Insulin-like growth factor binding protein-3 mediates IGF-I action in a bovine mammary epithelial cell line independent of an IGF interaction.
IGF-I is mitogenic for the bovine mammary epithelial cell line MAC-T. In addition, IGF-I specifically upregulates IGFBP-3 synthesis in these cells. To investigate this effect on cell growth and IGF-I responsiveness, cell lines were developed that constitutively express IGFBP-3. MAC-T cells transfected with IGFBP-3 (+BP3) or vector alone (Mock) grew similarly over 7 days in 10 or 1% fetal calf serum. Basal DNA synthesis was lower (70%) in +BP3 cells compared to Mock cells. However, DNA synthesis was increased by IGF-I (1-50 ng/ml) relative to untreated controls to a greater extent in +BP3 cells compared to Mock cells. IGF-I (20 ng/ml) increased DNA synthesis 11- and threefold in +BP3 and Mock cells, respectively. Additionally, +BP3 cells were more sensitive to the lower concentrations of IGF-I (1-5 ng/ml). In contrast, preincubation of Mock cells with exogenous IGFBP-3 did not enhance responsiveness or sensitivity to IGF-I. Basal DNA synthesis was unaffected by either an IGF neutralizing antibody or exogenous IGFBP3, indicating the differences observed between +BP3 and Mock cells were not attributable to sequestration of endogenous IGF-I by IGFBP-3. There were no differences between +BP3 and Mock cells in IGF-I receptor number or affinity. DNA synthesis was also increased in +BP3 cells, compared to controls, in response to 5 microg/ml insulin and 2.5 ng/ml Long R(3)IGF-I, indicating that the potentiated response did not require an interaction with IGFBP-3. These results suggest that IGF-I regulation of IGFBP-3 represents a regulatory loop, the function of which is to increase IGF-I bioactivity, using a mechanism that does require an IGF-I-IGFBP-3 interaction.
Grill
C J
CJ
Department of Nutritional Sciences, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901-8520, USA.
Cohick
W S
WS
D000818:Animals; D002417:Cattle; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D004247:DNA / Q000096:biosynthesis; D004847:Epithelial Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D005260:Female; D015870:Gene Expression; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008321:Mammary Glands, Animal / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D017526:Receptor, IGF Type 1 / Q000378:metabolism; D014162:Transfection
D018972:Insulin-Like Growth Factor Binding Protein 3; D007334:Insulin-Like Growth Factor I; D004247:DNA; D017526:Receptor, IGF Type 1