Exosome-Mediated Transfer of Cancer Cell Resistance to Antiestrogen Drugs

Exosomes are small vesicles which are produced by the cells and released into the surrounding space. They can transfer biomolecules into recipient cells. The main goal of the work was to study the exosome involvement in the cell transfer of hormonal resistance. The experiments were performed on in vitro cultured estrogen-dependent MCF-7 breast cancer cells and MCF-7 sublines resistant to SERM tamoxifen and/or biguanide metformin, which exerts its anti-proliferative effect, at least in a part, via the suppression of estrogen machinery. The exosomes were purified by differential ultracentrifugation, cell response to tamoxifen was determined by MTT test, and the level and activity of signaling proteins were determined by Western blot and reporter analysis. We found that the treatment of the parent MCF-7 cells with exosomes from the resistant cells within 14 days lead to the partial resistance of the MCF-7 cells to antiestrogen drugs. The primary resistant cells and the cells with the exosome-induced resistance were characterized with these common features: decrease in ERα activity and parallel activation of Akt and AP-1, NF-κB, and SNAIL1 transcriptional factors. In general, we evaluate the established results as the evidence of the possible exosome involvement in the transferring of the hormone/metformin resistance in breast cancer cells.

The phenomenon of acquired resistance to metformin in breast cancer cells: The interaction of growth pathways and estrogen receptor signaling

Metformin, a biguanide antidiabetic drug, is used to decrease hyperglycemia in patients with type 2 diabetes. Recently, the epidemiological studies revealed the potential of metformin as an anti-tumor drug for several types of cancer, including breast cancer. Anti-tumor metformin action was found to be mediated, at least in part, via activation of adenosine monophosphate-activated protein kinase (AMPK)-intracellular energy sensor, which inhibits the mammalian target of rapamycin (mTOR) and some other signaling pathways. Nevertheless, some patients can be non-sensitive or resistant to metformin action. Here we analyzed the mechanism of the formation of metformin-resistant phenotype in breast cancer cells and its role in estrogen receptor (ER) regulation. The experiments were performed on the ER-positive MCF-7 breast cancer cells and metformin-resistant MCF-7 subline (MCF-7/M) developed due to long-term metformin treatment. The transcriptional activity of NF-κB and ER was measured by the luciferase reporter gene analysis. The protein expression was determined by immunoblotting (Snail1, (phospho)AMPK, (phospho)IκBα, (phospho)mTOR, cyclin D1, (phospho)Akt and ERα) and immunohistochemical analysis (E-cadherin). We have found that: 1) metformin treatment of MCF-7 cells is accompanied with the stimulation of AMPK and inhibition of growth-related proteins including IκBα, NF-κB, cyclin D1 and ERα; 2) long-term metformin treatment lead to the appearance and progression of cross-resistance to metformin and tamoxifen; the resistant cells are characterized with the unaffected AMPK activity, but the irreversible ER suppression and constitutive activation of Akt/Snail1 signaling; 3) Akt/Snail1 signaling is involved into progression of metformin resistance. The results presented may be considered as the first evidence of the progression of cross-resistance to metformin and tamoxifen in breast cancer cells. Importantly, the acquired resistance to both drugs is based on the constitutive activation of Akt/Snail1/E-cadherin signaling that opens new perspectives to overcome the metformin/tamoxifen resistance of breast cancer.

Molecular mechanisms of hormone resistance of breast cancer

More than 70% malignant mammary tumors contain steroid hormone receptors; this suggests the possibility of hormone therapy in the majority of patients with breast cancer (BC). The main cause of inefficiency of hormone therapy in BC is hormone resistance (tumor resistance to hormonal cytostatics). Here we discuss the main mechanisms of hormone resistance of BC and the mechanisms underlying the formation of hormone resistance of the tumors are analyzed at the molecular level. The data on the signal pathways of estrogen receptors (ER), the key regulators of BC cell proliferation, are presented. The most important factors of BC hormone resistance are: high activity/expression of receptor tyrosine kinases; high activity of proteins regulating cell defense mechanisms (Akt PI3K, mTOR); changes in the activities of cell cycle regulator proteins (Myc, c-Fos, Cyclin D1). Our experiments have demonstrated that estrogen-independent BC cell growth is supported by VEGF/VEGFR2 and EGF/EGFR mitogenic signal pathways. Our data indicate that NF-kappaB transcription factor is directly involved in the regulation of hormone-resistant BC cell growth and survival, while NF-kappaB suppression determines cell sensitivity to apoptotic activity of antitumor compounds. On the whole, the results indicate good prospects of using EGFR, HER-2/neu, mTOR, VEGFR, PI3K/Akt molecular pathways as targets for BC therapy, including therapy for BC resistant forms.

Snail/beta-catenin signaling protects breast cancer cells from hypoxia attack

The tolerance of cancer cells to hypoxia depends on the combination of different factors--from increase of glycolysis (Warburg Effect) to activation of intracellular growth/apoptotic pathways. Less is known about the influence of epithelial-mesenchymal transition (EMT) and EMT-associated pathways on the cell sensitivity to hypoxia. The aim of this study was to explore the role of Snail signaling, one of the key EMT pathways, in the mediating of hypoxia response and regulation of cell sensitivity to hypoxia, using as a model in vitro cultured breast cancer cells. Earlier we have shown that estrogen-independent HBL-100 breast cancer cells differ from estrogen-dependent MCF-7 cells with increased expression of Snail1, and demonstrated Snail1 involvement into formation of hormone-resistant phenotype. Because Snail1 belongs to hypoxia-activated proteins, here we studied the influence of Snail1 signaling on the cell tolerance to hypoxia. We found that Snail1-enriched HBL-100 cells were less sensitive to hypoxia-induced growth suppression if compared with MCF-7 line (31% MCF-7 vs. 71% HBL-100 cell viability after 1% O2 atmosphere for 3 days). Snail1 knock-down enhanced the hypoxia-induced inhibition of cell proliferation giving the direct evidence of Snail1 involvement into cell protection from hypoxia attack. The protective effect of Snail1 was shown to be mediated, at least in a part, via beta-catenin which positively regulated expression of HIF-1-dependent genes. Finally, we found that cell tolerance to hypoxia was accompanied with the failure in the phosphorylation of AMPK - the key energy sensor, and demonstrated an inverse relationship between AMPK and Snail/beta-catenin signaling. Totally, our data show that Snail1 and beta-catenin, besides association with loss of hormone dependence, protect cancer cells from hypoxia and may serve as an important target in the treatment of breast cancer. Moreover, we suggest that the level of these proteins as well the level of AMPK phosphorylation may be considered as predictors of the tumor sensitivity to anti-angiogenic drugs.

The relationships between snail1 and estrogen receptor signaling in breast cancer cells

The loss of hormonal dependency of breast tumor cells is often accompanied with the appearance of epithelial-mesenchymal transition (EMT) features and increase in cell metastasis and invasiveness. The central role in the EMT belongs to transcription factors Snail responded for the decrease in E-cadherin expression and cell contacts, stimulation of cell mobility and invasiveness. Aim was to study the relationships between estrogen receptor machinery and Snail1 signaling, and mechanism of Snail1 regulation in hormone-resistant breast cancer cells. The experiments were performed on the estrogen-dependent MCF-7 breast cancer cells, estrogen-hyposensitive MCF-7/LS subline generated through long-term cultivation of the parental cells in steroid-free medium, and ER-negative estrogen-resistant HBL-100 cells. Snail1, estrogen receptor, p65 NF-κB, E-cadherin levels were analyzed by Western blot. We found that decrease in the estrogen dependency is correlated with increase in Snail1 expression and activity, we demonstrated the Snail1 involvement in the negative regulation of ER, and showed that Snail1 inhibition partially restores the sensitivity of the estrogen-hyposensitive cells to antiestrogen tamoxifen. Furthermore, NF-κB was found to serve as a positive regulator of Snail1 in breast cancer cells, and simultaneous inhibition of NF-κB and Snail1 resulted in additional increase in cell response to tamoxifen. In general, the results obtained demonstrate the phenomenon of Snail1 activation in the hormone-resistant breast cancer cells, and show that Snail1 and NF-κB may serve as an important targets in the treatment of breast cancer, both estrogen-dependent and estrogen-independent tumors.

[The role of transcription factor Snail1 in the regulation of hormonal sensitivity of in vitro cultured breast cancer cells]

The loss of hormonal dependency in breast tumor cells is often accompanied by epithelial-mesenchymal transition (EMT) features and an increase in cell metastasizing and invasiveness. Here we studied the role of transcription factors Snail1--the central mediator of EMT, in the progression of hormonal resistance of breast cancer cells. The experiments were performed on the estrogen receptor(ER)-positive estrogen-dependent MCF-7 breast cancer cells, ER-positive estrogen-resistant MCF-7/LS subline generated through long-term cultivation of the parental cells in steroid-free medium, and ER-negative estrogen-resistant HBL-100 breast cancer cells. We found that decrease in the estrogen dependency of breast cancer cells is accompanied by an increase in Snail1 expression and activity, and demonstrated the Snail1 involvement in the negative regulation of ER. NF-kappaB was found to serve as a positive regulator of Snail1 in breast cancer cells, and simultaneous inhibition of NF-kappaB and Snail1 by RNA interference resulted in marked increase of cell response to antiestrogen tamoxifen. In general, the results obtained demonstrate that direct inhibition of NF-kappaB and Snail1 partially restores the estrogen receptor machinery, and show that Snail1 and NF-kappaB may serve as an important targets in the treatment of breast cancer.

The breast cancer cells response to chronic hypoxia involves the opposite regulation of NF-kB and estrogen receptor signaling

The progression of cancer is associated with tumor's ability to outgrow the existing vasculature resulting in chronic hypoxic pressure, however the molecular mechanism of cancer cell response to chronic hypoxia is poorly understood. In this study we have analyzed the reorganization of estrogen receptor (ER) signaling in breast cancer cells under chronic hypoxia and examined the role of interrelations between ER and NF-kB signaling in cell adaptation to hypoxia. Using long-term culturing of MCF-7 breast cancer cells in hypoxia-mimetic conditions (cobalt chloride) we have established a hypoxia-tolerant subline characterized by HIF-1 hyperexpression that retained the tolerance to hypoxia even when the cells were returned to normoxic conditions. The hypoxia-tolerant cells were characterized by non-affected ER signaling, irreversible suppression of NF-kB activity, and increased sensitivity to cytokine-induced apoptosis. Estradiol treatment suppressed the NF-kB activity in both parent and hypoxia-tolerant MCF-7 cells. In contrast to MCF-7 cells, the exposure of estrogen-independent MCF-7/T2 subline to chronic hypoxia was not accompanied by noticeable changes in NF-kB activity or cell sensitivity to cytokines. Taken together, the results presented demonstrate the importance of interrelations between ER and NF-kB signaling in the response of estrogen-dependent breast cancer cells to chronic hypoxia.

NF-kappaB suppression provokes the sensitization of hormone-resistant breast cancer cells to estrogen apoptosis

The progression of breast cancer cells to estrogen-independent growth may be accompanied with the paradoxical cell sensitization to estrogen apoptotic action; however, the mechanism of this phenomenon is still unclear. In the present study, we have shown that the sensitization of hormone-resistant breast cancer cells to estrogen apoptotic action is accompanied with the gradual NF-kappaB suppression. Using the chemical inhibitors of NF-kappaB as well as the dominant-negative NF-kappaB constructs, we have proved the sufficiency of NF-kappaB inhibition for the sensitization of the resistant cells to estrogen apoptosis. Estradiol treatment results in the additional suppression of NF-kappaB, demonstrating the possible NF-kappaB involvement in the regulation of cell response to estrogens. Totally, the results presented suggest that the constitutive NF-kappaB suppression in the estrogen-independent cells may be considered as one of the factors resulting in a imbalance between pro- and anti-apoptotic pathways and enhancement in estrogen apoptotic action in the cells.

Mechanism of estrogen-induced apoptosis in breast cancer cells: role of the NF-kappaB signaling pathway

The ability of sex steroid hormones to up-regulate the apoptotic signaling proteins is well documented; however, the apoptotic potential of sex hormones is not remarkable and fully compensated by their growth stimulatory action to target cells. In the present study using the long-term cultivation of estrogen-dependent MCF-7 breast cancer cells in steroid-free medium, we have established a cell subline, designed as MCF-7/LS, which was characterized by the resistance to growth stimulatory estradiol action and hypersensitivity to estrogen-induced apoptosis. We have demonstrated that estrogen treatment of the cells does not influence on the level of TNF-R1 or Fas, but dramatically decreases the transcriptional activity of NF-kappaB. Importantly, the MCF-7/LS cells, which are insensitive to growth stimulatory estrogen action, retain the ability to decrease in the NF-kappaB activity in response to estrogen stimulus. Furthermore, the significant increase in the basal (in the absence of ligand) estrogen receptor (ER)-dependent transcriptional activity in the MCF-7/LS cells was revealed and reciprocal transcriptional antagonism between ER and NF-kappaB was demonstrated. Finally, we proved the possible involvement of phosphatidylinositol-3 kinase (PI3K) in the ligand-independent ER activation. In general, the results presented suggest that long-term growth of MCF-7 breast cancer cells in steroid-free medium is accompanied with the increase in the basal ER-dependent transcriptional activity as well as the maintenance of the negative regulatory loop ER-NF-kappaB. The latter may be considered as one of the factors resulting in a disbalance between pro- and anti-apoptotic pathways and enhancement in estrogen apoptotic action in the cells.

Phosphatidylinositol 3-kinase/AKT signalling pathway components in human breast cancer: clinicopathological correlations

BACKGROUND

The phosphatidylinositol 3-kinase (PI3K)/Akt signalling pathway plays a major role in the regulation of breast cancer growth and survival, but the clinical value of its components in human tumours is unclear.

PATIENTS AND METHODS

PI3K was analysed using Western blotting with monoclonal antibodies to the p85 subunit in tumour and adjacent mammary gland samples from 33 breast cancer patients. Activated Akt1 (pAkt1) expression was quantified in 46 sample pairs by a direct sandwich ELISA assay.

RESULTS

Tumour PI3K expression was increased in 79% of the investigated sample pairs and was not associated with the main clinico-pathological features. Only 49% of breast cancers had increased pAkt1, but the frequency of its elevation was positively associated with tumour size and histological grade, and controversially related to estrogen and progesterone receptor status.

CONCLUSION

Increased PI3K but not pAkt1, expression appears to be a widespread feature of human breast cancer indicating the different roles of the two components of one signalling system.

Development of differential sensitivity of CaOv ovarian adenocarcinoma cells to antitumor agents under conditions of hypoxia

We studied the role of VEGF signal pathway in autocrine regulation of tumor cell growth and survival under conditions of hypoxia. Hypoxia-resistant CaOv/H substrain with high level of VEGF-A secretion was obtained by long-term culturing of CaOv ovarian adenocarcinoma cells with CoCl2 (hypoxia inductor). VEGF-A directly participates in autocrine regulation of CaOv cell growth, including the maintenance of cell growth under conditions of hypoxia or cytostatic treatment. On the other hand, CaOv/H cells retain high apoptotic potential and are characterized by high expression of p27Kip1 (cyclin-dependent kinase inhibitor), which attests to possible involvement of this inhibitor into the regulation of apoptotic response of cells under conditions of hypoxia.

Activation of mitogenic pathways and sensitization to estrogen-induced apoptosis: two independent characteristics of tamoxifen-resistant breast cancer cells?

Paradoxical induction of apoptosis by estrogen has been described previously for estrogen-deprived and antiestrogen-resistant breast cancer cells. In this study we analyzed the possible interrelations between cell sensitization to estrogen apoptotic action and cell ability to (anti)estrogen-independent growth. Using tamoxifen-resistant sublines derived from the parent MCF-7 breast cancer cells by long-term tamoxifen treatment we demonstrated that resistant cells are characterized by increased level of EGF receptor and unexpected increase of VEGF receptor 2 (Flk-1/KDR) and its specific ligand, VEGF-A. The importance of the VEGF signaling in the autocrine regulation of cell growth was indicated by the ability of VEGF inhibitor, soluble fragment of Flt-1/Fc chimera, to suppress the phosphorylation of MAP kinases as well as to inhibit the estrogen-independent growth of MCF-7 cells. Sensitization of tamoxifen-resistant cells to estrogen-induced apoptosis required the additional continuous cultivation in steroid-depleted medium and did not depend on the activity of both EGF and VEGF pathways. Finally, we showed that treatment of the cells with 17beta-estradiol (10(-9) M) resulted in a marked increase in p53 level both in the resistant cells undergoing apoptosis and in the parent MCF-7 cells insensitive to apoptotic estrogen action. These data provide an important support for the existence of a disbalance between pro- and anti-apoptotic machinery in the resistant breast cancer cells that forms independently of the acquired ability to estrogen-independent growth.

Activation of mitogenic pathways and sensitization to estrogen-induced apoptosis: two independent characteristics of tamoxifen-resistant breast cancer cells?

Paradoxical induction of apoptosis by estrogen has been described previously for estrogen-deprived and antiestrogen-resistant breast cancer cells. In this study we analyzed the possible interrelations between cell sensitization to estrogen apoptotic action and cell ability to (anti)estrogen-independent growth. Using tamoxifen-resistant sublines derived from the parent MCF-7 breast cancer cells by long-term tamoxifen treatment we demonstrated that resistant cells are characterized by increased level of EGF receptor and unexpected increase of VEGF receptor 2 (Flk-1/KDR) and its specific ligand, VEGF-A. The importance of the VEGF signaling in the autocrine regulation of cell growth was indicated by the ability of VEGF inhibitor, soluble fragment of Flt-1/Fc chimera, to suppress the phosphorylation of MAP kinases as well as to inhibit the estrogen-independent growth of MCF-7 cells. Sensitization of tamoxifen-resistant cells to estrogen-induced apoptosis required the additional continuous cultivation in steroid-depleted medium and did not depend on the activity of both EGF and VEGF pathways. Finally, we showed that treatment of the cells with 17beta-estradiol (10(-9) M) resulted in a marked increase in p53 level both in the resistant cells undergoing apoptosis and in the parent MCF-7 cells insensitive to apoptotic estrogen action. These data provide an important support for the existence of a disbalance between pro- and anti-apoptotic machinery in the resistant breast cancer cells that forms independently of the acquired ability to estrogen-independent growth.

Sensitization of MCF-7 breast cancer cells to the apoptotic effect of estradiol

A new substrain of hormone-resistant MCF-7/T breast cancer cells was selected after long-term culturing of estrogen-dependent MCF-7 cells in the presence of tamoxifen. These cells were resistant to the growth-stimulating and cytostatic effects of estradiol and tamoxifen, respectively. MCF-7/T cells gained paradoxical sensitivity to the apoptotic effect of estradiol. Estradiol stimulated p53 expression and decreased DNA-binding activity of NF-kappaB. Our findings provide indirect evidence that these proteins are involved in the regulation of estrogen-induced apoptosis. These results indicate that tamoxifen-resistant breast cancer cells can be sensitized to the apoptotic effect of estradiol. The data form a basis for the development of new methods of endocrine therapy for breast cancer patients.

Role of phosphatidylinositol-3 kinase in regulation of differential sensitivity of melanoma cells to antitumor agents. A model for hormone resistance development in tumor cells

Phosphatidylinositol-3 kinase (PI3K) belongs to one of the most important cellular proteins involved in the transmission of anti-apoptotic signal and regulation of survival pathways in tumor cells. Earlier we have found that prolonged treatment of melanoma cells with dexamethasone results in formation of a cell subline which was resistant to growth inhibitory dexamethasone action. We showed that constitutive activation of PI3K can be considered as one of the factors that regulate cell resistance to dexamethasone. Here we demonstrate that increased level of PI3K protein in dexamethasone-resistant cells correlates with partial decrease in expression of down-stream target of PI3K--protein kinase B (PKB). Study of the cell's sensitivity to various damaging agents showed that the cells after prolonged dexamethasone treatment are characterized by increased level of the resistance to both hormonal drugs and hypoxia, and at the same time with high sensitivity to ultraviolet (UV) radiation or anti-tumor agents such as adriamycin. As revealed, hypoxic conditions or short-term dexamethasone treatment of the resistant cells lead to a substantial increase in the PKB level, whereas neither UV radiation nor adriamycin affects the PKB level in these cells. We demonstrate that long-term dexamethasone treatment of melanoma cells results in the accumulation of the active form of mitogen-transducing signaling protein STAT3 (Signal Transducer and Activator of Transcription-3), which also contributes to inducing the melanoma cell's resistance to antiproliferative action of dexamethasone. We suggest that decreased level of PKB in combination with an activation of PI3K/STAT3 signaling in the melanoma cells after prolonged dexamethasone treatment may be one of the mechanisms of different sensitivity of these cells to hormonal drugs and damaging agents. The model of the progression of hormonal resistance of in vitro cultured tumor cells is presented.

[The molecular mechanism responsible for the adaptation of malignant tumors to hormonal drugs: a role of phophatidylinositol-3-kinase and phosphoinositide-dependent proteins]

Phophatidylinositol-3-kinase (PI3K) is a major intracellular protein that is responsible for the transmission of an antiapoptotic signal and controls the survival of tumor cells upon exposure to damaging agents. Experiments using different tumor cell cultures have shown that the resistance of cells to the antiproliferative action of dexamethasone, caused by their long cultivation with the hormone, is associated with the activation of PI3K and the transcription factor STATS. The activation of PI3K and STAT3 in the dexamethasone-resistant cells correlates with the increase in the total thyrosine kinase activity and with the decrease in the sensitivity of cells to exogenous proliferative agents, such as 17beta-estradiol. The long exposure of hormone-sensitive cells to nonhormonal factors that activate the PI3K/STAT3 signaling pathway, hypoxia in particular, has been shown to suffice to reduce the degree of hormonal tumor cell dependence. VEGF-A, an angiogenic peptide whose action was partially realizes through the PI3K-signalling pathway, has been demonstrated to be involved in the maintenance of cell growth, including the growth of hormone-independent cells. The findings suggest that complex changes in the antiapoptotic and mitogenic signaling pathways associated with PI3K, which ensures the autonomic, hormone-independent growth of tumor cells, may underlie the decreased hormonal dependence of tumor cells. Whether PI3K may be used to suppress the growth of hormone-independent tumors is discussed.

[Signaling pathways that control the growth and survival of prostate tumor cells in the absence of androgens]

Androgen-dependent human prostate adenocarcinoma cell line LNCaP was used to study the effect of androgen deprivation on the cell response to TNF-related cytokines. Several signaling pathways were implicated in cell survival in the absence of androgens. In androgen-deprived LNCaP cells, TNF-alpha and TRAIL stimulated the cell growth and activated the mitogenic and antiapoptotic signaling pathways involving NF-kappa B, STAT3, PI3K, and beta-catenin. The results suggested a role of cytokines in the survival of prostate adenocarcinoma cells deprived of androgens in vitro.

[Role of phosphatidylinositol signalling path in developing hormonal resistance in tumor cells]

Phosphatidylinositol 3-kinase (PI3K) is a key regulatory protein which is responsible for anti-apoptotic signal transduction regulating cell survival during exposure to damaging factors. The report deals with the role of the PI3K signaling pathway in regulating cellular response to hormones and, particularly, in development of resistance as a result of long-term exposure of cells to steroid cytostatic hormones. In our study, even a short-term exposure of transformed fibroblasts of hamster (line 2PK) resulted in an activation of main PI3K effectors (MAP-kinase and protein kinase B (PKB)) which appeared against the background of hormone-induced inhibition of cellular growth. A long-term (3 months) cell culturing with dexamethasone was followed by formation of subpopulations of cells which were refractory to the growth inhibition by hormone and were characterized by high levels of activity of PI3K, MAP-kinases and PKB. Activation of PI3K and PI3K-dependent enzymes correlated with enhancement of synthesis of c-jun, a component of the AP-1 transcriptional factor, was observed both in short- and long-term application of dexamethasone. We believe that, during long-term exposure of cells to cytostatic hormones, continuous activation of PI3K and PI3K-dependent transcriptional factors may result in a significant restructuring of intracellular signal pathway, and, finally, constitutive PI3K-signal pathway and partial overcoming the proliferative block by cells.

Expression of phosphatidylinositol-3 kinase in lung cancer

The expression of phosphatidylinositol-3 kinase in tumors and homologous tissues from 29 patients with lung cancer, 5 patients with lung metastases of various tumors, and some non-tumorous pulmonary diseases was studied by Western blot analysis. The expression of phosphatidylinositol-3 kinase was increased in these tumors in comparison with histologically intact lung tissue in 5 patients with non-small-cell cancer. In 20 patients expression of phosphatidylinositol-3 kinase was the same as in homologous tissue and in 4 patients it was decreased. No relationship between phosphatidylinositol-3 kinase expression and clinical and morphological characteristics of lung cancer was revealed.

Phospatidylinositol 3-kinase expression in human breast cancer

Phospatidylinositol 3-kinase (PI 3-kinase) expression was analysed by Western blotting with monoclonal antibodies to the p85 subunit in a series of tumour and adjacent mammary gland samples collected at surgery from 33 breast cancer patients. Seventy-nine percent of the investigated pairs of the samples were characterised by an increased level of PI 3-kinase in the tumour in comparison with the adjacent mammary gland. PI 3-kinase activation was not associated with tumour steroid receptor status, histologic grade and other clinico-morphological characteristics. Furthermore, immunoblotting of epidermal growth factor receptor (EGFR) in the tumours with increased PI 3-kinase and corresponding adjacent tissues revealed no association between EGFR and PI 3-kinase activation. Thus, increased PI 3-kinase expression appears to be a widespread feature of breast cancer not associated with the main biological markers of its prognosis and hormone sensitivity.

The role of phosphatidylinositol 3-kinase in the regulation of cell response to steroid hormones

Phosphatidylinositol 3-kinase (PI-3 kinase) has been implicated in the regulation of many cellular processes, including growth and transformation. We describe the effect of glucocorticoids on cell growth, phosphoinositide formation and PI-3 kinase activity in Rous sarcoma virus-transformed hamster fibroblasts (HET-SR). Using a prolonged dexamethasone treatment of HET-SR cells we have selected a new glucocorticoid receptor-positive cell subline, HET-SR(h), that was resistant to growth inhibitory action of dexamethasone and/or non-hormonal drugs (vinblastine, adriamycin) and was characterized by higher levels of phosphoinositide formation and increased PI-3 kinase activity. Study of the short-term hormone action has shown that both dexamethasone-sensitive and -resistant sublines responded to hormone by a decrease in phospholipid turnover rate. At the same time, in both cell lines activation of PI-3 kinase after dexamethasone addition was revealed. Dexamethasone-dependent activation of PI-3 kinase was more significant and maintained for a longer period in HET-SR(h) cells than in parent HET-SR cells. Finally, by transfecting p110*, a constitutively active catalytic subunit of PI-3 kinase, into hormone-sensitive HET-SR cells, we have found a marked increase in cell resistance to growth inhibitory dexamethasone action. These results suggest that PI-3 kinase may serve as one of the factors providing cell resistance to cytostatic drugs.

[Effect of specific activation of phosphatidylcholine metabolism in hamster fibroblasts transformed by Rous sarcoma virus]

Transformation of embryonic hamster fibroblasts by the Rous sarcoma virus results in sharp increase of the turnover rate of one of cellular phospholipids-phosphatidylcholine. The decrease in the rate of virus-transformed cells (HETSR strain) during the monolayer formation is attended by additional activation of phosphatidylcholine turnover. A similar effect is observed after prolonged culturing of cells with dexamethasone. Addition of the tyrosine kinase inhibitor, genistein, to cells leads to selective inhibition of phosphatidylcholine synthesis without any effect of phosphoinositide synthesis. Immunoblotting analysis of p60-src, the product of the viral oncogen v-src related to the tyrosine kinase family failed to produce any significant changes in protein synthesis and activity during dexamethasone-induced inhibition of HETSR cell growth. The data obtained testify to selective activation of phosphatidylcholine metabolism in src-transformed cells which enhances with a decrease in the rate of cell growth. The presence in HETSR cells of p60-src whose synthesis is not controlled by dexamethasone may be responsible for increased phosphatidylcholine metabolism and sustaining cell growth under conditions of limited activity of growth-promoting compounds.

Comparative analysis of the sensitivity of endometrial cancer cells to epidermal growth factor and steroid hormones

BACKGROUND

Epidermal growth factor (EGF) and EGF-regulated processes play an important role in steroid signal transduction. Comparative analysis of EGF and steroid receptor expression and the sensitivity of early stages of proliferation induction, such as activation of phospholipid turnover to EGF and steroids, may provide a useful new approach to characterizing the sensitivity of endometrial cancer to hormone therapy.

METHODS

Progesterone (PR), estradiol (ER), and EGF receptor (EGFR) content was measured by radioligand competitive methods in surgically excised tumors from 26 patients with primary endometrial cancer. In short term cell cultures isolated from 11 of these tumors, the influence of a 10-minute treatment with 10(-8)M EGF either alone or combined with 10(-8)M progesterone on 32P-incorporation into phospholipids was studied. Phospholipids were fractionated by thin-layer chromatography and were located by autoradiography, and quantification of the labeled compounds was made by densitometric scanning of the autoradiograms.

RESULTS

Epidermal growth factor receptor was found in 15 of 26 (58%) endometrial cancer samples. Eighty-two percent of the tumors studied contained PR, and 81% contained ER. No significant correlations were revealed between EGFR and ER/PR status or concentration. Epidermal growth factor stimulated 32P-incorporation by more than 120% of the control level in five of seven EGFR-positive and in one of four EGFR-negative endometrial cancer samples. An inverse relationship was revealed between EGFR content and the percentage of EGF-induced stimulation of phospholipid turnover in endometrial cancer cells (r = -0.6; P = 0.15) and between EGFR content in EGFR-positive samples and the extent of progesterone suppression of EGF-induced turnover (r = -0.77; P = 0.04).

CONCLUSIONS

Determination of EGF sensitivity on a receptor and a functional level may provide important additional information about the hormonal sensitivity of endometrial cancer.

[Regulation of the cycle of phospholipid turnover in hamster fibroblasts transformed by v-src and N-ras oncogenes]

The phospholipid turnover has been studied in two lines of golden hamster cells: in cells transformed by the Rous sarcoma virus (line HET-SR) and in cells additionally transfected with the activated oncogene N-ras (line HET-SR-N-ras, clone 6). It has been found that HET-SR cells are distinguished by a high level of phosphatidylcholine turnover and a relatively low level of phosphoinositide turnover. Transfection of cells with the activated N-ras (line HET-SR-N-ras) leads to the inhibition of phosphatidylcholine synthesis and activation of phosphoinositide metabolism. Both cell lines preserve their sensitivity to serum growth factors stimulating the rate of phospholipid turnover. In both cell lines dexamethasone decreases the rate of DNA synthesis and inhibits the phosphatidylcholine and phosphoinositide turnover. At the same time, dexamethasone does not influence the predominant activation of phosphatidylcholine synthesis in HET-SR cells or the activation of phosphoinositide synthesis characteristic of HET-SR-N-ras cells. The data obtained suggest that the transmission of the mitogenic signal from growth factor in HET-SR and HET-SR-N-ras cells occurs via the activation of the phospholipid turnover and is controlled by steroid hormones. The role of v-src and N-ras oncogens in the transmission of the mitogenic signal seems to be insignificant; their activity is not controlled by dexamethasone.

Regulation of phospholipid turnover by steroid hormones in endometrial carcinoma and breast cancer cells

To study the early effects of steroid hormones on cells we investigated the influence of the sex steroids and tamoxifen on phospholipid turnover in endometrial carcinoma and breast cancer cells. Studies were performed on 19 human uterine adenocarcinomas and 29 breast cancer tumors. Progesterone in a final concentration of 10(-7) mol/l caused a twofold decrease of 32P incorporation into phospholipids (phosphatidylcholine and phosphoinositides) in 85% of the uterine adenocarcinomas where the progesterone receptor (PR) content was more than 100 nmol/kg and only in 30% of the tumors where the PR content was less than 100 nmol/kg. Treatment of the cells with 10(-8) mol/l 17 beta-estradiol or 10(-8) mol/l epidermal growth factor led to an increase in 32P incorporation into phospholipids. Analysis of the hormonal responsiveness of 29 human breast cancers showed that 17 beta-estradiol increased 32P incorporation into phospholipids in 47% of the tumors where the estradiol receptor (ER) content was more than 10 nmol/kg and in 21% of the receptor-negative tumors (ER < 10 nmol/kg) The results show that phospholipid turnover in uterine and breast cells can be regulated by sex steroids. Treatment of the breast cancer cells with the antiestrogen tamoxifen (10(-6) mol/l) led to an increase of 32P incorporation into phosphoinositides and a decrease of 32P incorporation into phosphatidylcholine. Addition of an activator of protein kinase C, i.e. 2 x 10(-7) mol/l 12-0-tetradecanoylphorbol-13-acetate, weakened the inhibitory effect of tamoxifen on phosphatidylcholine turnover. These findings suggest that tamoxifen action can be mediated via an alteration of the growth signal transducing system.

[The effect of progesterone and tamoxifen on EGF-dependent activation of phospholipid turnover in uterine and breast tumor cells]

The in vitro effects of the epidermal growth factor (EGF) and progesterone on phospholipid turnover in cells of 19 human adenocarcinomas (postsurgical material) have been studied. In 58% of tumours EGF increased the 32P incorporation into two basic cell phospholipids--phosphatidylcholine and phosphoinositides. In EGF-insensitive cells progesterone induced no noticeable changes in the basal level of phospholipid metabolism. However, in 10 out of 11 positively responding to EGF adenocarcinomas progesterone inhibited the EGF-dependent activation of 32P incorporation into the phospholipids already on the 15th min after its addition to the cells. Analysis of effects of EGF and the anti-estrogen drug tamoxifen on phospholipid turnover in 22 human mammary tumours did not reveal any significant differences in tamoxifen effect on tumour cells differing in their sensitivity to EGF. Independently of cell sensitivity to EGF, tamoxifen caused some decrease in the 32P incorporation into phosphatidylcholine but increased the label incorporation into phosphoinositides. Tamoxifen added to tumour cells prestimulated with EGF or 17 beta-estradiol failed to abrogate the effect of these compounds on phospholipid turnover. At the same time, treatment of cells with the protein kinase C activator 12-O-tetradecanoyl-phorbol-13-acetate fully inhibited the effect of tamoxifen on phospholipid metabolism. The results obtained suggest that the EGF-dependent activation of intracellular phospholipid turnover is under the negative control of progesterone. As for tamoxifen, its effect on cells is independent of EGF and consists, apparently, in the inhibition of protein kinase C activity.

[Biochemical paths for regulating hormone-sensitive cells]

The main causes of alterations in cell sensitivity to steroid hormones were studied during malignant growth and upon ageing. In many cases studied the decreased sensitivity of cells to steroids was reversible and unrelated to changes in the receptor system of the cell. Based on the data obtained, a hypothesis was proposed concerning the multifunctional regulation of cell sensitivity to hormones. Within the framework of this hypothesis the sensitivity of cells to hormones is regulated by both changes in the receptor system responsible for hormonal signal transmission and by changes in the activity of hormone-dependent intracellular enzymatic systems. The role of target cell microenvironment in the formation of the ultimate cell response to hormonal stimuli is discussed.

Glucocorticoid regulation of phospholipid turnover and protein kinase C activity in mouse hepatoma 22 cells

Glucocorticoids induce growth inhibition in certain sensitive hepatoma cells. To investigate how glucocorticoids interact with growth-factor-dependent pathways, we studied the effects of dexamethasone (Dex) on the DNA synthesis, protein kinase C (PKC) activity and phospholipid turnover in mouse hepatoma 22 cells. Dex was found to reduce DNA synthesis in slowly growing hepatoma cells, whereas exponentially growing cells were Dex-insensitive. Direct measurements of PKC activity in the hormone-sensitive hepatoma 22 cells showed a rapid inhibition (within 30 min) when treated with Dex. Dex addition to hormone-sensitive but not to hormone-insensitive hepatoma 22 cells for 30 min caused a significant decrease of 32P-incorporation into the major cellular phospholipids: phosphatidylcholine, phosphatidylglycerol and phosphoinositides. At the same time, the analysis of the correlation between changes in PKC activity and phospholipid turnover showed that synthesis of phosphatidylcholine and phosphatidylglycerol was under positive control of PKC activity. The data suggest that suppression of phospholipid turnover in hormone-sensitive hepatoma 22 cells is one of the early events caused by glucocorticoids, whereas the decrease of PKC activity induced by the hormone is mediated, probably, via changes in phospholipid metabolism.

[The role of peritoneal macrophages in realizing the growth inhibiting effect of glucocorticoids on reinoculated murine hepatoma 22 cells: the effect of hydrocortisone on the cytotoxic activity and metabolism of phospholipids by macrophages]

A single injection of hydrocortisone to rats with ascite hepatoma 22 had practically no effect on tumour growth. Inhibition of tumour growth was observed only after reinoculation of ascite hepatoma to mice that had received no less than 8 daily injections of the hormone. A single injection of hydrocortisone induced inhibition of the cytotoxic activity and decreased phospholipid metabolism in peritoneal macrophages. Contrariwise, long-term administration of the hormone caused marked activation of macrophage cytotoxicity. In this case incorporation of 32P into macrophage phospholipids was restored up to the control level. It is concluded that one of mechanisms underlying the inhibitory effect of glucocorticoids on macrophages is inhibition of phospholipid turnover. Presumably, long-term administration of the hormone promotes the formation of a new population of macrophages insensitive to the inhibitory effect of glucocorticoids and possessing a high cytostatic activity. The appearance of such activated macrophages may account for the enhancement of hydrocortisone effect on tumour cells upon prolonged administration of the hormone.

[Growth inhibiting effect of glucocorticoids and progestins on tumor cells in vitro: effect on the rate of phospholipid reversal]

The growth-inhibiting effect of dexamethasone was estimated by the ability of the hormone to inhibit the proliferative activity of in vitro cultured hepatoma 22 cells. The effect of another steroid proliferation inhibitor, progesterone, was studied in primary cell cultures of human uterine carcinoma. The cytostatic effect of dexamethasone was observed only in slowly proliferating cells and was rapidly reversed by stimulation of cell division with fresh sera. Dexamethasone did not induce any conspicuous changes in the rate of 32P incorporation into hepatoma phospholipids. In 9 out of 14 human uterine carcinomas progesterone inhibited, whereas 17 beta-estradiol stimulated the 32P incorporation into phospholipids (phosphatidylinositol and phosphatidylcholine); this effect was manifested already after 15-min incubation of cells with the hormone. The resistance of uterine carcinoma cells to steroids was paralleled, as a rule, with the increase in the basal level of 32P incorporation into the phospholipids typical of actively proliferating cells. It was assumed that the inhibition of phospholipid exchange rate is related to the earliest manifestations of the growth-inhibiting effect of steroid hormones, at least progestins. In its turn, stimulation of the proliferative activity of cells may relieve this effect of steroids, eventually resulting in a temporary decrease of the cell sensitivity to hormones.

[Inhibition of proliferation of normal and neoplastic liver cells under conditions of prolonged hormonal stimulation]

Single injections of rats with hydrocortisone led to the inhibition of regenerating liver cell proliferation and protooncogene++ Ha-ras mRNA synthesis within 48 hours of hormonal induction. Administration of hydrocortisone to rats daily for 10 days resulted in a persistent decrease of the liver cell capacity to proliferate in response to partial hepatectomy. This inhibiting effect was observed for at least 7 days after cessation of hormonal stimulation; the level of Ha-ras mRNA was thereby decreased. A marked inhibition of ascite hepatoma cell growth was demonstrated after injections of those cells to mice induced with hydrocortisone for 10 days. Such a persistent effect of hydrocortisone is thought to be due to the depletion of the hormone-dependent hepatotrophic factors. The effect of the glucocorticoid hormone in vivo can be supposed to involve both the direct and indirect regulation of target cell proliferation. The latter is mediated via the changes in the activity of exogenous factors which control cell growth and proliferation.

[Regulation of the transcription of the tyrosine aminotransferase gene by glucocorticoids in Morris hepatoma 7777 and 8994 cells]

Kinetics of tyrosine aminotransferase (TAT) mRNA synthesis in Morris rat hepatoma cell lines 7777 and 8994 after dexamethasone treatment (10(-7) M) was studied by molecular hybridization of the RNA with cloned fragments of TAT gene from rat liver cells. It was demonstrated that initiation of TAT gene transcription increased 20 minutes after glucocorticoid treatment. The level of TAT mRNA was not induced by dexamethasone in rat hepatoma cell line 8994. Actinomycin D prevented the deinduction of TAT by stabilization of TAT mRNA.

[Autogenic regulation of the sensitivity of liver cells to glucocorticoids during prolonged hormonal stimulation]

The mechanisms of reversible decrease of hormone-dependent induction of tyrosine aminotransferase (TAT) by rat liver cells after prolonged administration of the glucocorticoid was studied. It was shown that the main links of the glucocorticoid action mechanism (i.e., the formation of a cytoplasmic hormone-receptor complex and the hormone accumulation in the nuclei) do not change under these conditions. It was found also that one of the necessary prerequisites for the decrease of the hormone-dependent induction of TAT is the constant production by liver cells of large amounts of TAT irrespective of whether this process is induced by the glucocorticoid or by a non-hormonal inducer, e.g., tryptophan. Using the dot-hybridization technique, it was demonstrated that the inhibition of hormone-dependent induction of TAT is correlated with the reduction of mRNA TAT. It was supposed that the main links in the mechanism of inhibition of the hormone-dependent induction are the formation of a large excess of the inducible protein--TAT--in the cells as well as the accumulation of end products of the TAT-catalyzed transamination reaction which cause a feed-back repression of the de novo synthesis of TAT. Studies with cell cultures of Morris hepatoma which is known to be sensitive to glucocorticoids revealed the ability of glucose, the end product of gluconeogenesis reactions, to provide for selective inhibition of the hormone-induced accumulation of mRNA TAT in hepatoma cells.

[Effect of glucocorticoid hormones on DNA synthesis and expression of ras proto-oncogenes in proliferating rat liver cells]

A single injection of partially hepatectomized rats with glucocorticoids results in the blocking of DNA synthesis as well as in the inhibition of the protooncogene Ha-ras-1 mRNA accumulation in proliferating rat liver cells. The kinetics of the both hormone-induced effects differ from those observed for tyrosine aminotransferase induction. The effect of glucocorticoids persists for at least 48 hours and does not depend on the time of the hormone injection.

[Hormonal induction of tyrosine aminotransferase and RNA synthesis in the cells of Morris hepatoma strain 7777]

The transfer of Morris hepatoma cells induced by the hormone within 10-60 min in to a hormone-free medium is associated with the augmentation of tyrosine aminotransferase synthesis. The kinetics of this process does not differ from that of the hormone-induced enzyme. The return of tyrosine aminotransferase synthesis to the basal level occurs 15-20 hours after the hormone withdrawal from the medium, although the concentration of the intranuclear hormone sharply decreases already after 3 hours. It was demonstrated that the presence in the hepatoma cell nuclei of 20-25% of the initially bound hormone for at least 20 hours after the cell transfer to the hormone-free medium is not sufficient for maintaining a high level of tyrosine aminotransferase gene expression. Using two-dimensional electrophoresis of 3H-labeled hepatoma cell proteins, it was demonstrated that the observed high activity of tyrosine aminotransferase is due to the de novo synthesis of enzyme molecules rather than to the existence of preformed long-living tyrosine aminotransferase molecules inside the cell. Study of [14C]uridine incorporation into non-ribosomal nuclear RNA of hepatoma cells showed a long-term presence of the label in the RNA throughout the chase experiment. It was assumed that the high activity of the enzyme for 10-15 hours after the hormone release from the hepatoma cell nuclei is due to the accumulation in the nuclei of long-living pre-mRNA molecules synthesized after the hormone addition to the cells and during the first hours after the cell transfer to the hormone-free medium.(ABSTRACT TRUNCATED AT 250 WORDS)

[Interaction of triamcinolone acetonide with Morris hepatoma cells in the presence antibiotics--inhibitors of RNA synthesis]

Some regularities of [3H]triamcinolone acetonide (TA)binding to glucocorticoid-sensitive Morris hepatoma cell nuclei were studied. It was shown that part of the hormone incorporated into the nuclei form highly stable complexes with nuclear structures that are not destroyed during nuclei lysis with 0.25% SDS. Such complex formation is not practically suppressed by a 500-fold excess of non-labeled TA. As the time of incubation of Morris hepatoma cells with the hormone rises from 10 min to 24 hours, the specific binding of TA to the nuclei decreases, while the specific radioactivity of the [3H]TA-nuclei complexes resistant to 0.25% SDS increases. The stable complexes are eluted from Sepharose 6B together with the bulk of the nuclear proteins and do not contain DNA. Actinomycin D extrudes, to some extent, the [3H]TA from the complexes having specific binding sites that are localized in the nuclei and induces the accumulation of the steroid in the firmly bound nuclear complexes resistant to 0.25% SDS. The ability to suppress hormonal induction of tyrosine aminotransferase was detected only in the antibiotics with a high affinity for the GC-pairs of DNA. i.e., actinomycin D and mitramycin. It was assumed that high concentrations of TA specifically bound to the nuclei are necessary only at initial steps of hormonal induction. At later stages, gradual dissociation of the complexes takes place and the hormone is accumulation within the composition of the SDS-resistant firmly bound complexes.

[Biochemical criteria of mammalian cell sensitivity to glucocorticoids]

The resistance of Morris hepatoma cells strain 8994 to glucocorticoids which lack hormonal induction of tyrosine aminotransferase synthesis was studied. The cells of Morris hepatoma 7777 were used as a sensitive strain by a criterion of the enzyme synthesis. Using two-dimensional polyacrylamide gel electrophoresis, it was demonstrated that, i) in the cells of the "resistant" Morris hepatoma strain 8994 glucocorticoids change the rate of synthesis of at least five proteins, ii) two of these proteins are common for both cell strains, while the other ones are individual for each line, iii) in the cells of Morris hepatoma 8994 tyrosine aminotransferase is not controlled by glucocorticoids, iiii) glucocorticoids may not only control the activated genes but also the genes whose expression is suppressed. It was assumed that in the absence of disturbances in the receptor apparatus the resistance of any cell population to glucocorticoids can only be established by the use of a great variety of experimental approaches. The resistance of a cell population to the hormones cannot be judged upon by an analysis of the intensity of synthesis of one or even several proteins.

[Stages of the interaction of steroid hormones with intranuclear acceptor sites of Morris hepatoma cells]

The maximal amount of specifically bound triamsinolone acetonide (TA) penetrates into Morris hepatoma cells at initial steps of hormonal induction. This amount is gradually decreased during incubation of the cells with the hormone. As the incubation time rises, the inhibition of [3H]TA binding to the nuclear fraction induced by an excess of the non-labelled hormone is further decreased, which is paralleled with deceleration of the [3H]TA efflux from the nuclei to a hormone-free medium. After removal of the hormone the cells retain their ability to induce the synthesis of tyrosine aminotransferase for 10 hours, although after 3 hours the amount of the bound hormone falls down to 20-25% of the original level. The rate of further deinduction of tyrosine aminotransferase synthesis depends on the incubation time, i.e. in the cells preincubated with TA for 10 min the deinduction occurs at a slower rate than in the cells preincubated with the hormone for 48 hours. The increase in the amount of specifically bound TA 10 min after hormone addition leads to augmented synthesis of tyrosine aminotransferase which surpasses the synthesis providing for the maintenance of maximal induction. An addition of actinomycin D to a hormone-free medium containing the cells preincubated with TA for 48 hours prevents the deinduction. It is assumed that Morris hepatoma cells contain an actinomycin D-sensitive feed-back mechanism which controls the concentration and distribution of specifically bound intranuclear TA depending on hormonal response.

[Binding of glucocorticoid-receptor complexes with the acceptor zones of nuclei and effect of glucocorticoids on RNA synthesis in hormone-sensitive and hormone-resistant cell populations]

The binding of free radioactive glucocorticoid and the glucocorticoid-receptor complex to rat liver nuclei was studied in vitro. The binding is non-saturated and independent of preliminary injection of the "cold" hormone. In the course of DNA hydrolysis the amount of the radioactive hormone bound to the chromatin moiety in vivo remains practically unchanged relatively to the initial radioactivity of the protein. The liberation of the nuclei into a cell-free medium and the effect of DNAase I on the nuclei are associated with the redistribution of the hormone-receptor complex in the chromatin molecule and with the appearance of new, previously masked acceptor zones of the hormone binding. During the first 1-2 hours following the hormone injection the endogenous RNA-synthesizing activity of the nuclei is decreased. The increase of RNA synthesis in liver nuclei occurs not earlier than 3 hours after the injection. In Zajdela hepatoma nuclei the repression of RNA synthesis persists as long as 3 hours after the injection of dexamethasone. When RNA synthesis is determined in the nuclei in the presence of exogenous RNA-polymerase of E. coli in vitro, the increase in nuclear RNA synthesis can be observed beginning with the 30th min after the hormone injection. It is assumed that this effect is due to conformational changes in the chromatin structure, which are concomitant with the initial steps of association of the hormone-receptor complex.

[Sensitivity of intranuclear glucocorticoid complex from normal rat liver and Zajdela ascites hepatoma to ionic strength and nuclease treatment]

After 30 min of intraperitoneal injection of dexamethasone to adrenectomized rats about 15% of the label are incorporated into liver homogenate and only 1% of the cytosol-bound hormone is detected in the cell nuclei. The binding of the "in vitro" injected hormone by the nuclei does not obey the second-order reactions (the Scatchard plots). This is probably due to the existence of various ancillary mechanisms, which control the translocation of the hormone complex into cell nuclei at the level of cytoplasm and nuclear membranes. DNAase I, micrococcal nuclease and endogenous nucleolysis markedly increase the part of the nuclear hormone complex resistant to 0.4 M NaCl. In hepatocyte nuclei obtained by the collagenase method, the content of the 0.4 M NaCl-resistant receptor complex is also increased. The resistance of 0.4 M NaCl was also found in 80% of the glucocorticoid-insensitive nuclear complex from Zajdela ascite hepatoma cells. The changes in interaction of the hormone-receptor complex with nuclear acceptor sites eventually resulting in impaired sensitivity of host tissues to hormonal control can be due to the damage of chromatin structure induced by different influences and tumour growth.

[Interaction between the dexamethasone-receptor complex and Zajdela ascites hepatoma DNA]

The activated dexamethasone--receptor complex from rat liver cell cytoplasm and from Zajdela ascite hepatoma cell cytoplasm is bound to Sepharose-immobilized native DNA containing different numbers of recurrent sequences. The DNA-bound hormonereceptor complex is eluted as three peaks by varying concentrations of NaCl. The binding in independent of the number of DNA recurrent sequences and of the source of the cytoplasmic complex used.