Down-regulation of lactogenic hormone receptors in Nb2 lymphoma cells by cholera toxin.

Exposure of lactogen-dependent (Nb2-11C) and lactogen independent (Nb2-SP) lymphoma cells to cholera toxin (0.05-50 pM) resulted within 18-28 h in a 50% decrease in the binding capacity of the intact cells to iodinated human growth hormone, and 40% decrease in cell-homogenates. Scatchard analysis revealed that the reduction in binding resulted from loss of cell-surface receptors accompanied by degradation of intracellular receptors. No alterations in receptor binding affinity were observed. One to 3 h of exposure to the toxin was sufficient to reduce the binding to the level obtained after continuous incubation with the toxin for 28 h. Addition of dibutyryl cAMP (0.1mM) to the medium resulted in similar down-regulation of lactogenic receptors.