Expression of transcription factor AP-2 in colorectal adenomas and adenocarcinomas; comparison of immunohistochemistry and in situ hybridisation.

AIMS: To investigate whether the three different AP-2 isoforms are expressed differently in colorectal adenomas and carcinomas.
METHODS: The study comprised 43 randomly selected patients diagnosed and treated at Kuopio University Hospital in 1996 for colorectal adenocarcinoma (n = 30) and colorectal adenoma (n = 13). The expression of AP-2alpha, AP-2beta, and AP-2gamma was analysed by immunohistochemistry (IHC) and the mRNA status of AP-2alpha was determined by in situ hybridisation (ISH) and confirmed by reverse transcription polymerase chain reaction (RT-PCR). AP-2 expression patterns were correlated with clinicopathological variables.
RESULTS: In adenomas and carcinomas, AP-2beta cytoplasmic positivity was higher than that of AP-2alpha or AP-2gamma. AP-2alpha expression was reduced in advanced Dukes's stage carcinomas. In high grade carcinomas, both AP-2alpha and AP-2gamma expression was reduced. ISH demonstrated increased AP-2alpha values in high grade carcinomas. Seven of 30 carcinoma specimens displayed a moderate or strong mRNA signal, despite being negative for AP-2alpha protein. RT-PCR from AP-2alpha mRNA and protein positive tumours confirmed that the positive signal in ISH originated from the exon 2 of TFAP2A.
CONCLUSIONS: AP-2alpha was reduced in advanced Dukes's stage adenocarcinomas. Together with reduced AP-2gamma expression in high grade carcinomas, this might contribute to tumour progression. The discrepancy between mRNA and protein expression suggests that post-transcriptional regulatory mechanisms might modify the availability of functional AP-2alpha protein in colorectal carcinoma.