c-Fos overexpression increases the proliferation of human hepatocytes by stabilizing nuclear Cyclin D1

Abstract

flow cytometry showed that c-Fos accelerated the cell cycle kinetics. Following serum stimulation, Cyclin D1 was more abundantly expressed in c-Fos overexpressing cells. Cyclin D1 accumulation did not result from increased transcriptional activation, but from nuclear stabilization. Overexpression of c-Fos correlated with higher nuclear levels of inactive phosphorylated GSK-3β, a kinase involved in Cyclin D1 degradation and higher levels of EGF-R mRNA, and EGF-R protein compared to IHH-C both in serum starved, and in serum stimulated cells. Abrogation of EGF-R signalling in IHHFos by treatment with AG1478, a specific EGF-R tyrosine kinase inhibitor, prevented the phosphorylation of GSK-3β induced by serum stimulation and decreased Cyclin D1 stability in the nucleus. METHODS:IHHs stably transfected with c-Fos (IHHFos) or an empty vector (IHH-C) were grown in medium supplemented with 1% serum or stimulated with 10% serum. Cell proliferation was assessed by cell counts, 3H-thymidine uptake and flow cytometry analyses. The levels of cell cycle regulatory proteins (Cyclin D1, E, A) cyclin dependent kinases (cdk) cdk2, cdk4, cdk6, and their inhibitors p15, p16, p21, p27, total and phosphorylated GSK-3β and epidermal growth factor receptor (EGF-R) were assayed by Western blotting. Analysis of Cyclin D1 mRNA levels was performed by reverse transcription-polymerase chain reaction and real-time polymerase chain reaction (PCR) analysis. Stability of Cyclin D1 was studied by cycloheximide blockade experiments. RESULTS: Stable c-Fos overexpression increased cell proliferation under low serum conditions and resulted in a two-fold increase in [3 H]-thymidine incorporation following serum addition. Cell cycle analysis by

Mitogenic Regulation of p271 Gene Is Mediated by AP-1 Transcription Factors

“…Similar to this observation, Güller et al (37) recently reported that Fos overexpression can cause an increase in p27…”

EWS/ATF1 expression induces sarcomas from neural crest–derived cells in mice

“…The direct interaction of EWS/ATF1 at CRE may induce continuous transcriptional activation of Fos in EWS/ATF1-induced tumor cells. Previous studies have demonstrated a higher expression level of FOS to be involved in tumor growth in several cancers (42)(43)(44), and overexpression of Fos results in osteosarcoma formation in transgenic mice (45,46). Here we showed that FOS was also upregulated in CCS by the EWS/ATF1 fusion transcript and that the increased FOS promoted the growth of EWS/ATF1-related sarcomas.…”

Evidence for Homodimerization of the c-Fos Transcription Factor in Live Cells Revealed by Fluorescence Microscopy and Computer Modeling

“…c-Fos was overexpressed in some tamoxifen-resistant human breast tumors (20) and highly overexpressed in malignant oral tissues (21). It could also contribute to hepatocarcinogenesis (22). In a murine skin carcinogenesis model, c-Fos was shown to be required for malignant tumor conversion (23).…”