DIFFERENTIAL ACTIVATION OF IGF-II PROMOTERS P3 AND P4 IN CACO-2 CELLSDURING GROWTH AND DIFFERENTIATION

Background & Aims: Insulin-like growth factor (IGF)-II gene is overexp ressed in colon cancers, Transcriptional up-regulation may be the majo r mechanism contributing to its overexpression. IGF-II messenger RNA ( mRNA) levels are up-regulated during proliferation followed by a signi ficant decline during differentiation of Caco-2 cells. Mechanisms unde rlying transcriptional regulation of the IGF-II gene promoters (P1-P4) have yet to be examined in colon cancers, which was the basis for thi s study, Methods: Ribonuclease protection assay was used to measure IG F-II mRNA derived from P1-P4. To determine if changes in the IGF-II tr anscripts reflected differences in promoter activity, transient transf ection assays with the full-length P1-P4-luciferase expression vectors were performed. Results: Both P3- and P4-derived transcripts were sig nificantly up-regulated during the proliferative phase of the cells (d ays 3-6 in culture) and declined rapidly in cells undergoing different iation (days 7-10); conversely, P1- and P2-derived transcripts were no t detected. Similarly, transcriptional activity of P3 and P4 promoters reached peak levels by days 4-6 and declined rapidly thereafter, P1 a nd P2 were relatively inactive on all days. Conclusions: The activity of the P3 and P4 promoters may play a selective role in regulating IGF -II mRNA levels during growth and differentiation of colon cancer cell s.