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Insulin binding by normal and neoplastic colon tissue

Insulin receptors in human colon tumours, normal colon tissue and mesenteric fat removed at surgery have been identified by measuring the binding of labelled insulin to cell membrane preparations. Insulin binding sites were readily detected in all tissues, with mean ± SD binding site concentrations of 43 ± 41, 44 ± 39, and 44 ± 35fmol/mg membrane protein, and dissociation constants of 0.73 ± 0.61, 0.66 ± 0.41, and 0.78 ± 0.58nM for microsomal plasma membrane preparations of tumour (n = 25), normal colon (n = 25) and fat (n = 23) respectively. The specificity of binding of labelled insulin was similar in tumour and normal colon samples. Binding in normal colon preparations was highest in the epithelium (84fmol/mg membrane protein) and lower in lamina propria (19fmol/mg), submucosa (26fmol/mg) and muscle wall (13fmol/mg). Degradation of labelled insulin was similar in tumour and normal colon preparations. Mean receptor levels were not significantly different between microsomal membrane preparations and plasma membranes partially purified on discontinuous sucrose gradients, quantitated against either unit membrane protein or unit 5′‐nucleotidase specific activity. There was a significant negative correlation between insulin binding to fat membranes and fasting serum insulin levels, but no significant relationship was seen between serum insulin and receptor levels in either colon tumour or tissue preparations from full‐thickness normal colon wall. An inverse correlation between serum insulin and receptor levels was, however, apparent in preparations of colonic musosa. These data indicate that although insulin receptors in colon tumours share the same biochemical characteristics as those present in the normal colon, receptors in tumour tissue are less sensitive to down‐regulation by ambient insulin than receptors in mesenteric fat cells and normal colonic mucosa.