Constitutive overexpression of c-fos protein in rat liver epithelial cells decreases TGF-beta synthesis and increases TGF-beta 1 receptors.

We have previously shown that rat liver epithelial cells were more sensitive to TGF-beta 1 when they were transfected with c-fos cDNA. We analyzed the production of TGF-beta and TGF-beta 1 binding proteins in transfected and parental cells. TGF-beta-like activity released in the medium was reduced in c-fos expressing cells. TGF-beta 1 binding sites were more numerous in transfected cells (x3). Cross-linking studies confirmed that c-fos transfected cells showed increased binding of 125I-TGF-beta 1 to membrane binding sites corresponding to type I, II and III receptors. Transfected cells internalized and degraded 125I-TGF-beta 1 more rapidly than parental cells. TGF-beta 1 incubation rapidly down-regulated the receptors. In parental cells, the down-regulation was total, while in transfected cells, a few binding proteins could still be detected. The c-fos cell line is an interesting tool in analysing the mechanism of action of TGF-beta.

Apigenin and tangeretin enhance gap junctional intercellular communication in rat liver epithelial cells.

Two flavones, apigenin and tangeretin, were studied for their ability to modulate gap junctional intercellular communication (GJIC) in the rat liver epithelial cell line REL. Their cytotoxicity was first determined by cell density and neutral red uptake assays: neither apigenin nor tangeretin are cytotoxic at 10 and 25 microM, the concentrations used in our experiments. We then studied GJIC using the dye transfer assay and we observed that both apigenin and tangeretin enhance it, the maximum stimulation (x 1.7-1.8) being achieved at 25 microM for 24 h. When the dye transfer was enhanced, the amount of connexin 43 increased, which was demonstrated by Western blot and immunofluorescence analysis. For apigenin only, Northern blot analysis showed an accumulation of connexin 43 mRNA. In addition, the incubation of REL cells with the two compounds, for 1 or 24 h, prevented the inhibition of dye transfer by 12-O-tetradecanoylphorbol-13-acetate (1 or 10 ng/ml). The enhancement of GJIC by apigenin could be one of the major mechanisms responsible for apigenin's anti-tumour promoting action in vivo. As for tangeretin, its capacity to enhance GJIC completes its potential protective properties towards the post-initiation process.

Altered response to growth factors in rat epithelial liver cells overexpressing human c-Fos protein.

Human c-fos cDNA was transfected into normal rat liver epithelial (REL) cells to identify cellular modifications associated with high expression of c-Fos protein. Responses to EGF and TGF beta were examined in the different cell lines, under anchorage-dependent and -independent conditions. Sensitivity to both factors was modified in transfected cells. While parental cells in monolayer did not respond to EGF, c-fos containing cells growth was stimulated by this factor. Overexpression of c-Fos protein led to an enhanced TGF beta-induced growth inhibition under anchorage dependent conditions, and TGF beta abolished spontaneous growth in soft agar of the cell lines containing c-fos oncogene. The mechanisms underlying the increased sensitivity to TGF beta in c-fos transfected cells are still to be determined.