Gene Targets of Insulin-Like Growth Factor-I Receptor (IGF-IR) Signaling and Their Potential Role in Breast Cancer.

Abstract

Abstract BACKGROUND: Insulin-like Growth Factor-I Receptor (IGF-IR) has been implicated in a number of human tumors including breast cancer. IGF-IR is over-expressed and potentially hyper-phosphorylated in breast cancer. The specific gene targets of the IGF-IR signaling pathway, and the role of these genes in the development and progression breast cancer is not known.METHODS: MCF7B breast caner cells, and MCF10A immortalized mammary epithelial cells were treated with either serum-free medium or serum-free medium plus 100 ng/ml IGF-I for 3 hours and 24 hours. RNA was isolated and hybridized to U133A Affymetrix microarrays. Candidate genes were selected based on fold change and p-value. Q-RT-PCR was used to validate expression of potential candidate genes. 5,6-Dichlorobenzimidazole 1-β-D-ribofuranoside (DRB) was used (50µM) to block transcription. Cycloheximide (CHX) (20µg/mL) was used to block translation.RESULTS: Data from the microarray revealed 3673 and 3727 genes that were regulated by IGF-I treatment in MCF7B and MCF10A cells, respectively, at 3 hours. Various genes have been validated by Q-RT-PCR, and of these, Suppressor of Cytokine Signaling 2 (SOCS2) SOCS2 was repressed by IGF-I in MCF7B cells, but induced by IGF-I in MCF10A cells. The repression of SOCS2 by IGF-I was inhibited by pretreatment of MCF7B cells with a small molecule inhibitor of IGF-IR, but not by knockdown of IGF-IR by siRNA. This suggests that the repression of SOCS2 is specific to IGF-I signaling, but may be occurring through IGF-I stimulation of an alternate pathway other than IGF-IR. IGF-I doesn't alter levels of SOCS2 mRNA following blockade of transcription, suggesting that IGF-I does not have an effect on SOCS2 mRNA stability. Furthermore, IGF-I-mediated repression of SOCS2 does not require new protein synthesis, and is thus likely an immediate and direct response.CONCLUSION: SOCS2 is a candidate tumor suppressor gene in breast cancer, with loss of SOCS2 being related to cell proliferation and tumor growth. Here we show that SOCS2 is one of the gene targets of IGF-I signaling and is repressed by IGF-I stimulation in MCF7B breast cancer cells. Furthermore, the repression of SOCS2 by IGF-I appears to be via a direct inhibition of transcription. Together, this suggests that SOCS2 is a direct gene target of IGF-I signaling and that IGF-I repression of SOCS2 may be a crucial event that plays a role in driving the development or progression of breast cancer. Further studies are planned to determine how IGF-I affects SOCS2 function and the role of SOCS2 in breast cancer. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4167.