Development of High Affinity Selective VIP₁ Receptor Agonists

Abstract

Gourlet, P., A. Vandermeers, P. Vertongen, J. Rathe, P. De Neef, J. Cnudde, M. Waelbroeck and P. Robberecht. Development of high affinity selective VIP₁ receptor agonists. Peptides 18(10) 1539–1545, 1997.—The biological effects of VIP are mediated by at least two VIP receptors: the VIP₁ and the VIP₂ receptors that were cloned in rat, human and mice. As the mRNA coding for each receptor are located in different tissues, it is likely that each receptor modulates different functions. It is therefore of interest to obtain selective agonists for each receptor subtype. In the present work, we achieved the synthesis of two VIP₁ receptor selective agonists derived from secretin and GRF. [R¹⁶]chicken secretin had IC₅₀ values of binding of 1, 10,000, 20, and 3000 nM for the rat VIP₁-, VIP₂-, secretin- and PACAP receptors, respectively. This peptide, however, had a weaker affinity for the human VIP₁ receptor (IC₅₀ of 60 nM). The chimeric, substituted peptide [K¹⁵,R¹⁶,L²⁷]VIP(1-7)/GRF(8-27) had IC₅₀ values of binding of 1, 10,000, 10,000 and 30,000 nM for the rat VIP₁-, VIP₂-, secretin- and PACAP receptors, respectively. Furthermore, its also showed an IC₅₀ of 0.8 nM for the human VIP₁ receptor and a low affinity for the human VIP₂ receptor. It is unlikely that this GRF analogue interacted with a high affinity to the pituitary GRF receptors as it did not stimulate rat pituitary adenylate cyclase activity. The two described analogues stimulated maximally the adenylate cyclase activity on membranes expressing each receptor subtype.

Introduction

Vasoactive Intestinal Polypeptide (VIP) is a neuropeptide of 28 amino acids with a widespread distribution in both the central and peripheral nervous system [7]. VIP effects are mediated by high affinity Gs protein coupled receptors.

Molecular cloning revealed the existence of two distinct VIP receptors with seven transmembrane helices named the VIP₁ and the VIP₂ receptors.

The VIP₁ receptor was cloned from rat lung [13]and cancerous human colonic epithelial cells 4, 26; the VIP₂ receptor was cloned from the rat olfactory bulb [17]and cerebral cortex, from mouse pancreatic islets [12]and from human SUP T1 lymphoblasts [27]. In situ hybridization to rat organs revealed the expression of VIP₁ receptor mRNA [28]in pulmonary large and moderate size bronchi, small intestine, thymus, liver, adrenal medulla, uterine smooth muscle and within the brain in the cerebral cortex and hippocampus; VIP₂ receptor mRNA was visualized in mucosa and muscles from the stomach and duodenum, in spleen, thymus, pancreatic islets, the terminal bronchioles, testes [15], ovary, uterus, pituitary, and within the brain, in the thalamus and hypothalamic nuclei. VIP₁ receptor mRNA was expressed in human epithelial cell lines [20], rat pituitary cells and tumors [30]and occasionally in human brain tumors [31]and neuroblastomas [32]. VIP₂ receptor was also expressed in one glial tumor [33], and in several human cell lines of T origin [20].

VIP₁ and VIP₂ receptors are pharmacologically distinct: the VIP₂ receptor—previously named the helodermin-preferring receptor—had a higher affinity than the VIP₁ receptor for helodermin and a lower affinity for secretin and GRF [21]. Several VIP analogues modified in the amino-terminal part of the peptide recognized differently the VIP₁ and the VIP₂ receptors 9, 22. The affinity or selectivity of these peptides were too low to allow them to be useful tools for receptor classes identification.

We recently discovered [10]that the introduction of an arginine residue in position 16 of VIP or Pituitary Adenylate Cyclase Activating Peptide (PACAP) increased the peptide affinity for the VIP and PACAP receptors and also that the same substitution in secretin reduced its affinity for the secretin receptors. We took advantage of that finding to synthesize derivatives with a high affinity and selectivity for the VIP₁ receptors. We focused our attention on that receptor class as a high affinity and highly selective VIP₂ agonist was already available [11].