Bromocriptine-induced apoptosis in pituitary adenoma cells: relationship to p53 and bcl-2 expression

Long-term Discontinuation of Dopamine Agonist Treatment in Patients with Prolactinomas Revisited

The most common type of functioning pituitary adenomas is prolactinomas; unlike other types, they are treated medically with dopamine agonists (DA). This treatment aims to normalize PRL levels and decrease tumor size by 50% or more. These objectives are typically achieved by 90% of patients with microprolactinoma, two-thirds of those with macroprolactinomas, and about half of those with giant prolactinomas. Life-long pharmacological treatment implies costs, discomfort, and the possibility of side effects, therefore, it has been suggested that DA discontinuation could be attempted in some patients. Long-term remission seems more likely in who, after 2 years of therapy achieve clinical, biochemical, and imaging remission criteria: no evidence of hypogonadism, a normal PRL level (preferably <5 ng/mL), and a >50% of tumor size reduction. Long-term remission seems to be more likely if the patient has been treated with cabergoline (CBG) for a minimum of 2 years, the PRL levels have normalized, tumor size has decreased by at least 50%, and the DA dose can gradually be tapered down to 0.25-0.5 mg per week. After treatment withdrawal, about 65% of patients experience a recurrence of hyperprolactinemia within the first 12 months of DA discontinuation. Although in most patients in whom DA discontinuation has been attempted, the hyperprolactinemia will recur, not all of them will require re-initiation of treatment. A good clinical judgement is crucial to identify those patients who need life-long treatment.

Anticancer efficacy of endo- and exogenous potent ligands acting at dopaminergic receptor-expressing cancer cells

Cancer is one of the most common and dreaded diseases affecting the vastness of society. Unfortunately, still some people die especially when cancer is not diagnosed and thus caught early enough. On the other hand, using available chemo- or radiotherapy may result in serious side effects. Therefore, cancer-specific medications seem to be the most desired and safe therapy. Knowing that some cancers are characterized by overexpression of specific receptors on the cell surface, target-mediated drugs could serve as a unique and effective form of therapy. In line with this, recently dopaminergic receptors were presented important in cancer therapy as several dopaminergic ligands revealed their efficacy in tumor growth reduction as well as in apoptosis mediation. Unfortunately, the indication of whether DA receptor agonists or antagonists are the best choices in cancer treatment is quite difficult, since both of them may exert either pro- or anticancer effects. In this review, we analyze the therapeutic efficacy of compounds, both of exogenous and endogenous origin, targeting dopaminergic receptor-expressing cancers.

Atiprimod induce apoptosis in pituitary adenoma: Endoplasmic reticulum stress and autophagy pathways

Pituitary adenoma is the most common tumor with a high recurrence rate due to a hormone-dependent JAK/signal transducer and activator of transcriptions (STAT) signaling. Atiprimod, a novel compound belonging to the azaspirane class of cationic amphiphilic drugs, has antiproliferative, anticarcinogenic effects in multiple myeloma, breast, and hepatocellular carcinoma by blocking STAT3 activation. Therapeutic agents' efficiency depends on endoplasmic reticulum (ER) stress-autophagy regulation during drug-mediated apoptotic cell death decision. However, the molecular machinery of dose-dependent atiprimod treatment regarding ER stress-autophagy has not been investigated yet. Thus, our aim is to investigate the ER stress-autophagy axis in atiprimod-mediated apoptotic cell death in GH-secreting rat cell line (GH3) pituitary adenoma cells. Dose-dependent atiprimod treatment decreased GH3 cell viability, inhibited cell growth, and colony formation. Upregulation of Atg5, Atg12, Beclin-1 expressions, cleavage of LC-3II and formation of autophagy vacuoles were determined only after 1 µM atiprimod exposure. In addition, atiprimod-triggered ER stress was evaluated by BiP, C/EBP-homologous protein (CHOP), p-PERK upregulation, and Ca⁺² release after 1 µM atiprimod exposure. Concomitantly, increasing concentration of atiprimod induced caspase-dependent apoptotic cell death via modulating Bcl₂ family members. Moreover, by N-acetyl cycteinc pretreatment, atiprimod triggered reactive oxygen species generation and prevented apoptotic induction. Concomitantly, dose-dependent atiprimod treatment decreased both GH and STAT3 expression in GH3 cells. In addition, overexpression of STAT3 increased atiprimod-mediated cell viability loss and apoptotic cell death through suppressing autophagy and ER stress key molecules expression profile. In conclusion, a low dose of atiprimod exposure triggers autophagy and mild-ER stress as a survival mechanism, but increased atiprimod dose induced caspase-dependent apoptotic cell death by targeting STAT3 in GH3 pituitary adenoma cells.

The Mechanism and Pathways of Dopamine and Dopamine Agonists in Prolactinomas

Dopamine agonists such as bromocriptine and cabergoline are the predominant treatment drugs for prolactinoma by inhibiting prolactin secretion and shrinking tumor size. However, the pathways of either dopamine or its agonists that lead to the death of cells are incompletely understood and some are even conflicting conclusions. The main aim of this paper is to review the different pathways of dopamine and its agonists in prolactinomas to help to gain a better understanding of their functions and drug resistance mechanisms.

Relation among Aromatase P450 and Tumoral Growth in Human Prolactinomas

The pituitary gland is part of hypothalamic-pituitary–gonadal axis, which controls development, reproduction, and aging in humans and animals. In addition, the pituitary gland is regulated mainly by hormones and neurotransmitters released from the hypothalamus and by systemic hormones secreted by target glands. Aromatase P450, the enzyme responsible for the catabolization of aromatizable androgens to estrogens, is expressed in different parts of body, including the pituitary gland. Moreover, aromatase P450 is involved in sexual dimorphism where alteration in the level of aromatase can initiate a number of diseases in both genders. On the other hand, the direct actions of estrogens, mainly estradiol, are well known for stimulating prolactin release. Numerous studies have shown that changes in the levels of estrogens, among other factors, have been implicated in the genesis and development of prolactinoma. The pituitary gland can produce estradiol locally in several types of endocrine cells, and it is possible that aromatase could be responsible for the maintenance of the population of lactotroph cells and the modulation of the action of central or peripheral regulators. Aromatase overexpression due to inappropriate gene regulation has clinical effects such as the pathogenesis of prolactinomas. The present study reports on the synthesis of pituitary aromatase, its regulation by gonadal steroids, and the physiological roles of aromatase on pituitary endocrine cells. The involvement of aromatase in the pathogenesis of pituitary tumors, mainly prolactinomas, through the auto-paracrine production of estradiol is reviewed.

Dopamine receptor D2S gene transfer improves the sensitivity of GH3 rat pituitary adenoma cells to bromocriptine

Bromocriptine, a dopamine agonist (DA), has been used in the treatment of prolactinomas. Recent studies have indicated that dopamine 2 receptor short isoform (D2S) may play an important role in suppressing PRL synthesis and prolactinoma cell growth under DA treatment. In the current study, we investigated the role of D2S in the therapeutic action of bromocriptine in GH3 using both in vitro and in vivo approaches. Infection of adenovirus-D2S increased D2S expression in GH3 cells (P<0.05). D2S expression significantly decreased the GH3 cell viability subjected to bromocriptine treatment in vitro (P<0.05). In nude mice, adenovirus-D2S transfection sensitized GH3 xenograft to bromocriptine treatment evidenced by the significant inhibition of D2S expressed tumor growth as compared with vector control. Furthermore, decrease of Bcl-2 expression, increase of Bax, and active Caspase-3 were found in D2S expressed GH3 xenograft subjected to bromocriptine treatment. In summary, our study indicates that D2S expression plays a critical role in the therapeutic action of bromocriptine in pituitary adenomas and that adenovirus-mediated D2S gene transfer combined with bromocriptine may provide a novel treatment for DA-resistant prolactinomas.

Caveolin-1 sensitizes rat pituitary adenoma GH3 cells to bromocriptine induced apoptosis

BACKGROUND

Prolactinoma is the most frequent pituitary tumor in humans. The dopamine D2 receptor agonist bromocriptine has been widely used clinically to treat human breast tumor and prolactinoma through inhibition of hyperprolactinemia and induction of tumor cell apoptosis, respectively, but the molecular mechanism of bromocriptine induction of pituitary tumor apoptosis remains unclear. Caveolin-1 is a membrane-anchored protein enriched on caveolae, inverted flask-shaped invaginations on plasma membranes where signal transduction molecules are concentrated. Currently, caveolin-1 is thought to be a negative regulator of cellular proliferation and an enhancer of apoptosis by blocking signal transduction between cell surface membrane receptors and intracellular signaling protein cascades. Rat pituitary adenoma GH3 cells, which express endogenous caveolin-1, exhibit increased apoptosis and shrinkage after exposure to bromocriptine. Hence, the GH3 cell line is an ideal model for studying the molecular action of bromocriptine on prolactinoma.

RESULTS

The expression of endogenous caveolin-1 in GH3 cells was elevated after bromocriptine treatment. Transiently expressed mouse recombinant caveolin-1 induced apoptosis in GH3 cells by enhancing the activity of caspase 8. Significantly, caveolin-1 induction of GH3 cell apoptosis was sensitized by the administration of bromocriptine. Phosphorylation of caveolin-1 at tyrosine 14 was enhanced after bromocriptine treatment, suggesting that bromocriptine-induced phosphorylation of caveolin-1 may contribute to sensitization of apoptosis in GH3 cells exposed to bromocriptine.

CONCLUSION

Our results reveal that caveolin-1 increases sensitivity for apoptosis induction in pituitary adenoma GH3 cells and may contribute to tumor shrinkage after clinical bromocriptine treatment.

Expression of p53, Ki-67 and c-erb B2 in growth hormone-and/or prolactin-secreting pituitary adenomas

The subcellular events implicated on the formation and behavior of pituitary adenomas are not fully understood. In this study we investigated the presence of p53, Ki-67 and c-erb B2 in 38 pituitary adenomas with immunohistochemical positivity for GH and prolactin (n=26; 68.4%), for prolactin (n=9; 23.7%) and for GH (n=3. 7.8%). The analyses revealed the following results: 24 (63.2%) tumors expressed variable positivity for c-erb B2, 11 (28.9%) expressed p53 positivity and 11 (28.9%) tumors were variably positive for Ki-67. Our results demonstrated a high percentage of GH/prolactin-, prolactin- and GH-secreting tumors with immunohistochemical positivity for c-erb B2. Once this membrane receptor is related to growth factors EGF and TGFalpha and both have a definite effect on tumor growth, our data suggest a possible role for c-erb B2 on the evolution of these tumors.

Inhibitory effects of melatonin on the growth of pituitary prolactin-secreting tumor in rats

The in vivo effects of melatonin on proliferation and apoptosis of 17-beta-estradiol (E2)-induced pituitary prolactin-secreting tumor (prolactinoma) were investigated in rats kept in 12 L/12 D (lights on: 06:00-18:00 hr). As melatonin was shown to induce apoptosis of breast and liver tumor cells, we examined whether melatonin would induce apoptosis of rat pituitary prolactinoma cells. 0.125, 0.25, 0.50 or 1.0 mg melatonin/day/rat was administrated subcutaneously at 17:30-18:00 hr. The weight of prolactinomas was measured. Apoptosis was evaluated using the TdT-mediated dUTP nick-end labeling method. It was found that treatment with 0.25 and 0.50 mg melatonin for 97 days inhibited prolactinoma cell proliferation and increased prolactinoma cell apoptosis. Furthermore, melatonin induced mRNA expression of Bax and cytochrome c protein expression. Conversely, mRNA expression of Bcl-2, and mitochondrial membrane potential were inhibited by melatonin treatment. These results suggest that melatonin inhibits the proliferation and induces apoptosis of rat pituitary prolactin-secreting tumor via perturbation of mitochondria physiology.

A novel apoptosis gene identified in the pituitary gland

Although multiple different cancers have been described, it is likely that these tumour types share a small, and common, number of newly acquired functional capabilities. Tumours that arise within the pituitary gland are no exception with respect to these new functional capabilities. Although compelling evidence for self-sufficiency in growth signals is presented, loss of functional tumour suppressor genes by classic mechanisms has not been clearly established. However, and in this context, methylation-mediated or -associated gene silencing, in particular of tumour suppressor genes, has been reported by numerous investigators in this tumour type. More recently, a search for novel genes on the basis of their inappropriate methylation has led to identification of a novel pro-apoptotic gene. Its pituitary tumour derivation and role in drug-induced apoptosis resulted in the acronym PTAG (pituitary tumour apoptosis gene) being assigned to this gene. In a model pituitary tumour cell line, AtT20, expression of PTAG per se had no discernible effects on proliferation, cell cycle profile or viability. However, enforced expression was associated with a significantly increased sensitivity to the apoptotic effects induced by bromocriptine challenge. Apoptosis was mediated through caspase activation and associated with DNA fragmentation as determined by TUNEL labelling. Similar findings are also evident in the rodent pituitary cell line, GH3 and our data shows that drugs other than bromocriptine, and that engage characterized receptors, elicit a PTAG-augmented apoptotic response. The isolation of novel genes, on the basis of their methylation status, offers a significant advantage with respect to our understanding of tumorigenesis in both the pituitary and other tumour types. The reversal of apparent gene silencing may lead to tumour cell 'sensitisation' to chemo- and radiotherapeutic treatment strategies.

[Non-functioning pituitary adenomas: clinical features and immunohistochemistry]

Clinically non-functioning pituitary adenomas do not produce clinical signs of hormonal hypersecretion. Therefore, signs and symptoms will depend on the mass effect of these adenomas over the central nervous system. Their etiopathogeny is complex and their development is probably influenced by several factors, such as hypothalamic hormones (GHRH), growth factors (FGF), proliferation factors (PCNA, and KI-67), protein P53 and the proto-oncogene c-erb-B2.

OBJECTIVE

1) Determining the clinical features of a population of 117 patients treated for clinically non-functioning pituitary adenoma (age, sex, tumor size, number of surgical procedures, development of hormonal deficiency and hyperprolactinemia). 2) Identifying, after the patients had been clinically characterized, those with clinically non-functioning adenomas with positive immunohistochemistry for hypophyseal hormones (PRL, LH, FSH, GH, TSH and ACTH). 3) Determining if the immunohistochemistry of this population was positive for the cellular proliferation factor Ki-67, protein P53 and protein C-erb-B2 and establishing a correlation with tumor size and tumor invasiveness. This will help in the evaluation of the prognostic value of these proliferation factors. 4) Confronting the results of immunohistochemistry using a standard block with the results of immunohistochemistry using a tissue micro-array.

METHOD

Study of the clinical features of 117 patients with clinically non-functioning pituitary adenoma (age, sex, tumor size, number of surgical procedures, development of hormonal deficiency and hyperprolactinemia). Immunohistochemical study (H&E) of 39 patients for hypophyseal hormones, protein P53, protein C-erb-B2, Ki-67 to establish their correlation to tumor growth. The next step was a tissue micro-array of the 39 previously studied cases, using immunohistochemistry for hypophyseal hormones, protein P 53, protein C-erb-B2, Ki-67 to establish their correlation to tumor growth.

RESULTS

There was no statistically significant difference between males and females with regards to age, tumor size and number of surgical procedures (p=0.279, p=813, p=139 respectively). There is a statistically significant correlation between the size of the tumor, the number of surgical procedures and hormonal deficiency (p=0.032, p=0.223 respectively). There was no statistically significant correlation between a positive immunohistochemistry for protein P53, protein C-erb-B2, Ki-67 and tumor size (r=0.182, p=0.396; r=-0.181, p=0.397; r=0.272, p=0.199, respectively). The tissue micro-array also did not demonstrate a correlation between positive immunohistochemistry for Ki-67 and C-erb-B2 and tumor size, but it showed a statistically significant correlation between a positive immunohistochemistry for p53 and tumor size (r=-0.696; p=001).

CONCLUSION

The biological behavior of the clinically non-functioning adenoma is similar for both sexes. The larger the tumor the greater the number of surgical procedures needed. Hormonal deficiency also becomes more significant as the size of the tumor increases. This paper suggests that a positive immunohistochemistry for p53 is negatively correlated to tumor size, thus demonstrating that it has a predictor value. However, a positive immunohistochemistry for Ki-67 and protein C-erb-B2 does not seem to be a prognostic factor for clinically non-functioning pituitary adenomas, as is the case with other neoplasias.

Activation of caspase 8 in the pituitaries of streptozotocin-induced diabetic rats: implication in increased apoptosis of lactotrophs

Lactotroph cell death is increased in streptozotocin-induced diabetic rats. To determine the mechanism involved, cell death proteins were accessed in pituitaries of diabetic (streptozotocin at 65 mg/kg, 2 months evolution) and control male rats by Western blot analysis and double immunohistochemistry. The intact and cleaved forms of caspase 9 were increased in diabetic rat pituitaries compared with controls. Although the proforms of caspases 3, 6, and 7 were increased in diabetic rat pituitaries, their activated forms were either unchanged or decreased. Activation of these effector caspases may be blocked by the increased expression of X-chromosome-linked inhibitor of apoptosis protein (XIAP) in diabetic rat pituitaries. However, in diabetic rats, XIAP expression in lactotrophs was decreased, suggesting that this cell type is not protected. Caspase 8, p53, and nuclear factor kappaB were more highly activated in diabetic rat pituitaries, with caspase 8 colocalization in lactotrophs being increased. These results suggest that, in the pituitaries of diabetic rats, the cascades of normal cell turnover are partially inhibited, possibly via XIAP, and this may be cell specific. Furthermore, activation of the extrinsic cell-death pathway, including activation of caspase 8, may underlie the diabetes-associated increase in lactotroph death.

Isolation and characterization of a novel pituitary tumor apoptosis gene

To determine mechanisms for pituitary neoplasia we used methylation-sensitive arbitrarily primed-PCR to isolate novel genes that are differentially methylated relative to normal pituitary. We report the isolation of a novel differentially methylated chromosome 22 CpG island-associated gene (C22orf3). Sodium bisulfite sequencing of pooled tumor cohorts, used in the isolation of this gene, showed that only a proportion of the adenomas within the pools were methylated; however, expression analysis by quantitative RT-PCR of individual adenoma irrespective of subtype showed the majority (30 of 38; 79%) failed to express this gene relative to normal pituitary. Sodium bisulfite sequencing of individual adenomas showed that 6 of 30 (20%) that failed to express pituitary tumor apoptosis gene (PTAG) were methylated; however, genetic change as determined by loss of heterozygosity and sequence analysis was not apparent in the remaining tumors that failed to express this gene. In those cases where the CpG island of these genes was methylated it was invariably associated with loss of transcript expression. Enforced expression of C22orf3 in AtT20 cells had no measurable effects on cell proliferation or viability; however, in response to bromocriptine challenge (10-40 microm) cells expressing this gene showed a significantly augmented apoptotic response as determined by both acridine orange staining and TUNEL labeling. The apoptotic response to bromocriptine challenge was inhibited in coincubation experiments with the general caspase inhibitor z-VAD-fmk. In addition, in time course experiments, direct measurement of active caspases by fluorochrome-labeled inhibition of caspases, showed an augmented increase (approximately 2.4 fold) in active caspases in response to bromocriptine challenge in cells expressing C22orf3 relative to those harboring an empty vector control. The pituitary tumor derivation and its role in apoptosis of this gene led us to assign the acronym PTAG to this gene and its protein product. The ability of cells, showing reduced expression of PTAG, to evade or show a blunted apoptotic response may underlie oncogenic transformation in both the pituitary and other tumor types.

Effect of therapeutic doses of ionising radiation on the somatomammotroph pituitary cell line, GH3

Ionising radiation is used for the treatment of pituitary tumours as fractionated radiotherapy, where the total dose reaching the tumour area is in the range of 40-50 Gy, or during stereotactic radiosurgery, where the total dose reaching the tumour area during one session is in the range of 20-90 Gy. In this study, we investigated the effect of ionising radiation of (60)Co (dose rate of 3 Gy/min, similar to that used during gamma knife procedure) on the mode of cell death of the somatomammotroph pituitary cell line, GH3, an immortalized cell line derived from a rat pituitary adenoma. We found that the basic mechanism of cell death induced by irradiation of this GH3 cell line by gamma-rays was programmed cell death-apoptosis. Doses of 20-50 Gy were shown to inhibit proliferation in these cells. 24 hours after irradiation with a dose of 20 and 50 Gy, cells were shown to accumulate in the G(2)/M phase of cell cycle. This cell cycle arrest lasted for at least ten days. Apoptosis was detected 72 hours towards until the end of the study (10 days). However, a significant number of cells were still alive ten days following irradiation. We conclude that ionising radiation doses of 20 and 50 Gy induce pituitary GH3 cell apoptosis following cell cycle arrest in the G(2)/M phase.

Apoptotic cell death of oestrogen activated lactotrophs induced by tamoxifen

Stimulation and inhibition of lactotroph cells cause remarkable morphological and functional changes. In keeping with these changes, the size of the lactotroph cell population undergoes striking alterations due to proliferation or cell death. Factors involved in the induction of apoptosis of pituitary cells are not well established. We demonstrated earlier that oestrogens prevent lactotroph cells of female rats to die by apoptosis induced by bromocryptine treatment, a fact that can be reversed in ovariectomised rats. In this study, we developed experimental models for in vivo and in vitro studies to gain further insight on the survival effect of oestrogens on lactotrophs. In rats pretreated with oestrogens, tamoxifen generates a massive cell death by apoptosis as validated by the TUNEL technique and DNA electrophoresis of pituitary gland. On electron microscope observations, numerous lactotrophs exhibited progressive morphological changes in the nuclei compatible with the apoptotic process. The cells remaining intact also exhibit signs of inhibition due to a significant transformation of regular lactotrophs in atypical subtypes. In pituitary cell cultures exposed to tamoxifen and oestrogen simultaneously, most of the lactotrophs displayed features of apoptosis in the nucleus. The present reports gathered new evidences on the apoptogenic potential of tamoxifen on lactotroph cells, and corroborates the contribution of oestrogens to sustain both a balanced population of lactotrophs and a competent secretory activity. The concept that opposed activities, such as inhibition and stimulation, can activate apoptosis is also strengthen by these observations.

Increased tumour extracellular pH induced by Bafilomycin A1 inhibits tumour growth and mitosis in vivo and alters 5-fluorouracil pharmacokinetics

The aim was to determine if a specific inhibitor of vacuolar H(+)-ATPases (V-ATPases), Bafilomycin A1 (BFM), could increase the low extracellular pH (pHe) typical of solid tumours and thus inhibit their growth in vivo. BFM inhibited the proliferation of various human cells and rat pituitary GH3 tumour cells in vitro (IC50: 2.5-19.2 nM), and flow cytometry on GH3 cells showed a marked increase in S and G2M phases after 16-48 h, but no evidence of increased apoptosis. BFM caused significant inhibition of GH3 xenograft growth, and histomorphometry showed a 30% decrease in mitosis but no change in apoptosis. 31P-magnetic resonance spectroscopy (MRS) in vivo of GH3 xenografts showed that BFM increased pHe, but did not affect pHi, resulting in a decrease in the negative pH gradient (-delta pH). BFM decreased lactate formation suggesting a reduction in glycolysis. We suggest that BFM reduces extracellular H(+)-transport by inhibition of V-ATPases leading to an increase in pHe and decreased glycolysis, and thus reduced tumour cell proliferation. 19F-MRS in vivo showed that a smaller -delta pH was associated with decreased retention of 5-fluorouracil (5FU) which was consistent with our previous data in vivo implying the -delta pH controls tumour retention of 5 FU.

p27 deficiency desensitizes Rb-/- cells to signals that trigger apoptosis during pituitary tumor development

Low p27 expression in many human cancers is a prognostic indicator for poor outcome. While analysing the mechanism by which p27 deficiency contributed to tumor development in the Rb+/- mouse model, we identified a role for p27 as a proapoptotic tumor suppressor. We examined the cell cycle and apoptotic response of these pituitary tumor cells to the dopamine analog bromocriptine as well as the expression of Arf and other cell cycle and apoptotic regulators in these tumors. We also examined the expression of Arf and its function in mouse embryo fibroblasts either singly or doubly deficient for Rb and p27. From these studies, we concluded that the absence of p27 disabled the trigger for an Arf-dependent apoptotic response in Rb-/- tumor cells. This suggests a novel mechanism by which the loss of p27 may impact on tumor development.

Dominant negative ER induces apoptosis in GH(4) pituitary lactotrope cells and inhibits tumor growth in nude mice

The ER plays an important role in the proliferation and differentiation of lactotrope tumor cells. GH(4) cells were infected with adenoviral vectors (AdL540Q and Ad1-536) to investigate the ability of dominant negative ER mutants to affect the regulation of gene expression and cell growth by endogenous ER. The dominant negative mutants suppressed estradiol stimulation of an estrogen-responsive reporter gene and the PRL promoter in these cells. AdL540Q or Ad1--536 infection also inhibited GH(4) cell growth and induced apoptosis, increasing the expression of the proapoptotic Bax protein and decreasing the expression of antiapoptotic Bcl-2. AdwtER-infected cells also showed decreased Bcl-2 protein. E2-induced activation of p38 MAPK, an enzyme that may participate in apoptosis, was observed in cells infected with AdwtER, AdL540Q, and Ad1--536. Consistent with the apoptotic effects in vitro, infection of GH(4) cells with AdL540Q or Ad1--536 inhibited the ability of the cells to form tumors in nude mice. These results indicate that dominant negative ER mutants induce apoptosis of GH(4) cells and suppress tumor formation and development. The delivery of dominant negative ERs by adenoviral vectors may provide an alternative modality for the targeted therapy of pituitary lactotrope adenomas and other estrogen-responsive tumors.