Regulation of the Insulin-like Growth Factor 1-Secretory Clusterin Expression Axis in Genomic Instability and Cell Stress
Secretory clusterin (sCLU) is a pro-survival factor that is up-regulated in human tumors and after exposure to cell stress. Understanding the regulation of sCLU expression in cancer, and after exposure to therapeutic agents, could reveal new therapeutic targets for cancer treatment. A DNA damage induced signaling cascade leading from ATM to sCLU expression mediated by IGF-1/IGF-1R/MAPK activation was uncovered. IGF-1 ligand promoter activity, mRNA, and protein expression induced after exposure to ionizing radiation (IR), hydrogen peroxide, or topoisomerase I and II-alpha poisons matched sCLU expression. Elevated basal IGF-1-sCLU signaling was noted in genomically unstable cells, whether they were deficient in DNA repair factors or telomerase function. ATM function was necessary for induction of sCLU after IR, and for maintaining elevated expression of sCLU in genomically unstable cells. p53 suppressed IGF-1 promoter activity, leading to decreased mRNA and protein expression, and abrogated induction of IGF-1 and sCLU by IR. Loss of p53 by knockdown or knockout enhanced IGF-1 and sCLU induction. Mutations in the p53 DNA binding domain found in cancer did not repress IGF-1 and sCLU. An NF-Y binding site in the IGF-1 promoter was essential for p53 suppression, and both p53 and NF-YA bound to the IGF-1 promoter. Nutlin-3, an Mdm2-p53 inhibitor, stabilized p53 expression, leading to dramatically decreased sCLU expression. Nutlin-3 treatment sensitized wild-type p53 cells to IR exposure. Finally, exogenous IGF-1 exposure led to serine 1981 auto-phosphorylation of ATM, and enhanced DNA damage repair and abrogated cell death after IR exposure. These studies uncovered key molecules important for the regulation of IGF-1-sCLU expression axis after IR exposure, and supported the use of IGF-1 or sCLU expression inhibitors for cancer chemotherapy.