Comparisons of the stimulation of normal lymphocytes and lymphoma cells by tetrameric concanavalin A (Con A) and dimeric succinyl-Con A suggest that both stimulatory and inhibitory signals operate to modulate mitogenesis. Synergistic effects can be obtained for the stimulatory event using lectins, phorbol esters, and calcium ionophores, all of which are independently mitogenic for lymphocytes. The inhibitory effects of high doses of Con A could be mimicked by the simultaneous addition of the phorbol ester and Con A under conditions in which both reagents are optimally mitogenic when used alone. No inhibition of stimulation was found, however, when succinyl-Con A was used with phorbol ester under the same conditions. Moreover, when lymphocytes were cultured with Con A in the presence of succinyl-Con A, the inhibitory effect of the native lectin was seen at lower doses than in the absence of the derivative. These observations suggest that the stimulatory and inhibitory portions of the dose-response curve can be manipulated independently and may be mediated by two distinct signals. It is likely the signal for the inhibition of cell proliferation is regnlated by the same cell surface modulating assembly that controls the mobility of cell surface receptors.Since Nowell's discovery that phytohemagglutinin (PHA), the lectin from Phaseolus vulgarms, can induce blast transformation in normal leukocytes (1), the mitogenic stimulation of lymphocytes has been studied extensively as a model for both antigenic activation and growth control. Mitogens show characteristic unimodal dose--response curves for a given population of lymphocytes, i.e., concentrations significantly below or above the optimum show no stimulation. A typical example of this behavior is seen in the stimulation of lymphocytes by the lectin concanavalin A (Con A), whose structure (2) and binding specificity (3) have been extensively characterized.Much effort has been directed at understanding the stimulation induced by low doses of lectins and other agents. Although their causal connections remain to be established, it has been shown that calcium ionophores and agents capable of stimulating synthesis of guanosine 3': 5'-cyclic inonophosphate (cyclic GMP) (4), such as phorbol esters, are also mitogenic (5-7). Much less attention has been directed, however, at the inhibitory effects of high doses of lectins and these agents.In this paper, we describe several approaches to the analysis of the stimulatory and inhibitory effects responsible for the characteristic mitogenic dose-response curve. We have found that divalent succinvl-Con A shows stimulatory but not Abbreviations: PHA, phytohemagglutinin; Con A, concanavalin A; TPA, 12-0-tetradecanoyl-phorbol-13-0-acetate; cyclic GMP, guanosine 3' :5'-cyclic monophosphate; CSMA, cell surface modulating assembly.
1917inhibitory properties. Moreover, lectins, calcium ionophores, and phorbol esters are synergistic in stimulating mitogenesis. Addition of phorbol ester to higher doses of Con A, however, results in ...