IGF-I and IGF-II expression in human breast cancer xenografts: Relationship to hormone independence

Although the growth of some estrogen receptor (ER) positive breast cancers can initially be hormonally manipulated, all will eventually escape hormonal control. It is possible that the expression of polypeptide growth factors is initially under the control of steroid hormones, while the hormone unresponsive state is characterized by constitutive expression of growth factors. We studied the relationship between hormone responsiveness and IGF expression in xenograft models. The ER+ T61 xenograft was established from a primary breast cancer and has been continually passaged in athymic mice. ER+ MCF-7 cells and ER-MDA-MB-231 cells were grown in tissue culture and then inoculated into athymic mice. ER+ xenograft growth was regulated by estrogen, but with opposite results - T61 xenografts are inhibited by estrogen, while MCF-7 xenografts require estrogen for tumor formation. All xenografts expressed type I and II IGF receptors. Although T61 xenografts also express an alternatively spliced IGF-I mRNA, its expression was not regulated by estrogen. Both xenografts expressed IGF-II in a hormonally regulated manner - T61 levels were depressed by estrogen, while MCF-7 levels were increased. Thus, in these model systems, xenograft regulation of tumor growth is accompanied by parallel changes in IGF-II expression. In the estrogen independent MDA-MB-231 cells, IGF-II was constitutively expressed. These data show that IGF-II expression correlates with estrogen treatment, suggesting that autocrine expression of IGF-II may mediate estrogen-regulated cell growth.