Elevated levels of serum secretoneurin in patients with therapy resistant carcinoma of the prostate

PURPOSE

The majority of prostate cancers show some degree of neuroendocrine differentiation. It was previously demonstrated that chromogranin A, a constituent of large dense core vesicles of neuroendocrine cells, is frequently elevated in patients with metastatic prostate cancer. We evaluate the expression of secretoneurin, which is generated by proteolytic processing of secretogranin II (chromogranin C), in patients with prostate disease.

MATERIALS AND METHODS

Secretoneurin was measured in sera of 16 healthy men whose blood was drawn for prostate cancer screening (controls), and in 9 patients with prostatitis, 19 with benign prostate hyperplasia and 54 with prostate cancer detected by radioimmunoassay. Therapy resistant disease (clinical stage D3) was noted in 20 prostate cancer cases. Serum prostate specific antigen was measured in all patients and controls. In addition, chromogranin A, prostate acid phosphatase and interleukin-6 were determined in patients with D3 prostate cancer. Molecular properties of secretoneurin immunoreactivity were analyzed by gel filtration chromatography followed by radioimmunoassay.

RESULTS

Mean secretoneurin was 58.9+/-8 fmol./ml. in patients with therapy resistant prostate cancer. Levels were significantly higher than those measured in sera from controls and patients with prostatitis, benign prostatic hyperplasia and pT2 or pT3 prostate cancer. There was a statistically significant correlation between secretoneurin and chromogranin A in patients with endocrine therapy failure (r = 0.543, p<0.05). There was no correlation between serum secretoneurin and prostate specific antigen, prostate acid phosphatase or interleukin-6. Gel filtration chromatography analysis of sera of 3 patients with D3 prostate cancer revealed a peak of secretoneurin immunoreactivity where intact secretoneurin elutes, thus showing that the processed peptide is circulating in the blood.

CONCLUSIONS

Secretoneurin is elevated in sera of patients with endocrine therapy refractory prostate cancer. Our results support the concept that neuroendocrine differentiation is associated with prostate cancer progression.

Neuroendocrine secretory protein 55 (NESP55): alternative splicing onto transcripts of the GNAS gene and posttranslational processing of a maternally expressed protein

Recent studies established a novel genomically imprinted gene located 45 kb upstream of the human GNAS1 locus. This locus encoded for the Neuroendocrine Secretory Protein with an apparent molecular weight of 55,000 (NESP55), which is transcribed exclusively from the maternal allele. We sequenced rat and human NESP55 and investigated tissue-specific splicing of its mRNA and posttranslational modifications of the protein in various tissues. Alternative mRNA splicing of NESP55 was analyzed by sequencing of cDNA clones, RT-PCR and Northern blotting. Two main splice variants, which were generated in a tissue-specific manner, were identified: The open reading frame encoding NESP55 was spliced onto exons 2-13 of Gsalpha in the adrenal medulla, pituitary and the brain. In addition, in the pituitary a second shorter, prominent mRNA transcript was generated by splicing of NESP55 onto exons 2, 3 and N1 of Gsalpha. Several of the cDNA clones isolated contained inverted repeats of 50-150 bp at their 5' or 3' termini, which might form hairpin stems and thus alter mRNA stability. The NESP55 open reading frame encoded a hydrophilic protein of 28,018 Da (human) and 29,218 Da (rat), respectively, which resembled the class of acidic, neuroendocrine secretory proteins collectively called chromogranins. NESP55 is highly conserved among mammalian species. It is posttranslationally acidified by the addition of keratan sulfate glycosaminoglycan chains and differentially processed by endopeptidases in various endocrine and neuronal tissues.

Levels and molecular properties of secretoneurin-immunoreactivity in the serum and urine of control and neuroendocrine tumor patients

We have determined the levels of secretoneurin (SN), a novel 33-amino acid neuropeptide belonging to the class of chromogranins, in the serum and urine of healthy subjects and patients suffering from various tumors. SN serum levels averaged 22.1+/-1.1 fmol/mL. They were 5-fold higher in younger children and then declined continuously. SN levels were positively correlated with serum creatinine, suggesting an influence of renal function on the clearance of SN from the serum. In the urine 80.0 fmol/mL SN was present. In patients with endocrine tumors like gut carcinoids, endocrine pancreatic tumors, oat cell lung carcinomas, and pheochromocytomas, SN serum levels were elevated up to 45-fold. Patients suffering from neuroblastomas, insulinomas, pituitary adenomas including acromegaly, and solid nonendocrine tumors had concentrations in the normal range. In human serum, SN-immunoreactivity was confined to the free peptide SN; neither larger intermediate-sized forms nor the precursor secretogranin II were detected. An efficient removal of the small molecule SN from the serum by the kidney explains why SN serum levels are lower when compared to chromogranin A, which is present as large molecule in serum.

Localization of neuroendocrine secretory protein 55 messenger RNA in the rat brain

Neuroendocrine secretory protein 55 (NESP55) is a recently characterized secretory protein localized to large dense-core vesicles resembling the class of chromogranins. We investigated the distribution of the messenger RNA encoding for NESP55 in the rat brain by in situ hybridization with specific 35S-labelled oligonucleotides. NESP55 messenger RNA was detected only on neuronal but not glial cells. In the brain, expression of NESP55 messenger RNA was most prominent in several areas throughout the midbrain and brainstem, including the locus coeruleus, the raphe complex and the reticular formation. NESP55 messenger RNA-expressing cells were also found in many areas and nuclei throughout the hypothalamus. Neocortical areas, the hippocampus and the cerebellum were devoid of NESP55 messenger RNA-containing neurons. From this distribution pattern, a significant overlap of NESP55 expression with the noradrenergic, adrenergic and serotonergic transmitter systems was evident. The present study defines, for the first time, the cellular localizaton of NESP55 messenger RNA in the rat brain. The present results provide the basis for future studies defining the as yet obscure function of NESP55.

Presence of chromogranins and regulation of their synthesis and processing in a neuroendocrine prostate tumor cell line

BACKGROUND

Small-cell carcinoma and carcinoid tumors of the prostate display a neuroendocrine phenotype. To some extent, adenocarcinomas of the prostate also express neuroendocrine properties. Prostatic neuroendocrine tumors do not respond to androgen ablation therapy. The regulation of synthesis of chromogranins and their processing into neuropeptides have not yet been studied in neuroendocrine cells of the prostate. We used CRL-5813 cells which were derived from a metastasis from small-cell prostate cancer for studies on steroid receptor expression and chromogranin processing.

METHODS

The expression of steroid receptor mRNA in CRL-5813 cells was examined by polymerase chain reaction. The synthesis and secretion of chromogranin- and secretogranin II-derived peptides were investigated by radioimmunoassays and high-performance liquid chromatography in untreated cells and in cells treated with the protein kinase A activator forskolin or basic fibroblast growth factor (bFGF).

RESULTS

cDNA fragments for alpha-estrogen receptor and androgen receptor but not for beta-estrogen receptor, progesterone receptor, and glucocorticoid receptor were amplified from CRL-5813 cells. These cells were found to contain typical markers of large dense-core vesicles, i.e., chromogranins A and B and secretogranin II. Forskolin significantly stimulated the synthesis and secretion of the chromogranin B-derived peptide PE-11 and the secretogranin II-derived secretoneurin. bFGF significantly induced PE-11 protein levels in cell extracts. CONCLUSIONS. Our results demonstrate the expression of typical large dense-core vesicle proteins, i.e., chromogranins, in a small-cell prostate cancer cell line and their upregulation by a protein kinase A activator and, in part, by bFGF.

Molecular cloning and characterization of NESP55, a novel chromogranin-like precursor of a peptide with 5-HT1B receptor antagonist activity

The chromogranins comprise a class of acidic proteins that are secreted from large dense core vesicles and expressed in neuronal and endocrine tissues. We describe here the molecular characterization of NESP55 (neuroendocrine secretory protein of Mr 55,000), a novel member of the chromogranins. Several NESP55 cDNA clones were isolated from bovine chromaffin cell libraries. The cDNA sequence of NESP55 totals 1499 nucleotides. All of the clones that were isolated contained in their 3'-untranslated mRNA a sequence that was homologous to exon 2 of the G-protein Gsalpha. The open reading frame encodes for an acidic and hydrophilic protein of 241 amino acids with a predicted molecular mass of 27,494 Da. An antiserum directed against the C terminus of NESP55 labeled a band of Mr 55,000 with an acidic pI ranging from 4.4 to 5.2 in one- and two-dimensional immunoblots of secretory proteins from chromaffin granules. NESP55 is localized within the cell to the large dense secretory vesicles and is expressed, apart from the adrenal medulla, in the anterior and posterior pituitary and various regions of the brain. For the physiological function, one interesting factor has emerged. NESP55 is proteolytically processed within the chromaffin granule to smaller peptides that might be physiologically active. One tetrapeptide, Leu-Ser-Ala-Leu (LSAL), present in the NESP55 sequence and flanked by arginine residues suitable for cleavage by prohormone convertases, has been identified recently as an endogenous antagonist of the serotonergic 5-HT1B receptor subtype. Alterations in the serotonergic system are thought to play an important role in mental disorders, especially depression, and might be related to abnormal ethanol consumption. It is tempting to speculate that increased expression of NESP55 or its proteolytically derived peptide LSAL might contribute to the pathophysiology of the serotonergic transmission.