The regulation of insulin-like growth factor binding protein-5 production and secretion by ovarian hormones in human breast cancer cells

• Main Author: Turk, Carolina Nicole
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2008
• Subjects: Cellular biology; Molecular biology
• ISBN: 9781109035322; 1109035322

Insulin-like growth factor-I (IGF) is a regulator of cell survival in mammary epithelial cells (MEC). The bioactivity of IGF-I is modulated by six IGF-binding proteins (IGFBP). Of these, IGFBP-5 has high affinity for IGFs and induces apoptosis during involution of the mammary gland via its increased local expression. Paradoxically, expression of IGFBP-5 is increased in breast cancer cell lines and in breast cancer, raising important questions about its regulation and biological function(s). We examined the distribution of IGFBP-5 in the normal human breast and in estrogen receptor (ER) progesterone receptor (PR) positive and negative (ER+/PR+ and ER-/PR-) breast tumors by immunohistochemistry. In normal breast tissue, IGFBP-5 localized to the MEC and was also accumulated at the basolateral region of epithelial cells as determined by colocalization with the myoepithelium. The staining intensity for IGFBP-5 in ER+/PR+ tumors was greater than in ER-/PR- tumors. Microarray analysis of T47D breast cancer cells suggested that IGFBP-5 gene expression was downregulated in the presence of progesterone (P). As confirmed by qRT-PCR, P suppressed IGFBP-5 mRNA levels in T47D cells and downregulated the transcription of IGFBP-5 via the proximal promoter in a reporter assay. Conversely, serum- and estrogen (E)-induced proliferation of T47D cells was associated with increased secretion of IGFBP-5 while its intracellular production remained constant. Furthermore, IGFBP-5 mRNA expression was also upregulated by hormone-induced cellular proliferation in ovariectomized, E-treated pigs. A relationship between hormone-modulated cell proliferation and IGFBP-5 production was found to be consistent with increased transcription of IGFBP-5 mRNA in T47D cells during cell cycle progression. Finally, we successfully created a T47D cell line stably expressing shRNA against IGFBP-5. Our data indicate that IGFBP-5 production in the normal mammary gland and breast cancer is inconsistent with an exclusive pro-apoptotic function and that its production and secretion is upregulated during hormone-mediated cellular proliferation and tumor progression.