Changes in cAMP and fibronectin concentrations in adherent and suspension forms of a hybrid cell line

The coexistence of adherent (PCM3-M) and suspension (PCM3-S) cells in a hybrid cell line led to the investigation of changes in cellular adhesion. The clonal origin, stable karyotype and subculturing properties of this hybrid indicate that these cells represent different forms of an identical population. Time-lapse photography and mitotic studies signify that the PCM3-S cells were found in the mitotic phase of the cell cycle, while incorporation of [³H]thymidine into DNA by both PCM3-M and PCM3-S cells suggested that the adherent cells detach during the S phase. This was substantiated by inducing quiescence of the culture during the G2 phase, since a preferential increase in the suspension was subsequently observed. Thus the cell cycle studies are consistent with the proposal that floating cells are discernable in a culture of adherent cells because the point at which they round up for division occurs earlier in the cell cycle. Prostaglandin E₁ (2.8 μM) and 3-isobutyl-1-methylxanthine (0.1 mM) were shown to maximally stimulate cAMP production and incubating the hybrid in these agents dramatically decreased the percentage of floating cells, slowed the growth rate and increased the adhesion of the PCM3-M cells. However, determination of intracellular concentrations of cAMP indicated that the floating cells were not a manifestation of decreased cAMP concentrations. In addition, the cell surface glycoprotein, fibronectin, was found to be present on the PCM3-M cells but lost from the PCM3-S cells. Addition of extracted fibronectin to these floating cells resulted in a culture consisting exclusively of adherent PCM3-M cells, suggesting that an aberration in fibronectin-associated cell-substratum adhesion is responsible for the unusual growth of the hybrid.