Adrenomedullin inhibits prostate cancer cell proliferation through a cAMP-independent autocrine mechanism

Adrenomedullin - Current perspective on a peptide hormone with significant therapeutic potential

The peptide hormone adrenomedullin (ADM) consists of 52 amino acids and plays a pivotal role in the regulation of many physiological processes, particularly those of the cardiovascular and lymphatic system. Like calcitonin (CT), calcitonin gene-related peptide (CGRP), intermedin (IMD) and amylin (AMY), it belongs to the CT/CGRP family of peptide hormones, which despite their low little sequence identity share certain characteristic structural features as well as a complex multicomponent receptor system. ADM, IMD and CGRP exert their biological effects by activation of the calcitonin receptor-like receptor (CLR) as a complex with one of three receptor activity-modifying proteins (RAMP), which alter the ligand affinity. Selectivity within the receptor system is largely mediated by the amidated C-terminus of the peptide hormones, which bind to the extracellular domains of the receptors. This enables their N-terminus consisting of a disulfide-bonded ring structure and a helical segment to bind within the transmembrane region and to induce an active receptor confirmation. ADM is expressed in a variety of tissues in the human body and is fundamentally involved in multitude biological processes. Thus, it is of interest as a diagnostic marker and a promising candidate for therapeutic interventions. In order to fully exploit the potential of ADM, it is necessary to improve its pharmacological profile by increasing the metabolic stability and, ideally, creating receptor subtype-selective analogs. While several successful attempts to prolong the half-life of ADM were recently reported, improving or even retaining receptor selectivity remains challenging.

Adrenomedullin and tumour microenvironment

Adrenomedullin (AM) is a regulatory peptide whose involvement in tumour progression is becoming more relevant with recent studies. AM is produced and secreted by the tumour cells but also by numerous stromal cells including macrophages, mast cells, endothelial cells, and vascular smooth muscle cells. Most cancer patients present high levels of circulating AM and in some cases these higher levels correlate with a worst prognosis. In some cases it has been shown that the high AM levels return to normal following surgical removal of the tumour, thus indicating the tumour as the source of this excessive production of AM. Expression of this peptide is a good investment for the tumour cell since AM acts as an autocrine/paracrine growth factor, prevents apoptosis-mediated cell death, increases tumour cell motility and metastasis, induces angiogenesis, and blocks immunosurveillance by inhibiting the immune system. In addition, AM expression gets rapidly activated by hypoxia through a HIF-1α mediated mechanism, thus characterizing AM as a major survival factor for tumour cells. Accordingly, a number of studies have shown that inhibition of this peptide or its receptors results in a significant reduction in tumour progression. In conclusion, AM is a great target for drug development and new drugs interfering with this system are being developed.

Tumor-expressed adrenomedullin accelerates breast cancer bone metastasis

Introduction

Adrenomedullin (AM) is secreted by breast cancer cells and increased by hypoxia. It is a multifunctional peptide that stimulates angiogenesis and proliferation. The peptide is also a potent paracrine stimulator of osteoblasts and bone formation, suggesting a role in skeletal metastases—a major site of treatment-refractory tumor growth in patients with advanced disease.

Methods

The role of adrenomedullin in bone metastases was tested by stable overexpression in MDA-MB-231 breast cancer cells, which cause osteolytic bone metastases in a standard animal model. Cells with fivefold increased expression of AM were characterized in vitro, inoculated into immunodeficient mice and compared for their ability to form bone metastases versus control subclones. Bone destruction was monitored by X-ray, and tumor burden and osteoclast numbers were determined by quantitative histomorphometry. The effects of AM overexpression on tumor growth and angiogenesis in the mammary fat pad were determined. The effects of AM peptide on osteoclast-like multinucleated cell formation were tested in vitro. A small-molecule AM antagonist was tested for its effects on AM-stimulated ex vivo bone cell cultures and co-cultures with tumor cells, where responses of tumor and bone were distinguished by species-specific real-time PCR.

Results

Overexpression of AM mRNA did not alter cell proliferation in vitro, expression of tumor-secreted factors or cell cycle progression. AM-overexpressing cells caused osteolytic bone metastases to develop more rapidly, which was accompanied by decreased survival. In the mammary fat pad, tumors grew more rapidly with unchanged blood vessel formation. Tumor growth in the bone was also more rapid, and osteoclasts were increased. AM peptide potently stimulated bone cultures ex vivo; responses that were blocked by small-molecule adrenomedullin antagonists in the absence of cellular toxicity. Antagonist treatment dramatically suppressed tumor growth in bone and decreased markers of osteoclast activity.

Conclusions

The results identify AM as a target for therapeutic intervention against bone metastases. Adrenomedullin potentiates osteolytic responses in bone to metastatic breast cancer cells. Small-molecule antagonists can effectively block bone-mediated responses to tumor-secreted adrenomedullin, and such agents warrant development for testing in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-014-0458-y) contains supplementary material, which is available to authorized users.

Improving intermittent androgen deprivation therapy: lessons learned from basic and translational research

Intermittent androgen deprivation therapy (IADT) is an alternative to continuous androgen deprivation therapy (ADT) in prostate cancer patients with nonmetastatic disease. ADT is associated with numerous side effects such as hot flashes, sexual dysfunction, anemia, fatigue, loss of muscle mass, osteoporosis, metabolic syndrome and premature cardiovascular disease. IADT was developed with the intention of improving the quality of life and to delay progression of prostate cancer to castration resistance. The benefits of slightly improved quality of life by IADT compared to ADT were demonstrated in multiple clinical trials. IADT was noted to be noninferior to ADT in patients with biochemical recurrence of prostate cancer but in studies performed in patients with metastatic prostate cancer, the results were inconclusive. Our recent studies suggested that the administration of 5 alpha-reductase inhibitors during the off-cycle of IADT can significantly prolong the survival of mice bearing androgen-sensitive prostate tumors when off-cycle duration was short. This review discusses the survival benefit of 5 alpha-reductase inhibition in IADT in animal models and the potential translation of this finding into clinic.

Adrenomedullin enhances baroreceptor reflex response via cAMP/PKA signaling in nucleus tractus solitarii of rats

Adrenomedullin (ADM), a 52-amino acid peptide, elicits differential cardiovascular responses when it is administered systemically or directly to the brain. We evaluated in the present study the hypothesis that ADM may modulate baroreceptor reflex (BRR) response through an ADM receptor-mediated cAMP/ protein kinase A (PKA)-dependent mechanism in the nucleus tractus solitarii (NTS), the terminal site for primary baroreceptor afferents, using Sprague-Dawley rats. Our immunoblot and immunohistochemical results showed that the two component proteins of the ADM(1) receptor complex, calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein (RAMP)-2, were uniformly distributed and highly co-localized in the NTS. Site-specific microinjection of ADM (0.02-0.2pmol) unilaterally into the NTS significantly increased BRR response and sensitivity in a time- and dose-related manner, without affecting arterial pressure and heart rate. The BRR enhancing effect of ADM was also temporally correlated with an up-regulation of PKA(beta), the active form of PKA and an increase in PKA activity. In addition, the ADM-evoked BRR enhancement or PKA activation was abolished by co-microinjection with a selective ADM(1) receptor antagonist, ADM(22-52), an adenylyl cyclase inhibitor, SQ22536, or a PKA inhibitor, Rp-8-bromo-cAMP. These results suggest that ADM enhances BRR via activation of a cAMP/PKA-dependent mechanism by acting site-specifically on ADM(1) receptors in NTS.

Adrenomedullin is induced by hypoxia and enhances pancreatic cancer cell invasion

Adrenomedullin (ADM) is synthesized by different types of cells and acts by binding calcitonin receptor-like receptor (CRLR) and members of the receptor activity-modifying protein (RAMP) family. In this study, the expression and functional role of ADM and its signaling components were investigated in pancreatic adenocarcinoma (PDAC). By QRT-PCR, median mRNA levels of ADM and CRLR were 1.5- and 2.4-fold higher, respectively, in PDAC tissues compared to normal pancreatic tissues. By immunohistochemistry, ADM, CRLR, RAMP1 and RAMP2, but not RAMP3, were expressed in pancreatic cancer cells. ADM serum levels were significantly increased in PDAC patients compared to healthy controls and chronic pancreatitis (CP) patients, with an area under the ROC curve of 0.83 and 0.98, respectively. At a cut-off level of 30.6 ng/ml, the specificity of ADM to differentiate PDAC from controls and CP patients was 85.5 and 83.6%, with a sensitivity of 80 and 100%. All 5 evaluated pancreatic cancer cells lines expressed ADM, CRLR, RAMP1 and RAMP2, whereas RAMP3 was expressed in only 1/5 pancreatic cancer cell lines. ADM was strongly induced by hypoxia and significantly increased invasiveness in 3/5 human pancreatic cancer cells. Blocking of CRLR decreased invasiveness in 4/5 human pancreatic cancer cells. In addition, rADM slightly up-regulated vascular endothelial growth factor secretion in 3/5 cell lines. In conclusion, ADM is induced by hypoxia and over-expressed in PDAC and might therefore serve as a potential tumor marker. Furthermore, ADM increases invasiveness of some pancreatic cancer cells and might influence angiogenesis, suggesting that blocking this pathway might have a therapeutic potential.

Adrenomedullin: a new and promising target for drug discovery

Adrenomedullin (AM) is a 52 amino acid peptide that plays a critical role in several diseases such as hypertension, cancer, diabetes, cardiovascular and renal disorders, among others. Interestingly, AM behaves as a protective agent against some pathologies, yet is a stimulating factor for other disorders. Thus, AM can be considered as a new and promising target for the design of non-peptidic modulators that could be useful for the treatment of those pathologies, by regulating AM levels or the activity of AM. A full decade on from its discovery, much more is known about AM molecular biology and pharmacology, but this knowledge still needs to be applied to the development of clinically useful drugs.

Adrenomedullin prevents apoptosis in prostate cancer cells

The 52-aminoacid peptide adrenomedullin (AM) is expressed in the normal and malignant prostate. We have previously shown that prostate cancer cells produce and secrete AM, which acts as an autocrine growth inhibitory factor. We have evaluated in the present study the role of AM in prostate cancer cell apoptosis, induced either by serum deprivation or treatment with the chemotherapeutic agent etoposide (which acts as an inhibitor of topoisomerase II). For this purpose we over-expressed AM in PC-3, DU 145 and LNCaP cells, which were transfected with an expression vector carrying AM. We also treated the parental cell lines with synthetic AM in normal culture conditions and in conditions of induced-apoptosis. After serum removal, AM prevented apoptosis in DU 145 and PC-3 cells, but not in LNCaP cells. When treated with etoposide, AM prevented apoptosis in PC-3 and LNCaP cells, but not in DU 145 cells. Cell cycle analysis demonstrated a significant decrease in the percentage of AM-overexpressing PC-3 cells in the subG0/G1 phase after treatment with etoposide, as compared to the percentage of mock-transfected PC-3 treated cells. Western blot showed that protein levels of phosphorylated ERK1/2 increased in parental PC-3 cells after treatment with etoposide. In PC-3 cells overexpressing AM, phosphorylated ERK1/2 basal levels were lower than basal levels of parental PC-3 cells, and treatment with etoposide did not result in such an increase. Etoposide produced a significant increase in cleaved PARP in parental PC-3 cells. However, PC-3 clones overexpressing AM that were treated with etoposide only showed a mild increase in fragmented PARP. The ratio Bcl-2/Bax was reduced in parental or mock-transfected PC-3 cells after treatment with etoposide. On the contrary, this ratio was not reduced in PC-3 clones with AM overexpression that were treated with etoposide. All these data demonstrate that AM plays a protective role against induced apoptosis in prostate cancer cells. These results may have important implications in prostate cancer resistance to chemotherapeutic agents.

Calcitonin stimulates multiple stages of angiogenesis by directly acting on endothelial cells

Although a strong correlation between neuroendocrine differentiation and angiogenesis of prostate cancer has been reported, no mechanistic link between the two events has been established. Because neuropeptide calcitonin is secreted by prostate tumors and endothelial cells are known to express calcitonin receptor-like receptor, we examined the potential action of calcitonin on endothelial cells. The presence of calcitonin receptor, calcitonin receptor-like receptor, and receptor activity-modifying proteins in human microvessel endothelial-1 cells was tested by reverse transcriptase-PCR (RT-PCR). The proangiogenic action of calcitonin was examined in several in vitro models of angiogenesis using HMEC-1 cells and also in vivo using dorsal skinfold assays. Calcitonin expression of PC-3M cells was modulated, and its effect on angiogenesis was examined in in vitro as well as in vivo models. The results of RT-PCR and radioligand receptor assays showed the presence of functional calcitonin receptor in HMEC-1 cells. Calcitonin stimulated all phases of angiogenesis through the calcitonin receptor, but its effect on tube morphogenesis by endothelial cells occurred at the concentration of the Kd of calcitonin receptor. Silencing of calcitonin receptor expression in HMEC-1 cells abolished calcitonin-induced tube formation. Vascular endothelial growth factor antibodies attenuated but did not abolish calcitonin-induced tube morphogenesis. PC-3M prostate cancer cells induced angiogenesis in in vivo and in vitro models. Overexpression of calcitonin in PC-3M cells increased their angiogenic activity, whereas the silencing of calcitonin expression abolished it. These results show that prostate tumor-derived calcitonin may play an important role in prostate tumor growth by regulating intratumoral vascularization.

Apoptosis of pancreatic cancer BXPC-3 cells induced by indole-3-acetic acid in combination with horseradish peroxidase

AIM: To explore the mechanisms underlying the apoptosis of human pancreatic cancer BXPC-3 cells induced by indole-3-acetic acid (IAA) in combination with horseradish peroxidase (HRP).

METHODS: BXPC-3 cells derived from human pancreatic cancer were exposed to 40 or 80 µmol/L IAA and 1.2 µg/mL HRP at different times. Then, MTT assay was used to detect the cell proliferation. Flow cytometry was performed to analyze cell cycle. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay was used to detect apoptosis. 2,7-Dichlorofluorescin diacetate uptake was measured by confocal microscopy to determine free radicals. Level of malondialdehyde (MDA) and activity of superoxide dismutase (SOD) were measured by biochemical methods.

RESULTS: IAA/HRP initiated growth inhibition of BXPC-3 cells in a dose- and time-dependent manner. Flow cytometry revealed that the cells treated for 48 h were arrested at G1/G0. After exposure to 80 µmol/L IAA plus 1.2 µg/mL HRP for 72 h, the apoptosis rate increased to 72.5, which was nine times that of control. Content of MDA and activity of SOD increased respectively after treatment compared to control. Meanwhile, IAA/HRP stimulated the formation of free radicals.

CONCLUSION: The combination of IAA and HRP can inhibit the growth of human pancreatic cancer BXPC-3 cells in vitro by inducing apoptosis.