Identification of a BRAF/PA28 gamma/MEK1 signaling axis and its role in epithelial-mesenchymal transition in oral submucous fibrosis

Oral submucous fibrosis (OSF) is a chronic and insidious oral potentially malignant disorder associated with a 4-17% risk of oral squamous cell carcinoma (OSCC). Our previous study found that proteasomal activator 28 gamma (PA28 gamma) is frequently overexpressed in oral squamous cell carcinoma and negatively correlated with poor patient prognosis. However, the role of PA28 gamma in the occurrence and development of OSF remains unclear. Here, we screened PA28 gamma-related genes and investigated their function in OSF. We demonstrated that the expression of PA28 gamma was positively associated with MEK1 and gradually elevated from normal to progressive stages of OSF tissue. Arecoline, a pathogenic component of OSF, could upregulate the protein levels of PA28 gamma and phosphorylated MEK1 and contribute to epithelial to mesenchymal transition (EMT) in epithelial cells. Notably, PA28 gamma could interact with MEK1 and upregulate its phosphorylation level. Furthermore, arecoline upregulated BRAF, which can interact with PA28 gamma and upregulate its protein level. Additionally, BRAF, PA28 gamma, and MEK1 could form protein complexes and then enhance the MEK1/ERK signaling pathways. The concrete mechanism of the protein stability of PA28 gamma is that BRAF mediates its degradation by inhibiting its ubiquitination. These findings underscore the instrumental role of PA28 gamma in the BRAF/MEK1 pathway and enhanced EMT through MEK1/ERK activation in OSF.