FRA1 (FOSL1) suppresses spontaneous and radiation-induced transformation through transcriptional control of transformation-associated genes, remodeling of activator protein-1 composition, and regulation of cell cycle checkpoints

Abstract

FRA1 (FOSL1), a member of the Activator Protein-1 (AP-1) transcription factor complex, is implicated in various aspects of cancer biology. However, its role in early carcinogenesis, particularly during radiation-induced transformation, remains poorly understood. This thesis investigates the functional role of FRA1 in neoplastic transformation using the CGL1 human hybrid cell line, which is derived from the fusion of normal human fibroblasts and the cervical cancer HeLa cell line. This genetically stable model is well-suited for studying both spontaneous and radiation-induced transformation. Using CRISPR-based tools, we generated CGL1 cells with either FRA1 overexpression or knockout and assessed their phenotypic and molecular characteristics. FRA1 overexpression induced broad transcriptional changes, including modulation of genes involved in extracellular matrix organization, cell adhesion, and transcriptional regulation. AP-1 subunit composition was also altered, with increased c-JUN and decreased c-FOS and FOSL2 expression, suggesting that FRA1 reprograms the AP-1 complex toward a less transformation-permissive state. Moreover, FRA1 overexpressing cells exhibited reduced adhesion to fibronectin and collagen and showed modest changes in cell cycle dynamics. Functionally, FRA1 overexpression significantly suppressed both basal and radiation-induced neoplastic transformation, whereas FRA1 knockout dramatically increased transformation frequency, indicating a protective role in early tumorigenesis. These transformation effects were not attributable to differences in DNA double-strand break repair, as γH2AX foci resolution post-irradiation was comparable between lines. FRA1 knockout cells exhibited defective G2/M checkpoint activation following radiation, suggesting impaired cell cycle control. To further assess FRA1’s regulatory influence, we evaluated gene expression following serum stimulation. FRA1 overexpression blunted the induction of cFOS and cJUN, while knockout enhanced their expression, revealing that FRA1 modulates early transcriptional responses to mitogenic signaling. RNA sequencing also revealed reciprocal expression of transformation-associated genes across FRA1 knockout, control, and overexpression cell lines. Together, these findings establish FRA1 as a context-dependent tumor suppressor that limits neoplastic transformation through regulation of AP-1 composition, cell cycle checkpoint control, and early transcriptional responses to growth stimuli.