Estrogen mitogenic action. I. Demonstration of estrogen-dependent MTW9/PL2 carcinogen-induced rat mammary tumor cell growth in serum-supplemented culture and technical implications

Theories of carcinogenesis: an emerging perspective

Four decades ago Leslie Foulds remarked that "Experimental analysis has produced an alarming mass of empirical facts without providing an adequate language for their communication or effective concepts for their synthesis". Examining the relevance of the data avalanche we all generate and are subjected to in the context of the premises and predictions of the current cancer theories may help resolve this paradox. This goal is becoming increasingly relevant given the looming attempts to rigorously model and parameterize crucial events in carcinogenesis (microenvironmental conditions, cellular proliferation and motility), which will require the adoption of reliable premises on which to base those efforts. This choice must be made a priori, as premises are not testable, and data are not free of the theoretical frame used to gather them. In this review we provide a critical analysis of the two main currents in cancer research, one centered at the cellular level of biological organization, the somatic mutation theory, which conceptualizes carcinogenesis as a problem of cell proliferation control, and the other centered at the tissue level, the tissue organization filed theory, which considers carcinogenesis a process akin to organogenesis gone awry.

Estrogen-like effects of ultraviolet screen 3-(4-methylbenzylidene)-camphor (Eusolex 6300) on cell proliferation and gene induction in mammalian and amphibian cells

We tested the ultraviolet screen 3-(4-methylbenzylidene)-camphor (4-MBC; Eusolex 6300), which has been implicated as a potential endocrine disruptor, for its potential to bind to and activate endogenous estrogen receptors (ER) and to mediate ER-dependent changes in gene transcription, in hepatocytes of the water-dwelling South African clawed frog Xenopus laevis. We were able to confirm previous findings that 4-MBC accelerates cell proliferation in estrogen-dependent human breast cancer cells (MCF-7). Results of competitive binding assays of [3H]17beta-estradiol and 4-MBC using cytosolic protein preparations from Xenopus hepatocytes indicated that 4-MBC weakly binds to the ER. 4-MBC at a concentration of 100 micromol/L is not able to completely replace estradiol from the receptor. However, when 4-MBC was tested in a gene induction assay using the relative amount of ER transcript as a marker for ER-dependent transcriptional activation, we found that micromolar concentrations of this substance produced an increase in the amount of ER mRNA that was not different from the amount of mRNA that was observed upon activation of cells with 17beta-estradiol in concentrations above 1 nmol/L. The results indicate that 4-MBC has the potential to change physiological and developmental processes mediated by ER signaling mechanisms. It may therefore be a potentially harmful substance for water-dwelling animals when present in the environment at micromolar concentrations.

Establishment of two hormone-responsive mouse mammary carcinoma cell lines derived from a metastatic mammary tumor

We report the establishment of two mouse mammary cancer cell lines, MC7-2A and MC7-2B obtained from a mouse mammary carcinoma induced by medroxyprogesterone acetate (MPA) and maintained by syngeneic transplantation in BALB/c mice. They are epithelial (express cytokeratins) and express both estrogen receptors alpha (ERalpha) and progesterone receptors (PRs) isoforms A and B (western blots). In vitro, MPA inhibited 3H-thymidine uptake, starting from concentrations as low as 10(-13) M in MC7-2A and 10(10) M in MC7-2B; the antiprogestin RU 486 exerted a stimulatory effect at 10(-14) M in both cell lines; 17-beta-estradiol (E2) also exerted a stimulatory effect starting at 10(-10) M in MC7-2A and at 10(-13) M in MC7-2B. When transplanted in syngeneic mice, both cell lines originated adenocarcinomas that gave rise to lung metastases within 3 months. In in vivo studies, in MC7-2A, the antiprogestin inhibited completely tumor growth, E2 induced a slight although significant ( p < 0.05) stimulatory effect and MPA stimulated tumor growth while MC7-2B cells were unresponsive to all treatments. ER and PR were also expressed in tumors as assessed by immunohistochemistry. Two marker chromosomes were identified by FISH as translocations between chromosomes 4 and 7, and between chromosomes X and 2; the third marker chromosome remains unidentified. All these markers were also present in the parental tumor. A new marker, a centric fusion of chromosomes 2, was acquired in both cell lines. Considering that there are very few murine breast carcinoma responsive cell lines, these cells represent new tools in which the regulatory effect of hormones can be studied.

Estrogen mitogenic action. ii. negative regulation of the steroid hormone-responsive growth of cell lines derived from human and rodent target tissue tumors and conceptual implications

In an accompanying report (Moreno-Cuevas, J. E.; Sirbasku, D. A., In Vitro Cell. Dev. Biol.; 2000), we demonstrated 80-fold estrogen mitogenic effects with MTW9/PL2 rat mammary tumor cells in cultures supplemented with charcoal-dextran-treated serum. All sera tested contained an estrogen reversible inhibitor(s). The purpose of this report is to extend those observations to additional sex steroid-responsive human and rodent cell lines. Every line tested showed a biphasic response to hormone-depleted serum. Concentrations of < or = 10% (v/v) promoted substantive growth. At higher concentrations, serum was progressively inhibitory. With estrogen receptor-positive (ER+) human breast cancer cells, rat pituitary tumor cells, and Syrian hamster kidney tumor cells, 50% (v/v) serum caused significant inhibition, which was reversed by very low physiologic concentrations of estrogens. This same pattern was observed with the steroid hormone-responsive LNCaP human prostatic carcinoma cells. Because steroid hormone mitogenic effects are now easily demonstrable using our new methods, the identification of positive results has nullified our original endocrine estromedin hypothesis. We also evaluated autocrine/paracrine growth factor models of estrogen-responsive growth. We asked if insulin-like growth factors I and II, insulin, transforming growth factor alpha, or epidermal growth factor substituted for the positive effects of estrogens. Growth factors did not reverse the serum-caused inhibition. We asked also if transforming growth factor beta (TGFP) substituted for the serum-borne inhibitor. TGFbeta did not substitute. Altogether, our results are most consistent with the concept of a unique serum-borne inhibitor as has been proposed in the estrocolyone model. However, the aspect of the estrocolyone model related to steroid hormone mechanism of action requires more evaluation. The effects of sex steroids at picomolar concentrations may reflect mediation via inhibitor "activated" intracellular signaling pathways.

Estrogen mitogenic action. III. is phenol red a "red herring"?

The reported estrogenic action of phenol red and/or its lipophilic contaminants has led to the widespread use of indicator-free culture medium to conduct endocrine studies in vitro. Because we have recently developed methods to measure large-magnitude estrogen effects in the tissue culture medium containing phenol red, we concluded that the indicator issue required further evaluation. To do this, we selected nine estrogen receptor positive (ER+) cell lines representing four target tissues and three species. We investigated phenol red using five different experimental protocols. First, 17beta-estradiol (E2) responsive growth of all nine ER+ cells lines was compared in the medium with and without the indicator. Second, using representative lines we asked if phenol red was mitogenic in the indicator-free medium. The dose-response effects of phenol red were compared directly to those of E2. Third, we asked if tamoxifen-inhibited growth equally in phenol red-containing and indicator-free medium. This study was based on a report indicating that antiestrogen effects should be seen only in phenol red-containing medium. Fourth, we asked if phenol red displaced the binding of 3H-E2 using ERK intact human breast cancer cells. Fifth, we compared E2 and phenol red as inducers of the progesterone receptor using a human breast cancer cell line. All the experiments presented in this report support the conclusion that the concentration of phenol red contaminants in a standard culture medium available today is not sufficient to cause estrogenic effects. In brief, our studies indicate that the real issue of how to demonstrate estrogenic effects in culture resides elsewhere than phenol red. We have found that the demonstration of sex steroid hormone-mitogenic effects in culture depends upon conditions that maximize the effects of a serum-borne inhibitor(s). When the effects of the inhibitor are optimized, the presence or absence of phenol red makes no everyday difference to the demonstration of estrogen mitogenic effects with several target cell types from diverse species.