Synergistic interaction of growth factors and albumin in regulating estradiol synthesis in breast cancer cells

Insulin-like growth factor I enhances the expression of aromatase P450 by inhibiting autophagy

Aromatase, a key enzyme of estrogen biosynthesis, is transcriptionally regulated by many growth factors. IGF-I enhances aromatase activity in a variety of cells, but the mechanism of action has not been determined. We herein report our finding of a novel mechanism of action for IGF-I. IGF-I enhanced the dexamethasone (DEX)-induced aromatase activity by 30% in serum-starved THP-1 cells. The increase was associated with a corresponding increase in the level of aromatase protein but not with any change in the mRNA level. Metabolic labeling experiments revealed that IGF-I inhibited the degradation of aromatase. We identified pepstatin A as the most effective inhibitor of aromatase degradation by in vitro assay. Using a nontoxic concentration of pepstatin A, we examined IGF-I's action on aromatase distribution in microsomes and lysosomes. In the presence of pepstatin A, DEX caused an increase in the amount of aromatase in both microsomes and lysosomes, and IGF-I attenuated the DEX-induced accumulation of aromatase in lysosomes and, conversely, enhanced its accumulation in the microsomes. The addition of serum abolished the IGF-I-induced changes. The transport from microsome to lysosome was fluorescently traced in cells using a recombinant aromatase. IGF-I selectively reduced the aromatase signal in the lysosomes. Finally, we observed that IGF-I enhanced the aromatase activity by 50% as early as 1 h after treatment; furthermore, rapamycin, an enhancer of autophagy, completely negated the effect of IGF-I on the enzyme. These results indicate that IGF-I enhances aromatase by the inhibition of autophagy.

The regulation and inhibition of 17beta-hydroxysteroid dehydrogenase in breast cancer

17Beta-hydroxysteroid dehydrogenase Type 1 (17beta-HSD1) has a pivotal role in regulating the synthesis of oestradiol (E2) within breast tumours. In whole body studies in postmenopausal women with breast cancer the conversion of oestrone (E1) to E2 (4.4+/-1.1%) was much lower than the inactivation of E2 to E1 (17.3+/-5.0%). In contrast, an examination of in vivo oestrogen metabolism within breast tumours revealed that whereas little metabolism of E2 occurred, E1 was converted to E2 to a much greater extent in malignant (48+/-14%) than in normal (19+/-6%) breast tissue. Findings from these studies originally suggested that oestrogen metabolism within breast tumours may differ from the mainly oxidative direction found in most other body tissues and that the activity of 17beta-HSD1 might be regulated by tumour-derived factors. Several growth factors (e.g. IGF-I, IGF-II) and cytokines (e.g. IL-6, TNFalpha) have now been identified which can markedly stimulate the activity of 17beta-HSD1 and such a mechanism may account for the high concentrations of E2 found in most breast tumours. Cells of the immune system, which can infiltrate breast tumours, are thought to be a major source of the growth factors and cytokines which can modulate 17beta-HSD1 activity. Given the central role that 17beta-HSD1 has in regulating breast tumour E2 concentrations the development of potent inhibitors of this enzyme has recently attracted considerable attention. Our initial studies in this area explored the use of derivatives of E1 as inhibitors, with 2-ethyl- and 2-methoxy E1 being found to inhibit 17beta-HSD1 activity in T-47D breast cancer cells by 96+/-2 and 91+/-1% respectively at 10 microM, but with a lack of specificity. Using the E1 scaffold a number of potent, selective 17beta-HSD1 inhibitors have now been identified including E1- and 2-ethyl-E1 containing a side chain with a m-pyridylmethylamidomethyl functionality extending from the 16beta position of the steroid nucleus. At 10 microM these compounds both inhibited 17beta-HSD1 activity by >90%, however some inhibition of 17beta-HSD2 activity was exhibited by the E1 derivative (25%) but not the 2-ethyl analogue. It is now apparent that 17beta-HSD1 activity contributes to the high E2 concentrations found in most breast tumours. The identification of potent, selective novel 17beta-HSD1 inhibitors will allow their efficacy to be tested in in vitro and in vivo studies.

Characterization of a plasma membrane-resident albumin-binding protein associated with the proliferation of estrogen-target, serum-sensitive cells

Estrogens control the proliferation of their target cells through a receptor-mediated pathway. Recently presented evidence suggests that estradiol cancels the proliferative inhibition exerted by human albumin (HA) and recombinant human albumin (rHA) on estrogen-target serum-sensitive cells (indirect-negative hypothesis). We postulate that this mechanism requires the presence of a plasma membrane estrogen receptor (mER) and a plasma membrane albumin-binding protein (mABP). Direct evidence confirming the presence of mERalpha in MCF7 cells has recently been presented. Herein, we now show that Western blot analysis of purified T47D membrane proteins with the C542 ERalpha specific monoclonal antibody also revealed specific, multiple M(r) mERs (67, 110, and 130k M(r)). In addition, Western blot analysis with an ABP antiserum revealed a potential 60k M(r) ABP in both MCF7 and T47D plasma membrane extracts. No such evidence was observed in similar extracts from ER-negative, serum-insensitive MDA-MB231 cells. Ligand blot analysis of similar plasma membrane extracts with bovine serum albumin confirmed the presence of a 60k M(r) ABP in MCF7 and T47D cells; again, no such evidence was observed in comparable extracts from MDA-MB231 cells. Fluorescence and confocal microscopy of MCF7 cells fixed in 2.0% paraformaldehyde/0.1% glutaraldehyde identified specific membrane ABP antigenic sites by immunocytochemistry. Serum-insensitive MDA-MB231 cells fixed and labeled similarly did not exhibit this mABP. These results suggest that the proposed mABP is expressed only in serum-sensitive estrogen-target cells and is not expressed in cells insensitive to the proliferative inhibition of HA and rHA. Also, the present data suggest that the proposed mABP may be the recognition mechanism by which both HA and rHA inhibit MCF7 and T47D cell proliferation.

The role of cytokines in regulating estrogen synthesis: implications for the etiology of breast cancer

Cytokines, such as IL-6 and tumor necrosis factor (TNF)-alpha, have an important role in regulating estrogen synthesis in peripheral tissues, including normal and malignant breast tissues. The activities of the aromatase, estradiol 17beta-hydroxysteroid dehydrogenase and estrone sulfatase are all increased by IL-6 and TNF-alpha. Prostaglandin E2 may also be an important regulator of aromatase activity in breast tumors. Macrophages and lymphocytes, which invade many breast tumors, are thought to be an important source of factors that can stimulate estrogen synthesis in malignant breast tissues. The co-ordinated stimulation of the activities of the enzymes that are involved in estrogen synthesis offers an explanation for the high concentrations of estrogens that are present in breast tumors.

The role and proposed mechanism by which oestradiol 17 beta-hydroxysteroid dehydrogenase regulates breast tumour oestrogen concentrations

Synthesis of the biologically active oestrogen, oestradiol, within breast tumours makes an important contribution to the high concentrations of oestrogens which are present in malignant breast tissues. In breast tumours, oestrone is preferentially converted to oestradiol by the Type I oestradiol 17 beta-hydroxysteroid dehydrogenase (E2DH). Several growth factors, such as insulin-like growth factor Type I, and cytokines, such as Tumour Necrosis Factor alpha (TNF alpha), have been shown to stimulate E2DH activity in MCF-7 breast cancer cells. As little is known about the regulation of Type I E2DH expression and activity in other breast cancer cell lines, the expression and activity of this enzyme was examined in other oestrogen receptor positive and also oestrogen receptor negative breast cancer cell lines. As it is possible that E2DH activity may be limited by co-factor availability, the effects of exogenous co-factors on enzyme activity in these cell lines was also investigated. For T47D and BT20 breast cancer cells, the addition of exogenous co-factors was found to enhance enzyme activity. TNF alpha, in addition to stimulating E2DH activity in MCF-7 cells, also increased activity in T47D and MDA-MB-231 cells, although to a lesser extent than in MCF-7 cells. An investigation of signalling pathways involved in the regulation of E2DH activity revealed that stimulation of both the protein kinase C (PKC) and PKA pathways may be involved in regulation of E2DH activity. As several growth factors and cytokines have now been found to be involved in regulating E2DH activity, the role that macrophages and lymphocytes have in supplying these factors and the mechanism by which these factors may stimulate tumour growth, is also reviewed.

The role of cytokines and sulphatase inhibitors in regulating oestrogen synthesis in breast tumours

Synthesis of oestrogens within breast tissues makes an important contribution to the high concentrations of oestradiol which are found in breast tumours. The activities of the enzymes involved in oestrogen synthesis, i.e. the aromatase, oestradiol dehydrogenase (E2DH) and oestrone sulphatase (E1-STS), can be stimulated by several growth factors and cytokines. As it is possible that some of these factors may be derived from cells of the immune system (macrophages and lymphocytes), the effects of basic fibroblast growth factor (bFGF) and interleukin-2 (IL-2), which are produced by these cells, on E2DH activity was examined in MCF-7 cells. Treatment of these cells with bFGF resulted in a dose-dependent increase in E2DH reductive activity whereas IL-2 was inactive at the concentration tested. To obtain further evidence that factors produced by macrophages and lymphocytes can modulate the activities of enzymes involved in oestrogen synthesis, conditioned medium was collected from these cells and found to stimulate both E1-STS and E2DH activities. In addition to understanding the control of oestrogen synthesis in breast tumours an inhibitor to block the synthesis of oestrone via the oestrone sulphatase pathway was developed. Oestrone-3-O-sulphamate (EMATE) is a potent, irreversible, inhibitor of E1-STS. A single dose of EMATE (10 mg/kg) inhibited tissue E1-STS activity in rats by more than 95% for up to 7 days, indicating that this compound may have considerable therapeutic potential for the treatment of breast cancer. Evidence is also reviewed that another steroid sulphatase, dehydroepiandrosterone sulphate sulphatase, may have a crucial role in regulating cytokine production and that this may indirectly control tumour oestrogen synthesis.

Stimulation of aromatase activity in breast fibroblasts by tumor necrosis factor alpha

The conversion of androstenedione to estrone, the reaction mediated by the aromatase enzyme complex, may make an important contribution to the synthesis of estrogens in breast tissues. In the present study, the effect of the cytokine. TNF alpha, on aromatase activity was examined in breast fibroblasts derived from normal and malignant breast tissue. TNF alpha (2.5-10.0 ng/ml), in the presence of stripped fetal calf serum and dexamethasone, significantly stimulated fibroblast aromatase activity in a dose-dependent manner. IL-1 and IL-6 also stimulated fibroblast aromatase activity, but no marked synergism between TNF alpha and IL-1 or IL-6 was detected. Using a specific radioimmunoassay, significant concentrations of TNF alpha were detected in samples of breast cyst fluid and breast tumor cytosol, which had previously been shown to stimulate aromatase activity, but not in conditioned medium from breast tumor-derived fibroblasts. As TNF alpha may be preferentially expressed and produced in the adipose tissue component of the breast, this cytokine may have an important role in regulating estrogen synthesis in normal and malignant breast tissues.

The interaction of cytokines in regulating oestradiol 17 beta-hydroxysteroid dehydrogenase activity in MCF-7 cells

Oestradiol 17 beta-hydroxysteroid dehydrogenase (E2DH) has a pivotal role in the regulation of oestradiol (E2) concentrations in normal and malignant breast tissues. Previous studies have suggested that a number of cytokines can stimulate E2DH activity to increase the conversion of oestrone (E1) to E2. In this investigation we have examined the effect of TNF alpha, interleukin-1 beta (IL-1 beta) and IL-6 on E2DH activity in MCF-7 breast cancer cells. These cytokines may be produced by breast tumours and their presence in conditioned medium (CM) from tumour-derived fibroblasts was also measured to assess their possible contribution to its E2DH stimulatory activity. Treatment of MCF-7 cells with IL-1 beta and TNF alpha (5 ng/ml) significantly increased (P < 0.001) reductive E2DH (red-E2DH, the conversion of E1 to E2) activity. In contrast, IL-6 at a concentration of 100 ng/ml produced little, if any, stimulation of reductive activity. Combinations of all three cytokines acted synergistically to stimulate red-E2DH activity. No cytokine, either alone or in combination, affected oxidative (E2-->E1) activity. Significant concentrations of IL-6 and IL-1 beta were detected in CM, but the stimulation of red-E2DH activity was much greater than that which could be explained by their levels alone. It is concluded that these cytokines may play an important role in regulating E2DH activity in breast cancer cells and may act synergistically in vivo to enhance the formation of E2 in breast tumours.

Control of aromatase activity in breast cancer cells: the role of cytokines and growth factors

The aromatase complex has a key role in regulating oestrogen formation in normal and malignant breast tissues. Using dexamethasone-treated fibroblasts, derived from breast tumours, breast tumour cytosol and breast tumour-derived conditioned medium (CM) markedly stimulate aromatase activity. The cytokine, interleukin-6 (IL-6) has been identified as a factor present in CM which is capable of stimulating aromatase activity. To examine whether IL-6 may have a role in vivo in regulating breast tissue aromatase activity, IL-6 production and aromatase activity in breast tumour and adipose tissue from breast quadrants were examined. In 5/6 breasts examined so far, aromatase activity was highest in adipose tissue in the breast quadrant containing the tumour or on which the tumour impinged. There was a significant correlation (P < 0.05, Kendall's rank correlation) between IL-6 production and aromatase activity in these breast tissues. It is concluded that IL-6 may have an important role in regulating aromatase activity in breast tissues.