Disruption of growth hormone signaling retards prostate carcinogenesis in the Probasin/TAg rat

GH-dependent growth of experimentally induced carcinomas in vivo

Interest in investigating the role of the growth hormone (GH)/insulin-like growth factor 1 (IGF-1) axis in the initiation and progression of experimentally induced carcinomas has arisen due to several observations in the human population. First, subjects with Laron syndrome who lack GH signaling have significantly lower rates of cancer than people who have normal GH signaling. Second, epidemiologic studies have found strong associations between elevated circulating IGF-1 and the incidence of several common cancers. Third, women who bear children early in life have a dramatically reduced risk of developing breast cancer, which may be due to differences in hormone levels including GH. These observations have motivated multiple studies that have experimentally altered activity of the GH/IGF-1 axis in the context of experimental carcinoma models in mice and rats. Most of these studies have utilized carcinoma models for four organ systems that are also frequent sites of carcinomas in humans: the mammary gland, prostate gland, liver, and colon. This review focuses on these studies and describes some of the most common genetic models used to alter the activity of the GH/IGF-1 axis in experimentally induced carcinomas. A recurring theme that emerges from these studies is that manipulations that reduce the activity of GH or mediators of GH action also inhibit carcinogenesis in multiple model systems.

Clinical, Cellular, and Molecular Evidence of the Additive Antitumor Effects of Biguanides and Statins in Prostate Cancer

CONTEXT

Prostate cancer (PCa) is one of the leading causes of cancer-related death among the male population worldwide. Unfortunately, current medical treatments fail to prevent PCa progression in a high percentage of cases; therefore, new therapeutic tools to tackle PCa are urgently needed. Biguanides and statins have emerged as antitumor agents for several endocrine-related cancers.

OBJECTIVE

To evaluate: (1) the putative in vivo association between metformin and/or statins treatment and key tumor and clinical parameters and (2) the direct effects of different biguanides (metformin/buformin/phenformin), statins (atorvastatin/simvastatin/lovastatin), and their combination, on key functional endpoints and associated signalling mechanisms.

METHODS

An exploratory/observational retrospective cohort of patients with PCa (n = 75) was analyzed. Moreover, normal and tumor prostate cells (normal [RWPE-cells/primary prostate cell cultures]; tumor [LNCaP/22RV1/PC3/DU145 cell lines]) were used to measure proliferation/migration/tumorsphere-formation/signalling pathways.

RESULTS

The combination of metformin+statins in vivo was associated to lower Gleason score and longer biochemical recurrence-free survival. Moreover, biguanides and statins exerted strong antitumor actions (ie, inhibition of proliferation/migration/tumorsphere formation) on PCa cells, and that their combination further decreased; in addition, these functional parameters compared with the individual treatments. These actions were mediated through modulation of key oncogenic and metabolic signalling pathways (ie, AR/mTOR/AMPK/AKT/ERK) and molecular mediators (MKI67/cMYC/androgen receptor/cell-cycle inhibitors).

CONCLUSIONS

Biguanides and statins significantly reduced tumor aggressiveness in PCa, with this effect being more potent (in vitro and in vivo) when both compounds are combined. Therefore, given the demonstrated clinical safety of biguanides and statins, our results suggest a potential therapeutic role of these compounds, especially their combination, for the treatment of PCa.

Growth Hormone Receptor Regulation in Cancer and Chronic Diseases

The GHR signaling pathway plays important roles in growth, metabolism, cell cycle control, immunity, homeostatic processes, and chemoresistance via both the JAK/STAT and the SRC pathways. Dysregulation of GHR signaling is associated with various diseases and chronic conditions such as acromegaly, cancer, aging, metabolic disease, fibroses, inflammation and autoimmunity. Numerous studies entailing the GHR signaling pathway have been conducted for various cancers. Diverse factors mediate the up- or down-regulation of GHR signaling through post-translational modifications. Of the numerous modifications, ubiquitination and deubiquitination are prominent events. Ubiquitination by E3 ligase attaches ubiquitins to target proteins and induces proteasomal degradation or starts the sequence of events that leads to endocytosis and lysosomal degradation. In this review, we discuss the role of first line effectors that act directly on the GHR at the cell surface including ADAM17, JAK2, SRC family member Lyn, Ubc13/CHIP, proteasome, βTrCP, CK2, STAT5b, and SOCS2. Activity of all, except JAK2, Lyn and STAT5b, counteract GHR signaling. Loss of their function increases the GH-induced signaling in favor of aging and certain chronic diseases, exemplified by increased lung cancer risk in case of a mutation in the SOCS2-GHR interaction site. Insight in their roles in GHR signaling can be applied for cancer and other therapeutic strategies.

Androgen Receptor Regulation of Local Growth Hormone in Prostate Cancer Cells

Prostate cancer (PCa) growth is mainly driven by androgen receptor (AR), and tumors that initially respond to androgen deprivation therapy (ADT) or AR inhibition usually relapse into a more aggressive, castration-resistant PCa (CRPC) stage. Circulating growth hormone (GH) has a permissive role in PCa development in animal models and in human PCa xenograft growth. As GH and GH receptor (GHR) are both expressed in PCa cells, we assessed whether prostatic GH production is linked to AR activity and whether GH contributes to the castration-resistant phenotype. Using online datasets, we found that GH is highly expressed in human CRPC. We observed increased GH expression in castration-resistant C4-2 compared with castration-sensitive LNCaP cells as well as in enzalutamide (MDV3100)-resistant (MDVR) C4-2B (C4-2B MDVR) cells compared with parental C4-2B. We describe a negative regulation of locally produced GH by androgens/AR in PCa cells following treatment with AR agonists (R1881) and antagonists (enzalutamide, bicalutamide). We also show that GH enhances invasive behavior of CRPC 22Rv1 cells, as reflected by increased migration, invasion, and anchorage-independent growth, as well as expression of matrix metalloproteases. Moreover, GH induces expression of the AR splice variant 7, which correlates with antiandrogen resistance, and also induces insulinlike growth factor 1, which is implicated in PCa progression and ligand-independent AR activation. In contrast, blockade of GH action with the GHR antagonist pegvisomant reverses these effects both in vitro and in vivo. GH induction following ADT or AR inhibition may contribute to CRPC progression by bypassing androgen growth requirements.

TIMP3 Modulates GHR Abundance and GH Sensitivity

GH receptor (GHR) binds GH at the cell surface via its extracellular domain and initiates intracellular signal transduction, resulting in important anabolic and metabolic actions. GH signaling is subject to dynamic regulation, which in part is exerted by modulation of cell surface GHR levels. Constitutive and inducible metalloprotease-mediated cleavage of GHR regulate GHR abundance and thereby modulate GH action. We previously demonstrated that GHR proteolysis is catalyzed by the TNF-α converting enzyme (TACE; ADAM17). Tissue inhibitors of metalloproteases-3 (TIMP3) is a natural specific inhibitor of TACE, although mechanisms underlying this inhibition are not yet fully understood. In the current study, we use two model cell lines to examine the relationships between cellular TACE, TIMP3 expression, GHR metalloproteolysis, and GH sensitivity. These two cell lines exhibited markedly different sensitivity to inducible GHR proteolysis, which correlated directly to their relative levels of mature TACE vs unprocessed TACE precursor and indirectly to their levels of cellular TIMP3. Our results implicate TIMP3 as a modulator of cell surface GHR abundance and the ability of GH to promote cellular signaling; these modulatory effects may be conferred by endogenous TIMP3 expression as well as exogenous TIMP3 exposure. Furthermore, our analysis suggests that TIMP3, in addition to regulating the activity of TACE, may also modulate the maturation of TACE, thereby affecting the abundance of the active form of the enzyme.

Growth hormone enhances LNCaP prostate cancer cell motility

PURPOSE

Prostate cancer cells are responsive to multiple hormones and growth factors that can affect cell function. These effects may include modulating cell proliferation and apoptosis, but the ability to impinge on the metastatic potential of prostate cancer cells by affecting cell motility should also be considered, as prostate tumor metastasis correlates with limited therapeutic options and poor prognosis. Human growth hormone (hGH) can affect the growth and survival of prostate cancer cells, but the effect of hGH on prostate cancer cell motility is unknown. In the present study, the potential for exogenous and autocrine hGH to directly affect prostate cancer cell motility was addressed.

MATERIALS AND METHODS

The effects of exogenous and autocrine hGH on the chemokinesis and chemotaxis of LNCaP prostate cancer cells were tested using cell monolayer wound healing and Boyden chamber invasion assays. The signaling pathways underlying these effects were resolved with chemical inhibitors and the correlation with cytoskeletal actin reorganization evaluated microscopically by staining cells with fluor-conjugated phalloidin.

RESULTS

Both exogenous and autocrine hGH augmented the migration and invasion of LNCaP cells, and hGH itself acted as a chemoattractant. This activity was dependent upon the STAT5, MEK1/2 and PI3K signaling pathways, and was accompanied by an alteration in cellular actin organization.

CONCLUSIONS

hGH may enhance the metastatic potential of prostate cancer cells, both as a stimulant of cellular motility and invasiveness and as a chemoattractant.

Growth hormone and reproduction: a review of endocrine and autocrine/paracrine interactions

The somatotropic axis, consisting of growth hormone (GH), hepatic insulin-like growth factor I (IGF-I), and assorted releasing factors, regulates growth and body composition. Axiomatically, since optimal body composition enhances reproductive function, general somatic actions of GH modulate reproductive function. A growing body of evidence supports the hypothesis that GH also modulates reproduction directly, exerting both gonadotropin-dependent and gonadotropin-independent actions in both males and females. Moreover, recent studies indicate GH produced within reproductive tissues differs from pituitary GH in terms of secretion and action. Accordingly, GH is increasingly used as a fertility adjunct in males and females, both humans and nonhumans. This review reconsiders reproductive actions of GH in vertebrates in respect to these new conceptual developments.

Dynamic analysis of GH receptor conformational changes by split luciferase complementation

The transmembrane GH receptor (GHR) exists at least in part as a preformed homodimer on the cell surface. Structural and biochemical studies suggest that GH binds GHR in a 1:2 stoichiometry to effect acute GHR conformational changes that trigger the activation of the receptor-associated tyrosine kinase, Janus kinase 2 (JAK2), and downstream signaling. Despite information about GHR-GHR association derived from elegant fluorescence resonance energy transfer/bioluminescence resonance energy transfer studies, an assessment of the dynamics of GH-induced GHR conformational changes has been lacking. To this end, we used a split luciferase complementation assay that allowed detection in living cells of specific ligand-independent GHR-GHR interaction. Furthermore, GH treatment acutely augmented complementation of enzyme activity between GHRs fused, respectively, to N- and C-terminal fragments of firefly luciferase. Analysis of the temporal pattern of GH-induced complementation changes, pharmacological manipulation, genetic alteration of JAK2 levels, and truncation of the GHR intracellular domain (ICD) tail suggested that GH acutely enhances proximity of the GHR homodimer partners independent of the presence of JAK2, phosphorylation of GHR-luciferase chimeras, or an intact ICD. However, subsequent reduction of complementation requires JAK2 kinase activity and the ICD tail. This conclusion is in contrast to existing models of the GHR activation process.

Extrapituitary growth hormone in the chicken reproductive system

Increasing evidence shows that growth hormone (GH) expression is not limited to the pituitary, as it can be produced in many other tissues. It is known that growth hormone (GH) plays a role in the control of reproductive tract development. Acting as an endocrine, paracrine and/or autocrine regulator, GH influences proliferation, differentiation and function of reproductive tissues. In this review we substantiate the local expression of GH mRNA and GH protein, as well as the GH receptor (GHR) in both male and female reproductive tract, mainly in the chicken. Locally expressed GH was found to be heterogeneous, with a 17 kDa variant being predominant. GH secretagogues, such as GHRH and TRH co-localize with GH expression in the chicken testis and induce GH release. In the ovarian follicular granulosa cells, GH and GHR are co-expressed and stimulate progesterone production, which was neutralized by a specific GH antibody. Both testicular and follicular cells in primary cultures were able to synthesize and release GH to the culture medium. We also characterized GH and GH mRNA expression in the hen's oviduct and showed that it had 99.6% sequence identity with pituitary GH. Data suggest local reproductive GH may have important autocrine/paracrine effects.

Insulin-like growth factor-I induces CLU expression through Twist1 to promote prostate cancer growth

Clusterin (CLU) is cytoprotective molecular chaperone that is highly expressed in castrate-resistant prostate cancer (CRPC). CRPC is also characterized by increased insulin-like growth factor (IGF)-I responsiveness which induces prostate cancer survival and CLU expression. However, how IGF-I induces CLU expression and whether CLU is required for IGF-mediated growth signaling remain unknown. Here we show that IGF-I induced CLU via STAT3-Twist1 signaling pathway. In response to IGF-I, STAT3 was phosphorylated, translocated to the nucleus and bound to the Twist1 promoter to activate Twist1 transcription. In turn, Twist1 bound to E-boxes on the CLU promoter and activated CLU transcription. Inversely, we demonstrated that knocking down Twist1 abrogated IGF-I induced CLU expression, indicating that Twist1 mediated IGF-I-induced CLU expression. When PTEN knockout mice were crossed with lit/lit mice, the resultant IGF-I deficiency suppressed Twist1 as well as CLU gene expression in mouse prostate glands. Moreover, both Twist1 and CLU knockdown suppressed prostate cancer growth accelerated by IGF-I, suggesting the relevance of this signaling not only in an in vitro, but also in an in vivo. Collectively, this study indicates that IGF-I induces CLU expression through sequential activation of STAT3 and Twist1, and suggests that this signaling cascade plays a critical role in prostate cancer pathogenesis.

Functional collaboration of insulin-like growth factor-1 receptor (IGF-1R), but not insulin receptor (IR), with acute GH signaling in mouse calvarial cells

GH signals through the GH receptor (GHR), a cytokine receptor linked to Janus kinase 2 (JAK2). GH activates signal transducer and activator of transcription 5 (STAT5), causing expression of genes including IGF-I. IGF-I binds IGF-I receptor (IGF-IR), a heterotetrameric (α2-β2) tyrosine kinase growth factor receptor similar to insulin receptor (IR). In addition to this GH -> GHR -> IGF-I -> IGF-IR pathway, GH induces a complex including GHR, JAK2, and IGF-IR and deletion of floxed IGF-1R in primary murine calvarial cells with Cre-recombinase-expressing adenovirus (Ad-Cre) desensitizes cells to GH for STAT5 activation and IGF-I mRNA accumulation. Diminished GH-induced STAT5 phosphorylation in Ad-Cre-treated cells is rescued by adenoviruses encoding either IGF-IR or IGF-IR lacking the β-chain intracellular domain. Reasoning that IGF-IR's extracellular portion (α or extracellular β) mediates functional interaction with GH signaling, we pursued reconstitution studies. Although structurally related to IGF-IR, IR expressed adenovirally did not rescue GH-induced STAT5 phosphorylation in Ad-Cre-treated cells. We thus created chimeras, swapping homologous IR extracellular regions into IGF-IR. IR and IGF-IR possess N-terminal L1, cysteine-rich (CR), and L2 α-chain domains. We created Ad-IGF-IR/IR-L1 and Ad-IGF-IR/IR-L1-CR-L2, in which L1 alone or L1, CR, and L2 of IR replace corresponding IGF-IR regions, respectively. Ad-IGF-IR/IR-L1, but not Ad-IGF-IR/IR-L1-CR-L2, rescued GH-induced STAT5 phosphorylation in Ad-Cre-treated cells. Additionally, medium containing a soluble IGF-IR (including only L1-CR-L2) dampened GH-induced STAT5 phosphorylation in calvarial cells and two other GH-responsive cell lines. Thus, an extracellular determinant(s), likely in CR-L2, specifically allows IGF-IR to collaborate with GHR and JAK2 for robust GH-induced acute STAT5 phosphorylation.

Human metastatic melanoma cell lines express high levels of growth hormone receptor and respond to GH treatment

Accumulating evidence implicates the growth hormone receptor (GHR) in carcinogenesis. While multiple studies show evidence for expression of growth hormone (GH) and GHR mRNA in human cancer tissue, there is a lack of quantification and only a few cancer types have been investigated. The National Cancer Institute's NCI60 panel includes 60 cancer cell lines from nine types of human cancer: breast, CNS, colon, leukemia, melanoma, non-small cell lung, ovarian, prostate and renal. We utilized this panel to quantify expression of GHR, GH, prolactin receptor (PRLR) and prolactin (PRL) mRNA with real-time RT qPCR. Both GHR and PRLR show a broad range of expression within and among most cancer types. Strikingly, GHR expression is nearly 50-fold higher in melanoma than in the panel as a whole. Analysis of human metastatic melanoma biopsies confirmed GHR gene expression in melanoma tissue. In these human biopsies, the level of GHR mRNA is elevated in advanced stage IV tumor samples compared to stage III. Due to the novel finding of high GHR in melanoma, we examined the effect of GH treatment on three NCI60 melanoma lines (MDA-MB-435, UACC-62 and SK-MEL-5). GH increased proliferation in two out of three cell lines tested. Further analysis revealed GH-induced activation of STAT5 and mTOR in a cell line dependent manner. In conclusion, we have identified cell lines and cancer types that are ideal to study the role of GH and PRL in cancer, yet have been largely overlooked. Furthermore, we found that human metastatic melanoma tumors express GHR and cell lines possess active GHRs that can modulate multiple signaling pathways and alter cell proliferation. Based on this data, GH could be a new therapeutic target in melanoma.

SOCS2 mediates the cross talk between androgen and growth hormone signaling in prostate cancer

Anabolic signals such as androgens and the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis play an essential role in the normal development of the prostate but also in its malignant transformation. In this study, we investigated the role of suppressor of cytokine signaling 2 (SOCS2) as mediator of the cross talk between androgens and GH signals in the prostate and its potential role as tumor suppressor in prostate cancer (PCa). We observed that SOCS2 protein levels assayed by immunohistochemistry are elevated in hormone therapy-naive localized prostatic adenocarcinoma in comparison with benign tissue. In contrast, however, castration-resistant bone metastases exhibit reduced levels of SOCS2 in comparison with localized or hormone naive, untreated metastatic tumors. In PCa cells, SOCS2 expression is induced by androgens through a mechanism that requires signal transducer and activator of transcription 5 protein (STAT5) and androgen receptor-dependent transcription. Consequentially, SOCS2 inhibits GH activation of Janus kinase 2, Src and STAT5 as well as both cell invasion and cell proliferation in vitro. In vivo, SOCS2 limits proliferation and production of IGF-1 in the prostate in response to GH. Our results suggest that the use of GH-signaling inhibitors could be of value as a complementary treatment for castration-resistant PCa.

SUMMARY

Androgen induced SOCS2 ubiquitin ligase expression and inhibited GH signaling as well as cell proliferation and invasion in PCa, whereas reduced SOCS2 was present in castration-resistant cases. GH-signaling inhibitors might be a complementary therapeutic option for advanced PCa.

Pharmacologic inhibition of Jak2-Stat5 signaling By Jak2 inhibitor AZD1480 potently suppresses growth of both primary and castrate-resistant prostate cancer

PURPOSE

Progression of prostate cancer to the lethal castrate-resistant stage coincides with loss of responsiveness to androgen deprivation and requires development of novel therapies. We previously provided proof-of-concept that Stat5a/b is a therapeutic target protein for prostate cancer. Here, we show that pharmacologic targeting of Jak2-dependent Stat5a/b signaling by the Jak2 inhibitor AZD1480 blocks castrate-resistant growth of prostate cancer.

EXPERIMENTAL DESIGN

Efficacy of AZD1480 in disrupting Jak2-Stat5a/b signaling and decreasing prostate cancer cell viability was evaluated in prostate cancer cells. A unique prostate cancer xenograft mouse model (CWR22Pc), which mimics prostate cancer clinical progression in patients, was used to assess in vivo responsiveness of primary and castrate-resistant prostate cancer (CRPC) to AZD1480. Patient-derived clinical prostate cancers, grown ex vivo in organ explant cultures, were tested for responsiveness to AZD1480.

RESULTS

AZD1480 robustly inhibited Stat5a/b phosphorylation, dimerization, nuclear translocation, DNA binding, and transcriptional activity in prostate cancer cells. AZD1480 reduced prostate cancer cell viability sustained by Jak2-Stat5a/b signaling through induction of apoptosis, which was rescued by constitutively active Stat5a/b. In mice, pharmacologic targeting of Stat5a/b by AZD1480 potently blocked growth of primary androgen-dependent as well as recurrent castrate-resistant CWR22Pc xenograft tumors, and prolonged survival of tumor-bearing mice versus vehicle or docetaxel-treated mice. Finally, nine of 12 clinical prostate cancers responded to AZD1480 by extensive apoptotic epithelial cell loss, concurrent with reduced levels of nuclear Stat5a/b.

CONCLUSIONS

We report the first evidence for efficacy of pharmacologic targeting of Stat5a/b as a strategy to inhibit castrate-resistant growth of prostate cancer, supporting further clinical development of Stat5a/b inhibitors as therapy for advanced prostate cancer.

Targeted expression of Escherichia coli purine nucleoside phosphorylase and Fludara® for prostate cancer therapy

BACKGROUND

Previous studies have shown that Herpes Simplex Virus thymidine kinase (HSV-tk)/ganciclovir (GCV) comprised the most commonly used suicide gene therapy for prostate cancer, with modest results being obtained. However, novel suicide genes, such as Escherichia coli purine nucleoside phosphorylase (PNP), have been utilized to demonstrate more potent tumor killing and an enhanced bystander effect on local, non-expressing cells compared to HSV-tk.

METHODS

PNP/fludarabine (Fludara®; fludarabine phosphate; Berlex Labs, Richmond, CA, USA) was deliveried by prostate-specific, rat probasin-based promoter, ARR2PB. After infection of various cell lines with ADV.ARR(2) PB-PNP and administration of androgen analog, R1881, expression of PNP mRNA was detected; in vivo, the antitumor effect of the ARR(2) PB-PNP/Fludara system was monitored and analyzed, as well as animal survival.

RESULTS

After in vitro infection with ADV.ARR(2) PB-PNP (multiplicity of infection  =  10), LNCaP cells were more sensitive to a lower concentration Fludara (LD(50) , approximately 0.1 µg/ml) in the presence of R1881. Furthermore, robust bystander effects after R1881/Fludara treatment were observed in LNCaP cells after infection with bicistronic vector ADV.ARR2PB/PNP-IRES-EGFP in contrast to a much weaker effect in cells treated with ADV.CMV-HSV-tk/GCV. In vivo, tumor size in the ADV.ARR2PB-PNP/Fludara treatment group was dramatically smaller than in the control groups, and the mice treated with our system had a significantly prolonged survival, with three of eight mice surviving up to the 160-day termination point, as well as no systemic toxicity.

CONCLUSIONS

The ARR(2) PB-PNP/Fludara system induced massive tumor cell death and a prolonged life span without systemic cytotoxicity; therefore, it might be a more attractive strategy for suicide gene therapy of prostate cancer.

The role of prolactin receptor in GH signaling in breast cancer cells

GH and prolactin (PRL) are structurally related hormones that exert important effects in disparate target tissues. Their receptors (GHR and PRLR) reside in the cytokine receptor superfamily and share signaling pathways. In humans, GH binds both GHR and PRLR, whereas PRL binds only PRLR. Both hormones and their receptors may be relevant in certain human and rodent cancers, including breast cancer. GH and PRL promote signaling in human T47D breast cancer cells that express both GHR and PRLR. Furthermore, GHR and PRLR associate in a fashion augmented acutely by GH, even though GH primarily activates PRLR, rather than GHR, in these cells. To better understand PRLR's impact, we examined the effects of PRLR knockdown on GHR availability and GH sensitivity in T47D cells. T47D-ShPRLR cells, in which PRLR expression was reduced by stable short hairpin RNA (shRNA) expression, were compared with T47D-SCR control cells. PRLR knockdown decreased the rate of GHR proteolytic turnover, yielding GHR protein increase and ensuing sensitization of these cells to GHR signaling events including phosphorylation of GHR, Janus kinase 2, and signal transducer and activator of transcription 5 (STAT5). Unlike in T47D-SCR cells, acute GH signaling in T47D-ShPRLR cells was not blocked by the PRLR antagonist G129R but was inhibited by the GHR-specific antagonist, anti-GHR(ext-mAb). Thus, GH's use of GHR rather than PRLR was manifested when PRLR was reduced. In contrast to acute effects, GH incubation for 2 h or longer yielded diminished STAT5 phosphorylation in T47D-ShPRLR cells compared with T47D-SCR, a finding perhaps explained by markedly greater GH-induced GHR down-regulation in cells with diminished PRLR. However, when stimulated with repeated 1-h pulses of GH separated by 3-h washout periods to more faithfully mimic physiological GH pulsatility, T47D-ShPRLR cells exhibited greater transactivation of a STAT5-responsive luciferase reporter than did T47D-SCR cells. Our data suggest that PRLR's presence meaningfully affects GHR use in breast cancer cells.

Insulin-like growth factors and insulin: at the crossroad between tumor development and longevity

Numerous lines of evidence indicate that insulin-like growth factor signaling plays an important role in the regulation of life span and tumor development. In the present paper, the role of individual components of insulin-like growth factor signaling in aging and tumor development has been extensively analyzed. The molecular mechanisms underlying aging and tumor development are frequently overlapping. Although the link between reduced insulin-like growth factor signaling and suppressed tumor growth and development is well established, it remains unclear whether extended life span results from direct suppression of insulin-like growth factor signaling or this effect is caused by indirect mechanisms such as improved insulin sensitivity.

Inhibitory GH receptor extracellular domain monoclonal antibodies: three-dimensional epitope mapping

GH receptor (GHR) mediates the anabolic and metabolic effects of GH. We previously characterized a monoclonal antibody (anti-GHR(ext-mAb)) that reacts with subdomain 2 of the rabbit GHR extracellular domain (ECD) and is a conformation-specific inhibitor of GH signaling in cells bearing rabbit or human GHR. Notably, this antibody has little effect on GH binding and also inhibits inducible metalloproteolysis of the GHR that occurs in the perimembranous ECD stem region. In the current study, we demonstrate that anti-GHR(ext-mAb) inhibits GH-dependent cellular proliferation and also inhibits hepatic GH signaling in vivo in mice that adenovirally express rabbit GHR, as assessed with our noninvasive bioluminescence hepatic signaling assay. A separate monoclonal antibody (anti-GHR(mAb 18.24)) is a sister clone of anti-GHR(ext-mAb). Here, we demonstrate that anti-GHR(mAb 18.24) also inhibits rabbit and human GHR signaling and inducible receptor proteolysis. Further, we use a random PCR-generated mutagenic expression system to map the three-dimensional epitopes in the rabbit GHR ECD for both anti-GHR(ext-mAb) and anti-GHR(mAb 18.24). We find that each of the two antibodies has similar, but nonidentical, discontinuous epitopes that include regions of subdomain 2 encompassing the dimerization interface. These results have fundamental implications for understanding the role of the dimerization interface and subdomain 2 in GHR activation and regulated GHR metalloproteolysis and may inform development of therapeutics that target GHR.

Attenuation of IGF-I receptor signaling inhibits serum-induced proliferation of prostate cancer cells

OBJECTIVE

Several studies showed that high serum levels of insulin-like growth factor-I (IGF-I) correlate with an increased risk for prostate cancer, although the causal role of IGF-I remains to be established. In this study, we addressed the role of IGF-I as a serum factor on the growth of two androgen-independent cell lines (Du145 and PC3) and one androgen-dependent cell line (LNCaP).

DESIGN

We investigated the effects of a blocking antibody against the IGF-I receptor (αIR3) on DNA synthesis in prostate cancer cells cultured in the presence of recombinant human IGF-I or normal human serum (NHS).

RESULTS

We show that in all three prostate cancer cell lines, NHS exerts a markedly stronger stimulating effect on DNA synthesis than IGF-I, and that the effect of NHS can be completely abrogated by an antibody against the IGF-I receptor (αIR3). Using pharmacological inhibitors of the two canonical IGF-I receptor signaling pathways, we show that the phosphatidylinositol-3'-kinase (PI3K) and the Mek-Erk pathways are not required for the stimulating effect of NHS.

CONCLUSION

Our observations indicate that the stimulating effect of NHS is completely dependent on IGF-I receptor signaling transduction and that IGF-I stimulates DNA synthesis in prostate cancer cells in strong synergy with other serum factors. We speculate that the role of other serum factors could explain the discrepancy between the results observed in different animal models to study the function of IGF-I in prostate cancer.

Growth hormone signaling in human T47D breast cancer cells: potential role for a growth hormone receptor-prolactin receptor complex

GH receptor (GHR) and prolactin (PRL) receptor (PRLR) are structurally similar cytokine receptor superfamily members that are highly conserved among species. GH has growth-promoting and metabolic effects in various tissues in vertebrates, including humans. PRL is essential for regulation of lactation in mammals. Recent studies indicate that breast tissue bears GHR and PRLR and that both GH and PRL may impact development or behavior of breast cancer cells. An important facet of human GH (hGH) and human PRL (hPRL) biology is that although hPRL interacts only with hPRLR, hGH binds well to both hGHR and hPRLR. Presently, we investigated potential signaling effects of both hormones in the estrogen receptor- and progesterone receptor-positive human T47D breast cancer cell line. We found that this cell type expresses ample GHR and PRLR and responds well to both hGH and hPRL, as evidenced by activation of the Janus kinase 2/signal transducer and activator of transcription 5 pathway. Immunoprecipitation studies revealed specific GHR-PRLR association in these cells that was acutely enhanced by GH treatment. Although GH caused formation of disulfide-linked and chemically cross-linked GHR dimers in T47D cells, GH preferentially induced tyrosine phosphorylation of PRLR rather than GHR. Notably, both a GHR-specific ligand antagonist (B2036) and a GHR-specific antagonist monoclonal antibody (anti-GHR(ext-mAb)) failed to inhibit GH-induced signal transducer and activator of transcription 5 activation. In contrast, although the non-GHR-specific GH antagonist (G120R) and the PRL antagonist (G129R) individually only partially inhibited GH-induced activation, combined treatment with these two antagonists conferred greater inhibition than either alone. These data indicate that endogenous GHR and PRLR associate (possibly as a GHR-PRLR heterodimer) in human breast cancer cells and that GH signaling in these cells is largely mediated by the PRLR in the context of both PRLR-PRLR homodimers and GHR-PRLR heterodimers, broadening our understanding of how these related hormones and their related receptors may function in physiology and pathophysiology.

Extrapituitary growth hormone

Pituitary somatotrophs secrete growth hormone (GH) into the bloodstream, to act as a hormone at receptor sites in most, if not all, tissues. These endocrine actions of circulating GH are abolished after pituitary ablation or hypophysectomy, indicating its pituitary source. GH gene expression is, however, not confined to the pituitary gland, as it occurs in neural, immune, reproductive, alimentary, and respiratory tissues and in the integumentary, muscular, skeletal, and cardiovascular systems, in which GH may act locally rather than as an endocrine. These actions are likely to be involved in the proliferation and differentiation of cells and tissues prior to the ontogeny of the pituitary gland. They are also likely to complement the endocrine actions of GH and are likely to maintain them after pituitary senescence and the somatopause. Autocrine or paracrine actions of GH are, however, sometimes mediated through different signaling mechanisms to those mediating its endocrine actions and these may promote oncogenesis. Extrapituitary GH may thus be of physiological and pathophysiological significance.

Pegvisomant: current and potential novel therapeutic applications

BACKGROUND

Pegvisomant is a genetically engineered molecule, which exhibits specific growth hormone (GH) antagonism by directly interacting with the GH receptor. It is currently licensed for the treatment of acromegaly where surgery and medical therapy with somatostatin analogues have failed.

OBJECTIVE

To delineate the role of pegvisomant in the treatment of acromegaly and its novel application in other areas of clinical medicine where suppression of GH action may be of therapeutic benefit.

METHODS

A literature review from PubMed- and EMBASE-listed publications and the web-sites of licensing organisations for medicinal products.

CONCLUSION

Pegvisomant is currently used as a second line therapy in the management of acromegaly. It is highly effective in suppressing the metabolic effects of elevated GH levels when used alone or in combination with somatostatin analogues. However, its long term efficacy and safety for this indication has yet to be established. Preliminary data indicate that pegvisomant may have a role in management of type 1 diabetes with beneficial effects on insulin sensitivity and in preventing the progression of microvascular complications. Additional roles as an adjunct to cancer chemotherapy regimens and for the diagnosis of GH deficiency have been proposed, but have yet to be confirmed.

Growth hormone excess and the development of growth hormone receptor antagonists

In 1990, a single amino acid substitution in the growth hormone (GH) gene at position 119 was found to transform the consequent protein from an agonist to an antagonist at the growth hormone receptor (GHR). Further amino acid substitutions plus prolongation of the half-life of the protein by pegylation resulted in the first clinically effective GHR antagonist, pegvisomant. Following extensive clinical trials, this medication has emerged as the most efficacious therapy for treatment-resistant acromegaly. Subsequent advances in our understanding of GH-GHR interactions and downstream GH signalling pathways suggest that pegvisomant binds to preformed GHR dimers and prevents rotational changes within the receptor-GH complex necessary for intracellular signalling to occur. This article reviews the discovery of pegvisomant, from initial experimental data to successful licensing of the drug for treatment-resistant acromegaly, and discusses its other potential therapeutic uses in diseases with abnormalities in the GH-IGF-I axis.

Modulation of growth hormone receptor abundance and function: roles for the ubiquitin-proteasome system

Growth hormone plays an important role in regulating numerous functions in vertebrates. Several pathways that negatively regulate the magnitude and duration of its signaling (including expression of tyrosine phosphatases, SOCS and PIAS proteins) are shared between signaling induced by growth hormone itself and by other cytokines. Here we overview downregulation of the growth hormone receptor as the most specific and potent mechanism of restricting cellular responses to growth hormone and analyze the role of several proteolytic systems and, specifically, ubiquitin-dependent pathways in this regulation.