Inhibition of the proliferation of Nb2 cells by femtomolar concentrations of cholera toxin and partial reversal of the effect by 12‐O‐tetradecanoyl‐phorbol‐13‐acetate

One hour of exposure to cholera toxin is sufficient to elicit a significant delay in the initiation of DNA synthesis and cell division in lactogenic hormone‐dependent Nb2‐11C lymphoma cells. The inhibitory effect occurs already at very low concentrations of cholera toxin (5–50 fM), at which it is not accompanied by a detectable increase in intracellular cAMP, or ADP‐ribosylation of the alpha subunit of G_s, the stimulatory guanine nucleotide binding protein of adenylate cyclase; IBMX, the phosphodies‐terase inhibitor, acts synergistically to cholera toxin, indicating that a minute increase in cAMP may be sufficient for the inhibition. This indication is substantiated by the finding that dibutyryl cAMP also inhibits cell proliferation. Phorbol diester reverses partially the inhibitory activity of cholera toxin. It is most likely that this effect does not result from blocking the increase in cAMP, but rather from some subsequent, yet unidentified, events. The inhibitory effect of cholera toxin is not dependent on the concentration of the proliferation‐stimulating lactogenic hormone and cannot be abolished or reduced by excess of the hormone. Cholera toxin also inhibits the autonomous proliferation of a lactogenic hormone‐independent cell line (Nb2‐SP); however, in this case the inhibition is not affected by TPA.