Control of the membrane sex hormone-binding globulin-receptor (SHBG-R) in MCF-7 cells: effect of locally produced SHBG.
The interaction between plasma sex hormone-binding globulin (SHBG) and its receptor (SHBG-R) inhibits estradiol-induced proliferation of MCF-7 cells (human estrogen-dependent breast cancer) through cAMP and PKA. Thus, SHBG can modulate estradiol action in breast cancer, but the implications of this require a more detailed knowledge of the SHBG-R. To this end, we have transfected MCF-7 cells with an expression vector carrying the human SHBG cDNA (S-MCF-7) and studied the effects of this on both SHBG-R binding and cell proliferation. Control cells were parental MCF-7 (P-MCF-7) and MCF-7 cells transfected with the beta-galactosidase gene (B-MCF-7). Transfections were mediated by lipofectin followed by selection of transfected cells with G418. The amounts of SHBG in culture medium were evaluated by IRMA assay, with only S-MCF-7 cells shown to secrete SHBG; SHBG-R levels were evaluated by tracer binding technique. In P-MCF-7 and B-MCF-7 cells, SHBG-R was detectable as a two-binding site receptor, but no binding of SHBG was observed in S-MCF-7 cells. Proliferation of cells treated with estradiol was evaluated by [3H]thymidine incorporation in the three cell lines and in cells pretreated with SHBG (1 nM) purified from human serum or with conditioned medium from S-MCF-7 cells (medium S). In all three lines, cell proliferation increased after estradiol treatment. Preincubation with purified SHBG was effective in reducing estrogen-induced cell proliferation to basal levels in P-MCF-7 and B-MCF-7 but not in S-MCF-7 cells. The estradiol effect was also inhibited in P-MCF-7 cells treated with medium S. In conclusion, 1) SHBG inhibits estradiol-induced proliferation in cells containing a functional SHBG-R, whereas it has no detectable effect in cells in which the SHBG-R is either absent or not available to bind SHBG; and 2) S-MCF-7 cells are insensitive to SHBG (locally produced or exogenous) because their SHBG-R is occupied by SHBG.
Fortunati
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N
Laboratorio di Endocrinologia, Azienda Ospedaliera S.Giovanni B.ta, Torino, Italy. fortunat@iol.it
Raineri
M
M
Cignetti
A
A
Hammond
G L
GL
Frairia
R
R
D001943:Breast Neoplasms / Q000235:genetics / Q000473:pathology; D002455:Cell Division / Q000502:physiology; D004967:Estrogens / Q000502:physiology; D006801:Humans; D011956:Receptors, Cell Surface / Q000502:physiology*; D012738:Sex Hormone-Binding Globulin / Q000494:pharmacology / Q000502:physiology*; D014162:Transfection; D014407:Tumor Cells, Cultured
D004967:Estrogens; D011956:Receptors, Cell Surface; D012738:Sex Hormone-Binding Globulin; C063572:sex hormone-binding globulin receptor