Serine‐227 in the N‐terminal kinase domain of RSK2 is a potential therapeutic target for mantle cell lymphoma

Abstract

RSK2 is a serine/threonine kinase downstream signaling mediator in the RAS/ERK signaling pathway and may be a therapeutic target in mantle cell lymphoma (MCL), an almost incurable disease subtype of non‐Hodgkin lymphoma. In this study, serine‐227 (RSK2 Ser227 ) in the N‐terminal kinase domain (NTKD) of RSK2 was found to be ubiquitously active in five MCL‐derived cell lines and in tumor tissues derived from five MCL patients. BI‐D1870, an inhibitor specific to RSK2‐NTKD, caused RSK2 Ser227 dephosphorylation, and thereby, induced dose‐dependent growth inhibition via G 2 /M cell cycle blockade and apoptosis in four of the five cell lines, while one cell line showed only modest sensitivity. In addition, RSK2 gene knockdown caused growth inhibition in the four BI‐D1870‐sensitive cell lines. Comparative gene expression profiling of the MCL‐derived cell lines showed that inhibition of RSK2 Ser227 by BI‐D1870 caused downregulation of oncogenes, such as c‐MYC and MYB; anti‐apoptosis genes, such as BCL2 and BCL2L1; genes for B cell development, including IKZF1, IKZF3, and PAX5; and genes constituting the B cell receptor signaling pathway, such as CD19, CD79B, and BLNK. These findings show that targeting of RSK2 Ser227 enables concomitant blockade of pathways that are critically important in B cell tumorigenesis. In addition, we found favorable combinatory growth inhibitory effects of BI‐D1870 with inhibitors of BTK (ibrutinib), AKT (ipatasertib), and BCL2 (venetoclax) in cell characteristic‐dependent manners. These results provide a rationale for RSK2 Ser227 in the NTKD as a potential therapeutic target in MCL and for future development of a novel bioavailable RSK2 NTKD‐specific inhibitor.

The Rationale for the Dual-Targeting Therapy for RSK2 and AKT in Multiple Myeloma

“…In addition, with these growth inhibitory effects, treatment with BI-D1870 repressed the phosphorylation of RSK2 Ser227 , while treatment with ipatasertib for 48 h exerted the optimal inhibition of AKT activity as shown by the dephosphorylation of PRAS40 Thr246 , which is the substrate of AKT. Unlike other cancers [ 33 , 34 , 35 , 36 ], both BI-D1870 and ipatasertib showed no cross-reaction to their respective target molecules. In addition, the combination of the two agents did not result in further inactivation of RSK2 and AKT compared with the effect induced by a single agent in the two HMCLs ( Figure 1 D).…”

“…WB analysis was performed as previously stated ( Supplementary Information ) [ 5 , 23 , 29 , 30 , 32 , 33 ]. The antibodies used are also described in Supplementary Information .…”

“…Importantly, while the activation of PDPK1 is visible in clonal plasma cells of asymptomatic MM, the activation of RSK2-NTKD becomes more visible when the disease progresses to the symptomatic phase, suggesting the involvement of RSK2 activation in MM progression to the treatment-necessary disease stage [ 29 ]. Furthermore, the inactivation of RSK2-NTKD causes apoptosis in myeloma cells regardless of their cytogenetic/molecular features [ 32 ], while there is a compensative interaction between RAS/ERK/RSK2-mediated signal and AKT, which is another major oncogenic substrate of PDPK1 in various types of cancer cells [ 33 , 34 , 35 , 36 ]. These findings prompted us to investigate the therapeutic efficacy and mechanism of action of dual blockade of two major PDPK1 substrates, RSK2-NTKD and AKT, in MM.…”

KRAS and RAS-MAPK Pathway Deregulation in Mature B Cell Lymphoproliferative Disorders

“…RSK2, a kinase downstream to RAS-MAPK signaling, was frequently found to be constitutively active in the MCL cell line and primary samples, independently from RAS/MEK/ERK activation. BI-D1870, a specific inhibitor of RSK2 N-terminal kinase domain (NTKD), induced growth inhibition and apoptosis in MCL lines, suggesting RSK2 NTKD as a potential therapeutic target in MCL [190].…”

The role of Bruton's tyrosine kinase in the immune system and disease

“…Matsumura‐Kimoto et al demonstrated the potential of a combination of the serine/threonine kinase ribosomal protein S6 kinase (RSK2) and the BTK inhibitor ibrutinib. The inhibition of RSK2 was reported to affect downstream proteins involved in the BCR signalling pathway such as BLNK and CD19, as well as proteins from other pathways, blocking the B‐cell pathogenesis [80].…”