Identifying requirements for RSK2 specific inhibitors

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. RSK regulates various homeostatic processes including those involved in transcription, translation and ribosome biogenesis, proliferation and survival, cytoskeleton, nutrient sensing, excitation and inflammation. RSK also acts as a major negative regulator of ERK1/2 signaling. RSK is associated with numerous cancers and has been primarily studied in the context of transformation and metastasis. The development of specific RSK inhibitors as cancer therapeutics has lagged behind that of other members of the mitogen-activated protein kinase signaling pathway. Importantly, a pan-RSK inhibitor, PMD-026, is currently in phase I/1b clinical trials for metastatic breast cancer. However, there are four members of the RSK family, which have overlapping and distinct functions that can vary in a tissue specific manner. Thus, a problem for transitioning a RSK inhibitor to the clinic may be the necessity to develop isoform specific inhibitors, which will be challenging as the NTKDs are very similar to each other. CTKD inhibitors have limited use as therapeutics as they are not able to inhibit the activity of the NTKD but could be used in the development of proteolysis-targeting chimeras.

P90 ribosomal S6 kinases: A bona fide target for novel targeted anticancer therapies?

The 90 kDa ribosomal S6 kinase (RSK) family of proteins is a group of highly conserved Ser/Thr kinases. They are downstream effectors of the Ras/ERK/MAPK signaling cascade. ERK1/2 activation directly results in the phosphorylation of RSKs, which further, through interaction with a variety of different downstream substrates, activate various signaling events. In this context, they have been shown to mediate diverse cellular processes like cell survival, growth, proliferation, EMT, invasion, and metastasis. Interestingly, increased expression of RSKs has also been demonstrated in various cancers, such as breast, prostate, and lung cancer. This review aims to present the most recent advances in the field of RSK signaling that have occurred, such as biological insights, function, and mechanisms associated with carcinogenesis. We additionally present and discuss the recent advances but also the limitations in the development of pharmacological inhibitors of RSKs, in the context of the use of these kinases as putative, more efficient targets for novel anticancer therapeutic approaches.

RSK inhibitors as potential anticancer agents: Discovery, optimization, and challenges

Ribosomal S6 kinase (RSK) family is a group of serine/threonine kinases, including four isoforms (RSK1/2/3/4). As a downstream effector of the Ras-mitogen-activated protein kinase (Ras-MAPK) pathway, RSK participates in many physiological activities such as cell growth, proliferation, and migration, and is intimately involved in tumor occurrence and development. As a result, it is recognized as a potential target for anti-cancer and anti-resistance therapies. There have been several RSK inhibitors discovered or designed in recent decades, but only two have entered clinical trials. Low specificity, low selectivity, and poor pharmacokinetic properties in vivo limit their clinical translation. Published studies performed structure optimization by increasing interaction with RSK, avoiding hydrolysis of pharmacophores, eliminating chirality, adapting to binding site shape, and becoming prodrugs. Besides enhancing efficacy, the focus of further design will move towards selectivity since there are functional differences among RSK isoforms. This review summarized the types of cancers associated with RSK, along with the structural characteristics and optimization process of the reported RSK inhibitors. Furthermore, we addressed the importance of RSK inhibitors' selectivity and discussed future drug development directions. This review is expected to shed light on the emergence of RSK inhibitors with high potency, specificity, and selectivity.

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). The interrelationship and the feedback mechanisms between these pathways occurs at the level of transcription, translation, and posttranslational modification. Identifying how ERK1/2-RSK2 and estrogen signaling cooperate in homeostasis and disease may lead to novel therapeutic approaches in estrogen-dependent disorders.