Increased Expression after X‐Irradiation of MUC1 in Cultured Human Colon Carcinoma HT‐29 Cells

The effect of X‐irradiation on production of MUC1 was studied with human colon carcinoma HT‐29 cells. As evaluated by immunocytochemical staining, the percentages of MUC1‐positive cells in cells at 4 days after 6 Gy irradiation and in unirradiated control cells were 52±3.5% (n=6) and 26±2.8% (n=6), respectively. Flow‐cytometric analysis of living cells showed that MUC1 began to rise from day 1, reaching a plateau by day 4 after 6 Gy irradiation. Western blot analysis with monoclonal antibody MY.1E12 against glycosylated MUC1 (mature form) showed dose‐dependent increases of two bands (500 and 390 kDa) corresponding to two polymorphic MUC1 alleles. Premature forms of MUC1 (350 and 240 kDa) were detectable with monoclonal antibody HMFG‐2 only in irradiated cells, suggesting that new core protein synthesis had been induced. The transcriptional activity of the MUC1 gene was analyzed in terms of transient expression of MUC1‐CAT reporter plasmids containing 5′‐flanking sequences of the MUC1 gene fused to the bacterial chloramphenicol acetyltransferase (CAT) gene. The results of CAT assay indicate that enhanced expression of MUC1 in irradiated HT‐29 cells was due to upregulation of MUC1 transcription, and required the upstream promoter.