Molecular characteristics and evidence for internalization of vasoactive‐intestinal‐peptide (VIP) receptors in the tumoral rat‐pancreatic acinar cell line AR 4‐2 J

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Abstract
1 Vasoactive intestinal peptide (VIP) receptors were investigated in the tumoral acinar cell line AR 4-2 J derived from rat pancreas [125I]Iodo-VIP binding to cell membranes showed the following IC50 values for unlabeled peptides: VIP, 0.3 nM; peptide His-IleNH2, 2 nM; helodermin, 30 nM; secretin, 100 nM. After incubation with 20 nM dexamethasone, the binding capacity increased twofold but affinities were unchanged. External [125I]iodo-VIP binding to intact cells reached steady state after 5 min at 37°C, while the sequestration-internalization of the [125I]iodo-VIP -receptor complex (tested by cold acid washing) increased progressively, reaching 75% of total binding after 1 h. This phenomenon was blocked at 4°C. Further data with dexamethasone, tunicamycin, cycloheximide, low temperature, and/or phenylarsine oxide, suggested a half-life of 2 days for VIP receptors and the necessity of N-glycosylation for proper translocation. 2 For chemical [125I]iodo-VIP cross-linking bis[2-(succinimidooxycarbonyloxy)ethyl]sulfone gave the best yield when compared with five other bifunctional reagents. In membranes, the main specifically cross-linked peptide had MT 66000 under nonreducing conditions, and migrated with lower velocity (–5%) under reducing conditions. Cross-linking was suppressed by VIP, peptide His-IleNH2 and helodermin (competitively) and also by GTP. In intact cells, the Mr of [125I]iodo-VIP-cross-linked peptides depended on the mode of cell solubilization. After direct solubilization, the major cross-linked radioactivity migrated as a smear of Mr 130000–180000 but an MT-66000 peptide was also detectable. In contrast, the solubilization of cross-linked cells detached by mild trypsinisation gave mainly the MT-66000 labeled peptide. This suggests that most VIP receptors in intact, attached cells were in a high-MT complex and that mild cell treatment was sufficient to disrupt this complex.

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