Abstract
Cyclin D1 regulates G1 progression and is important in the development and proliferation of various human cancers. Cyclin D1 gene expression is activated by the Ras kinase cascade. Nuclear cyclin D1 levels are dependent on cytoplasmic degradation of cyclin D1 via ubiquitin-mediated proteolysis. We sought to determine whether the important MAPK signaling pathway, in the cyclin D1 cascade, including FBXW8, Cullin1, and the ubiquitination pathway mediated these effects. Ursolic acid (UA) treatment of SNG-2 cells, an endometrial cancer cell line, decreased cyclin D1, pERK1/2, FBXW8, and Cullin1 levels in a dose- and time-dependent manner. RING-type E3 ligase consists of CulIin1, Rbx, Skp1, and a member of the F-box protein family. In SNG-2, both dose- and time-dependent inhibition of Rbx 1 were observed following treatment with UA. Moreover, in HEC108 cells, another endometrial cancer cell line, UA treatment decreased cyclin D1, pERK1/2, and Cullin1 levels in a dose- and time-dependent manner and UA markedly inhibited FBXW8. Treatment of HEC108 cells moderately decreased Rbx1 in a dose- and-time-dependent fashion. In contrast, UA treatment increased ubiquitinated proteins in a dose- and time-dependent manner in both cell lines. RING-type E3 ligase accumulated in the cytoplasm following UA treatment of SNG-2cells. That in turn prevented cytoplasmic degradation of cyclin D1 via RING-type E3 (SCF E3s) ligase. In conclusion, our study found inhibition of the MAPK- cyclin D1 pathway and RING type E3 ligase (SCF E3s) in both endometrial cancer cell lines. Furthermore, CD36 was noted as a cell surface receptor for UA.