REGULATION OF ERCC-1 MRNA EXPRESSION BY INSULIN

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Summary of work: The notion that hormones and growth factors may control expression of DNA repair genes in mammalian cells is of great interest because impairment in DNA repair may result in aberrant gene expression leading to degenerative disorders and cell death. In the first study, we investigated whether the expression of ERCC1, a member of the nucleotide excision repair (NER) family, was under the control of insulin, and, if so, determined the signal transduction pathway(s) responsible for this action of insulin. It was found that insulin enhanced 2 to 3-fold the expression of ERCC1 mRNA both in Chinese hamster ovary (CHO) cells overexpressing human insulin receptors and in fully-differentiated 3T3-L1 adipocytes, a cell model known for its high responsiveness to insulin. Using a variety of pharmacological tools known to inhibit distinct signaling pathways, we demonstrated that insulin enhanced ERCC1 mRNA levels by activation of the Ras-ERK-dependent pathway without involvement of the phosphatidylinositol 3'-kinase/pp70 S6 kinase. There is a correlation between expression of DNA repair genes and cell survival, as evidenced by the fact that defects in repair capacity sharply increase cellular sensitivity to a variety of lesions to DNA. We have found that treatment of CHO cells with insulin-like growth factor 1 (IGF-1) led to increased expression of ERCC1 and XPB/ERCC3, a gene product that encodes a subunit of the basal transcription factor bTFIIH, which is implicated in NER and in cell cycle regulation. In light of the fact that IGF-1 has been recently identified as an antiapoptotic agent, we examined the possibility that activation of the IGF-1 receptor may offer protection against apoptotic cell death as a result of its role in NER. Apoptosis or programmed cell death plays a major role in normal turnover of cells. Wild-type and repair-deficient CHO cells with specific defects in ERCC1 or XPB/ERCC3 were used in our study, and the results indicated that the protective effect of IGF-1 did not require functional NER. Furthermore, it appears that IGF-1's ability to confer protection against apoptotic death was dependent on phosphatidylinositol 3' kinase activation. The possibility remains that other DNA lesion-repair mechanisms, which include mismatch repair and DNA glycosylases, may be required in mediating the antiapoptotic function of IGF-1. These studies have led to the publication of two papers in first-rate peer-reviewed journals.