Aberrant expression of human mucin gene MUC5B in gastric carcinoma and cancer cells - Identification and regulation of a distal promoter

In gastric cancer, altered expression of MUC1, MUC2, MUC5AC, and MUC6 mucin genes has already been described. We show in this report by the means of i n situ hybridization, reverse transcriptase-polymerase chain reaction, and transfection assays that MUC5B is also abnormally expressed in gastric carc inomatous tissues and cell lines. We thus undertook to elucidate the molecu lar mechanisms that regulate the transcription of MUC5B in gastric cancer c ells. To this end, high expressing (KATO-III) and low expressing (AGS) gast ric cancer cell lines were chosen to study human mucin gene MUC5B expressio n and promoter activity, Sequencing of the promoter region revealed a dista l TATA box located 1 kilobase upstream of the proximal TATA box. Functional activity of the promoter was addressed by using deletion mutants covering 2044 nucleotides upstream of the MUC5B transcription start site, We identif ied a distal promoter 10 times more active than the proximal promoter in KA TO-III cells, In AGS cells, both promoters, much less active, showed the sa me range of activity. Binding assays allowed us to show that the transcript ion factor ATF-1 binds to a cis-element present in the distal promoter. Sp1 , which binds to both promoters specifically transactivates the proximal pr omoter. Treatment of transfected cells with PMA, cholera toxin A subunit, a nd calcium ionophore A23187 showed that only PMA led to a substantial activ ation of the distal promoter, MUC5B 5'-flanking region having a high GC con tent, influence of methylation on the MUC5B expression was assessed, Our re sults indicate that repression of MUC5B expression visualized in AGS cells is due in part to the presence of numerous methylated cytosine residues thr oughout the 5'-flanking region, Altogether these results demonstrate that M UC5B expression in gastric cancer cells is governed by a highly active dist al promoter that is up-regulated by protein kinase C and that repression is under the influence of methylation.