Vol. 4, 697-71/. Marc/i 1998 Clinical Cancer Research 697 Tamoxifen-resistant Fibroblast Growth Factor-transfected MCF-7 Cells Are Cross-Resistant in Vivo to the Antiestrogen IC! 182,780 and Two Aromatase Inhibitors1 Sandra W. McLeskey, Lurong Zhang, Dorraya El-Ashry, Bruce J. Trock, Cecilia A. Lopez, Samir Kharbanda, Christopher A. Tobias, Lori A. Lorant, Rachel S. Hannum, Robert B. Dickson, and Francis G. Kern2 Lombardi Cancer Center [S. W. M., L. Z., B. J. T., C. A. L., S. K.. C. A. T.. R. S. H.. D. E-A.. R. B. D.. F. G. K.l. Departments of Biochemistry and Molecular Biology ID. E-A.. F. G. K.]. Cell Biology [R. B. D.. L. Z.], Medicine [B. J. TI, and Pharmacology [S. W. M.]. and the School of Nursing [S. W. M.], Georgetown University Medical Center, Washington. D. C. 20007 ABSTRACT Although the antiestrogen tamoxifen has been the mainstay of therapy for estrogen receptor (ER)-positive breast cancer, successful treatment of responsive tumors is often followed by the acquisition of tamoxifen resistance. Subsequently, only 30-40% of patients have a positive re- sponse to second hormonal therapies. This lack of response might be explained by mechanisms for tamoxifen resistance that sensitize ER pathways to small amounts of estrogenic activity present in tamoxifen or that bypass ER pathways completely. To elucidate one possible mechanism of tamox- ifen resistance, we treated ovariectomized tumor-bearing mice injected with fibroblast growth factor (FGF)-trans- fected MCF-7 breast carcinoma cells with the steroidal an- tiestrogen ICI 182,780 or one of two aromatase inhibitors, 4-OHA or letrozole. These treatments did not slow estrogen- independent growth or prevent metastasis of tumors pro- duced by FGF-transfected MCF-7 cells in ovariectomized nude mice. FGF-transfected cells had diminished responses to ICI 182,780 in vitro, suggesting that autocrine activity of the transfected FGF may be replacing estrogen as a mito- Received 7/3/97: revised 1 1/26/97; accepted 12/10/97. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertiseinepit in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. I This work was supported by NIH Grants CA50376 (to F. G. K.). CA09218 (to F.G.K. and S.W.M.), CA53185 (to F.G.K. and R. B. D.), CA66154 (to S. W. M.). CA71465 (to D. E-A.), and Cancer Center Grant CA5IOO8; American Cancer Society Grant IRG-193 (to S. W. M.): U.S. Army Medical Research and Material Command Grants DAMD 17-94-4172 (to D. E-A.) and DAMD l7-94-J-4173 (to S. W. M.): and a Susan Komen Foundation Fellowship (to L. Z.). 2 To whom requests for reprints should be addressed, at Southern Research Institute, P. 0. Box 55305, 2000 Ninth Avenue South, Bir- mingham. AL 35255-5305. Phone: (205)581-2480: Fax: (205)581- 2877: E-mail: kern@sri.org. genic stimulus for tumor growth. ER levels in FGF trans. fectants were not down-regulated, and basal levels of tran- scripts for estrogen-induced genes or of ER-mediated transcription of estrogen response element (ERE) luciferase reporter constructs in the FGF expressing cells were not higher than parental cells, implying that altered hormonal responses are not due to down-regulation of ER or to FGF- mediated activation of ER. These studies indicate that estro- gen independence may be achieved through FGF signaling pathways independent of ER pathways. If so, therapies di- rected at the operative mechanism might produce a thera- peutic response or allow a response to a second course of antiestrogen treatment. INTRODUCTION Because conventional therapy is not usually curative in clinical breast cancer, development of tamoxifen resistance. in which breast tumors previously growth-inhibited by tamoxifen become refractory, represents an important therapeutic di- lemma. However, the development of tamoxifen resistance is not necessarily associated with progression to an ER3-negative phenotype. In many cases of clinical tamoxifen resistance, ER expression may be retained ( 1-4), implying that the resistance is due an alteration in activity of the tamoxifen/ER complex. Tamoxifen resistance in such a case could result from three possible mechanisms that, according to present knowledge, would not preclude successful treatment with an alternative hormonal therapy. First. alterations in the ER could arise, which might diminish or extinguish inhibitory responses to tamoxifen, leaving only its partial agonist effects to predominate (5-8). Second, tamoxifen resistance arising in the setting of an intact ER could be a result of altered intratumoral tamoxifen metab- olism, which might produce more estrogenic metabolites locally (7, 9-1 1). Third, available tamoxifen could be sequestered by an increase in antiestrogen binding sites not associated with ERs (12). As mentioned, in each of these three instances. substitution of a hormonal therapy different from tamoxifen might result in a clinical response. Two such alternative therapies used in this report are steroidal estrogen antagonists, such as IC! 182,780, which lack the partial agonist activity of tamoxifen, and aro- matase inhibitors, which inhibit endogenous estrogen produc- tion by all tissues, depriving the ER of its bigand. Although the mechanisms of tamoxifen resistance de- 3 The abbreviations used are: ER. estrogen receptor; FGF. fibroblast growth factor: IMEM, improved minimal essential medium: X-gal, 5-bromo-4-chloro-3-indoyl- 3-D-galactopyranoside: FBS. fetal bovine serum: 4-OHA. 4-hydroxyandrostenedione: NK, natural killer: CCS. charcoal-stripped calf serum: ERE. estrogen response element: CAT. chloramphenicol acetyltransferase: RT. reverse transcription. Research. on November 29, 2021. © 1998 American Association for Cancer clincancerres.aacrjournals.org Downloaded from