| Abstract: | The membrane effects of a potent tumour promoter, 12-O-tetradecanoyl phorbol-13-acetate (TPA), were studied in a series of cultured rat liver epithelial cell lines. Treatment with TPA resulted in the formation of strand-like aggregates (ridges) of viable cells over the monolayer of IAR 6-1 cells, but not of three other cell lines tested (IAR 20, IAR 6, IAR 6-7). The morphological response of IAR 6-1 to TPA was investigated by determination of phorbol ester receptors, analysis of cellular fucoproteins, surface galactoproteins and iodinatable surface proteins, and specific immunofluorescence for components of the extracellular matrix (fibronectin, laminin-entactin, procollagen type III). A class of specific, saturable, high-affinity receptors for phorbol esters was demonstrated in all four cell lines employing a conventional [20(-3)H]phorbol-12,13-dibutyrate ([3H]PDBu)-binding assay. The dissociation constants were similar in four lines, but the number of receptors per cell in IAR 6-1 cells was about twice that in other lines. Down-regulation of receptors was demonstrated in IAR 20 and IAR 6-1 cells with similar characteristics. Iodinatable surface proteins and galactose-containing surface glycoproteins did not respond to TPA. The distribution of fibronectin, laminin-entactin and procollagen type III was not affected by TPA. A TPA-responsive cell line, IAR 6-1, contained considerably less laminin-entactin than did the other lines. TPA had no influence on metabolic labelling of [3H]fucose-containing cellular glycoprotein in IAR 6-1 cells. One specific protein, with molecular mass of 78 kD, was more heavily labelled with [3H]fucose in IAR 6-1 cells than in the other cell lines. Taken together, the results of this study show that the responsive cells (IAR 6-1) differed from non-responsive ones in having more phorbol ester receptors, increased fucosylation of a specific glycoprotein and decreased deposition of laminin-entactin in the extracellular matrix. These surface properties of IAR 6-1 cells may contribute to their ability to respond to TPA. |
