Abstract

Vasoactive intestinal peptide receptors 1 (VPAC1) and 2 (VPAC2) have been identified in humans. Cell lines expressing only VPAC1 (HT-29) or VPAC2 (Molt-4b) were identified using real-time reverse transcriptase polymerase chain reaction. Vasoactive intestinal peptide (VIP) and related peptides, VIP_–6–28, VIP_4–28, and VIP_10–28, previously isolated from cultures of human leukocytes, were evaluated for their ability to bind to VPAC1 and VPAC2 and to increase the levels of cAMP in HT-29 and Molt-4b cells. VIP bound to membranes of HT-29 colon carcinoma cells and Molt-4b lymphoblasts with high affinity (K_D = 1.6 ± 0.2 and 1.7 ± 0.9 nM, respectively). VIP_4–28 also demonstrated high-affinity binding (K_D = 1.7 ± 0.2 and 1.7 ± 0.7 nM in HT-29 and Molt-4b, respectively). VIP and VIP_4–28 are potent VPAC1 agonists, inducing maximal 200- and 400-fold increases in cAMP, respectively. VIP demonstrated weak VPAC2 agonist activity, inducing a maximal 14-fold increase in cAMP. VIP_4–28 had no VPAC2 agonist activity but demonstrated potent VPAC2 antagonist activity. VIP_4–28 inhibited VPAC2-mediated increases in cAMP in Molt-4b cells up to 95%, but had no antagonistic effect on VPAC1. Lymphoblasts did not hydrolyze VIP_4–28 to a form with VPAC1 antagonist activity. VIP_4–28 thus is a lymphocyte-generated VIP fragment with potent agonist activity for VPAC1 and potent antagonist activity for VPAC2.