Expression of c‐myc in human colonic tissue in response to β‐carotene supplementation

Abstract

Dietary supplementation with β‐carotene at 30 mg/day results in an increased serum trans‐retinoic acid concentration in patients with a prior colonic polyp. In a number of human cell lines, trans‐retinoic acid upregulates c‐myc mRNA expression. We assayed for changes in c‐myc mRNA expression in colonic mucosa by reverse transcription polymerase chain reaction and correlated the results with serum concentrations of all‐trans‐ (ATRA), 75‐cis‐ (13‐cRA), and total retinoic acid. Serum and colonic biopsy samples were obtained before and 90 days after administration of a placebo (n = 7) or 30 mg of β‐carotene (n = 5) daily. An increase in c‐myc expression after supplementation was observed in 6 of 12 subjects, but 5 of these 6 subjects had decreased total serum retinoic acid concentration and 4 had decreased ATRA concentration. In addition, five of the six subjects with increased c‐myc expression had received a placebo. Conversely, c‐myc expression was increased in only two of five paired samples from subjects whose total serum retinoic acid concentration increased during the 90‐day supplementation period. We conclude that c‐myc expression is not correlated with ATRA, 13‐cRA, or total retinoic acid concentration in vivo and that increased serum retinoic acid secondary to increased tissue β‐carotene is not sufficient to activate c‐myc transcription.