KSR1 Mediates Small Cell Lung Carcinoma Tumor Initiation and Cisplatin Resistance.

Small cell lung cancer (SCLC) has a dismal 5-year survival rate of less than 7%, with limited advances in first-line treatment over the past four decades. Tumor-initiating cells (TIC) contribute to resistance and relapse, a major impediment to SCLC treatment. In this study, we identify kinase suppressor of Ras 1 (KSR1), a molecular scaffold for the Raf/MEK/ERK signaling cascade, as a critical regulator of SCLC TIC formation and tumor initiation in vivo. We further show that KSR1 mediates cisplatin resistance in SCLC. Whereas 50% to 70% of control cells show resistance after 6-week exposure to cisplatin, CRISPR/Cas9-mediated KSR1 knockout prevents resistance in >90% of SCLC cells in ASCL1, NeuroD1, and POU2F3 subtypes. KSR1 knockout significantly enhances the ability of cisplatin to decrease SCLC TICs via in vitro extreme limiting dilution analysis, indicating that KSR1 disruption enhances the cisplatin toxicity of cells responsible for therapeutic resistance and tumor initiation. The ability of KSR1 disruption to prevent cisplatin resistance in H82 tumor xenograft formation supports this conclusion. Previous studies indicate that ERK activation inhibits SCLC tumor growth and development. We observe a minimal effect of pharmacologic ERK inhibition on cisplatin resistance and no impact on TIC formation via in vitro extreme limiting dilution analysis. However, mutational analysis of the KSR1 DEF domain, which mediates interaction with ERK, suggests that ERK interaction with KSR1 is essential for KSR1-driven cisplatin resistance. These findings reveal KSR1 as a key regulatory protein in SCLC biology and a potential therapeutic target across multiple SCLC subtypes. IMPLICATIONS: Genetic manipulation of the molecular scaffold KSR1 in SCLC cells reveals its contribution to cisplatin resistance and tumor initiation.