RSK1 and RSK2 as therapeutic targets: an up-to-date snapshot of emerging data.

Introduction: The four members of the p90 ribosomal S6 kinase (RSK) family are serine/threonine protein kinases, which are phosphorylated and activated by ERK1/2. RSK1/2/3 are further phosphorylated by PDK1. Receiving inputs from two major signaling pathways places RSK as a key signaling node in numerous pathologies.

Chronic High-Salt Diet Activates Tumor-Initiating Stem Cells Leading to Breast Cancer Proliferation.

Several chronic inflammatory diseases have been linked to high-salt (HS) diets. Chronic inflammation is an established causative hallmark of cancer. However, a direct role of HS diets in tumorigenesis is yet to be defined.

Therapeutic targeting of p90 ribosomal S6 kinase.

The Serine/Threonine protein kinase family, p90 ribosomal S6 kinases (RSK) are downstream effectors of extracellular signal regulated kinase 1/2 (ERK1/2) and are activated in response to tyrosine kinase receptor or G-protein coupled receptor signaling. RSK contains two distinct kinase domains, an N-terminal kinase (NTKD) and a C-terminal kinase (CTKD). The sole function of the CTKD is to aid in the activation of the NTKD, which is responsible for substrate phosphorylation.

Identifying the effectiveness of 3D culture systems to recapitulate breast tumor tissue in situ.

Purpose: Breast cancer heterogeneity contributes to chemotherapy resistance and decreased patient survival. To improve patient outcomes it is essential to develop a technology that is able to rapidly select the most efficacious therapy that targets the diverse phenotypes present within the tumor. Breast cancer organoid technologies are proposed as an attractive approach for evaluating drug responses prior to patient therapy.

ERK1/2-RSK2 Signaling in Regulation of ERα-Mediated Responses.

Signaling via extracellular regulated kinase 1/2 (ERK1/2) and p90 ribosomal S6 kinase (RSK), a downstream effector, mediates numerous processes. For example, ERK1/2-RSK signaling is essential for estrogen homeostasis in the mammary gland and uterus to maintain physiological responsiveness. This review will focus on the coordination of ERK1/2-RSK2 and estrogen signaling through estrogen receptor alpha (ERα).

ERK1/2-RSK regulation of oestrogen homeostasis.

The molecular mechanisms regulating oestrogen homeostasis have been primarily studied in the mammary gland, which is the focus of this review. In the non-pregnant adult, the mammary gland undergoes repeated cycles of proliferation and apoptosis in response to the fluctuating levels of oestrogen that occur during the reproductive stage. Oestrogen actions are mediated through the steroid hormone receptors, oestrogen receptor α and β and through a G-protein coupled receptor.

Identifying requirements for RSK2 specific inhibitors.

Identifying isoform-specific inhibitors for closely related kinase family members remains a substantial challenge. The necessity for achieving this specificity is exemplified by the RSK family, downstream effectors of ERK1/2, which have divergent physiological effects. The natural product, SL0101, a flavonoid glycoside, binds specifically to RSK1/2 through a binding pocket generated by an extensive conformational rearrangement within the RSK N-terminal kinase domain (NTKD).

FACS protocol for direct comparison of cell populations isolated from mice.

A FACS protocol is described that eliminates isolation and staining artifacts to allow accurate comparison between cell populations isolated from organs obtained from disparate mouse groups. This protocol was validated by characterizing the estrogen receptor positive cells within the mammary gland of transgenic mice with different genotypes at different stages of the estrous cycle. We include protocols necessary to batch stage animals within the cycle to proceed directly to FACS, which provides optimal RNA yields for RNA-seq.

RSK2 and ERα comrades-in-arms in homeostasis and transformation.

The physiological response to estrogen differs according to the developmental stage. We show, in the adult, estrogen-responsiveness is driven by ERK1/2 (extracellular signal-regulated kinase 1/2) whereas its downstream effector, RSK2 (p90 ribosomal S6 kinase 2), prevents continuous ERK1/2 activity through regulation of oxidative stress. Bioinformatic analysis revealed RSK2 association with breast cancer risk and oral contraceptives.

RSK2 Maintains Adult Estrogen Homeostasis by Inhibiting ERK1/2-Mediated Degradation of Estrogen Receptor Alpha.

In response to estrogens, estrogen receptor alpha (ERα), a critical regulator of homeostasis, is degraded through the 26S proteasome. However, despite the continued presence of estrogen before menopause, ERα protein levels are maintained. We discovered that ERK1/2-RSK2 activity oscillates during the estrous cycle.

Synthesis and Biological Evaluation of 4'-Substituted Kaempfer-3-ols.

The synthesis of two series of five kaempfer-3-ols was described. The first set all have a -3 hydroxyl group and the second has a carboxymethoxy ether at the -3 position. Both series have variable substitution at the -4' position (i.

The affinity of RSK for cylitol analogues of SL0101 is critically dependent on the B-ring C-4'-hydroxy.

Five cyclitol analogues of SL0101 with variable substitution at the C-4' position (i.e., OH, Cl, F, H, OMe) were synthesized.

Regioselective Synthesis of a -4'' Carbamate,-6'' -Pr Substituted Cyclitol Analogue of SL0101.

An asymmetric synthesis of two analogues of SL0101 () has been achieved. The effort is aimed at the discovery of inhibitors of the p90 ribosomal S6 kinase (RSK) with improved bioavailability. The route relies upon the use of the Taylor catalyst to regioselectively install -3″ acetyl or carbamate functionality.

ERα-Mediated Nuclear Sequestration of RSK2 Is Required for ER Breast Cancer Tumorigenesis.

Although ribosomal protein S6 kinase A3 (RSK2) activation status positively correlates with patient responses to antiestrogen hormonal therapies, the mechanistic basis for these observations is unknown. Using multiple and models of estrogen receptor-positive (ER) breast cancer, we report that ERα sequesters active RSK2 into the nucleus to promote neoplastic transformation and facilitate metastatic tumor growth. RSK2 physically interacted with ERα through its N terminus to activate a proneoplastic transcriptional network critical to the ER lineage in the mammary gland, thereby providing a gene signature that effectively stratified patient tumors according to ERα status.

Stereoselective Synthesis and Evaluation of C6″-Substituted 5a-Carbasugar Analogues of SL0101 as Inhibitors of RSK1/2.

A convergent synthesis of 5a-carbasugar analogues of the n-Pr-variant of SL0101 is described. The analogues were synthesized in an effort to find compounds with potent in vivo efficacy in the inhibition of p90 ribosomal s6 kinase (RSK1/2). The synthesis derived the desired C-4 L-rhamnose stereochemistry from quinic acid and used a highly selective cuprate addition, NaBH reduction, Mitsunobu inversion, and alkene dihydroxylation to install the remaining stereochemistry.

Development of a RSK Inhibitor as a Novel Therapy for Triple-Negative Breast Cancer.

Metastatic breast cancer is an incurable disease and identification of novel therapeutic opportunities is vital. Triple-negative breast cancer (TNBC) frequently metastasizes and high levels of activated p90RSK (RSK), a downstream MEK-ERK1/2 effector, are found in TNBC. We demonstrate, using direct pharmacologic and genetic inhibition of RSK1/2, that these kinases contribute to the TNBC metastatic process in vivo Kinase profiling showed that RSK1 and RSK2 are the predominant kinases targeted by the new inhibitor, which is based on the natural product SL0101.

Ribosomal S6 kinase (RSK) modulators: a patent review.

Introduction: The p90 ribosomal S6 kinases (RSK) are a family of Ser/Thr protein kinases that are downstream effectors of MEK1/2-ERK1/2. Increased RSK activation is implicated in the etiology of multiple pathologies, including numerous types of cancers, cardiovascular disease, liver and lung fibrosis, and infections.

Areas Covered: The review summarizes the patent and scientific literature on small molecule modulators of RSK and their potential use as therapeutics.

Synthesis and Structure-Activity Relationship Study of 5a-Carbasugar Analogues of SL0101.

The Ser/Thr protein kinase, RSK, is associated with oncogenesis, and therefore, there are ongoing efforts to develop RSK inhibitors that are suitable for use in vivo. SL0101 is a natural product that demonstrates selectivity for RSK inhibition. However, SL0101 has a short biological half-life in vivo.

De novo synthesis and biological evaluation of C6″-substituted C4″-amide analogues of SL0101.

In an effort to improve upon the in vivo half-life of the known ribosomal s6 kinase (RSK) inhibitor SL0101, C4″-amide/C6″-alkyl substituted analogues of SL0101 were synthesized and evaluated in cell-based assays. The analogues were prepared using a de novo asymmetric synthetic approach, which featured Pd-π-allylic catalyzed glycosylation for the introduction of a C4″-azido group. Surprisingly replacement of the C4″-acetate with a C4″-amide resulted in analogues that were no longer specific for RSK in cell-based assays.

Improving the affinity of SL0101 for RSK using structure-based design.

Enhanced activity of the Ser/Thr protein kinase, RSK, is associated with transformation and metastasis, which suggests that RSK is an attractive drug target. The natural product, SL0101 (kaempferol 3-O-(3″,4″-di-O-acetyl-α-L-rhamnopyranoside), has been shown to be a RSK selective inhibitor. However, the Ki for SL0101 is 1 μM with a half-life of less than 30 min .

Insights into the inhibition of the p90 ribosomal S6 kinase (RSK) by the flavonol glycoside SL0101 from the 1.5 Å crystal structure of the N-terminal domain of RSK2 with bound inhibitor.

The p90 ribosomal S6 family of kinases (RSK) are potential drug targets, due to their involvement in cancer and other pathologies. There are currently only two known selective inhibitors of RSK, but the basis for selectivity is not known. One of these inhibitors is a naturally occurring kaempferol-α-L-diacetylrhamnoside, SL0101.

Analogs of the RSK inhibitor SL0101: optimization of in vitro biological stability.

The Ser/Thr protein kinase, RSK, is important in the etiology of tumor progression including invasion and motility. The natural product kaempferol-3-O-(3″,4″-di-O-acetyl-α-l-rhamnopyranoside), called SL0101, is a highly specific RSK inhibitor. Acylation of the rhamnose moiety is necessary for high affinity binding and selectivity.

Sustained activation of the HER1-ERK1/2-RSK signaling pathway controls myoepithelial cell fate in human mammary tissue.

Human mammary glands arise from multipotent progenitor cells, which likely respond both to cell-autonomous and to extrinsic cues. However, the identity of these cues and how they might act remain unclear. We analyzed HER1 ligand effects on mammary morphogenesis using a three-dimensional organoid model generated from human breast tissue that recapitulates both qualitatively and quantitatively the normal ductal network in situ.

A chromatin-bound kinase, ERK8, protects genomic integrity by inhibiting HDM2-mediated degradation of the DNA clamp PCNA.

Proliferating cell nuclear antigen (PCNA) acts as a scaffold, coordinator, and stimulator of numerous processes required for faithful transmission of genetic information. Maintaining PCNA levels above a critical threshold is essential, but little is known about PCNA protein turnover. We now show that ERK8 (extracellular signal-regulated kinase 8) is required for PCNA protein stability.

RSK in tumorigenesis: connections to steroid signaling.

The Ser/Thr kinase family, RSK, has been implicated in numerous types of hormone-dependent and -independent cancers. However, there has been little consideration of RSKs as downstream mediators of steroid hormone non-genomic effects or of their ability to facilitate steroid receptor-mediated gene expression. Steroid hormone signaling can directly stimulate the MEK/ERK/RSK pathway to regulate cellular proliferation and survival in transformed cells.