RESUMO
The CT-mediated signaling mechanisms have been widely used as a tool for helping the knowledge of the more complex mechanisms regulating cell growth and proliferation in which gangliosides are involved as receptors and cAMP as second messenger. In the present study we compare the susceptibility of two murine cell lines (SR-4987 stromal cells and L1210 leukemic cells) to inhibitory effect of cholera toxin (CT) on cell growth and correlate their sensitivity to CT with ganglioside content and intracellular cAMP accumulation. The results indicate a very different response of the two cell lines to CT treatment. L1210 cells (which contain GM1a ganglioside) are sensitive to the inhibiting activity of CT (IC50 in the clonogenic assay = 10(-9) M) but no cAMP accumulation was observed after the treatment. SR-4987 cells (which lack GM1a) show a dramatic increase of intracellular cAMP without any inhibition of cell growth following the CT treatment until 10(-8) M. However, after SR4987 cells have incorporated GM1a they became susceptible to CT (with a IC50 value = 10(-11) M). The comparison of these results with our previous studies on WEHI-3B leukemia cells confirms the remarkable heterogeneity of cell sensitivity to the growth inhibition by CT by emphasizing that this inhibition is the final event of very different mechanisms in which CT binding to a specific ganglioside seems to be necessary and sufficient whereas cAMP accumulation may not be coupled with the antiproliferative effect of CT.