Bioactive fatty acids: role in bone biology and bone cell function

Malignant human breast tumours contain high levels of prostaglandin E2 (PGE2). However, the mechanisms controlling PGE2 production in breast cancer are unknown. This in vitro study investigates the capacity for PGE2 synthesis and metabolism in several human breast cancer cell lines and early passage human breast fibroblasts and seeks to identify potential regulatory factors which may control these pathways. Basal PGE2 production rose up to 30-fold in breast fibroblast lines on addition of exogenous arachidonic acid (10 microM), whereas no such changes were observed in six out of seven cancer cell lines, with the exception of modest increases in MDA-MB-231 cells. Interleukin 1 beta (IL-1 beta) also induced PGE2 production in breast fibroblasts in the presence of excess substrate, consistent with cyclo-oxygenase induction by the cytokine. Under these conditions only Hs578T cells and MDA-MB-231 cells demonstrated large increases in PGE2 in response to IL-1 beta or phorbol ester; no such responses were seen in MCF-7, T47-D, ZR-75-1, BT-20 or CLF-90-1 cells. In the absence of added arachidonate, bradykinin (BK) and endothelin-1 (ET-1), potentiated PGE2 production in IL-1 beta-treated fibroblasts, possibly by mobilising endogenous substrate. PGE2 also stimulated ET-1 production by breast cancer cells. In co-cultures with T47-D cells both basal and stimulated PGE2 production by breast fibroblasts was greatly reduced. This appeared to be due to metabolic inactivation by the cancer cell since T47-D cells readily converted PGE2 to 15-keto-PGE2. This apparent 15-hydroxy-PG dehydrogenase activity was stimulated by TPA and inhibited by cycloheximide. In conclusion, breast fibroblasts, particularly under the influence of inflammatory mediators, provide a potentially rich source for PGE2 production in breast tumours, whereas significant contributions from the epithelial tumour component may be restricted to cancer cells exhibiting an invasive phenotype. Metabolic inactivation by the cancer cells may also play an important role in the regulation of breast tumour PGE2 levels.

Prostaglandin E2 production and metabolism in human breast cancer cells and breast fibroblasts. Regulation by inflammatory mediators.

Celecoxib: a potent cyclooxygenase-2 inhibitor in cancer prevention.

Cholera toxin (which increases intracellular cAMP levels) significantly (p less than 0.05) increased the growth of MCF-7, T47-D and Hs578T human breast carcinoma cells in vitro. The effect of cholera toxin on growth of MCF-7 and T47-D cells was more pronounced in the presence of 17 beta-estradiol (p less than 0.05), indicating a synergism between cAMP and estradiol in growth control of estrogen-receptor-positive breast carcinoma cells. This interaction was not observed in the estrogen-receptor-negative cell line Hs578T. Daily injections of cholera toxin into female athymic nude mice bearing MCF-7 or Hs578T tumors resulted in significantly (p less than 0.05) increased growth of the tumors. Cholera toxin treatments, in addition, significantly (p less than 0.05) increased cAMP levels in tumor cells and tumor tissue, in vitro and in vivo, respectively. The results of this study clearly demonstrated that an increase in cAMP levels via cholera toxin treatment causes enhanced growth (in vitro and in vivo) of estrogen-receptor-positive and -negative human breast carcinoma cells and, although estrogen alone was not mitogenic to the estrogen-receptor-positive breast carcinoma cells in vitro, the steroid was mitogenic to these cells in the presence of elevated cellular cAMP levels.

Cholera-toxin-enhanced growth of human breast cancer cell lines in vitro and in vivo: interaction with estrogen.

Effect of hormonal status on the expression of the cyclooxygenase 1 and 2 genes and prostaglandin synthesis in rat mammary glands.

Prostaglandin content in 7,12-dimethylbenz[a]anthracene-induced rat mammary tumors was correlated with growth responses to alterations in endogenous estrogen and prolactin. Prostaglandin E2 (PGE2) content varied inversely with tumor latency. Ovariectomy of tumor-bearing Sprague-Dawley rats resulted in regression of tumors in 90% of the castrated rats. PGE2 content in tumors from ovariectomized rats was elevated twofold (P less than 0.05) compared to PGE2 content in tumors from intact controls. Haloperidol treatment promoted tumor growth even after ovariectomy and returned tumor PGE2 values to control levels. Tumors grouped by growth status irrespective of hormone status showed an inverse relationship between PGE2 content and growth response.

Hormone Dependence of 7,12-Dimethylbenz[a]anthracene-Induced Mammary Tumor Growth: Correlation With Prostaglandin E2 Content

The in vivo relationship was studied between these biochemical parameters which previous studies have separately implicated in the regression of hormone-dependent rat mammary tumors. Upon depletion of estrogen and suppression of prolactin levels by ovariectomy, there was a marked increase in the production of prostaglandin E2 (PGE2)(fourfold) and in the cyclic AMP (cAMP) content (twofold) in the regressing 7,12-dimethylbenz[a]anthracene-induced primary tumors. These two parameters appeared to be coupled since, in addition to this correlation, PGE2 stimulated adenylate cyclase and raised cAMP levels in both this primary tumor system and in another hormone-dependent transplantable rat mammary tumor (MTW9-A). Furthermore both the sensitivity of the adenylate cyclase system to PGE2 and the number of membrane binding sites for PGE2 increased upon induction of regression in these tumors. Under conditions where PGE2 and cAMP were elevated, (i.e., in regressing, but not growing, hormone-dependent mammary tumors), there was significant phosphorylation in the intact tissue of a 75,000-dalton nuclear protein, which appeared to be identical to the regression-associated protein shown by Cho-Chung and co-workers to undergo increased phosphorylation in response to elevated cAMP levels.

M. Katherine Foecking

Papers

Hormone Dependence of 7,12-Dimethylbenz[a]anthracene-Induced Mammary Tumor Growth: Correlation With Prostaglandin E2 Content

Concurrent Changes in Growth-Related Biochemical Parameters During Regression of Hormone-Dependent Rat Mammary Tumors