Expression and processing of parathyroid hormone-related protein in a pancreatic endocrine cell tumour associated with hypercalcaemia

What Lies behind Paraneoplastic Hypercalcemia Secondary to Well-Differentiated Neuroendocrine Neoplasms? A Systematic Review of the Literature

BACKGROUND

Neuroendocrine neoplasms (NEN) originate from neuroendocrine cells ubiquitously spread throughout the body. Hypercalcemia associated with cancer is the most common life-threatening metabolic disorder in patients with advanced stage cancer. Paraneoplastic hypercalcemia is more commonly associated with hematological malignancies, renal and breast carcinomas, and squamous cell carcinomas, but it has also been described in patients with well-differentiated NEN, where it often remains undiagnosed. Among its causes, systemic secretion of parathyroid hormone-related protein (PTHrP) and ectopic production of 1,25-dihydroxyvitamin D and parathyroid hormone (PTH) may be considered paraneoplastic causes of hypercalcemia. In order to clarify the diagnostic work up of paraneoplastic hypercalcemia in patients with NEN, we perform a systematic review, which is lacking in the literature.

METHODS

We performed a data search using MEDLINE and SCOPUS including papers from 1961 to 2021. We selected articles on paraneoplastic hypercalcemia associated with well-differentiated NEN.

RESULTS

The search led to the selection of 78 publications for a total of 114 patients. Pooled data showed that the most frequent primary tumor site associated with paraneoplastic hypercalcemia was pancreatic NEN, followed by Pheochromocytoma. In most cases, paraneoplastic hypercalcemia was caused by PTHrP production and secretion. In more than two thirds of cases, paraneoplastic hypercalcemia was present at the time of NEN diagnosis and, in metachronous cases, was related to local recurrence, distant metastasis development, or tumor progression. In most patients, a combination of therapeutic approaches was employed, and reduction of the tumor burden was essential to control the paraneoplastic syndrome.

DISCUSSION

The onset of hypercalcemia associated with cancer in patients with well-differentiated NEN represents a major clinical challenge. The complex clinical and therapeutical management of paraneoplastic hypercalcemia implies the need for a multidisciplinary approach, aimed at controlling the clinical syndrome and tumor growth.

Proteolytic Regulation of Parathyroid Hormone-Related Protein: Functional Implications for Skeletal Malignancy

Parathyroid hormone-related protein (PTHrP), with isoforms ranging from 139 to 173 amino acids, has long been implicated in the development and regulation of multiple tissues, including that of the skeleton, via paracrine and autocrine signaling. PTHrP is also known as a potent mediator of cancer-induced bone disease, contributing to a vicious cycle between tumor cells and the bone microenvironment that drives the formation and progression of metastatic lesions. The abundance of roles ascribed to PTHrP have largely been attributed to the N-terminal 1-36 amino acid region, however, activities for mid-region and C-terminal products as well as additional shorter N-terminal species have also been described. Studies of the protein sequence have indicated that PTHrP is susceptible to post-translational proteolytic cleavage by multiple classes of proteases with emerging evidence pointing to novel functional roles for these PTHrP products in regulating cell behavior in homeostatic and pathological contexts. As a consequence, PTHrP products are also being explored as potential biomarkers of disease. Taken together, our enhanced understanding of the post-translational regulation of PTHrP bioactivity could assist in developing new therapeutic approaches that can effectively treat skeletal malignancies.

A case of a giant glucagonoma with parathyroid hormone-related peptide secretion showing an inconsistent postsurgical endocrine status

A 53-year-old woman was admitted because of a giant pancreatic tumor. Hypercalcemia and a high serum parathyroid hormone-related peptide (PTHrP) level were observed. A hypoglycemic attack occurred during pancreatectomy, and the surgical specimen revealed a PTHrP-secreting glucagonoma. Liver metastases developed 1 and 5.5 years later, and bone metastases appeared 6 years after surgery. Her serum PTHrP concentrations remained normal after surgery, despite re-elevation of the serum glucagon concentration after recurrence. The clinical course of this case illustrates the process of development of neuroendocrine tumors secreting two or more hormones.

Somatostatin receptor expression in a parathyroid hormone-related peptide-secreting pancreatic neuroendocrine tumour causing severe hypercalcaemia

BACKGROUND

Humoral hypercalcaemia is a common complication of malignancy with parathyroid hormone-related protein (PTHrP) as a major cause. Breast and lung cancer are relatively common sources of ectopic PTHrP secretion leading to increased osteoclastic bone resorption.

CASE REPORT

We report the rare case of a 40-year-old man with severe hypercalcaemia due to a PTHrP-secreting poorly differentiated endocrine carcinoma of the pancreas. On immunohistochemistry, the tumour was positive for PTHrP and somatostatin receptors sst1, sst2, and sst3, whereas sst4 and sst5 were not detected. We demonstrate the transient improvement of hypercalcaemia after adding octreotide to the treatment mainstays in hypercalcaemia of malignancy (fluid repletion, administration of bisphosphonates, loop diuretics, and glucocorticoids).

CONCLUSION

To the best of our knowledge, this is the first report showing somatostatin receptor expression in a PTHrP-secreting pancreatic neuroendocrine tumour.

PTHrP, calcitonin and calcitriol in a case of severe, protracted and refractory hypercalcemia due to a pancreatic neuroendocrine tumor

A patient with a primary neuroendocrine tumor of the pancreas, presented with severe hypercalcemia. This hypercalcemia of malignancy (HCM) failed to respond to intensive bisphosphonate treatment and needed continuous enhanced diuresis. Only after successful antitumor therapy did the hypercalcemia subside. Hypercalcemia was associated with increased concentrations of plasma PTHrP, calcitonin and 1,25-(OH)(2)D(3). Bone mineral density was markedly increased. We demonstrated the presence of both PTHrP and calcitonin in the tumor at the mRNA and protein level, using RT-PCR, immunohistochemistry and Western blotting. The high levels of plasma PTHrP and the demonstrated predominant renal mechanism in this case of HCM are suspected to be the cause for its refractoriness to bone resorption inhibitors. Our findings furthermore suggest that the tumoral production of calcitonin and PTHrP might have contributed to the increased bone mineral storage of calcium and thus probably attenuated the development of frank hypercalcemia.

Parathyroid hormone-related protein as a novel tumor marker in pancreatic adenocarcinoma

INTRODUCTION

Parathyroid hormone-related protein (PTHrP) can act as an oncoprotein to regulate the growth and proliferation of many common malignancies, including pancreatic cancer. Previous studies have shown that PTHrP is produced by human pancreatic cancer cell lines, can be shown in the cytoplasm and nucleus of paraffin-embedded pancreatic adenocarcinoma tumor specimens, and is secreted into the media of cultured pancreatic adenocarcinoma cells. We hypothesized that PTHrP could serve as a tumor-marker for growth of pancreatic cancer in vivo.

AIM AND METHODOLOGY

To test this hypothesis, we used an orthotopic model developed in our laboratory of the PTHrP-producing human pancreatic cancer line, BxPC-3. This tumor was stably transduced with green fluorescence protein (GFP) to facilitate visualization of tumor growth and metastases. At early (5 weeks) and late (13 weeks) time points after surgical orthotopic implantation, serum PTHrP was measured and primary and metastatic tumor burden was determined for each mouse by assessing GFP expression.

RESULTS

By 5 weeks after surgical orthotopic implantation (early group), the mean serum PTHrP level was 33.3 pg/mL. In contrast, by 13 weeks after surgical orthotopic implantation (late group), the mean serum PTHrP level increased to 158.5 pg/mL. These differences were highly significant (p < 0.001, Student t test). Numerous metastatic lesions were readily visualized by GFP in the late group. Serum PTHrP levels measured by immunoassay correlated with primary pancreatic tumor weights and serum calcium levels (p <0.01). PTHrP levels were not detectable (<21 pg/mL) in any of the 10 control mice with no tumor. Western blotting of BxPC-3-GFP tumor lysates confirmed the presence of PTHrP. BxPC-3-GFP tumor tissue stained with antibody to PTHrP.

CONCLUSION

These results indicate that PTHrP can serve as a tumor marker in animal models of pancreatic cancer and may be a useful tumor marker for clinical pancreatic adenocarcinoma.

Parathyroid hormone-related peptide in pancreatic neuroendocrine tumours associated with hypercalcaemia

BACKGROUND

Hypercalcaemia is a common paraneoplastic syndrome. In the context of pancreatic neuroendocrine tumours, it is occasionally caused by secretion of parathyroid hormone-related peptide (PTH-rP).

CASE OUTLINES

Two patients are reported in whom persistent hypercalcaemia was traced to a large neuroendocrine pancreatic tumour hypersecreting PTH-rP. Resection of the tumour reduced serum levels of calcium and PTH-rP transiently in each case until the patient developed bulky metastatic disease. A 33-year-old woman remained hypercalcaemic after the removal of all four hyperplastic parathyroid glands had rendered circulating parathormone levels undetectable. Radical distal pancreatectomy was followed over the next 4 years by operative debulking of liver metastases, multiple hepatic artery embolisations, octreotide injections and repeated admissions for intravenous fluid and biphosphonate therapy. A 41-year-old man presented with hypercalcaemia as well as features of somatostatinoma syndrome. Symptomatic improvement after radical distal pancreatectomy was short-lived, and hepatic artery embolisation failed to control his rapidly progressive disease.

DISCUSSION

Malignant hypercalcaemia associated with a neuroendocrine pancreatic tumour hypersecreting PTH-rP is difficult to treat and can be life-threatening. Aggressive surgical treatment is recommended initially, while somatostatin analogues and hepatic artery embolisation are alternative therapeutic options for metastatic disease.

Abordagem clínico-laboratorial no diagnóstico diferencial de hipercalcemia

A hipercalcemia é anormalidade metabólica comum, porém pouco diagnosticada por ser freqüentemente assintomática. Aproximadamente 90% dos casos são decorrentes de hiperparatireoidismo primário (HPT) ou doença maligna complicada por hipercalcemia, estando prevalentes em pacientes ambulatoriais e hospitalizados, respectivamente. Laboratorialmente, o HPT e hipercalcemia humoral maligna apresentam algumas semelhanças, tais como: aumento do AMPc nefrogênico, hipofosfatemia e hipercalciúria. Porém, o quadro clínico da hipercalcemia associada à malignidade é mais severo e, geralmente, o paciente apresenta-se clinicamente debilitado pela doença, com múltiplas metástases. A dosagem sérica de PTH intacto (PTHi) é fundamental para o diagnóstico definitivo, estando o PTHi elevado ou normal no HPT primário, e suprimido na malignidade. Os mecanismos de hipercalcemia da doença maligna são: secreção de fatores humorais que alteram a homeostase do cálcio e fatores locais produzidos pelos tumores metastáticos ou hematológicos no osso, causando aumento da reabsorção osteoclástica. A proteína relacionada ao hormônio da paratireóide (PTHrP) tem sido implicada na maioria dos casos de hipercalcemia devido a tumores sólidos. Outros fatores como interleucina-6, fator de crescimento tumoral, fator de necrose tumoral e interleucina-1 podem modular os efeitos do PTHrP nos órgãos-alvo, e em alguns tumores, ativam diretamente o osteoctastos como por exemplo no mieloma múltiplo. A hipercalcemia pode estar menos freqüentemente associada a algumas doenças endócrinas como tireotoxicose, feocromocitoma, doença de Addison e neoplasia endócrino múltipla tipos I e IIA. Algumas drogas podem causar esse distúrbio metabólica, merecendo destaque a vitamina D, os diuréticos tiazídicos e o lítio. A sarcoidose é exemplo de doença granulomatosa que pode associar-se à hipercalcemia em 10% dos casos e hipercalciúria em 50%. O diagnóstico diferencial das hipercalcemias é essencial para que haja uma abordagem terapêutica eficaz dessa anormalidade metabólica.

Identification of a novel bone/calcium metabolism-regulating factor in porcine pancreas

We purified from porcine pancreas a hypocalcemic peptide clearly distinguishable from other pancreatic osteotropic factors such as amylin, calcitonin, and glucagon. Porcine pancreas was processed by acetone extraction, anion exchange chromatography, isoelectric focusing, and reverse-phase high performance liquid chromatography. Fractions were assayed for their inhibitory effects on bone resorption in vitro. Amino acid sequence of a homogeneous 28-kDa protein revealed 92% homology to a human elastase IIIB in the N terminus. Recombinant human elastase IIIB (rhEIIIB) inhibited bone resorption in organ culture stimulated by 1,25-dihydroxyvitamin D3 at concentrations as low as 75 ng/ml. Antibodies to rhEIIIB recognized purified pancreatic factor in Western blots and blocked its inhibitory effect on bone resorption. This antiresorptive activity was abolished by phenylmethylsulfonyl fluoride, suggesting the importance of elastase proteolytic activity for inhibition of bone resorption. In vivo, rhEIIIB and purified pancreatic factor significantly decreased recombinant human interleukin-1alpha-induced hypercalcemia. In conclusion, a novel naturally occurring inhibitor of bone resorption and calcium-lowering peptide has been identified in porcine pancreas. Because this pancreatic peptide has systemic effects on bone resorption and blood ionized calcium at low concentrations, it may represent a physiological regulator of normal bone remodeling and calcium homeostasis.

Distribution and functions of parathyroid hormone-related protein in vertebrate cells

Parathyroid hormone-related protein (PTHrP) was isolated from tumors and identified as the agent of humoral hypercalcemia of malignancy (HHM) in 1987. Since then its gene structure in several mammalian and an avian species has been analyzed and its gene expression demonstrated in many adult and embryonic tissues derived from all three germ layers. The composition and structure of PTHrP peptide depends on both differential gene splicing and posttranslational processing, which result in a range of peptides of potentially diverse functions. This chapter describes the distribution of PTHrP in both normal and neoplastic adult and embryonic tissues. PTHrP is of fundamental importance to cell survival because the absence of the gene is fatal; this aspect of PTHrP function in cell physiology becomes overwhelmingly important in neoplasia. Intracrine or paracrine actions for PTHrP seem to be most likely in mammalian and avian physiology, but in fishes high circulating levels suggest classic endocrine functions as well. Much remains to be learned of the biology of this fascinating protein.

Parathyroid hormone related protein in oral squamous cell carcinomas invading the mandible

AIM

To assess parathyroid hormone related protein (PTHrP) as a candidate biochemical marker of invasion of the mandible by oral squamous cell carcinoma.

METHODS

Tumour PTHrP concentrations were quantitated by immunoassay, and PTHrP was detected by immunohistochemistry, in a cohort of 24 primary squamous cell carcinomas of the mandible.

RESULTS

PTHrP was identified in all tumours examined, but no correlation was found between scores of the intensity and/or consistency of staining or tumour PTHrP concentrations and the histological classification of tumour invasion.

CONCLUSION

Although PTHrP was present in all squamous tumours studied, there was no correlation between PTHrP expression and pattern of tumour invasion. However, tumour derived PTHrP may act locally to influence tumour growth and differentiation and resorption of bone.

Parathyroid hormone-related protein (PTHrP) in breast cancer and benign breast tissue

Parathyroid hormone-related protein (PTHrP) 1-86 was quantified by immunoassay in extracts of 132 breast cancers, 27 samples of normal breast tissue and four fibroadenomas. PTHrP 1-86, was detected in 68% of primary tumours (range 40-302,000 fmol/g), 33% of normal breast tissues (range 100-1800 fmol/g), and all four fibroadenomas (range 110-11,600 fmol/g). PTHrP displayed molecular heterogeneity on gel filtration chromatography, and 1-86, 1-34 and 37-67 immunoreactivity eluted as 25-27 kDa together with a peak of 19-21 kDA containing only 37-67 activity. Tumour PTHrP 1-86 levels correlated inversely with age (P < 0.05) and were higher in premenopausal women (P = 0.05). The proportion of tumours containing PTHrP was higher in axillary node positive premenopausal women (P < 0.05). These data suggest that oestrogen may regulate expression of PTHrP in breast cancer and that production of PTHrP may be linked to development of axillary node metastases.