Opposing actions of two transforming growth factor-beta isoforms on pituitary lactotropic cell proliferation

The Balance of PI3K and ERK Signaling Is Dysregulated in Prolactinoma and Modulated by Dopamine

Prolactin-secreting adenomas, or prolactinomas, cause hypogonadism, osteoporosis, and infertility. Although dopamine agonists (DAs) are used clinically to treat prolactinoma and reduce prolactin secretion via cAMP inhibition, the precise mechanism by which DAs inhibit lactotrope proliferation has not been defined. In this study, we report that phosphatidylinositol 3-kinase (PI3K) signals through AKT and mTOR to drive proliferation of pituitary somatolactotrope GH4T2 cells. We demonstrate that the DA cabergoline reduces activity of the mTOR effector s6K and diminishes GH4T2 cell proliferation primarily via activation of the long isoform of the dopamine D2 receptor (D2R). Dysfunctional D2R-mediated signaling and/or downregulated D2R expression is thought be the primary mechanism of DA resistance, which is observed in 10% to 20% of prolactinoma tumors. Dopamine-mediated D2R activation results in ERK stimulation and PI3K inhibition, suggesting that these two pathways act in an inverse manner to maintain lactotrope homeostasis. In this study, we found that ERK1/2-mediated prolactin transcription is inhibited by PI3K/CDK4-driven cell cycle progression, emphasizing that the ERK and PI3K signaling pathways oppose one another in lactotrope cells under homeostatic conditions. Lastly, we show that both ERK1/2 and AKT are activated in prolactinoma, demonstrating that the balance of ERK and AKT is dysregulated in human prolactinoma. Our findings reveal a potential use for dual pharmacological inhibitors of ERK and AKT as an alternative treatment strategy for DA-resistant prolactinomas.

Transcriptome analysis of muskrat scented glands degeneration mechanism

The scented gland, a musk-secreting organ of male muskrats, shows clear seasonal changes. When entering the secreting season in March, scented glands gradually increase in size and active secretion starts. In September, scented glands become gradually smaller and secretion decreases. By November, scented glands are gradually replaced by adipose tissue. In this study, six healthy adult male muskrats were analysed: three from the secreting season (March) and three from the non-secreting season (November). Using RNA-Seq analysis, gene expression profiles of scented glands from both seasons were determined. Using the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, we found that genes involved in calcium and TGF-beta signalling pathways were significantly more expressed in the non-secreting than in the secreting season. These changes in gene expression correlated with alterations in scented gland size. Both calcium and TGF-beta signalling pathways are important regulators of cell apoptosis, which may thus be involved in muskrat scented gland degeneration.

Involvement of MEK/ERK1/2 and PI3K/Akt pathways in the refractory behavior of GH3B6 pituitary tumor cells to the inhibitory effect of TGFβ1

Pituitary tumor cells have a poor response to the growth inhibitory effect of TGFβ1, possibly resulting from the cross talk of TGFβ/Smads signal with other signaling pathways, an undescribed mechanism in these tumoral cells. To address this hypothesis, we investigated whether the mitogen-activated extracellular signal-regulated kinase (MEK)/ERK1/2 and phosphoinositide-3 kinase/protein kinase B (PI3K/Akt) pathways were able to regulate the antimitogenic effect of TGFβ1 on GH3B6 cells. TGFβ1 treatment decreased the cell proliferation and induced an activation of mothers against decapentaplegic homolog 2/3 (Smad2/3), effects that were potentiated by MEK and PI3K inhibitors, thus indicating the existence of a cross talk between TGFβ1/Smad with the MEK/ERK1/2 or PI3K/Akt pathways. In addition, through immunoprecipitation assays, a direct interaction was observed between Smad2/3-ERK1/2 and Smad2/3-Akt, which decreased when the GH3B6 cells were incubated with TGFβ1 in the presence of MEK or PI3K inhibitors, thereby suggesting that the ERK1/2- and Akt-activated states were involved. These Smad2/3-ERK1/2 and Smad2/3-Akt associations were also confirmed by confocal and transmission electron microscopy. These findings indicate that the TGFβ1-antimitogenic effect in GH3B6 cells was attenuated by the MEK/ERK1/2 and PI3K/Akt pathways via modulating Smad2/3 phosphorylation. This molecular mechanism could explain in part the refractory behavior of pituitary tumor cells to the inhibitory effect of TGFβ1.

Signaling pathways regulating pituitary lactotrope homeostasis and tumorigenesis

Dysregulation of the signaling pathways that govern lactotrope biology contributes to tumorigenesis of prolactin (PRL)-secreting adenomas, or prolactinomas, leading to a state of pathological hyperprolactinemia. Prolactinomas cause hypogonadism, infertility, osteoporosis, and tumor mass effects, and are the most common type of neuroendocrine tumor. In this review, we highlight signaling pathways involved in lactotrope development, homeostasis, and physiology of pregnancy, as well as implications for signaling pathways in pathophysiology of prolactinoma. We also review mutations found in human prolactinoma and briefly discuss animal models that are useful in studying pituitary adenoma, many of which emphasize the fact that alterations in signaling pathways are common in prolactinomas. Although individual mutations have been proposed as possible driving forces for prolactinoma tumorigenesis in humans, no single mutation has been clinically identified as a causative factor for the majority of prolactinomas. A better understanding of lactotrope-specific responses to intracellular signaling pathways is needed to explain the mechanism of tumorigenesis in prolactinoma.

Cellular and molecular specificity of pituitary gland physiology

The anterior pituitary gland has the ability to respond to complex signals derived from central and peripheral systems. Perception of these signals and their integration are mediated by cell interactions and cross-talk of multiple signaling transduction pathways and transcriptional regulatory networks that cooperate for hormone secretion, cell plasticity, and ultimately specific pituitary responses that are essential for an appropriate physiological response. We discuss the physiopathological and molecular mechanisms related to this integrative regulatory system of the anterior pituitary gland and how it contributes to modulate the gland functions and impacts on body homeostasis.

Tissue-Specific Effects of Loss of Estrogen during Menopause and Aging

The roles of estrogens have been best studied in the breast, breast cancers, and in the female reproductive tract. However, estrogens have important functions in almost every tissue in the body. Recent clinical trials such as the Women's Health Initiative have highlighted both the importance of estrogens and how little we know about the molecular mechanism of estrogens in these other tissues. In this review, we illustrate the diverse functions of estrogens in the bone, adipose tissue, skin, hair, brain, skeletal muscle and cardiovascular system, and how the loss of estrogens during aging affects these tissues. Early transcriptional targets of estrogen are reviewed in each tissue. We also describe the tissue-specific effects of selective estrogen receptor modulators (SERMs) used for the treatment of breast cancers and postmenopausal symptoms.

Molecular interaction of BMP-4, TGF-beta, and estrogens in lactotrophs: impact on the PRL promoter

The regulatory role of estrogen, bone morphogenetic protein-4 (BMP-4), and TGF-beta has a strong impact on hormone secretion, gene transcription, and cellular growth of prolactin (PRL)-producing cells. In contrast to TGF-beta, BMP-4 induces the secretion of PRL in GH3 cells. Therefore, we studied the mechanism of their transcriptional regulation. Both BMP-4 and TGF-beta inhibited the transcriptional activity of the estrogen receptor (ER). Estrogens had no effect on TGF-beta-specific Smad protein transcriptional activity but presented a stimulatory action on the transcriptional activity of the BMP-4-specific Smads. BMP-4/estrogen cross talk was observed both on PRL hormone secretion and on the PRL promoter. This cross talk was abolished by the expression of a dominant-negative form for Smad-1 and treatment with ICI 182780 but not by point mutagenesis of the estrogen response element site within the promoter, suggesting that Smad/ER interaction might be dependent on the ER and a Smad binding element. By serial deletions of the PRL promoter, we observed that indeed a region responsive to BMP-4 is located between -2000 and -1500 bp upstream of the transcriptional start site. Chromatin immunoprecipitation confirmed Smad-4 binding to this region, and by specific mutation and gel shift assay, a Smad binding element responsible site was characterized. These results demonstrate that the different transcriptional factors involved in the Smad/ER complexes regulate their transcriptional activity in differential ways and may account for the different regulatory roles of BMP-4, TGF-beta, and estrogens in PRL-producing cells.

Ethanol alters production and secretion of estrogen-regulated growth factors that control prolactin-secreting tumors in the pituitary

BACKGROUND

Chronic administration of ethanol increases plasma prolactin levels and enhances estradiol's mitogenic action on the lactotropes of the pituitary gland. The present study was conducted to determine whether ethanol's lactotropic cell-proliferating action, like estradiol's, is associated with alteration in the production of 3 peptides that regulate cell growth: transforming growth factor beta 1 (TGF-beta1), TGF-beta3 and basic fibroblast growth factor (bFGF).

METHODS

Using ovariectomized Fischer-344 female rats, we determined ethanol's and estradiol's actions on lactotropic cell proliferation and growth-regulatory peptide production and release in the pituitary gland during tumorigenesis.

RESULTS

Ethanol increased basal and estradiol-enhanced mitosis of lactotropes in the pituitary glands of ovariectomized rats. The level of growth-inhibitory TGF-beta1 was reduced in the pituitary following ethanol and/or estradiol treatment for 2 and 4 weeks. In contrast, ethanol and estradiol alone as well as together increased levels of growth-stimulatory TGF-beta3 and bFGF in the pituitary at 2 and 4 weeks. In primary cultures of pituitary cells, both ethanol and estradiol reduced TGF-beta1 release and increased TGF-beta3 and bFGF release at 24 hours. Ethanol's effect on growth factor levels in the pituitary or growth factor release from the pituitary cells was less than that of estradiol. When ethanol and estradiol were applied together, their individual effects on these growth factors were amplified.

CONCLUSIONS

These results confirm estradiol's modulation of pituitary growth factor production and release, and provide evidence that ethanol, like estradiol, alters the production and secretion of growth-regulatory peptides controlling lactotropic cell proliferation.

Global analysis of gene expression in the estrogen induced pituitary tumor of the F344 rat

The F344 rat rapidly forms large prolactinomas in response to chronic estrogen treatment. To identify genes expressed in the course of this estrogen induced pituitary tumor growth, we performed microarray analysis on the F344 rat pituitary after chronic estrogen treatment and on untreated controls. At a significance level set to minimize type I error, some 72 genes were found to be differentially expressed between estrogen treated and untreated. Of those genes, 70 have not been reported previously as being affected by estrogen in the F344 rat pituitary. Since many other investigators have studied the effect of estrogen on specific gene expression in rat pituitary, we also examined the mRNA expression of the 36 genes that have been previously reported as having their expression affected by estrogen in the rat pituitary. Of these, 13 were found to have their expression affected by estrogen treatment in the same direction as had been reported by others.

Genesis of prolactinomas: studies using estrogen-treated animals

Prolactin-secreting adenomas (prolactinomas) are the most prevalent form of pituitary tumors in humans. Our knowledge of the formation of these tumors is limited. Experimental work in animal has uncovered that estradiol exposure leads to prolactinoma formation via orchestrated events involving dopamine D2 receptors, transforming growth factor-beta(TGF-beta) isoforms and their receptors, as well as factors secondary to TGF-beta action. Additionally, these studies determined that TGF-beta and b-FGF interact to facilitate the communication between lactotropes and folliculo-stellate cells that is necessary for the mitogenic action of estradiol. The downstream signaling that governs lactotropic cell proliferation involves activation of the MAP kinase p44/42-dependent pathway.

Dopamine, dopamine D2 receptor short isoform, transforming growth factor (TGF)-beta1, and TGF-beta type II receptor interact to inhibit the growth of pituitary lactotropes

The neurotransmitter dopamine is known to inhibit prolactin secretion and the proliferation of lactotropes in the pituitary gland. In this study, we determined whether dopamine and TGFbeta1 interact to regulate lactotropic cell proliferation. We found that dopamine and the dopamine agonist bromocriptine stimulated TGFbeta1 secretion and TGFbeta1 mRNA expression but inhibited lactotropic cell proliferation both in vivo and in vitro. The dopamine's inhibitory action on lactotropic cell proliferation was blocked by a TGFbeta1-neutralizing antibody. We also found that PR1 cells, which express low amounts of the dopamine D2 receptor, demonstrated reduced expression of TGFbeta1 type II receptor and TGFbeta1 mRNA levels and had undetectable levels of TGFbeta1 protein. These cells showed a reduced TGFbeta1 growth-inhibitory response. Constitutive expression of the D2 receptor short isoform, but not the D2 receptor long isoform, induced TGFbeta1 and TGFbeta1 type II receptor gene expression and recovered dopamine- and TGFbeta1-induced growth inhibition in PR1 cells. The constitutive expression of D2 receptor short isoform also reduced the tumor cell growth rate. These data suggest that a TGFbeta1 system may mediate, in part, the growth-inhibitory action of dopamine on lactotropes.

Antagonic effects of oestradiol in interaction with IGF-1 on proliferation of lactotroph cells in vitro

The effects of IGF-1, 17 beta oestradiol and its functional interaction on lactotrophs cell proliferation were evaluated. In addition we investigated the involvement of PKC alpha, epsilon and phosphorilated ERK, in the mitogenic process. Primary cell cultures of adenohypophysis from female Wistar rats were studied in serum free conditions. The proliferation of lactotrophs was determined by double immunostaining for BrdU and PRL. The incubation with IGF-1 5, 30 or 100 ng/ml during 48 or 72 h increased lactotrophs proliferation two-threefold depending on IGF-1 concentration. Co-incubation of IGF-1 (30 ng/ml) with genistein (25 microM) or BIM (0.5 or 2 microM), lowered of tyrosine kinase receptor or of PKC respectively, inhibited the induced IGF-1 lactotrophs proliferation. 17 beta oestradiol (1, 10 or 100 nM) had not mitogenic effect, whereas in the presence of serum PRL cells proliferation was stimulated. Co-incubation with 1 nM oestradiol and IGF-1 significantly decreased the lactotroph BrdU-labelling achieved with IGF-1. PKC alpha, epsilon and ERK1/2 levels measured by western blot augmented in the presence of IGF-1 and were inhibited with the addition of genistein, supporting a participation of these enzymes in the proliferate process. Co-incubation of IGF-1 with 1 nM oestradiol decreased both PKC isoforms and activated ERK1/2 levels, suggesting that oestradiol would exert its antiproliferative effect by acting on the signalling pathway of IGF-1. The results revealed antagonic effects of oestradiol on lactotroph proliferation depending on its concentration and the presence of IGF-1.

Folliculostellate cells determine the susceptibility of lactotropes to estradiol's mitogenic action

Estradiol is known to increase lactotropic cell proliferation, but estradiol susceptibility varies among human populations and among various strains of rats. We had reported that folliculostellate (FS) cells regulate estradiol's mitogenic action on lactotropes; therefore, we studied their role in determining the susceptibility to estradiol in a high estradiol-responsive rat strain, Fischer 344 (F344), and in a low-responsive strain, Sprague Dawley (SD). Determination of total S-100-positive FS cells in the pituitary revealed that F344 rats have significantly more FS cells than do SD rats. Estradiol treatment did not change the number of FS cells in both F344 and SD rats. When cotransplanted with F344 pituitaries under the kidney capsule or cocultured with F344-derived lactotropes in vitro, FS cells derived from F344 rats increased estradiol's mitogenic action. They also increased estradiol's mitogenic action on SD-derived lactotropes in primary cultures. However, SD-derived FS cells failed to increase estrogen's action on F344- or SD-derived lactotropes. The levels of basic fibroblast growth factor production and secretion by TGF-beta 3 and estradiol were much higher in F344-derived FS cells than in SD-derived FS cells. However, the lactotropes' growth response to basic fibroblast growth factor was similar in both strains. These data suggest that cell-cell interaction between FS cells and lactotropes regulates estradiol's mitogenic action on lactotropes and also determines lactotrope susceptibility to the steroid.

Involvement of protein kinase C-dependent mitogen-activated protein kinase p44/42 signaling pathway for cross-talk between estradiol and transforming growth factor-beta3 in increasing basic fibroblast growth factor in folliculostellate cells

We have recently shown that TGF-beta3, in the presence of estradiol, increases the release of basic fibroblast growth factor (bFGF) from folliculostellate (FS) cells in the pituitary. We determined the interactive effects of TGF-beta3 and estradiol on bFGF production and release from FS cells, and the role of the MAPK pathway in TGF-beta3 and estradiol interaction. We found that TGF-beta3 and estradiol alone moderately increased cell content and release of bFGF from FS cells; but together, they markedly increased the peptide. Estradiol and TGF-beta3 alone moderately activated MAPK p44/42; together they produced marked activation of MAPK p44/42. Pretreatment of FS cells with an MAPK kinase 1/2 inhibitor or with protein kinase C inhibitors suppressed the activation of MAPK p44/42, bFGF release, and protein level increases, all of which were induced by TGF-beta3 and estradiol. Estradiol and TGF-beta3, either alone or in combination, increased the levels of active Ras. Furthermore, bFGF induction by TGF-beta3 and estradiol was blocked by overexpression of Ras N17, a dominant negative mutant of Ras p21. Estrogen receptor blocker ICI 182,780 failed to prevent estrogen's and TGF-beta3's effects on bFGF. These data suggest that an estradiol receptor-independent protein kinase C- activated Ras-dependent MAPK pathway is involved in the cross-talk between TGF-beta3 and estradiol to increase bFGF production and/or release from FS cells.

Rat strain specific attenuation of estrogen action in the anterior pituitary gland by dietary energy restriction

The purpose of this study was to compare the effects of a 40% restriction of dietary energy consumption, relative to that consumed by rats allowed to feed ad libitum, on the ability of 17beta-estradiol (E2) to induce pituitary tumorigenesis in two inbred rat strains, ACI and Copenhagen (COP), which are very closely related genetically. Ovary-intact ACI and COP rats were fed either a control or an energy-restricted diet beginning at 8 wk of age. Continuous treatment with E2, released from subcutaneous Silastic tubing implants, was initiated at 9 wk of age and the animals were killed 12 wk later. Estrogen-induced pituitary tumorigenesis is associated with rapid induction of lactotroph hyperplasia, increased pituitary mass, and hyperprolactinemia. E2 significantly increased pituitary mass and circulating prolactin (PRL) in both ACI and COP rats, and this response was significantly greater in ACI rats relative to COP. Dietary energy restriction did not inhibit E2-induced pituitary growth in the ACI rat. By contrast, E2-induced pituitary growth in COP rats was attenuated by dietary energy restriction, as evidenced by quantification of pituitary mass, pituitary weight to body weight ratio, circulating PRL, and pituitary cell proliferation. This study indicates that sensitivity to the inhibitory actions of dietary energy restriction on E2-induced pituitary tumorigenesis is genetically determined.

Lack of prolactin receptor signaling in mice results in lactotroph proliferation and prolactinomas by dopamine-dependent and -independent mechanisms

Hypothalamic dopamine inhibits pituitary prolactin secretion and proliferation of prolactin-producing lactotroph cells by activating lactotroph dopamine D2 receptors (D2Rs). Conversely, prolactin (PRL) stimulates hypothalamic dopamine neurons via PRL receptors (PRLRs) in a short-loop feedback circuit. We used Drd2(-/-) and Prlr(-/-) mutant mice to bypass this feedback and investigate possible dopamine-independent effects of PRL on lactotroph function. The absence of either receptor induced hyperprolactinemia and large prolactinomas in females. Small macroadenomas developed in aged Prlr(-/-) males, but only microscopic adenomas were found in Drd2(-/-) male mice. Pharmacologic studies in Prlr(-/-) mice with D2R agonists and antagonists demonstrated a significant loss of endogenous dopamine tone, i.e., constitutive inhibitory signaling by the D2R, in the pituitary. However, Prlr(-/-) mice exhibited more profound hyperprolactinemia and larger tumors than did age-matched Drd2(-/-) mice, and there were additive effects in compound homozygous mutant male mice. In vitro, PRL treatment markedly inhibited the proliferation of wild-type female and male Drd2(-/-) lactotrophs, but had no effect on female Drd2(-/-) lactotrophs, suggesting a downregulation or desensitization of PRLR in response to chronic hyperprolactinemia. We conclude that PRL inhibits lactotrophs by two distinct mechanisms: (a) indirectly by activation of hypothalamic dopamine neurons and (b) directly within the pituitary in a dopamine-independent fashion.

Expression of differentially expressed nucleolar transforming growth factor-beta1 target (DENTT) in adult mouse tissues

Differentially expressed nucleolar TGF-beta1 target (DENTT) is a novel member of the TSPY/TSPY-L/SET/NAP-1 (TTSN) superfamily that we have previously identified in human lung cancer cells. Here, we have investigated the expression of this protein in the adult mouse. By Western analysis, DENTT is highly expressed in the pituitary gland and moderately in the adrenals, brain, testis, and ovary. Immunohistochemical staining analysis for DENTT showed differential cytoplasmic and nuclear staining patterns in several cell types. The pituitary gland showed the highest level of immunostaining for DENTT, with strong cytoplasmic immunoreactivity in the anterior lobe, moderate levels in the posterior lobe, and a few cells showing nuclear staining in the intermediate lobe. In contrast, the intermediate lobe of the pituitary showed intense cytoplasmic staining for TGF-beta1. Nuclear and cytoplasmic staining for DENTT was present in the islets of Langerhans in the pancreas. Cytoplasmic staining for DENTT was particularly intense in the cortex of the adrenal gland, whereas the medulla showed weak nuclear staining. In the nervous system, the choroid plexus showed the highest immunoreactivity, with cortical motoneurons and Purkinje cells having relatively high levels of staining for DENTT as well. DENTT immunoreactivity was found in Leydig interstitial cells, Sertoli cells, and primary spermatocytes in the testis. In the female reproductive system, DENTT immunoreactivity was present in oocytes, thecal cells, and corpora lutea. The bronchial epithelium of the lung showed moderate levels of staining for DENTT localized to the cell nucleus. Additionally, three rodent pituitary cell lines (AtT20, GH3, and alphaT3-1, representing corticotropes, lactotropes, and gonadotropes, respectively) showed expression of DENTT. Addition of TGF-beta1 or serum to AtT20 cells increased DENTT protein production by 4 hr and, after reaching maximal levels at 2.4-fold above basal level by 8 hr, decreased, whereas no more than a 1.5-fold increase in DENTT protein occurred in GH3 or alphaT3-1 cells. Transient transfection studies showed that ectopic DENTT expression significantly increased the level of p3TP-Lux reporter transcription in AtT20 cells, but not in GH3 or alphaT3-1 cells. Interestingly, addition of TGF-beta1 had no significant effect on the ability of DENTT expression to influence p3TP-Lux reporter transcription in AtT20 cells. This report is the first detailed immunohistochemical examination of a member of the TTSN superfamily in the adult mouse. Expression of DENTT in endocrine tissues, nervous system, lung, oocytes, and thecal cells, in addition to the testis, suggests new roles for the TTSN superfamily. The differential patterns of expression of DENTT and TGF-beta1 in some tissues, including the pituitary, suggest that other factors are likely to be regulators of DENTT besides TGF-beta1.

Transforming growth factor-beta regulation of estradiol-induced prolactinomas

Prolactin-secreting adenomas (prolactinomas) are the most prevalent form of pituitary tumors in humans, and increased tumor growth under estrogenic influence in female patients is often of clinical concern. Extensive experimental work has uncovered the roles of estrogen receptors and various growth-regulatory peptides in estradiol action on lactotropes. However, it is only recently that we are beginning to gain insight into how these growth factors interact to regulate estradiol action on lactotrope cell proliferation. Recent studies have identified the regulatory role of TGF-beta-related peptides in estradiol action on lactotropes. Additionally, these studies determined that TGF-beta and FGF interact to facilitate the communication between lactotropes and folliculostellate cells that is necessary for the mitogenic action of estradiol. This review addresses the role of estradiol in prolactinoma formation and summarizes data that support a novel concept: Alterations in the expression and action of TGF-beta isoforms are crucial steps in estradiol-induced tumorigenesis.

Effect of antisense suppression of transforming growth factor-beta3 gene on lactotropic cell proliferation

The mechanism by which oestrogen regulates lactotropic cell proliferation is not well understood. Recently it has been shown that a transforming growth factor beta (TGF-beta) gene-related peptide, TGF-beta3, is produced in the lactotropes and stimulates lactotropic cell proliferation in vitro. In this study, the role of this growth factor in oestrogen-induced lactotropic cell proliferation was determined in vitro using oligonucleotide designed to inhibit TGF-beta3 gene expression. We used the oligonucleotide in an antisense orientation, which is complementary to regions in the TGF-beta3 message. Oligonucleotides in sense and missense orientation were also used. We found that the antisense sequence that was effectively incorporated into pituitary cells produced a marked inhibition of the stimulatory action of oestrogen on lactotropic cell proliferation. Sense and missense oligonucleotides produced no significant effects on oestrogen-stimulated lactotropic cell proliferation. The growth-inhibitory effect of antisense oligonucleotide was blocked by TGF-beta3 peptide. These results suggest that TGF-beta3 may be involved in the processes that mediate oestrogen-regulated lactotropic cell proliferation.