INHIBITION OF VASOACTIVE-INTESTINAL-PEPTIDE (VIP) BINDING ON HUMAN-MELANOMA CELLS IGR39 BY NITRIC-OXIDE - CGMP IS NOT INVOLVED

Nitric oxide (NO) and the NO generating agent nitroprusside (SNP), inhibit the binding of [I-125] vasoactive intestinal peptide (VIP) to its receptor at the surface of IGR39 human melanoma cells. Cysteine (10 mM) increases the sensitivity of the system to SNP while N-acetylcysteine (10 mM) decreases it. The NO gas as well as SNP inhibits the [I-125]VIP binding capacity. These observations sustain an effect of SNP-generated NO rather than an effect of the SNP molecule per se or the cyanoferrate portion of the molecule. The inhibitory effect of NO is time and concentration dependent and is fully reversible. Affinity constants of high and low affinity VIP receptors of SNP-treated IGR39 cells are not modified while maximal binding capacity (Bmax) of both receptor types are decreased to the same extent. Production of cGMP by SNP treated cells is time and concentration dependent and the maximum amount of cGMP obtained reaches 13 times the basal level. The cAMP production is not affected by SNP. However, the SNP effects on the [I-125]VIP binding are not mimicked by the membrane permeant cGMP analogs dibutyryl cGMP and I-bromo cGMP even at concentrations as high as 0.5 mM. Taken altogether, these data demonstrate a regulatory action of NO on VIP binding capacity of IGR39 melanoma cells which is not cGMP mediated. They also evidence a new step which could be involved in the NO-VIP interaction.