Estrogen-induced post-transcriptional modulation of c-myc proto-oncogene expression in human breast cancer cells

Extricating the Association Between the Prognostic Factors of Colorectal Cancer

PURPOSE

Colorectal cancer (CRC) is one of the recurring and lethal gastrointestinal tract disease rankings as the primary cause of worldwide morbidity and mortality. In general, the tumour node metastasis (TNM) and Dukes classification assist in diagnosis, prognosis and treatments of CRC along with haematological examinations and tumour demographic characterisations in patients.

METHODS

The present investigation is carried out on clinically acknowledged sixty-five CRC patients based on haematological findings and are sorted into stages using TNM and Dukes. The present study is to find the association between haematological findings, demographic characters, differentiation position, lymph node invasion and tumour node metastasis in CRC patients in accordance with their age.

RESULTS

We observed significant (p < 0.05) nexus between lymph node metastasis and tumour node metastasis on the basis of tumour's differentiation demographic positioning and age of the individuals.

CONCLUSION

Earlier location tracing and medicinal treatment or surgery lessen the chance of CRC morbidity and mortality along with prolonging survival rate via prognostic factors and disease position determination.

SRC Increases MYC mRNA Expression in Estrogen Receptor-Positive Breast Cancer via mRNA Stabilization and Inhibition of p53 Function

The transcription factor gene MYC is important in breast cancer, and its mRNA is maintained at a high level even in the absence of gene amplification. The mechanism(s) underlying increased MYC mRNA expression is unknown. Here, we demonstrate that MYC mRNA was stabilized upon estrogen stimulation of estrogen receptor-positive breast cancer cells via SRC-dependent effects on a recently described RNA-binding protein, IMP1 with an N-terminal deletion (ΔN-IMP1). We also show that loss of the tumor suppressor p53 increased MYC mRNA levels even in the absence of estrogen stimulation. However, in cells with wild-type p53, SRC acted to overcome p53-mediated inhibition of estrogen-stimulated cell cycle entry and progression. SRC thus promotes cell proliferation in two ways: by stabilizing MYC mRNA and by inhibiting p53 function. Since estrogen receptor-positive breast cancers typically express wild-type p53, these studies establish a rationale for p53 status to be predictive for effective SRC inhibitor treatment in this subtype of breast cancer.

MYC functions are specific in biological subtypes of breast cancer and confers resistance to endocrine therapy in luminal tumours

BACKGROUND

MYC is amplified in approximately 15% of breast cancers (BCs) and is associated with poor outcome. c-MYC protein is multi-faceted and participates in many aspects of cellular function and is linked with therapeutic response in BCs. We hypothesised that the functional role of c-MYC differs between molecular subtypes of BCs.

METHODS

We therefore investigated the correlation between c-MYC protein expression and other proteins involved in different cellular functions together with clinicopathological parameters, patients' outcome and treatments in a large early-stage molecularly characterised series of primary invasive BCs (n=1106) using immunohistochemistry. The METABRIC BC cohort (n=1980) was evaluated for MYC mRNA expression and a systems biology approach utilised to identify genes associated with MYC in the different BC molecular subtypes.

RESULTS

High MYC and c-MYC expression was significantly associated with poor prognostic factors, including grade and basal-like BCs. In luminal A tumours, c-MYC was associated with ATM (P=0.005), Cyclin B1 (P=0.002), PIK3CA (P=0.009) and Ki67 (P<0.001). In contrast, in basal-like tumours, c-MYC showed positive association with Cyclin E (P=0.003) and p16 (P=0.042) expression only. c-MYC was an independent predictor of a shorter distant metastases-free survival in luminal A LN+ tumours treated with endocrine therapy (ET; P=0.013). In luminal tumours treated with ET, MYC mRNA expression was associated with BC-specific survival (P=0.001). In ER-positive tumours, MYC was associated with expression of translational genes while in ER-negative tumours it was associated with upregulation of glucose metabolism genes.

CONCLUSIONS

c-MYC function is associated with specific molecular subtypes of BCs and its overexpression confers resistance to ET. The diverse mechanisms of c-MYC function in the different molecular classes of BCs warrants further investigation particularly as potential therapeutic targets.

MiR-17~92 ablation impairs liver regeneration in an estrogen-dependent manner

As one of the most important post‐transcriptional regulators, microRNAs (miRNAs) participate in diverse biological processes, including the regulation of cell proliferation. MiR‐17~92 has been found to act as an oncogene, and it is closely associated with cell proliferation. However, its role in liver regeneration is still unclear. We generated a hepatocyte‐specific miR‐17~92‐deficient mouse and used a mouse model with 70% partial hepatectomy (PH) or intraperitoneal injection of carbon tetrachloride to demonstrate the role of MiR‐17~92 in liver regeneration. In quiescent livers, the expression of the miR‐17~92 cluster showed a gender disparity, with much higher expression in female mice. The expression of four members of this cluster was found to be markedly reduced after 70% PH. The ablation of miR‐17~92 led to obvious regeneration impairment during the early‐stage regeneration in the female mice. Ovariectomy greatly reduced miR‐17~92 expression but significantly promoted liver regeneration in wild‐type mice. In addition, early regeneration impairment in miR‐17~92‐deficient livers could be largely restored following ovariectomy. The proliferation suppressors p21 and Pten were found to be the target effectors of miR‐17~92. MiR‐17~92 disruption resulted in elevated protein levels of p21 and Pten in regenerating livers. MiR‐17~92 functions as a proliferation stimulator and acts in an oestrogen‐dependent manner. The loss of this miRNA results in increases in p21 and Pten expression and therefore impairs liver regeneration in female mice.

Identification of miR-26 as a key mediator of estrogen stimulated cell proliferation by targeting CHD1, GREB1 and KPNA2

Introduction

Estrogen signaling is pivotal in the progression of estrogen receptor positive breast cancer primarily by the regulation of cell survival and proliferation. Micro (mi)RNAs have been demonstrated to be regulated by estrogen to mediate estrogenic effects. Herein, we determined the role of estrogen regulated miR-26 and its underlying molecular mechanisms associated with estrogen receptor (ER)+ breast cancer proliferation.

Methods

The expression of miR-26a and miR-26b was evaluated by real-time quantitative (RT)-PCR. The expression of miR-26a or miR-26b was modulated in ER+ breast cancer cells (MCF-7 and T47D) and tumor cell growth in vitro and an in vivo xenograft model was determined. Bioinformatics analyses were utilized to screen for estrogen responsive genes, which were also predicted to be targeted by miR-26. Luciferase reporter assays were performed to confirm miR-26 regulation of the 3' UTR of target genes. The levels of miR-26 target genes (CHD1, GREB1 and KPNA2) were evaluated by western blotting and immunohistochemistry.

Results

Estrogen reduced the expression of miR-26a and miR-26b in ER+ breast cancer cells. Forced expression of miR-26a or miR-26b significantly inhibited the estrogen stimulated growth of ER+ breast cancer cells and tumor growth in xenograft models, whereas miR-26a/b depletion increased the growth of ER+ breast cancer cells in the absence of estrogen treatment. Screening of estrogen responsive genes, which were also predicted to be targeted by miR-26, identified GREB1 and nine other genes (AGPAT5, AMMECR1, CHD1, ERLIN1, HSPA8, KPNA2, MREG, NARG1, and PLOD2). Further verification has identified nine genes (AGPAT5, CHD1, ERLIN1, GREB1, HSPA8, KPNA2, MREG, NARG1 and PLOD2) which were directly targeted by miR-26 via their 3′ UTR. Functional screening suggested only three estrogen regulated miR-26 target genes (CHD1, GREB1 and KPNA2) were involved in the regulation of estrogen promoted cell proliferation. Depletion of either CHD1, GREB1 or KPNA2 significantly abrogated the enhanced growth of ER+ breast cancer cells due to miR-26 depletion. We further demonstrated that estrogen stimulated c-MYC expression was both sufficient and necessary for the diminished expression of miR-26a and miR-26b.

Conclusions

We have identified a novel estrogen/MYC/miR-26 axis that mediates estrogen stimulated cell growth via CHD1, GREB1 and KPNA2.

Cohesin is required for activation of MYC by estradiol

Cohesin is best known as a multi-subunit protein complex that holds together replicated sister chromatids from S phase until G2. Cohesin also has an important role in the regulation of gene expression. We previously demonstrated that the cohesin complex positively regulates expression of the oncogene MYC. Cell proliferation driven by MYC contributes to many cancers, including breast cancer. The MYC oncogene is estrogen-responsive and a transcriptional target of estrogen receptor alpha (ERα). Estrogen-induced cohesin binding sites coincide with ERα binding at the MYC locus, raising the possibility that cohesin and ERα combine actions to regulate MYC transcription. The objective of this study was to investigate a putative role for cohesin in estrogen induction of MYC expression. We found that siRNA-targeted depletion of a cohesin subunit, RAD21, decreased MYC expression in ER-positive (MCF7 and T47D) and ER-negative (MDA-MB-231) breast cancer cell lines. In addition, RAD21 depletion blocked estradiol-mediated activation of MYC in ER-positive cell lines, and decreased ERα binding to estrogen response elements (EREs) upstream of MYC, without affecting total ERα levels. Treatment of MCF7 cells with estradiol caused enrichment of RAD21 binding at upstream enhancers and at the P2 promoter of MYC. Enriched binding at all sites, except the P2 promoter, was dependent on ERα. Since RAD21 depletion did not affect transcription driven by an exogenous reporter construct containing a naked ERE, chromatin-based mechanisms are likely to be involved in cohesin-dependent MYC transcription. This study demonstrates that ERα activation of MYC can be modulated by cohesin. Together, these results demonstrate a novel role for cohesin in estrogen-mediated regulation of MYC and the first evidence that cohesin plays a role in ERα binding.

Estradiol-activated estrogen receptor α does not regulate mature microRNAs in T47D breast cancer cells

Breast cancers are sensitive to hormones such as estrogen, which binds to and activates estrogen receptors (ER) leading to significant changes in gene expression. microRNAs (miRNA) have emerged as a major player in gene regulation, thus identification of miRNAs associated with normal or disrupted estrogen signaling is critical to enhancing our understanding of the diagnosis and prognosis of breast cancer. We have previously shown that 17β-estradiol (E2) induced activation of ERα in T47D cells results in significant changes in the expression of protein-coding genes involved in cell cycle, proliferation, and apoptosis. To identify miRNAs regulated by E2-activated ERα, we analysed their expression in T47D cells following E2-activation using both dual-color microarrays and TaqMan Low Density Arrays, and validations were carried out by real-time PCR. Although estrogen treatment results in altered expression of up to 900 protein-coding transcripts, no significant changes in mature miRNA expression levels could be confirmed. Whereas previous studies aiming to elucidate the role of miRNA in ER-positive breast cancers cell lines have yielded conflicting results, the work presented here represents a thorough investigation of and significant step forward in our understanding of ERα mediated miRNA regulation.

Gene regulation by cohesin in cancer: is the ring an unexpected party to proliferation?

Cohesin is a multisubunit protein complex that plays an integral role in sister chromatid cohesion, DNA repair, and meiosis. Of significance, both over- and underexpression of cohesin are associated with cancer. It is generally believed that cohesin dysregulation contributes to cancer by leading to aneuploidy or chromosome instability. For cancers with loss of cohesin function, this idea seems plausible. However, overexpression of cohesin in cancer appears to be more significant for prognosis than its loss. Increased levels of cohesin subunits correlate with poor prognosis and resistance to drug, hormone, and radiation therapies. However, if there is sufficient cohesin for sister chromatid cohesion, overexpression of cohesin subunits should not obligatorily lead to aneuploidy. This raises the possibility that excess cohesin promotes cancer by alternative mechanisms. Over the last decade, it has emerged that cohesin regulates gene transcription. Recent studies have shown that gene regulation by cohesin contributes to stem cell pluripotency and cell differentiation. Of importance, cohesin positively regulates the transcription of genes known to be dysregulated in cancer, such as Runx1, Runx3, and Myc. Furthermore, cohesin binds with estrogen receptor α throughout the genome in breast cancer cells, suggesting that it may be involved in the transcription of estrogen-responsive genes. Here, we will review evidence supporting the idea that the gene regulation function of cohesin represents a previously unrecognized mechanism for the development of cancer.

Estrogen deficiency reversibly induces telomere shortening in mouse granulosa cells and ovarian aging in vivo

Estrogen is implicated as playing an important role in aging and tumorigenesis of estrogen responsive tissues; however the mechanisms underlying the mitogenic actions of estrogen are not fully understood. Here we report that estrogen deficiency in mice caused by targeted disruption of the aromatase gene results in a significant inhibition of telomerase maintenance of telomeres in mouse ovaries in a tissue-specific manner. The inhibition entails a significant shortening of telomeres and compromised proliferation in the follicular granulosa cell compartment of ovary. Gene expression analysis showed decreased levels of proto-oncogene c-Myc and the telomerase catalytic subunit, telomerase reverse transcriptase (TERT), in response to estrogen deficiency. Estrogen replacement therapy led to increases in TERT gene expression, telomerase activity, telomere length and ovarian tissue growth, thereby reinstating ovary development to normal in four weeks. Our data demonstrate for the first time that telomere maintenance is the primary mechanism mediating the mitogenic effect of estrogen on ovarian granulosa cell proliferation by upregulating the genes of c-Myc and TERT in vivo. Estrogen deficiency or over-activity may cause ovarian tissue aging or tumorigenesis, respectively, through estrogen regulation of telomere remodeling.

Telomerase in the ovary

Telomerase, an enzyme complex that binds the chromosome ends (telomeres) and maintains telomere length and integrity, is present in germ cells, proliferative granulosa cells, germline stem cells, and neoplastic cells in the ovary, but it is absent in differentiated or aged cells. Activation of telomerase in the ovary underpins both benign and malignant cell proliferation in several compartments, including the germ cells, membrana granulosa, and the ovarian surface epithelium. The difference in telomerase operation between normal and abnormal cell proliferations may lie in the mechanisms of telomerase activation in a deregulated manner. Recent studies have implicated telomerase activity in ovarian cancer as well as oogenesis and fertility. Inhibition of telomerase and the shortening of telomeres are seen in occult ovarian insufficiency. Studies of how telomerase operates and regulates ovary development may provide insight into the development of both germ cells for ovarian reproductive function and neoplastic cells in ovarian cancer. The current review summarizes the roles of telomerase in the development of oocytes and proliferation of granulosa cells during folliculogenesis and in the process of tumorigenesis. It also describes the regulation of telomerase by estrogen in the ovary.

The estrogen receptor-alpha-induced microRNA signature regulates itself and its transcriptional response

Following estrogenic activation, the estrogen receptor-alpha (ERalpha) directly regulates the transcription of target genes via DNA binding. MicroRNAs (miRNAs) modulated by ERalpha have the potential to fine tune these regulatory systems and also provide an alternate mechanism that could impact on estrogen-dependent developmental and pathological systems. Through a microarray approach, we identify the subset of microRNAs (miRNAs) modulated by ERalpha, which include upregulation of miRNAs derived from the processing of the paralogous primary transcripts (pri-) mir-17-92 and mir-106a-363. Characterization of the mir-17-92 locus confirms that the ERalpha target protein c-MYC binds its promoter in an estrogen-dependent manner. We observe that levels of pri-mir-17-92 increase earlier than the mature miRNAs derived from it, implicating precursor cleavage modulation after transcription. Pri-mir-17-92 is immediately cleaved by DROSHA to pre-miR-18a, indicating that its regulation occurs during the formation of the mature molecule from the precursor. The clinical implications of this novel regulatory system were confirmed by demonstrating that pre-miR-18a was significantly upregulated in ERalpha-positive compared to ERalpha-negative breast cancers. Mechanistically, miRNAs derived from these paralogous pri-miRNAs (miR-18a, miR-19b, and miR-20b) target and downregulate ERalpha, while a subset of pri-miRNA-derived miRNAs inhibit protein translation of the ERalpha transcriptional p160 coactivator, AIB1. Therefore, different subsets of miRNAs identified act as part of a negative autoregulatory feedback loop. We propose that ERalpha, c-MYC, and miRNA transcriptional programs invoke a sophisticated network of interactions able to provide the wide range of coordinated cellular responses to estrogen.

Oestrogen, telomerase, ovarian ageing and cancer

1. Oestrogen plays an important role in ageing and ageing-related development. Lack of oestrogen prompts endocrine cell ageing of the ovary, whereas oestrogen overflow impacts on epithelial cell neoplastic development. 2. Recent studies indicate that oestrogen regulates cell proliferative fates by a mechanism of reprogramming the size of telomeres (ends of chromosomes) in the oestrogen target cells. This is achieved by upregulating the telomerase reverse transcriptase (TERT) gene in a temporal and spatial manner. 3. Currently, the relationship between oestrogen and telomerase activity in regulating productive cell development and function remains elusive. A number of lines of evidence suggest that telomerase is a downstream target of oestrogen in oestrogen-dependent reproductive ageing and neoplastic development. 4. The present minireview discusses our current understanding of the mechanisms by which telomerase maintenance of telomere homeostasis mediates oestrogen-induced ageing and tumourigenesis in the ovary under physiological and pathological conditions.

Estrogen deficiency leads to telomerase inhibition, telomere shortening and reduced cell proliferation in the adrenal gland of mice

Estrogen deficiency mediates aging, but the underlying mechanism remains to be fully determined. We report here that estrogen deficiency caused by targeted disruption of aromatase in mice results in significant inhibition of telomerase activity in the adrenal gland in vivo. Gene expression analysis showed that, in the absence of estrogen, telomerase reverse transcriptase (TERT) gene expression is reduced in association with compromised cell proliferation in the adrenal gland cortex and adrenal atrophy. Stem cells positive in c-kit are identified to populate in the parenchyma of adrenal cortex. Analysis of telomeres revealed that estrogen deficiency results in significantly shorter telomeres in the adrenal cortex than that in wild-type (WT) control mice. To further establish the causal effects of estrogen, we conducted an estrogen replacement therapy in these estrogen-deficient animals. Administration of estrogen for 3 weeks restores TERT gene expression, telomerase activity and cell proliferation in estrogen-deficient mice. Thus, our data show for the first time that estrogen deficiency causes inhibitions of TERT gene expression, telomerase activity, telomere maintenance, and cell proliferation in the adrenal gland of mice in vivo, suggesting that telomerase inhibition and telomere shortening may mediate cell proliferation arrest in the adrenal gland, thus contributing to estrogen deficiency-induced aging under physiological conditions.

Overexpression of c-Myc and Bcl-2 during progression and distant metastasis of hormone-treated breast cancer

The aim of this study was to identify molecules involved in the proliferation and survival of recurrent estrogen receptor (ER)-positive breast cancer at the site of metastasis. Most studies of biomarkers are done using the initial primary breast tumor whereas pathological studies of breast cancer lesions after distant recurrence are scarce. Here we evaluated the expression of the oncogenes c-Myc and Bcl-2, mediators of estrogen-dependent proliferation and survival, during breast cancer progression and relapse after adjuvant hormonal therapy. Using a preclinical model of tamoxifen-resistant growth, we found overexpression of c-Myc in all (3/3) and of Bcl-2 in most (2/3) tamoxifen resistant-breast cancer variants. To determine whether c-Myc and Bcl-2 are expressed during breast cancer progression in the clinics we identified breast cancer patients who had received adjuvant hormonal therapy for the treatment of their localized disease and had later experienced relapse. From 583 patients who had received adjuvant hormonal therapy a total of 82 experienced recurrence. Nevertheless, only 22 patients had had a biopsy of their metastatic lesion done after relapse. Twenty-one biopsies were useful for this biomarker study. These biopsies were obtained mostly (20) from breast cancer patients who had received tamoxifen as their adjuvant hormonal therapy. One patient had received an aromatase inhibitor instead. Our results showed that almost all (20) metastatic recurrences expressed ER. Expression of c-Myc was observed in 18 out of 19 metastatic lesions scored while expression of Bcl-2 was detected in 17 out of 21 metastatic tumors. A correlation between ER expression and Bcl-2, but not with c-Myc, was found in these recurrent metastatic lesions. In addition, c-Myc expression was correlated with the nuclear grade of the metastatic lesion. Thus, the frequent expression of c-Myc and Bcl-2 in metastatic breast cancer recurrences suggests that combining hormonal therapy with strategies to block c-Myc and Bcl-2 may prevent growth of ER-positive breast cancer at the site of metastasis.

Frequency, prognostic impact, and subtype association of 8p12, 8q24, 11q13, 12p13, 17q12, and 20q13 amplifications in breast cancers

Background

Oncogene amplification and overexpression occur in tumor cells. Amplification status may provide diagnostic and prognostic information and may lead to new treatment strategies. Chromosomal regions 8p12, 8q24, 11q13, 17q12 and 20q13 are recurrently amplified in breast cancers.

Methods

To assess the frequencies and clinical impact of amplifications, we analyzed 547 invasive breast tumors organized in a tissue microarray (TMA) by fluorescence in situ hybridization (FISH) and calculated correlations with histoclinical features and prognosis. BAC probes were designed for: (i) two 8p12 subregions centered on RAB11FIP1 and FGFR1 loci, respectively; (ii) 11q13 region centered on CCND1; (iii) 12p13 region spanning NOL1; and (iv) three 20q13 subregions centered on MYBL2, ZNF217 and AURKA, respectively. Regions 8q24 and 17q12 were analyzed with MYC and ERBB2 commercial probes, respectively.

Results

We observed amplification of 8p12 (amplified at RAB11FIP1 and/or FGFR1) in 22.8%, 8q24 in 6.1%, 11q13 in 19.6%, 12p13 in 4.1%, 17q12 in 9.9%, 20q13^Z (amplified at ZNF217 only) in 9.9%, and 20q13^Co (co-amplification of two or three 20q13 loci) in 8.5% of cases. The 8q24, 12p13, and 17q12 amplifications were correlated with high grade. The most frequent single amplifications were 8p12 (9.8%), 8q24 (3.3%) and 12p13 (3.3%), 20q13^Z and 20q13^Co (1.6%) regions. The 17q12 and 11q13 regions were never found amplified alone. The most frequent co-amplification was 8p12/11q13. Amplifications of 8p12 and 17q12 were associated with poor outcome. Amplification of 12p13 was associated with basal molecular subtype.

Conclusion

Our results establish the frequencies, prognostic impacts and subtype associations of various amplifications and co-amplifications in breast cancers.

Survival signals generated by estrogen and phospholipase D in MCF-7 breast cancer cells are dependent on Myc

Estrogens, which have been strongly implicated in the development of breast cancer, enhance proliferation of mammary epithelial cells and, importantly, estrogen receptor (ER)-positive breast cancer cells. In the absence of serum growth factors, the ER-positive MCF-7 breast cancer cell line undergoes apoptosis. Estrogens, most commonly 17-beta-estradiol (E2), can suppress apoptosis in MCF-7 cells deprived of serum. While E2 stimulated a short-term transient increase in Myc expression, E2 stimulated a sustained increase in Myc expression that was detectable at 48 h and pronounced at 5 days, the point where increased proliferation of MCF-7 cells in the absence of serum could be detected. The delayed increase in Myc expression was not dependent upon transcription of the Myc gene. Suppression of Myc expression reversed the survival effects of E2. The Myc-dependent survival signal generated by E2 was dependent upon basal levels of mTOR (mammalian target of rapamycin) and two upstream regulators of mTOR, phosphatidylinositol 3-kinase and phospholipase D (PLD). Stable elevated expression of PLD2 also increased Myc expression and provided a Myc-dependent survival signal in the absence of E2. These data provide evidence that E2 promotes survival signals in breast cancer cells through an mTOR-dependent increase in Myc expression. The data also suggest that elevated PLD expression, which is common in breast cancer, confers E2 independence.

Hormones and growth factors regulate telomerase activity in ageing and cancer

Telomerase is a specialised reverse transcriptase that synthesises and preserves telomeres (the ends of chromosomes), thereby playing a key role in regulating the lifespan of cell proliferation. Telomerase activity is critically involved in cell development, ageing and tumourigenesis. Activation of telomerase to maintain telomeres is required for self renewal and proliferative expansion of a number of cell types, including stem cells, activated lymphocytes and cancerous cells. However, recent studies show that the safeguard mechanisms and the modes of regulation of telomerase are more revealing than thought under various physiological and pathological conditions. Considerable evidence suggests that hormones and growth factors are crucially involved in regulating telomerase activity and gene expression of telomerase reverse transcriptase (TERT). This review briefly summarises our current understanding of how hormones and growth factors regulate the telomerase and telomere network and how deregulation can induce ageing and related diseases such as cancer.

Transcriptional and posttranscriptional regulation of fibulin-1 by estrogens leads to differential induction of messenger ribonucleic acid variants in ovarian and breast cancer cells

Fibulin-1 is an extracellular matrix protein overexpressed in epithelial ovarian and breast cancers. In estrogen receptor (ER)-positive ovarian and breast cancer cell lines, fibulin-1 mRNA levels are markedly increased by estrogens. Transfection experiments using fibulin-1 promoter constructs indicate that 17beta-estradiol (E2) increases fibulin-1 gene transcription and that ERalpha is more potent than ERbeta to mediate E2 regulation of the transfected fibulin-1 promoter. Using SL2 cells devoid of specificity protein 1 (Sp1) and site-directed mutagenesis of GC boxes, we evidenced that the E2 regulation occurs through a proximal specificity protein 1 binding site. In addition, we show that fibulin-1C and -1D mRNAs, the two major fibulin-1 splicing variants, are differentially induced by E2. The induction of both mRNAs variants is direct and independent of a newly synthesized protein intermediate. Interestingly, actinomycin D chase experiments demonstrate that E2 treatment selectively shortens the fibulin-1D mRNA half-life. This indicates that estrogens affect differentially the stability of fibulin-1 variants and may explain the lower accumulation of fibulin-1D mRNA on E2 treatment. In conclusion, our data show that estrogens, via ERalpha, are key regulators of fibulin-1 expression at both the transcriptional and posttranscriptional levels. The preferential induction of the fibulin-1C variant, which is overexpressed in ovarian and breast cancer, might play an important role in estrogen-promoted carcinogenesis.

Glycolysis and glucose transporter 1 as markers of response to hormonal therapy in breast cancer

Estrogen plays a key role in the development and progression of breast cancer; hence, antiestrogens, such as tamoxifen, have a marked impact on the treatment and outcome of breast cancer patients. Estrogen-induced growth requires continuous replenishment of energy, predominantly generated by glycolysis. Previous work from this laboratory demonstrated estrogen induction and tamoxifen inhibition of glycolysis in MCF7 human breast cancer cells in vitro (Furman et al., J Steroid Biochem Mol Biol 1992;43:189-95). We present here studies of estrogen vs. tamoxifen regulation of glycolysis in orthotopic MCF7 human breast cancer xenografts in vivo. In addition we investigated mediation of this metabolic regulation through glucose transporter 1, in the same cells, in vitro, as well as in 2 other hormone-responsive human breast cancer cells. Tumor response and glycolysis were monitored noninvasively by means of magnetic resonance imaging and 13C spectroscopy, respectively. During estrogen-stimulated tumor growth (from approximately 0.5 to approximately 1.3 cm3 in 10 days), the rate of glucose metabolism through glycolysis in vivo was high at 40 +/- 4 micromole/g/min. However, treatment for 10 days with tamoxifen induced growth arrest and a concomitant decrease of 2-fold in the rate of glycolysis. In congruence, glucose transporter 1 expression was stimulated by estrogen, reaching after 72 hr a 2- to 3-fold higher level of expression relative to that in tamoxifen-treated cells. Thus, estrogen-induced changes in glycolysis appeared to be mediated via its regulation of glucose transporter 1 expression. The in vivo monitoring of glycolysis may serve as a tool to expose hormonal regulation of glucose transporter 1 expression in breast cancer tumors, as well as to assess response to hormonal therapy.

Phospholipase D confers rapamycin resistance in human breast cancer cells

mTOR (mammalian target of rapamycin) is a protein kinase that regulates cell cycle progression and cell growth. Rapamycin is a highly specific inhibitor of mTOR in clinical trials for the treatment of breast and other cancers. mTOR signaling was reported to require phosphatidic acid (PA), the metabolic product of phospholipase D (PLD). PLD, like mTOR, has been implicated in survival signaling and the regulation of cell cycle progression. PLD activity is frequently elevated in breast cancer. We have investigated the effect of rapamycin on breast cancer cell lines with different levels of PLD activity. MCF-7 cells, with relatively low levels of PLD activity, were highly sensitive to the growth-arresting effects of rapamycin, whereas MDA-MB-231 cells, with a 10-fold higher PLD activity than MCF-7 cells, were highly resistant to rapamycin. Elevating PLD activity in MCF-7 cells led to rapamycin resistance; and inhibition of PLD activity in MDA-MB-231 cells increased rapamycin sensitivity. Elevated PLD activity in MCF-7 cells also caused rapamycin resistance for S6 kinase phosphorylation and serum-induced Myc expression. These data implicate mTOR as a critical target for survival signals generated by PLD and suggest that PLD levels in breast cancer could be a valuable indicator of the likely efficacy of rapamycin treatment.

Estrogen and Myc negatively regulate expression of the EphA2 tyrosine kinase

Estrogen receptor and c-Myc are frequently overexpressed during breast cancer progression but are downregulated in many aggressive forms of the disease. High levels of the EphA2 tyrosine kinase are consistently found in the most aggressive breast cancer cells, and EphA2 overexpression can increase metastatic potential. We demonstrate, herein, that estrogen and Myc negatively regulate EphA2 expression in mammary epithelial cells. These data reveal EphA2 as a downstream target of estrogen and Myc and suggest a mechanism by which estrogen and Myc may regulate breast cancer.

Tibolone: what does tissue specific activity mean?

Preclinical studies has found that tibolone can display a weak estrogenic, progestational and androgenic activity. The effect produced depends mainly on the target tissue involved. Clinical data indicate that tibolone produces the hormonal effects needed to treat climacteric symptoms and to prevent long-term effects of the menopause without stimulating breast and endometrial tissues. This clinical profile would be produced by the tissue specific activity of tibolone and its main metabolites. Tibolone's tissue specific activity may be explained by the interplay of several mechanisms, such as the metabolic conversion of tibolone to compounds with different biological activity, the classical interaction with the steroid receptor and the specific local metabolism, within the target tissue. Therefore, the tissue response and thus the clinical effect produced by tibolone in a given tissue seem to depend on the predominating mechanisms and interactions present in that tissue.

Determination of c-myc amplification and overexpression in breast cancer patients: evaluation of its prognostic value against c-erbB-2, cathepsin-D and clinicopathological characteristics using univariate and multivariate analysis

C-myc and c-erbB-2 amplification and/or overexpression as well as total cathepsin-D (CD) concentration have been reported to be associated with poor prognosis in breast cancer. The prognostic significance, however, remains somewhat controversial, partly because of discrepancies among the different methodologies used. We determined the amplification and overexpression of c-myc oncogene in 152 breast cancer patients and examined its prognostic value in relation to c-erbB-2 amplification and overexpression, high concentration of CD (> or = 60 pmol mg(-1) protein) and standard clinicopathological prognostic factors of the disease. High CD concentration, as well as c-myc amplification and overexpression, proved to be the best of the new variables examined for prediction of early relapse (ER; before 3 years). After multivariate analysis only CD remained significant, which suggests that the prognostic power of these variables is similar. Using univariate analysis we proved that c-myc amplification and overexpression were highly significant for disease-free survival (DFS) (P = 0.0016 and P = 0.0001 respectively) and overall survival (OS) (P < 0.0001 and P = 0.0095 respectively), although by multivariate analysis c-myc overexpression was statistically significant only for DFS (P = 0.0001) and c-myc amplification only for OS (P = 0.0006). With regard to c-erbB-2, only its overexpression appeared to be significant for DFS and OS, although after multivariate analysis its prognostic power was weaker (P = 0.030 and P = 0.024 respectively). c-myc amplification and overexpression exhibited a tendency for locoregional recurrence (LRR) (P = 0.0024 and P = 0.0075 respectively), however, their prognostic value was lower after multivariate analysis and only CD remained significant.

Differential display competitive polymerase chain reaction: an optimal tool for assaying gene expression

Gene discovery, i.e. detection of genes whose expression is affected in diseases or by different treatments of cells or animals, has become the focus of much genetic research. The technologies that are used to detect changes in expression level include polymerase chain reaction (PCR)-based subtraction methods, arrays of cDNA clones on chips or filters, serial analysis of gene expression, and differential display. In this paper we show that differential display can be used to investigate global gene expression in situations where a few genes change expression levels such as exposure of MCF7 cells to estradiol, and in more complex situations such as neuronal differentiation of human NTERA2 cells which affects a large number of genes. Furthermore, we show that differential display can replace Northern blotting and RNase protection as a tool to study the expression level of a specific gene in many samples. Results obtained by differential display can be stored in databases, where the identity of a band (gene or mRNA name) can be linked with information about the primer combination displaying the band and a gel image showing the band pattern, which is all the information that is needed to compare the expression level of this gene in other samples.

The growth inhibitory effect of conjugated linoleic acid on MCF-7 cells is related to estrogen response system

Conjugated linoleic acid (CLA) has been shown to have a direct oncostatic action on MCF-7 human breast cancer cells in culture. However, the mechanism involved is not fully elucidated. In this study we have examined whether the inhibitor action is related to the estrogen responsiveness of MCF-7 cells. Our results demonstrate that CLA selectively inhibits proliferation of ER positive MCF-7 cells as compared with ER negative MDA-MB-231 cells. Cell cycle studies indicated that a higher percentage of CLA treated MCF-7 cells remained in the G0/G1 phase as compared to control and those treated with linoleic acid (LA). CLA also inhibited expression of c-myc in MCF-7 cells. These results demonstrate that CLA may inhibit MCF-7 cell growth by interfering with the hormone regulated mitogenic pathway. We are reporting for the first time the involvement of CLA, a dietary factor, in the regulation of hormone mediated mitogenic pathways in ER positive breast cancer cell proliferation in vitro.

Growth arrest associated changes of mRNA levels in breast cancer cells measured by semi-quantitative RT-PCR: potential early indicators of treatment response

To find early and sensitive indicators of treatment response in breast cancer, we studied the mRNA levels of proliferation-related genes during growth arrest of the human breast cancer cell lines T47D and MCF7. A sensitive reverse transcriptase-PCR (RT-PCR) technique was used in order to monitor gene expression in small samples of cells. Estrogen-depletion and treatment with tamoxifen effectively induced a G1-arrest in both cell lines, accompanied by a decrease of the mRNA levels of histone H4, cyclin A, cyclin D1, and c-myc. Cyclin A expression decreased most strongly: up to 32-fold within 7 days. The expression of c-fos and WAF1 increased during growth arrest. In conclusion, significant changes of the levels of proliferation-related mRNAs, induced by growth arrest, can be measured in small samples of breast carcinoma cells using RT-PCR. Especially the decrease of the cyclin A mRNA level seems a potential early indicator of clinical response to tamoxifen therapy in breast cancer patients.

Estradiol stimulates c-myc proto-oncogene expression in normal human breast epithelial cells in culture

The proto-oncogene c-myc is involved in the stimulation of cell proliferation, and its expression is known to be stimulated by estradiol (E2) in human breast cancer cell lines and various non-cancerous E2-dependent tissues. However, little information is currently available concerning its expression and regulation in normal human breast tissue. We therefore studied c-myc expression and hormone modulation in normal human breast epithelial (HBE) cells in culture, routinely obtained in our laboratory and which remain hormone-dependent. On these normal HBE cells, E2 induced a biphasic increase in c-myc mRNA level, with a first peak as early as 30 min, and a secondary increase after 2 h of treatment; this stimulation was dose-dependent, with an optimal concentration of 10 nM E2. Its primary action is probably at the transcriptional level since the half-life of c-myc mRNA measured in the presence of actinomycin D (12 +/- 3 min) was not modified by E2 treatment. In addition, E2 stimulation of c-myc mRNA does not require protein synthesis since it was not suppressed by cycloheximide treatment. Western blot studies of c-myc protein in HBE cells revealed the same biphasic pattern of stimulation, with a first peak after 60 min and a second one after 2 h of E2 treatment. In conclusion, the c-myc proto-oncogene is expressed in normal HBE cells, as in breast cancer cells. Moreover, E2 stimulates c-myc expression which, therefore, may partly mediate the growth-promoting effect of E2.

Transcriptional activation of c-myc proto-oncogene by estrogen in human ovarian cancer cells

NIH:OVCAR-3 is a human ovarian cancer cell line that expresses a moderate amount of estrogen receptors, 28 fmol/mg protein. We have found that estrogen at a concentration of 10(-7) M induced a 2.3-fold increase in the growth of NIH:OVCAR-3 cells after 48 h stimulation. A 4-fold increase in c-myc mRNA expression at 30 min and a 7.5-fold increase at 50 min post-induction with estradiol were observed. Nuclear run-on analysis indicated that c-myc transcripts increased 4-fold within 10 min of estrogen addition. The half-life of c-myc mRNA was 64 min +/- 5 min and was not affected by estrogen. Antisense oligonucleotide to c-myc specifically inhibited the estrogen stimulated c-myc protein expression as well as the growth of NIH:OVCAR-3 cells. A control ovarian cancer cell line OC-3-VGH that had few estrogen receptors (1 fmol/mg protein) did not respond to estrogen in growth; however, these cells respond to estrogen with a 1.5-fold increase in c-myc mRNA. The stability of c-myc mRNA of these cells was not affected by estrogen. Our results indicate that transcriptional induction of c-myc expression by estrogen plays a critical role in the proliferation of NIH:OVCAR-3 cells.

Modulation of Bcl-2 and Ki-67 expression in oestrogen receptor-positive human breast cancer by tamoxifen

The expression of the bcl-2 proto-oncogene, which is associated with prolonged cell survival and prevention of programmed cell death, was investigated in human primary breast carcinomas prior to and following endocrine therapy with the anti-oestrogen, tamoxifen. Using the BCL-2-100 antibody, a 26-kD protein was detected by western immunoblot in the cytosols of oestrogen receptor (ER)+ve human breast cancers. In a cross-sectional study, the immunohistochemical expression of Bcl-2 was observed in 32% of invasive breast cancers, but in 65% of tumours treated with tamoxifen (P = 0.009). There was a significant association of Bcl-2 with ER status, with 64% of untreated and 88% of tamoxifen-treated Bcl-2-positive tumours being ER+ve. A significantly lower Ki-67 score was found in tamoxifen-treated tumours which were Bcl-2-positive compared with Bcl-2-negative (9.3 versus 24.6%, P = 0.01). In a separate series of sequential Trucut biopsies from 18 patients, the frequency of Bcl-2 expression was increased in ER+ve tumours from 3/12 to 8/11 following tamoxifen (P = 0.04). This was also associated with a significant reduction in mean Ki-67 score from 32 to 12% (P = 0.0004). The observations from this study clearly indicate that Bcl-2 in human breast cancer is associated with ER status, and that expression is enhanced in ER+ve tumours following tamoxifen, in association with reduced cell proliferation.

Sex differences in the incidence of colorectal cancer: an exploration of oestrogen and progesterone receptors

Sex differences exist in the site specific incidences of colorectal cancer. The increased incidence of colonic cancer in women with breast cancer and the protective effect of increasing parity suggest a role for sex hormones. To explore the molecular basis, the expression of messenger RNA for oestrogen and progesterone receptors in the large bowel has been studied. With northern and dot blot analyses mRNA coding for oestrogen receptor and progesterone receptor in large bowel cancers and corresponding normal mucosa and in adenomatous polyps has been identified. There were no significant differences in receptor mRNA concentrations between males and females or between cancers, normal mucosae, and polyps, except for rectal cancers, which had higher progesterone receptor concentrations than corresponding normal tissue. Oestrogen and progesterone receptor mRNA concentrations were strongly correlated in both cancers and normal tissues. Enzyme immunoassay for oestrogen receptor gave values of 1.2-7.4 fmol/mg total protein, an amount similar to that seen in normal breast tissue. Oestrogen receptor protein and mRNA for oestrogen receptor and progesterone receptor are present in the large bowel.

Activated oncogenes and putative tumor suppressor genes involved in human breast cancers

Cytogeneticists first proposed that the karyotypic abnormalities identified on chromosomes 1, 3, 6, 11, 13, 16, 17, and 18 supported a genetic basis for breast cancer. Such abnormal banding patterns, however, may represent either loss-of-function or gain-of-function molecular events. RFLP analyses have since confirmed that 20-60% of primary and spontaneous human breast tumors exhibit allelic losses on these same chromosomes, although the exact genes involved at these chromosomal sites remain largely unknown. Knowledge gained about the Rb-1 and p53 tumor suppressor genes at 13q14 and 17p13 in breast and other human tumors supports the paradigm that for any chromosomal locus, allelic loss associated with a mutation in the remaining tumor allele signifies an involved tumor suppressor gene. Given this paradigm, there are nearly a dozen putative breast tumor suppressor genes under active investigation, with most investigators now focusing on various chromosome 17 loci. Among the known proto-oncogenes found activated in breast cancer, amplification of c-erbB-2 at 17q21 is the most widely studied and clinically significant gain-of-function event uncovered to date, occurring in about 20% of all primary breast tumors. The involvement of this overexpressed membrane receptor has engendered interest in related tyrosine kinase receptors, such as EGFR, IR, and IGF-I-R, as well as their respective ligands, which may be overexpressed in a greater fraction of tumors, contributing to the autocrine and paracrine regulation of breast cancer growth and metastasis. New attention is being given to the potentially oncogenic function of structurally altered nuclear transactivating steroid hormone receptors, such as ER, whose overexpression has long been used to determine endocrine therapy and prognosis for individual breast cancer patients. While c-myc was one of the first known proto-oncogenes to be found amplified and overexpressed in human breast cancers, the actual incidence and clinical significance of its activation remain disputed and in need of further study. Lastly, we can expect greater clarification about the importance of various 11q13 genes found coamplified in nearly 20% of primary breast cancers, and pursuit into the intriguing possibility that a cyclin-encoding gene represents the overexpressed locus of real interest in this amplicon. Virtually all of these important genetic abnormalities identified thus far are associated with but not restricted to human breast cancers. The absence of identifiable molecular defects relating to the tissue specificity of this malignancy must be considered a substantial gap in our basic understanding of breast carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Towards a molecular basis for tamoxifen resistance in breast cancer

Breast cancer patients who acquire tamoxifen resistance may respond to second-line hormonal therapy or progress to true endocrine resistance. The biological basis for these processes are poorly understood. Following successful therapy with tamoxifen there is little evidence at relapse for change in either the host endocrine environment or drug metabolic profile to account for the development of acquired resistance. Many tamoxifen resistant tumours still retain a structurally and functionally normal oestrogen receptor (ER) and yet will grow independent of oestrogen. The oestrogen-regulated molecular events which normally govern the growth of hormone-sensitive breast cancer involve a complex autocrine and paracrine interaction between several peptide growth factors (including TGF alpha, IGF-1 and TGF beta), their receptors and signal transduction pathways. Evidence now exists that constitutive activity of many of these mediators of the mitogenic signal can bypass the cell's dependence on oestrogen and provide a mechanism for hormone-independent growth. Research into these molecular mechanisms may result in a better understanding of how to overcome the clinical problem of tamoxifen resistance.

Estrogen-dependent, tamoxifen-resistant tumorigenic growth of MCF-7 cells transfected with HER2/neu

Since the poor prognosis associated with HER2 amplified breast cancers might be explained by a mechanistic association between p185HER2 overexpression and therapeutic resistance, we assessed the chemo-endocrine sensitivity of estrogen receptor (ER) containing MCF-7 breast cancer cells transfected with full-length HER2 cDNA. Of the 36 isolated MCF/HER2 subclones, 7 were found to overexpress p185HER2 surface receptor at levels 3 to 45-fold greater than parental or control transfected cells (MCF/neo). The overexpressing transfectants possessed increased inositol-1,4,5-triphosphate-3'-kinase activity comparable to enzyme activity in the endogenously HER2 amplified breast cancer cell lines SK-Br-3 and BT-474. The anti-p185HER2 monoclonal antibody and receptor-specific partial agonist, muMAb4D5 (4D5), known to inhibit growth of SK-Br-3 and BT-474 cells, produced no significant growth inhibitory effect on any of the transfectants including the 45-fold overexpressing MCF/HER2-18 cells which were studied in greater detail. MCF/HER2-18 cells contained at least partially functioning exogenous receptor since 4D5 (3 micrograms/ml) specifically stimulated phosphorylation of p185HER2 and its co-precipitating ptyr56 substrate within 5 min, and this was followed at 1 h by a transient induction of c-myc but not c-fos mRNA. ER content and the in vitro sensitivity of MCF/HER2-18 cells to 5-fluorouracil and adriamycin were identical to those of control transfectants and parental cells. However, these highly overexpressing transfectants had acquired low level (2 to 4-fold) resistance to cisplatin and were no longer sensitive to the antiestrogen tamoxifen (TAM). To compare the hormone-dependent tumorigenicity of the HER2 transfectants, MCF/HER2-18 and control cells (MCF, MCF/neo-3) were implanted into ovariectomized athymic nude mice. No tumors were produced in the absence of estradiol (E2) administration. In E2 supplemented mice, MCF/HER2-18 tumors grew most rapidly. When E2 treatment was stopped and daily TAM injections were initiated, MCF-7 and MCF/neo-3 tumor growth ceased immediately, while MCF/HER2-18 tumors continued to show an accelerated growth rate lasting weeks. This pattern of hormone-dependent, TAM-resistant growth exhibited by the MCF/HER2-18 tumors in nude mice supports the possibility that p185HER2 overexpression in human breast cancers may be linked to therapeutic resistance.

Inhibitory effect of 17 beta -estradiol on prostaglandin E2-induced phosphoinositide hydrolysis in osteoblast-like cells

We examined the effect of estradiol on PGE2-induced phosphoinositide hydrolysis and cAMP production in cloned osteoblast-like MC3T3-E1 cells. 17 beta -Estradiol pretreatment significantly inhibited the formation of inositol phosphates induced by 10 microM PGE2 in a dose-dependent manner between 1 pM and 10 nM. This effect of 17 beta -estradiol was dependent on the time of pretreatment and submaximum inhibition was observed at 4 h. However, 17 beta -estradiol had little effect on the formation of inositol phosphates induced by 20 mM NaF, a GTP-binding protein activator. The cAMP production induced by PGE2 was not influenced by 17 beta -estradiol. These results suggest that 17 beta -estradiol modulates the signal transduction by PGE2 and that the effect seems to be exerted between PGE2 receptor and the GTP-binding protein coupled to phospholipase C in osteoblast-like MC3T3-E1 cells.

1,25-Dihydroxyvitamin D3 stimulates both alkaline phosphatase gene transcription and mRNA stability in human bone cells

We have previously reported that 1,25(OH)2D3 stimulated the cellular alkaline phosphatase (ALP) activity and increased the steady-state level of ALP mRNA in a human osteosarcoma cell line (TE-85), under serum-free conditions. To define the molecular mechanism by which 1,25(OH)2D3 acts to stimulate ALP activity, the time courses of the increases in ALP activity and in the steady-state ALP mRNA level in response to 1,25(OH)2D3 were evaluated. 1,25(OH)2D3 progressively increased the steady-state level of ALP mRNA from 5 to 24 h of treatment, at which time a plateau was observed. In contrast, no significant increase in ALP-specific activity was detected until after 10 h of treatment, at which time the activity increased linearly with time up to 72 h. These time courses are consistent with the premise that the increased ALP activity was the result of increased gene expression. Nuclear runoff analysis indicated that the transcription rate of the ALP gene was more than five-fold higher in the 1,25(OH)2D3-treated cells than in the control cells. In addition, it was found that 1,25(OH)2D3 treatment increased ALP mRNA stability. The 1,25(OH)2D3-induced increase in ALP mRNA stability was not due to an interaction of the 1,25(OH)2D3-receptor complex with the ALP mRNA, since the removal of 1,25(OH)2D3 did not abolish its stabilizing effect. In the presence of cycloheximide, the stabilizing effect of 1,25(OH)2D3 was abolished, suggesting that a 1,25(OH)2D3-inducible protein factor was involved. Based on these findings, we have proposed a model in which 1,25(OH)2D3 stimulated ALP activity in human bone cells through mechanisms involving both (1) increased transcription of the ALP gene and (2) increased stability of ALP mRNA, an effect which requires the de novo synthesis of a protein, a putative ALP mRNA "stabilizing factor."

C-myc expression is maintained during the G1 phase cell cycle block produced by beryllium

Salts of the toxic metal beryllium have been shown previously to prevent the synthesis of several enzymes essential for DNA replication in proliferating rat hepatic cells in vivo, and to inhibit the division of rat liver-derived BL9L epithelial cells in vitro, specifically during the G1 phase of the cell cycle. The present study shows, however, that exposure of serum-stimulated sub-confluent monolayer cultures of synchronized BL9L cells to inhibitory concentrations of the beryllium salt BeSO4 (50 microM) did not impair expression of the cell proliferation associated nuclear proto-oncogene c-myc. On the contrary, the increased c-myc mRNA levels normally observed during the G1 phase were maintained by continuous exposure of the cells to BeSO4. This response was specific in that other colloid forming metal salts (ZnSO4 and ZrSO4), which did not inhibit cell division, had no affect on c-myc expression, and mRNA levels for the constantly expressed H-2Kb major histocompatibility complex gene (3'Kb) were unaltered by BeSO4 treatment of the cells. The prevention by Be2+ of the down-regulation of c-myc expression in serum-stimulated BL9L cells appears to result from a modulation of the endogenous transcriptional control process for c-myc, which allows a maintained expression of the gene.

Glucocorticoids repress ribosome biosynthesis in lymphosarcoma cells by affecting gene expression at the level of transcription, posttranscription and translation

Growth arrest of P1798 murine lymphosarcoma cells by glucocorticoids is accompanied by a remarkable decrease in transcription of rRNA and translation of mRNAs encoding basic ribosomal proteins (rps). Here we report that the expression of other genes involved in ribosome biogenesis is repressed in dexamethasone-treated P1798 cells. These include posttranscriptionally regulated decline in the abundance of the mRNA and primary transcript of nucleolin; abrupt drop in the transcription rate of U3 small nucleolar RNA; and inhibition of translation of mRNAs coding for P2 and L5, acidic and basic rps, respectively. Normal expression of these genes is resumed upon hormonal withdrawal.

Thyroid and steroid hormone regulation of proto-oncogene expression

In contrast to the established effects of peptide growth factors on specific proto-oncogene expression, the actions of steroid and thyroid hormones are less clearly defined. However, there is increasing evidence that these hormones, acting through structurally related DNA-binding nuclear receptor proteins, influence proto-oncogene expression. This influence may determine the function of steroid and thyroid hormones in regulation of cell proliferation and maturation, and provide insight into the role of these hormones in oncogenesis.