Signaling pathways regulating pituitary lactotrope homeostasis and tumorigenesis

Is prolactin receptor signaling a target in dopamine-resistant prolactinomas?

The hypothalamic neuroendocrine catecholamine dopamine regulates the lactotroph function, including prolactin (PRL) secretion, proliferation, and apoptosis. The treatment of PRL-secreting tumors, formerly known as prolactinomas, has relied mainly on this physiological characteristic, making dopamine agonists the first therapeutic alternative. Nevertheless, the group of patients that do not respond to this treatment has few therapeutical options. Prolactin is another physiological regulator of lactotroph function, acting as an autocrine/paracrine factor that controls PRL secretion and cellular turnover, inducing apoptosis and decreasing proliferation. Furthermore, the signaling pathways related to these effects, mainly JAK/STAT and PI3K/Akt, and MAPK, have been extensively studied in prolactinomas and other tumors as therapeutic targets. In the present work, the relationship between PRL pathophysiology and prolactinoma development is explored, aiming to comprehend the value of PRL and PRLR-associated pathways as exploratory fields alternative to dopamine-related approaches, which are worth physiological characteristics that might be impaired and can be potentially restored or upregulated to provide more options to the patients.

The dopaminergic control of Cushing's syndrome

Cushing's Syndrome (CS), or chronic endogenous hypercortisolism, is a rare and serious disease due to corticotroph pituitary (Cushing's disease, CD) and extra-pituitary (ectopic CS) tumours overproducing ACTH, or cortisol-secreting adrenal tumours or lesions (adrenal CS). The first-line treatment for CS is represented by the surgical removal of the responsible tumour, but surgery might be unfeasible or ineffective and medical treatment can be required in a relevant percentage of patients with CS, especially CD and ectopic CS. Corticotroph pituitary and extra-pituitary tumours, as well as adrenal tumours and lesions responsible for CS express dopamine receptors (DRs), which have been found to mediate inhibition of hormone secretion and/or cell proliferation in experimental setting, suggesting that dopaminergic system, particularly DRs, might represent a target for the treatment of CS. Dopamine agonists (DAs), particularly cabergoline (CAB), are currently used as off-label treatment for CD, the most common form of CS, demonstrating efficacy in controlling hormone secretion and tumour growth in a relevant number of cases, with the improvement of clinical picture, and displaying good safety profile. Therefore, CAB may be considered a reasonable alternative treatment for persistent or recurrent CD after pituitary surgery failure, but occasionally also before pituitary surgery, as adjuvant treatment, or even instead of pituitary surgery as first-line treatment in case of surgery contraindications or refusal. A certain beneficial effect of CAB has been also reported in ectopic CS. However, the role of DAs in the clinical management of the different types of CS requires further evaluations.

Resistance to Dopamine Agonists in Pituitary Tumors: Molecular Mechanisms

Pituitary neuroendocrine tumors (PitNET) are commonly benign tumors accounting for 10-25% of intracranial tumors. Prolactin-secreting adenomas represent the most predominant type of all PitNET and for this subtype of tumors, the medical therapy relies on the use of dopamine agonists (DAs). DAs yield an excellent therapeutic response in reducing tumor size and hormonal secretion targeting the dopamine receptor type 2 (D2DR) whose higher expression in prolactin-secreting adenomas compared to other PitNET is now well established. Moreover, although DAs therapy does not represent the first-line therapy for other PitNET, off-label use of DAs is considered in PitNET expressing D2DR. Nevertheless, DAs primary or secondary resistance, occurring in a subset of patients, may involve several molecular mechanisms, presently not fully elucidated. Dopamine receptors (DRs) expression is a prerequisite for a proper DA function in PitNET and several molecular events may negatively modify DR membrane expression, through the DRs down-regulation and intracellular trafficking, and DR signal transduction pathway. The current mini-review will summarise the presently known molecular events that underpin the unsuccessful therapy with DAs.

Gestational dioxin exposure suppresses prolactin-stimulated nursing in lactating dam rats to impair development of postnatal offspring

A number of epidemiological studies have implicated environmental chemicals including dioxins in the induction of negative effects on child development. To clarify the underlying mechanisms, almost all toxicologists have concentrated on effects on the offspring themselves. We examined an alternative hypothesis that gestational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a highly-toxic dioxin, targets factors related to maternal childcare to disturb offspring development. Oral administration of TCDD (1 µg/kg) to pregnant rats on gestational day 15 suppressed maternal licking behavior, a nursing behavior, and mammary gland maturation during the lactational stage, as well as the body weight and short-term memory of postnatal offspring. In support of these findings, maternal production of prolactin, a pituitary hormone essential for nursing including milk production, was decreased during the same period. Intracerebroventricular infusion of prolactin to dioxin-exposed dams restored or tended to restore many of the above defects observed both in mothers and offspring. The TCDD-dependent defects in maternal nursing behaviors can be due to a direct action on aryl hydrocarbon receptor (AHR) of lactating dams, because they did not emerge in AHR-knockout dams or control dams with TCDD-exposed offspring. Further examinations revealed that TCDD induces transforming growth factor β1 expression, which suppresses prolactin-producing cell proliferation, in a nursing period-specific manner. In agreement with this, the number of prolactin-positive cells in nursing dams was decreased by TCDD. These results provide novel evidence that gestational dioxin exposure attenuates prolactin-stimulated nursing in lactating dams to impair offspring development, and that immaturity of prolactin-producing cells can contribute to them.

Cabergoline may act as a radioprotective agent in Cushing's disease

CONTEXT

Conventional fractionated radiotherapy (CRT) achieves control of pathological hypercortisolism in 75%-80% of patients with persistent or recurrent Cushing's disease (CD), over a mean period of 18-24 months. Medical therapy is recommended as bridge therapy while awaiting RT effect.

OBJECTIVE

To determine long-term outcome of CRT and its predictors in CD patients.

DESIGN, SETTING AND PATIENTS

This is a retrospective case record analysis of 42 patients with CD who received CRT as a treatment modality and had at least 12 months post-RT follow-up. The dose delivered was 45 Gy in 25 fractions over 5 weeks. Demographic details, hormonal evaluation and radiological data were extracted from case records. Dexamethasone suppressed cortisol at cut-off of 1.8 µg/dL was used to define remission or recurrence. Possible predictors for remission and recurrence were analysed.

RESULTS

The mean age at the time of CRT administration was 23.7 ± 10.7 (range: 12-48) years. A total of 29 (69%) patients achieved remission 26.5 ± 28.5 (median: 18, range: 3-120) months after RT, while 13 (31%) patients had persistent disease at last follow-up. There were no significant predictors of disease remission after CRT. Six (20.7%) patients had recurrence after a documented initial remission. Recurrence occurred 66.6 ± 25.9 (median: 74; range: 18 to 90) months after documented remission. Recurrence of the disease was exclusively seen in patients who received peri-RT cabergoline. Peri-CRT use of cabergoline was significantly associated with increased recurrence rates (P = .016).

CONCLUSION

Use of cabergoline in the peri-CRT period did not affect initial remission after CRT but was associated with increased recurrence after initial remission in CD.

A Natural Isoquinoline Alkaloid With Antitumor Activity: Studies of the Biological Activities of Berberine

Coptis, a traditional medicinal plant, has been used widely in the field of traditional Chinese medicine for many years. More recently, the chemical composition and bioactivity of Coptis have been studied worldwide. Berberine is a main component of Rhizoma Coptidis. Modern medicine has confirmed that berberine has pharmacological activities, such as anti-inflammatory, analgesic, antimicrobial, hypolipidemic, and blood pressure-lowering effects. Importantly, the active ingredient of berberine has clear inhibitory effects on various cancers, including colorectal cancer, lung cancer, ovarian cancer, prostate cancer, liver cancer, and cervical cancer. Cancer, ranked as one of the world’s five major incurable diseases by WHO, is a serious threat to the quality of human life. Here, we try to outline how berberine exerts antitumor effects through the regulation of different molecular pathways. In addition, the berberine-mediated regulation of epigenetic mechanisms that may be associated with the prevention of malignant tumors is described. Thus, this review provides a theoretical basis for the biological functions of berberine and its further use in the clinical treatment of cancer.

JAK2/STAT5 Pathway Mediates Prolactin-Induced Apoptosis of Lactotropes

Prolactinomas are increasingly viewed as a "problem of signal transduction." Consequently, the identification of factors and signaling pathways that control lactotrope cell turnover is needed in order to encourage new therapeutic developments. We have previously shown that prolactin (PRL) acts as a proapoptotic and antiproliferative factor on lactotropes, maintaining anterior pituitary cell homeostasis, which contrasts with the classical antiapoptotic and/or proliferative actions exerted by PRL in most other target tissues. We aimed to investigate the PRLR-triggered signaling pathways mediating these nonclassical effects of PRL in the pituitary. Our results suggest that (i) the PRLR/Jak2/STAT5 pathway is constitutively active in GH3 cells and contributes to PRL-induced apoptosis by increasing the Bax/Bcl-2 ratio, (ii) PRL inhibits ERK1/2 and Akt phosphorylation, thereby contributing to its proapoptotic effect, and (iii) the PI3K/Akt pathway participates in the PRL-mediated control of lactotrope proliferation. We hypothesize that the alteration of PRL actions in lactotrope homeostasis due to the dysregulation of any of the mechanisms of actions described above may contribute to the pathogenesis of prolactinomas.

Pituitary somatolactotropes evade an oncogenic response to Ras

Distinct cell types have been shown to respond to activated Ras signaling in a cell-specific manner. In contrast to its pro-tumorigenic role in some human epithelial cancers, oncogenic Ras triggers differentiation of pheochromocytoma cells and medullary thyroid carcinoma cells. Furthermore, we have previously demonstrated that in pituitary somatolactotropes, activated Ras promotes differentiation and is not sufficient to drive tumorigenesis. These findings demonstrate that lactotrope cells have the ability to evade the tumorigenic fate that is often associated with persistent activation of Ras/ERK signaling, and suggest that there may be differential expression of inhibitory signaling molecules or negative cell cycle regulators that act as a brake to prevent the tumorigenic effects of sustained Ras signaling. Here we aim to gain further insight into the mechanisms that allow GH4T2 cells to evade an oncogenic response to Ras. We show that Ral, but likely not menin, plays a key role in directing Ras-mediated differentiation of somatolactotropes, which may allow these cells to escape the tumorigenic fate that is often associated with activated Ras signaling. We also show that dominant negative Ras expression results in reduced GH4T2 cell proliferation and transformation, but does not influence differentiation. Taken together, the data presented here begin to shed light on the mechanisms by which pituitary somatolactotropes evade an oncogenic response to persistently activated Ras signaling and suggest that the architecture of the Ras signaling cascade in some endocrine cell types may be distinct from that of cells that respond to Ras in an oncogenic manner.

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.

Consider the context: Ras/ERK and PI3K/AKT/mTOR signaling outcomes are pituitary cell type-specific

Conserved signaling pathways are critical regulators of pituitary homeostasis and, when dysregulated, contribute to adenoma formation. Pituitary adenomas are typically benign and rarely progress to malignant cancer. Pituitary and other neuroendocrine cell types often display non-proliferative responses to ERK and PI3K, in contrast to non-endocrine cell types which typically proliferate in response to ERK and PI3K activation. These differences likely contribute to the infrequent progression to malignancy in many endocrine tumors. In this review, we highlight the Ras/ERK and PI3K/AKT/mTOR signaling pathways in each pituitary cell type, as well as in other endocrine tissues. Furthermore, we provide evidence that a balance of ERK and PI3K signaling is required to maintain pituitary homeostasis. It is unlikely that one sole oncogene will be identified as being responsible for sporadic pituitary adenoma formation. This review emphasizes the necessity to consider endocrine cell-specific contexts and the interplay of signaling pathways to define the mechanisms underlying pituitary tumorigenesis.

Notch4 inhibition reduces migration and invasion and enhances sensitivity to docetaxel by inhibiting Akt/fascin in pancreatic cancer cells

Overexpression of Notch4 is associated with a variety of tumor types. Only sparse information exists on Notch4 expression in pancreatic cancer (PC). The present study demonstrated that Notch4 expression was significantly upregulated in PC cell lines compared with a non-transformed pancreatic epithelial cell line, HPDE6c-7. To investigate the possible role of Notch4 in PC cells, an RNA interference approach was used to silence Notch4 expression. The results revealed that small interfering RNA (siRNA) targeting Notch4 significantly impeded the viability, migration and invasion abilities of PC cells in vitro. Downregulation of Notch4 with siRNA sensitized cells to the action of docetaxel. Furthermore, Notch4 downregulation enhanced the inhibition of Akt activation and the fascin expression induced by docetaxel in PC cells. Together, these data provide insight into the function of Notch4 and suggest that Notch4 may represent a new potential target for gene therapy in PC.

Inhibition of SKP2 Sensitizes Bromocriptine-Induced Apoptosis in Human Prolactinoma Cells

PURPOSE

Prolactinoma (prolactin-secreting pituitary adenoma) is one of the most common estrogen-related functional pituitary tumors. As an agonist of the dopamine D2 receptor, bromocriptine is used widely to inhibit prolactinoma progression. On the other hand, it is not always effective in clinical application. Although a dopamine D2 receptor deficiency contributes to the impaired efficiency of bromocriptine therapy to some extent, it is unknown whether there some other underlying mechanisms leading to bromocriptine resistance in prolactinoma treatment. That is the main point addressed in this project.

MATERIALS AND METHODS

Human prolactinoma samples were used to analyze the S-phase kinase associated protein 2 (SKP2) expression level. Nutlin-3/adriamycin/cisplatin-treated GH3 and MMQ cells were used to analyze apoptosis in SKP2 overexpression or knockdown cells. SKP2 expression and the interaction partners of SKP2 were also detected after a bromocriptine treatment in 293T. Apoptosis was analyzed in C25 and bromocriptine-treated GH3 cells.

RESULTS

Compared to normal pituitary samples, most prolactinoma samples exhibit higher levels of SKP2 expression, which could inhibit apoptosis in a p53-dependent manner. In addition, the bromocriptine treatment prolonged the half-life of SKP2 and resulted in SKP2 overexpression to a greater extent, which in turn compromised its pro-apoptotic effect. As a result, the bromocriptine treatment combined with C25 (a SKP2 inhibitor) led to the maximal apoptosis of human prolactinoma cells.

CONCLUSION

These findings indicated that SKP2 inhibition sensitized the prolactinoma cells to bromocriptine and helped promote apoptosis. Moreover, a combined treatment of bromocriptine and C25 may contribute to the maximal apoptosis of human prolactinoma cells.

Relationship between antipsychotic medication, serum prolactin levels and osteoporosis/osteoporotic fractures in patients with schizophrenia: a critical literature review

INTRODUCTION

Using an antipsychotic medication can increase prolactin (PRL) levels, causing hyperprolactinemia (HPRL). Although the occurrence of osteoporosis within the population of patients with schizophrenia has been recognized, the precise nature of the association between antipsychotic treatment, PRL, osteoporosis, and the disease itself seems to be elusive.

AREAS COVERED

The aim of this review is to critically review the literature regarding the association between osteoporosis and PRL and to summarize the available evidence with respect to the impact of PRL-elevating antipsychotics on bone mineral density (BMD) and fractures in non-elderly patients with schizophrenia.

EXPERT OPINION

Although long-standing HPRL can have an impact on the rate of bone metabolism and, when associated with hypogonadism, may lead to decreased bone density in both female and male subjects, the relative contribution of antipsychotic-induced HPRL in bone mineral loss in patients with schizophrenia remains unclear. Methodological shortcomings of existing studies, including the lack of prospective data and the focus on measurements of BMD instead of bone turnover markers, preclude definitive conclusions regarding the relationship between PRL-raising antipsychotics and BMD loss in patients with schizophrenia. Therefore, more well conducted prospective trials of these biomarkers are necessary to establish the precise relationship between antipsychotics, PRL levels and osteoporosis/osteoporotic risk.