Transcriptional regulation of insulin-like growth factor-I receptor gene expression in prostate cancer cells

A marked decrease in the type 1 insulin-like growth factor (IGF) receptor ( IGF-IR) occurs in prostate epithelial cells during transformation from the benign to the metastatic state. One of the principal regulators of IGF-IR g ene expression, the WT1 tumor suppressor, is expressed in prostate cancer a nd in prostate cancer cell lines. The purpose of this study was to determin e whether the decrease in IGF-IR expression was transcriptionally regulated , and whether WT1 action may be involved in the repression of the IGF-IR ge ne in prostate cancer cells. The P69 cell line was derived by immortalizati on of human primary prostate epithelial cells with simian virus-40 T antige n and is rarely tumorigenic, The M12 line was derived from the P69 line by selection for tumor formation in nude mice and is tumorigeneic and metastat ic. P69 cells express 20,000 IGF-IR/cell, whereas M12 cells express 3,500 I GF-IR/cell. These differences in receptor number are reflected in proportio nal differences in IGF-IR mRNA levels. To assess IGF-IR promoter activity i n these cell lines, each was transiently transfected with luciferase report er vectors containing the IGF-IR gene transcription start site and 476 bp o f 5'-flanking sequence, 640 bp of 5'-untranslated region sequence, or both regions. The promoter activity of the full-length construct was 50% lower ( P < 0.01) in M12 cells compared with P69 cells, the activity of the 5'-flan king region construct was 53% lower (P < 0.0001), and that of the 5'-untran slated region construct was 36% lower (P = 0.01). P69 clones stably transfe cted with a WT1 expression vector exhibited decreased expression of the end ogenous IGF-IR gene and decreased promoter activity in transient transfecti on assays with IGF-IR promoter constructs containing multiple WT1 binding s ites. The observed reduction in endogenous IGF-IR expression was sufficient to inhibit IGF-I-stimulated cell proliferation. These data suggest that mo st of the decreased expression of the IGF-IR seen in malignant prostate epi thelium is the result of transcriptional repression of the IgF-IR gene, and that this repression may be due in part to the increased expression of the WT1 tumor suppressor in metastatic prostate cancer.