The p90 ribosomal S6 kinase (RSK) inhibitor BI-D1870 prevents gamma irradiation-induced apoptosis and mediates senescence via RSK- and p53-independent accumulation of p21WAF1/CIP1

Abstract

The p90 ribosomal S6 kinase (RSK) family is a group of highly conserved Ser/Thr kinases that promote cell proliferation, growth, motility and survival. As they are almost exclusively activated downstream of extracellular signal-regulated kinases 1 and 2 (ERK1/2), therapeutic intervention by RSK inhibition is less likely to produce such severe side effects as those observed following inhibition of the upstream master regulators Raf, MEK and ERK1/2. Here, we report that BI-D1870, a potent small molecule inhibitor of RSKs, induces apoptosis, although preferentially, in a p21-deficient background. On the other hand, BI-D1870 also induces a strong transcription- and p53-independent accumulation of p21 protein and protects cells from gamma irradiation (γIR)-induced apoptosis, driving them into senescence even in the absence of γIR. Although we identified p21 in in vitro kinase assays as a novel RSK substrate that specifically becomes phosphorylated by RSK1-3 at Ser116 and Ser146, RNA-interference, overexpression and co-immunoprecipitation studies as well as the use of SL0101, another specific RSK inhibitor, revealed that BI-D1870 mediates p21 accumulation via a yet unknown pathway that, besides its off-site targets polo-like kinase-1 and AuroraB, also does also not involve RSKs. Thus, this novel off-target effect of BI-D1870 should be taken into serious consideration in future studies investigating the role of RSKs in cellular signaling and tumorigenesis.

Elevated ERK/p90 ribosomal S6 kinase activity underlies audiogenic seizure susceptibility in fragile X mice

“…6B). These findings suggest that elevated RSK activity contributes, at least in part, to the seizure susceptibility observed in fragile X mice, although we cannot rule out the possibility that BI-D1870 is acting on other pathways (29).…”

p63 threonine phosphorylation signals the interaction with the WW domain of the E3 ligase Itch

“…15 Much of the research in the quest for E3 inhibitors has been finalized for the p53-HDM2 pathway since p53 is an extremely powerful transcription factor crucial in DNA damage response. [16][17][18][19][20][21] Here, the cellular defense to DNA damage is based on sensors and effectors that activates the cell death pathway, [22][23][24] via p53 [25][26][27][28][29] or its family members. [30][31][32][33][34][35][36][37] Like for our recent Itch inhibitor screening, 14 we believe that in a near future innovative E3 inhibitors will be developed.…”

Two widely used RSK inhibitors, BI-D1870 and SL0101, alter mTORC1 signaling in a RSK-independent manner

“…For example, SL0101 and BI-D1870 were both observed to inhibit glucose uptake, yet by different mechanisms [23]. Moreover, as mentioned above, BI-D1870 can also induce senescence via a p21-dependent mechanism, which is an action not observed for SL0101 [24]. These results suggest that at least one of the inhibitors mediates off-target effects.…”

“…Therefore, data obtained using these substrates may not be accurate. In another recent article, 10 μM BI-D1870 was shown to induce p21 expression in a RSK-independent manner, and to induce senescence [24]. Furthermore, in vitro studies have shown that PLK1 can be inhibited when BI-D1870 is present at slightly higher concentrations.…”

“…Unlike BI-D1870, the concentrations reported for studies of SL0101 are rather variable, with 100 μM being the most used concentration (Fig. 1C) [24]. There was also a high proportion of papers that described the use of SL0101 as the unique tool to demonstrate the involvement of RSK (Fig.…”