Specific receptors for phorbol esters in lymphoid cell populations: role in enhanced production of T-cell growth factor.

Abstract

Phorbol ester tumor promoters act synergistically with concanavalin A to cause production of T cell growth factor by normal human peripheral blood lymphocytes. A specific, saturable, binding component which may mediate the phorbol ester effect was identified by using [20-3H]phorbol 12,13-dibutyrate in a whole-cell binding assay. Specific binding is maximal within 5 min at 37 or 23.degree. C but the level of bound ligand rapidly decreases to .apprx. 50% within 1 h. At 4.degree. C, 2 h are required to reach maximal binding and the binding is stable for at least 20 h. Binding is reversible at 37 and 4.degree. C with time courses similar to those for initial binding at the respective temperatures. Saturation of the specific binding occurs at a concentration (.apprx. 30 nM) consistent with that producing maximal T cell growth factor activity. Scatchard analysis of the binding after 30 min at 37.degree. C demonstrates a lower Kd (9 nM) than that determined after 2 h at 4.degree. C (22 nM). The median number of sites per cell for 6 donors was 2 .times. 105 (range, 1.3-4 .times. 105). Other tumor-promoting phorbol esters compete for [20-3H]phorbol 12,13-dibutyrate binding in approximate proportion to their activity in stimulating T cell growth factor production. Phorbol, 4-.alpha.-phorbol didecanoate, dexamethasone, retinoic acid, butyric acid and dimethyl sulfoxide do not compete for specific binding.