Genomic Loss and Epigenetic Silencing of the FOSL1 Tumor Suppressor Gene in Radiation-induced Neoplastic Transformation of Human CGL1 Cells Alters the Tumorigenic Phenotype In Vitro and In Vivo

The CGL1 human hybrid cell system has been utilized for many decades as an excellent cellular tool for investigating neoplastic transformation. Substantial work has been done previously implicating genetic factors related to chromosome 11 to the alteration of tumorigenic phenotype in CGL1 cells. This includes candidate tumor suppressor gene FOSL1, a member of the AP-1 transcription factor complex which encodes for protein FRA1. Here we present novel evidence supporting the role of FOSL1 in the suppression of tumorigenicity in segregants of the CGL1 system. Gamma-induced mutant (GIM) and control (CON) cells were isolated from 7 Gy gamma-irradiated CGL1s. Western, Southern and Northern blot analysis were utilized to assess FOSL1/FRA1 expression as well as methylation studies. GIMs were transfected to re-express FRA1 and in vivo tumorigenicity studies were conducted. Global transcriptomic microarray and RT-qPCR analysis were used to further characterize these unique cell segregants. GIMs were found to be tumorigenic in vivo when injected into nude mice whereas CON cells were not. GIMs show loss of Fosl/FRA1 expression as confirmed by Western blot. Southern and Northern blot analysis further reveals that FRA1 reduction in tumorigenic CGL1 segregants is likely due to transcriptional suppression. Results suggest that radiation-induced neoplastic transformation of CGL1 is in part due to silencing of the FOSL1 tumor suppressor gene promoter by methylation. The radiation-induced tumorigenic GIMs transfected to re-express FRA1 resulted in suppression of subcutaneous tumor growth in nude mice in vivo. Global microarray analysis and RT-qPCR validation elucidated several hundred differentially expressed genes. Downstream analysis reveals a significant number of altered pathways and enriched Gene Ontology terms genes related to cellular adhesion, proliferation, and migration. Together these findings provide strong evidence that FRA1 is a tumor suppressor gene deleted and epigenetically silenced after ionizing radiation-induced neoplastic transformation in the CGL1 human hybrid cell system.

Transcriptomic profiling of gamma ray induced mutants from the CGL1 human hybrid cell system reveals novel insights into the mechanisms of radiation-induced carcinogenesis

BACKGROUND

Somatic cell hybrid systems generated by combining cancerous with non-cancerous cells provide useful model systems to study neoplastic transformation. Combined with recent advances in omics-based technologies, novel molecular signatures that drive radiation-induced carcinogenesis can be analyzed at an exceptional global level.

METHODS

Here, we present a complete whole-transcriptome analysis of gamma-induced mutants (GIM) and gamma irradiated control (CON) segregants isolated from the CGL1 (HeLa x normal fibroblast) human hybrid cell-system exposed to high doses of radiation. Using the Human Transcriptome Array 2.0 microarray technology and conservative discrimination parameters, we have elucidated 1067 differentially expressed genes (DEGs) between tumorigenic and non-tumorigenic cells.

RESULTS

Gene ontology enrichment analysis revealed that tumorigenic cells demonstrated shifts in extracellular matrix (ECM) and cellular adhesion profiles, dysregulation of cyclic AMP (cAMP) signaling, and alterations in nutrient transport and cellular energetics. Furthermore, putative upstream master regulator analysis demonstrated that loss of TGFβ1 signaling due to reduced SMAD3 expression is involved in radiation-induced carcinogenesis.

CONCLUSIONS

Taken together, this study presents novel insights into specific gene expression and pathway level differences that contribute to radiation-induced carcinogenesis in a human cell-based model. This global transcriptomic analysis and our published tumor suppressor gene deletion loci analyses will allow us to identify and functionally test candidate nexus upstream tumor suppressor genes that are deleted or silenced after exposure to radiation.