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

Circadian rhythms in ischaemic heart disease: key aspects for preclinical and translational research: position paper of the ESC working group on cellular biology of the heart

Circadian rhythms are internal regulatory processes controlled by molecular clocks present in essentially every mammalian organ that temporally regulate major physiological functions. In the cardiovascular system, the circadian clock governs heart rate, blood pressure, cardiac metabolism, contractility, and coagulation. Recent experimental and clinical studies highlight the possible importance of circadian rhythms in the pathophysiology, outcome, or treatment success of cardiovascular disease, including ischaemic heart disease. Disturbances in circadian rhythms are associated with increased cardiovascular risk and worsen outcome. Therefore, it is important to consider circadian rhythms as a key research parameter to better understand cardiac physiology/pathology, and to improve the chances of translation and efficacy of cardiac therapies, including those for ischaemic heart disease. The aim of this Position Paper by the European Society of Cardiology Working Group Cellular Biology of the Heart is to highlight key aspects of circadian rhythms to consider for improvement of preclinical and translational studies related to ischaemic heart disease and cardioprotection. Applying these considerations to future studies may increase the potential for better translation of new treatments into successful clinical outcomes.

Differential regulation of cyclic AMP synthesis by estrogen in MCF7 cells

The classical view of the molecular actions of estrogen is described by its interaction with the intracellular estrogen receptor (ER), the binding of hormone receptor complex to the estrogen response element (ERE) on the DNA and followed by the alterations of gene expressions. Recently it has been reported that membrane estrogen receptor (mER) exist and it is suggested to be G protein linked receptor. In this report we show that under steroid-free culture conditions supplemented with low percentage of charcoal-stripped serum, differential estrogen treatments of human breast cancer MCF7 cells induce different responses of cyclic AMP (cAMP) productions. Treating [2-(3)H]adenine-labeled MCF7 cells with 1 nM estrogen for 30 min stimulates cAMP production by measuring the ratio of [3H]cAMP:Total [3H]adenine nucleotides (ATP+ADP+cAMP), as determined by column chromatography, when compared with the control. This short-term estrogen treatment also significantly enhanced forskolin stimulated cAMP production when compared with the ratio of cAMP/Total measured in cells stimulated with forskolin alone. Pre-treating MCF7 cells with the same concentration of estrogen for 24h before the assay, on the contrary, significantly decreased the basal cAMP level and it also suppressed cAMP production stimulated with forskolin when compared with its respective value under short-term estrogen treatment. Estrogen receptor antagonist ICI 182780 abolished both the stimulatory and suppressive effect of estrogen on cAMP synthesis indicating both effects were mediated through ER. Pre-treating cells with pertussis toxin relieved the suppression of cAMP synthesis by chronic estrogen treatment. Our data suggest that estrogen exerts differential effects on the cAMP production in MCF7 cells, involving the activations Galpha(i) and Galpha(s) family of G proteins, depending on the length of time of hormone treatment.

Therapeutic targeting in the estrogen receptor hormonal pathway

Estrogens work along with genetic changes to promote the development and growth of breast cancers. Because estrogenic hormones act via the estrogen receptors (ERs), ER-alpha and ER-beta, and the ER is present in more than half of breast tumors, this receptor has been the most widely targeted protein in breast cancer therapy. The presence of the ER in breast tumors predicts improved disease-free survival and response to selective ER modulators (SERMs), such as tamoxifen, or other forms of endocrine therapy. Suppression of ER activity by SERMs has proven to be a great benefit in the treatment of breast cancers and also in the prevention of breast cancer in women at high risk for the disease. The Study of Tamoxifen and Raloxifene trial comparing tamoxifen versus raloxifene effectiveness in breast cancer prevention is currently under way. To understand the balance of beneficial and undesirable effects of SERMs and to optimize their effectiveness, current investigations seek to characterize the genes activated or suppressed by these agents. Elucidation of the gene networks and cell signaling pathways under estrogen and SERM regulation and a clearer definition of the respective roles of ER-alpha and ER-beta and their coregulators in the actions of selective ER ligands, should enable the identification of new gene targets for therapeutic intervention and the development of novel drugs for the optimal treatment and prevention of breast cancer.

17beta-estradiol inhibits forskolin-induced vascular endothelial growth factor promoter in MCF-7 breast adenocarcinoma cells

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor whose expression is induced by the cAMP-dependent signalling pathway in several cell types, and by estrogens in some human breast cancer cells. Here, we investigated the cross-talk between estrogens and cAMP/PKA-dependent signalling pathway in human breast cancer MCF-7 cells. The results show that, in the absence of any CRE and ERE, forskolin induces whereas estrogens have no effect on VEGF promoter. Moreover, estrogens, through estrogen receptors, partly inhibit the forskolin-induced VEGF promoter in MCF-7 human breast cancer cells. Therefore, in breast cancers, estrogens could partly inhibit the effect of ligand-activated G protein-coupled receptors on VEGF expression.

Involvement of cyclic AMP response element binding protein (CREB) and estrogen receptor phosphorylation in the synergistic activation of the estrogen receptor by estradiol and protein kinase activators

Estrogen receptor (ER) and cAMP signaling pathways interact in a number of estrogen target tissues including mammary and uterine tissues. One aspect of this interaction is that estradiol and protein kinase A (PKA) activators can cooperate synergistically to activate ER-mediated transcription of both endogenous genes and reporter genes containing only estrogen response elements. The purpose of this study was to investigate the molecular mechanism of this interaction between signaling pathways. Site-directed mutagenesis of the potential PKA phosphorylation sites in the ER indicated that phosphorylation of these sites was not necessary for the observed transcriptional synergy. In transient transfection assays in two different cell lines using reporter constructs containing either cAMP response elements, estrogen response elements or both types of elements, with the addition or absence of cAMP response element binding protein (CREB) expression plasmid, we observed that only one of these cell lines exhibited estrogen/PKA transcriptional synergy. Experiments demonstrated that CREB itself was involved in the transcriptional synergy, and that transfection of CREB restored transcriptional synergy in the cell line in which it was lacking. A functional interaction between ER and CREB was also demonstrated using a mammalian cell protein interaction assay; a dominant negative mutant of CREB did not exhibit this interaction. Therefore, these data indicate that CREB protein is required for the transcriptional synergy between cAMP and estrogen signaling pathways. Furthermore, CREB cooperated with the ER on genes that did not contain cAMP response elements, but contained only estrogen response elements. We propose that activated CREB is recruited to estrogen responsive genes by an ER--coactivator complex containing proteins such as CREB binding protein (CBP) and that the interaction of CREB with ER may assist in stabilizing its interaction with CBP and in promoting estrogen-ER and PKA transcriptional synergy.

Estrogen receptor transcription and transactivation: Estrogen receptor alpha and estrogen receptor beta: regulation by selective estrogen receptor modulators and importance in breast cancer

Estrogens display intriguing tissue-selective action that is of great biomedical importance in the development of optimal therapeutics for the prevention and treatment of breast cancer, for menopausal hormone replacement, and for fertility regulation. Certain compounds that act through the estrogen receptor (ER), now referred to as selective estrogen receptor modulators (SERMs), can demonstrate remarkable differences in activity in the various estrogen target tissues, functioning as agonists in some tissues but as antagonists in others. Recent advances elucidating the tripartite nature of the biochemical and molecular actions of estrogens provide a good basis for understanding these tissue-selective actions. As discussed in this thematic review, the development of optimal SERMs should now be viewed in the context of two estrogen receptor subtypes, ERalpha and ERbeta, that have differing affinities and responsiveness to various SERMs, and differing tissue distribution and effectiveness at various gene regulatory sites. Cellular, biochemical, and structural approaches have also shown that the nature of the ligand affects the conformation assumed by the ER-ligand complex, thereby regulating its state of phosphorylation and the recruitment of different coregulator proteins. Growth factors and protein kinases that control the phosphorylation state of the complex also regulate the bioactivity of the ER. These interactions and changes determine the magnitude of the transcriptional response and the potency of different SERMs. As these critical components are becoming increasingly well defined, they provide a sound basis for the development of novel SERMs with optimal profiles of tissue selectivity as medical therapeutic agents.

Growth factor-induced transcription via the serum response element is inhibited by cyclic adenosine 3',5'-monophosphate in MCF-7 breast cancer cells

The effect of increased intracellular cAMP on MCF-7 breast cancer cell growth was examined by treating cells with either forskolin, an activator of adenylate cyclase, or 8-[4-chlorophenylthio]-cAMP (8-CPT-cAMP), a cAMP analog. Compared to cells maintained in control medium, treatment with either 1 or 10 microM forskolin decreased cell growth by 17% and 68%, respectively, whereas treatment with 250 microM 8-CPT-cAMP decreased cell growth by 29%. To determine whether this effect of cAMP on cell growth was mediated by inhibition of the activity of extracellular signal-regulated kinases 1 and 2 (ERK1 and -2), two mitogen-activated protein kinases, the effect of cAMP on growth factor-induced ERK activity in MCF-7 cells was examined. Treatment with either insulin-like growth factor I (IGF-I) or epidermal growth factor (EGF) for 10 min stimulated a 4- to 8-fold increase in ERK1 and -2 activity. This effect of IGF-I and EGF was not inhibited by increased intracellular cAMP generated by pretreatment of the cells with 10 microM forskolin. Similarly, 10 microM forskolin had no effect on IGF-I- or EGF-induced ERK activity in cells treated with growth factor for 30 min. To determine whether cAMP inhibits other growth factor-mediated effects, its effect on the activity of the serum response element (SRE), a DNA promoter element whose activity is regulated by a variety of growth-promoting events, was examined. For these assays, MCF-7 cells were transiently transfected with pTK81-SRE-Luc, a luciferase fusion gene that contains the SRE cloned 5' to a minimal thymidine kinase promoter and the luciferase gene. Treatment with either IGF-I or EGF increased pTK81-SRE-Luc activity in a dose-dependent fashion. Pretreatment of cells with 10 microM forskolin decreased IGF-I- and EGF-stimulated luciferase activity by approximately 75%. An intermediate effect was observed using 1 microM forskolin. When intracellular cAMP levels were increased using 8-CPT-cAMP, similar results were obtained. SRE activity is dependent upon the activation by phosphorylation of a ternary complex factor; included among the ternary complex factors is Elk-1. When MCF-7 cells were cotransfected with a vector that expresses a Gal4/Elk-1 fusion protein and UAS-TK-Luc, a plasmid that contains two Gal4 DNA recognition sites cloned 5' to a thymidine kinase promoter and the luciferase gene, treatment with forskolin partially inhibited the activation of Elk-1 by IGF-I and EGF. These data demonstrate that in MCF-7 breast cancer cells, cAMP has no effect on IGF-I- or EGF-induced ERK activity, but it inhibits growth factor-induced transcription. Taken together with the effects of cAMP on IGF-I- and EGF-induced Elk-1 activation, these data suggest that the effect of cAMP on SRE activity occurs distal to ERK activation, possibly via inhibition of an ERK-independent pathway. Finally, these data indicate that the effect of increased intracellular cAMP on breast cancer growth may be mediated through inhibition of specific growth factor-induced effects, including gene transcription.

Interaction between estradiol and cAMP in the regulation of specific gene expression

The mRNA levels of LIV-1 and pS2, two estrogen-responsive genes, are increased by the agents, cholera toxin (CT) plus 3-isobutyl-l-methylxanthine (IBMX), which cause an increase in cAMP in MCF-7 human breast cancer cells. The simultaneous addition of estradiol and CT/IBMX results in a synergistic induction of the two mRNAs. The changes in mRNA reflect changes in transcription of the two genes. Interestingly, the addition of CT/IBMX to estradiol not only causes a greater increase in transcription rate but the increase is longer-lasting that seen with the hormone alone. Stimulation of mRNA levels by CT/IBMX, but not by estradiol, was prevented by cycloheximide. Stimulation by both estradiol and by CT/IBMX was prevented by the antiestrogen, ICI 164387. Transcription of LIV-1 and pS2 genes is by both estradiol and cAMP, via separate mechanisms both requiring the estrogen receptor.

Estrogen action via the cAMP signaling pathway: stimulation of adenylate cyclase and cAMP-regulated gene transcription

Estrogenic hormones, believed to exert most of their effects via the direct interaction of their receptors with chromatin, are found to increase cAMP in target breast cancer and uterine cells in culture and in the intact uterus in vivo. Increases in intracellular cAMP are evoked by very low concentrations of estradiol (half maximal at 10 pM) and by other physiologically active estrogens and antiestrogens, but not by an inactive estrogen stereoisomer. These increases in cAMP result from enhanced membrane adenylate cyclase activity by a mechanism that does not involve genomic actions of the hormones (are not blocked by inhibitors of RNA and protein synthesis). The estrogen-stimulated levels of cAMP are sufficient to activate transcription from cAMP response element-containing genes and reporter plasmid constructs. Our findings document a nongenomic action of estrogenic hormones that involves the activation of an important second-messenger signaling system and suggest that estrogen regulation of cAMP may provide an additional mechanism by which this steroid hormone can alter the expression of genes.

Stimulation of rat Schwann cell proliferation by estradiol: synergism between the estrogen and cAMP

Schwann cells dissociated from sciatic nerves of 4- to 5-day-old rats were established in long-term primary cultures. After treatment with anti-Thy-1.1 and complement, pure Schwann cell cultures were obtained as confirmed by indirect immunofluorescence staining of the S-100 antigen. The effect of estradiol on Schwann cell proliferation was tested. The estrogen alone had no effect on cell multiplication, but in the presence of forskolin (5 microM) or dibutyryl cyclic AMP (dbcAMP, 1 microM), estradiol (100 nM) became a potent mitogen. This effect was specific since when the anti-estrogen ICI-164,384 (200 nM) was added to the cultures, it completely inhibited the mitogenic effect of estradiol. In the absence of the estrogen, ICI-164,384 did not inhibit dbcAMP-induced cell growth. The effect of progesterone was also tested in the presence of forskolin, and no influence on cell multiplication was seen. Specific and saturable binding sites for estradiol were measured in Schwann cells by whole cell assay after labeling cells at 37 degrees C with [3H]estradiol and about 3000 specific binding sites per cell were found. Treatment of the cultures with forskolin (5 microM) for 48 or 72 h, but not for shorter times, resulted in a two-fold increase in estradiol binding. Estrogen receptors were visualized inside Schwann cells with a monoclonal anti-ER antibody by indirect immunofluorescence staining. Double-label immunofluorescence of ER and S-100 showed that ER immunostaining was restricted to spindle-shaped Schwann cells expressing the S-100 antigen.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative abilities of athymic nude mice and severe combined immune deficient (SCID) mice to accept transplants of induced rat mammary carcinomas: enhanced transplantation efficiency of those rat mammary carcinomas that have elevated expression of neu oncogene

Grafts of primary carcinogen (DMBA)-induced mammary carcinomas from Sprague-Dawley rats have a poor transplantation efficiency in athymic nude mice. Further compromising these mice immunologically via whole-body irradiation and/or splenectomy, or the administration of hormonal growth factors (estrogen and progesterone) to these mice, did not significantly alter transplantation efficiency. Use of strains of mice that are more immune-impaired than the athymic nude mouse, i.e., the athymic nude-beige-XID mouse (T-cell and LAK-cell deficient) or mice with severe combined immune deficiency (SCID) (which lack functional T cells and B cells) also failed to improve transplantation efficiency. In contrast, transplantation efficiency was sharply increased when primary neu-induced rat mammary carcinomas from female Sprague-Dawley rats were used. These mammary carcinomas, unlike the DMBA-induced rat mammary carcinomas, have a very high level of expression of neu; transplantation of these tumors to either athymic nude mice or SCID mice was considerably more efficient. Thus, these data provide evidence that enhanced expression of neu confers heightened efficiency in the transplantation of primary rat mammary carcinomas to immune-deficient mice (athymic-nude or SCID). Increased neu expression was a greater determinant than more compromised immune states in the transplantation of these rat mammary carcinomas. This biological characteristic of neu expression in mammary carcinomas provides new, additional insight into the importance of this oncogene in mammary tumorigenic processes and may explain, at least in part, the reported inverse relationship between human breast carcinoma neu expression and patient prognosis.

Growth of MCF-7 human breast carcinoma in severe combined immunodeficient mice: growth suppression by recombinant interleukin-2 treatment and role of lymphokine-activated killer cells

The severe combined immunodeficient (SCID) mouse, lacking functional T and B lymphocytes, has been considered by many groups to be a prime candidate for the reconstitution of a human immune system in a laboratory animal. In addition, this immuno-deficient animal would appear to have excellent potential as a host for transplanted human cancers, thus providing an exceptional opportunity for the study of interactions between the human immune system and human cancer in a laboratory animal. However, because this animal model is very recent, few studies have been reported documenting the capability of these mice to accept human cancers, and whether or not the residual immune cells in these mice (e.g. natural killer, NK, cells; macrophages) possess antitumor activities toward human cancers. Thus, the purpose of this study was (a) to determine whether or not a human breast carcinoma cell line (MCF-7) can be successfully transplanted to SCID mice, (b) to determine whether or not chronic treatment of SCID mice with a potent lymphokine (recombinant interleukin-2, rIL-2) could alter MCF-7 carcinoma growth, and (c) to assess whether or not rIL-2-activated NK cells (LAK cells) are important modulators of growth of MCF-7 cells in SCID mice. To fulfill these objectives, female SCID mice were implanted s.c. with MCF-7 cells (5 x 10(6) cells/mouse) at 6 weeks of age. Six weeks later, some of the mice were injected i.p. twice weekly with rIL-2 (1 x 10(4) U mouse-1 injection-1). Results clearly show that MCF-7 cells can grow progressively in SCID mice; 100% of the SCID mice implanted with MCF-7 cells developed palpable measurable tumors within 5-6 weeks after tumor cell inoculation. In addition, MCF-7 tumor growth was significantly (P less than 0.01) suppressed by rIL-2 treatment. rIL-2 treatment was non-toxic and no effect of treatment on body weight gains was observed. For non-tumor-bearing SCID mice, splenocytes treated in vitro with rIL-2 (lymphokine-activated killer, LAK, cells) or splenocytes derived from rIL-2-treated SCID mice (LAK cells) had significant (P less than 0.01) cytolytic activity toward MCF-7 carcinoma cells in vitro. In contrast, splenocytes (LAK cells) derived from tumor(MCF-7)-bearing rIL-2-treated SCID mice lacked cytolytic activities toward MCF-7 cells in vitro. No significant concentration of LAK cells in MCF-7 human breast carcinomas ws observed nor did rIL-2 treatment significantly alter growth of MCF-7 cells in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of quail oviduct adenylate cyclase activity by estradiol and progesterone

Oviduct adenylate cyclase activity of the quail was measured by radiochemical analysis following different hormonal treatments. A single injection of estradiol benzoate (EB) to immature female quails resulted in a prereplicative surge of adenylate cyclase activity. A second surge of enzyme activity was observed during the proliferative phase induced by EB. Estradiol-17 alpha, estrone, estriol and testosterone were ineffective. Tamoxifen completely inhibits the growth-promoting effect of EB and the second surge of adenylate cyclase activity but does not inhibit the prereplicative increase of enzyme activity. This prereplicative increase of adenylate cyclase activity was also observed, even in the absence of increased plasma estradiol, when estradiol-17 beta (E2) was perfused through the hepatic portal vein. Moreover, E2 had no effect on enzyme activity when added directly to the oviduct homogenate preparation, at concentrations ranging from 10(-9) to 10(-7) M. In response to progesterone injection, oviduct adenylate cyclase activity followed a different pattern, beginning its increase after 3 h and remaining elevated up to 24 h. The activation by estradiol was independent of the presence of guanylylimidodiphosphate. Moreover, the enzyme was more sensitive to forskolin at submaximal concentration in estradiol treated birds than in control. These results demonstrate that transient activation of adenylate cyclase at the early stages of the action of estradiol does not occur through the classic nuclear receptor-gene activation pathway or a membrane receptor mediated process, but involves an indirect pathway, yet to be defined.

Transplantation of human breast epithelia to mammary-gland-free fat-pads of athymic nude mice: influence of mammotrophic hormones on growth of breast epithelia

Normal human breast tissue was enzymatically dissociated and the cells were injected into the gland-free fat-pads of athymic nude mice. Within 30 days, small, spherical, duct-like epithelial elements (organoids) formed in 68% of the fat-pads inoculated (0-23 organoids/fat-pad). Short-term (30-day) treatment of the host mice with mammotrophic hormones [secretions from a chorionic, soamto-mammotrophin-secreting transplantable human choriocarcinoma (JEG-3), secretions from a prolactin- and growth-hormone-secreting transplantable rat pituitary tumor (GH3), estrogen and/or progesterone] and/or cAMP inducers (cholera toxin) significantly (p less than 0.05) increased the size of the human breast organoids but did not increase organoid number or induce extensive and expansive growth (extensive duct elongation and branching) of these structures. Such treatments induced intense proliferation of the host mouse mammae resembling that which occurs during late pregnancy. The results of this study, therefore, provide evidence that normal human breast epithelium can be readily accepted by and maintained in the gland-free fat-pad of the athymic nude mouse, and the epithelium, within 30 days, forms spherical duct-like structures (organoids). The human breast organoids are hormone-responsive, as they respond to a mammotrophic growth stimulus by an increase in size. The failure of the human breast organoids to grow expansively in the gland-free fat-pad of this immunologically deficient mouse does not appear to be due to the absence of an appropriate hormonal growth stimulus.

1,25-Dihydroxycholecalciferol induces an increase in PGE1- and forskolin-stimulated cyclic-AMP production in T47D human breast cancer cell line

The effect of 1,25-dihydroxycholecalciferol [1,25(OH)2D3], the active form of vitamin D3, on cell growth, clonogenicity, and cyclic adenosine monophosphate (cAMP) production was examined in human breast cancer cell line T47D. 1,25(OH)2D3 markedly inhibited proliferation of T47D cells in a time- and concentration-dependent manner. 1,25(OH)2D3 5 X 10(-7) reduced to 70% [3H]thymidine incorporation into DNA. Specific high affinity nuclear receptors for 1,25(OH)2D3 were present in this cell line. The cAMP produced by T47D cells was measured during 10 min stimulation by effectors (prostaglandin E1 or forskolin). Without effector, T47D cells produced similar amounts of cAMP in control and 1,25(OH)2D3-treated cells. After 3 days in the presence of 1,25(OH)2D3, cAMP production was significantly increased compared to control cells when stimulated by 10(-4) M prostaglandin E1 or 5 X 10(-7) M forskolin (3.2- and 2.4-fold increase, respectively). This cAMP increase was concentration dependent within the same range that inhibited cell growth and clonogenicity. These results suggest that 1,25(OH)2D3 may indirectly affect cAMP production by modulating the target cell response to stimulatory agents of cAMP production.

Influence of hormone and growth factor interactions on the proliferative potential of normal rat mammary epithelial cells in vitro

These experiments were aimed at using a recently developed serum-free culture system for growth of normal rat mammary epithelial (RME) cells in vitro to examine the interactions of specific hormones and growth factors on the proliferative potential of these cells. RME cells were obtained by enzymatic dissociation of mammary tissues of Lewis rats. Primary cultures were started by plating 2 X 10(5) RME cells per 60-mm type I collagen-coated tissue culture dish. Cultures were maintained in a basal medium that consisted of Ham's F-12 medium supplemented with bovine serum albumin (BSA), ethanolamine (EA), and transferrin (Tf), which, by itself, did not support RME cell proliferation. Insulin (I), hydrocortisone (HC), and epidermal growth factor (EGF), when added to the basal medium interacted synergistically to stimulate RME cell proliferation, but this effect was dependent on the additional presence of cholera toxin (CT). Under these conditions a greater-than-tenfold increase in cell number over a 10-day culture period was obtained. Insulin could be replaced by physiological levels of insulin-like growth factor-I (IGF-I). CT could be replaced by other agents that elevate intracellular levels of cyclic adenosine 3':5' monophosphate (cAMP) such as dibutyryl-cAMP (db-cAMP), prostaglandin E1 (PGE-1), and/or isobutylmethylxanthine (IBMX). Prolactin (M) or progesterone (P) potentiated the effect of I, HC, EGF, and CT, resulting in an additional twofold increase in cell number over that found in their absence. However, addition of both hormones was no more effective than either one alone. Furthermore, addition of M or P in the absence of EGF had no effect on RME cell proliferation. Addition of 17-B-estradiol (E2) to the I-, HC-, EGF-, and CT-containing medium also resulted in enhanced RME cell proliferation. These results point to a number of hormone and growth factor interactions that influence the proliferation of normal RME cells in vitro.