Abstract 3906: The mRNA translational plasticity of small cell lung carcinoma is associated with its phenotypic transdifferentiation
Authors: Peng Haoning , Wang Mengyao , Li Lu , Liu Lunxu , Shen Shensi 
Published: April 2023

Abstract
Small cell lung carcinoma(SCLC), classified as a recalcitrant cancer type, accounts for 13-15% of all lung cancers with a median overall survival less than 10 months in patients with extensive stage SCLC. SCLC was once considered as molecularly homogeneous due to nearly universal loss of TP53 and RB1, whereas subsets of MYC-driven, non-neuroendocrine (non-NE) SCLC variants, possess chemoresistance, suggesting its high plasticity and intertumoral heterogeneity related to rapid refractory to first-line chemotherapies. Current pathognomonic classification stratifies SCLC into two NE subtypes (ASCL1+, NEUROD1+) and two non-NE subtypes (POU2F3+ and triple negative), associated with different expression patterns of MYC family proteins. MYC is a master regulator of mRNA translation, whether translational regulation actively tunes SCLC plasticity and the associated chemosensitivity remains an enigma. Here, we delineate the translation landscape of NE and non-NE SCLC cells by using paired human NE SCLC cell line and constitutive or induced non-NE SCLC cells.
As expected, non-NE SCLC cells are more resistant to the clinically relevant first-line chemotherapy than NE SCLC cells. With polysome profiling in combination with polysome RNA-seq, we showed differential expression at both transcription- and translation-level. However, transcription-level changes minimally overlapped with the translational changes between NE and non-NE SCLC cells, with a global reduction of translation activity observed in non-NE SCLC cells. Through analyzing the polysome profiles, we observed a striking accumulation of 60S monosomes and diminishment of 80S ribosomes derived from non-NE SCLC cells compared to those from NE SCLC cells. This unique translational pattern in non-NE SCLC cells was correlated with an increased expression of eIF6, a ‘sitting on the bench’ member of eukaryotic initiation factor (eIF) family. The eIF6 couples 60S monosome maturation and recycling, enrolling a quintessential role in functional 80S ribosome formation. Positive correlation between eIF6 expression and non-NE SCLC tumors was observed in 81 tumor samples derived from patients with limited stage SCLC and CCLE cell consortium, in which c-MYC expression was also positively correlated with eIF6 level. By using both ultracentrifugation and fraction analysis from polysome profiling, we found that less association of eIF6 with 60S may mitigate 80S ribosome assembly in non-NE SCLC cells. In addition, in vivo xenograft models showed that eIF6 expression was higher in residual tumors upon chemotherapy.
Taken together, these data revealed an eIF6-dependent translational regulation in non-NE transdifferentiation of SCLC subtypes, suggesting an underappreciated function of translational regulation of SCLC plasticity and the potential therapeutic value of eIF6 for overcoming SCLC resistance to chemotherapy.
Citation Format: Haoning Peng, Mengyao Wang, Lu Li, Lunxu Liu, Shensi Shen. The mRNA translational plasticity of small cell lung carcinoma is associated with its phenotypic transdifferentiation. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3906.