Saikosaponin‑D triggers cancer death by targeting the PIM1/c-Myc axis to rewire oncogenic alternative splicing

Saikosaponins (SSs, including SSA, SSB, SSC, and SSD), the major bioactive compounds in the traditional medicine Radix Bupleuri (RB), are emerging agents exhibiting anti-tumor efficacy in several cancers. However, the respective anti-tumor efficacy of these agents and mechanisms in cancers such as gastric and prostate cancer remains unclear. In this study, we demonstrated that SSD, among SSs, possessed a significant anti-tumor role in vivo and in vitro. Transcriptome analysis revealed a down-regulation of alternative splicing factors and rewiring of oncogenic alternative splicing events such as CYP1A1, whose transcript-specific ablation with CIRSPR-Cas13 significantly abolished SSD-induced cell growth inhibition. Specifically, we showed that SSD binds to PIM1 and down-regulates its expression, thus restraining PIM1-mediated serine 62 phosphorylation of Myc, which is critical for its transcription activity. This results in decreasing the expression of Myc-governed alternative splicing factors that are responsible for the regulation of oncogenic alternative splicing events. These studies demonstrated the potent anti-tumor efficacy of SSD and exposed a PIM1/Myc pathway that drives the expression of an essential alternative splicing regulatory network that mediates SSD’s anti-tumor role in cancers.