Differential activation of IGF-II promoters P3 and P4 in Caco-2 cells during growth and differentiation

Insulin-like growth factor (IGF)-II gene is overexpressed in colon cancers. Transcriptional up-regulation may be the major mechanism contributing to its overexpression. IGF-II messenger RNA (mRNA) levels are up-regulated during proliferation followed by a significant decline during differentiation of Caco-2 cells. Mechanisms underlying transcriptional regulation of the IGF-II gene promoters (P1-P4) have yet to be examined in colon cancers, which was the basis for this study.

Ribonuclease protection assay was used to measure IGF-II mRNA derived from P1-P4. To determine if changes in the IGF-II transcripts reflected differences in promoter activity, transient transfection assays with the full-length P1-P4-luciferase expression vectors were performed.

Both P3- and P4-derived transcripts were significantly up-regulated during the proliferative phase of the cells (days 3-6 in culture) and declined rapidly in cells undergoing differentiation (days 7-10); conversely, P1- and P2-derived transcripts were not detected. Similarly, transcriptional activity of P3 and P4 promoters reached peak levels by days 4-6 and declined rapidly thereafter. P1 and P2 were relatively inactive on all days.

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 cells.