Induction of insulin-like growth factor binding protein expression by ICI 182,780 in a tamoxifen-resistant human breast cancer cell line.
Earlier studies in our laboratory demonstrated that the steroidal antiestrogen ICI 182,780 is very effective in abolishing the tamoxifen-resistant proliferation of MCF 7/5-23 cells. In addition, preliminary binding studies showed that ICI 182,780 increased the binding of insulin-like growth factor (IGF)-I to the MCF 7/5-23 cells, although this finding was not the result of an increase in the expression of the insulin-like growth factor-I receptor (IGF-IR). Hence, we reasoned that the inhibition of tamoxifen-resistant cell growth by ICI 182,780 might have been due to increased expression of insulin-like growth factor binding proteins (IGFBPs). We observed the up-regulation of non-insulin-suppressible IGF-I binding in both the tamoxifen-sensitive MCF 7/5-21 cell line (1.5-fold) and the tamoxifen-resistant MCF 7/5-23 cell line (2.5-fold) after 5 days of treatment with ICI 182,780 (10(-7) M) in serum-free medium, suggesting a role for cell-associated IGFBPs. Affinity cross-linking experiments confirmed the presence of an IGF-I:IGFBP complex of approximately 38-kDa in tamoxifen or ICI 182,780-treated cells. Western ligand blots showed higher levels of a soluble 30-kDa IGFBP in media conditioned by either of the subclones that had been treated with ICI 182,780, an effect consistently opposed by estrogen (E2: 10(-9) M). RT-PCR showed higher levels of IGFBP-5 mRNA than any of the other known IGFBPs, suggesting that this was the major IGFBP subtype. The protein was subsequently identified by Western immunoblotting as IGFBP-5. In conclusion, we postulate that this may be a mechanism contributing to the greater potency of ICI 182,780 in the growth inhibition of the MCF 7/5-23, tamoxifen-resistant cell line.
Parisot
J P
JP
Division of Haematology & Medical Oncology, Peter MacCallum Cancer Institute, Melbourne, Australia.
Leeding
K S
KS
Hu
X F
XF
DeLuise
M
M
Zalcberg
J R
JR
Bach
L A
LA
D000970:Antineoplastic Agents / Q000494:pharmacology*; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism*; D019008:Drug Resistance, Neoplasm; D004958:Estradiol / Q000031:analogs & derivatives* / Q000494:pharmacology; D004965:Estrogen Antagonists / Q000494:pharmacology*; D004967:Estrogens / Q000494:pharmacology; D000077267:Fulvestrant; D006801:Humans; D018969:Insulin-Like Growth Factor Binding Proteins / Q000096:biosynthesis*; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis / Q000378:metabolism; D007457:Iodine Radioisotopes; D011485:Protein Binding; D012333:RNA, Messenger / Q000096:biosynthesis / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013629:Tamoxifen / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D000970:Antineoplastic Agents; D004965:Estrogen Antagonists; D004967:Estrogens; D018969:Insulin-Like Growth Factor Binding Proteins; D007457:Iodine Radioisotopes; D012333:RNA, Messenger; D013629:Tamoxifen; D000077267:Fulvestrant; D004958:Estradiol; D007334:Insulin-Like Growth Factor I