse effects in patients. Combination therapies including PI3K/Akt/mTOR inhibitors show improved patient response and clinical outcome, albeit developed resistance has been reported. In this review, we focus on revealing the mechanisms leading to the hyperactivation of PI3K/Akt/mTOR signaling in cancer and summarizing efforts for developing PI3K/Akt/mTOR inhibitors as either mono-therapy or combination therapy in different cancer settings. We hope that this review will facilitate further understanding of the regulatory mechanisms governing dysregulation of PI3K/Akt/mTOR oncogenic signaling in cancer and provide insights into possible future directions for targeted therapeutic regimen for cancer treatment, by developing new agents, drug delivery systems, or combination regimen to target the PI3K/Akt/mTOR signaling pathway. This information will also provide effective patient stratification strategy to improve the patient response and clinical outcome for cancer patients with deregulated PI3K/Akt/mTOR signaling.

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.

N-Terminal Proteoforms in Human Disease

The collection of chemically different protein variants, or proteoforms, by far exceeds the number of protein-coding genes in the human genome. Major contributors are alternative splicing and protein modifications. In this review, we focus on those proteoforms that differ at their N termini with a molecular link to disease. We describe the main underlying mechanisms that give rise to such N-terminal proteoforms, these being splicing, initiation of protein translation, and protein modifications. Given their role in several human diseases, it is becoming increasingly clear that several of these N-terminal proteoforms may have potential as therapeutic interventions and/or for diagnosing and prognosing their associated disease.

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.