Analysis of p27Kip1 Expression in Insulinomas Developed in Pancreatic β-cell Specific Men1 Mutant Mice

Expression of p27Kip1 and p18Ink4c in human multiple endocrine neoplasia type 1-related pancreatic neuroendocrine tumors

PURPOSE

Pancreatic neuroendocrine tumors are a major manifestation of multiple endocrine neoplasia type 1 (MEN1). This tumor syndrome is caused by germline mutations in MEN1, encoding menin. Insight into pathogenesis of these tumors might lead to new biomarkers and therapeutic targets for these patients. Several lines of evidence point towards a role for p27^Kip1 and p18^Ink4c in MEN1-related tumor development in animal models for MEN1, but their contribution to human MEN1-related pancreatic neuroendocrine tumor development is not known.

METHODS

In this study, we characterized protein expression of p27^Kip1 and p18^Ink4c in human MEN1-related PanNETs by immunohistochemistry. From the nationwide DutchMEN1 Study Group database including > 90% of the Dutch MEN1 population, MEN1-patients, who underwent pancreatic surgery, were selected. A tissue micro-array was constructed with available paraffin tissue blocks, and PanNETs from 61 MEN1 patients were eligible for analysis.

RESULTS

Expression of p27^Kip1 was high in 57 (93%) PanNETs and 67% of the tumors showed low expression of p18^Ink4c (67.3%). No association was found between expression of either p27^Kip1 or p18^Ink4c and clinic-pathological characteristics.

CONCLUSIONS

These findings indicate that loss of p18^Ink4c, but not p27^Kip1, is a common event in the development of MEN1-related PanNETs. Restoration of p18^Ink4c function through CDK4/6 inhibitors could be a therapeutic option for MEN1-related PanNETs.

Akt/p27kip1 Pathway Is Not Involved in Human Insulinoma Tumorigenesis

Insulinomas are pancreatic neuroendocrine tumors (pNET), usually benign. Akt/p27^kip1 is an intracellular pathway overexpressed in many pNET. There are no data regarding its expression in human insulinomas. We aimed to investigate the expression of Akt and p27^kip1 in 24 human insulinomas and to compare them to their expression in normal surrounding islets. Staining was performed on embedded paraffin tissue using polyclonal antibodies against total Akt, p-Akt, p27^kip1, and pp27^kip1. p-Akt was the predominant form in insulinomas; they presented lower Akt and p-Akt expression than normal islets in 83.3% and 87.5% of tumors, respectively. p27^kip1 and pp27^kip1 were mainly cytoplasmic in both insulinomas and normal tissue. Cytoplasmic pp27^kip1 staining was higher in insulinomas and surprisingly nearly half of the insulinomas also presented nuclear p27^kip1 (p = 0.029). No differences were observed in the subcellular localization of p27^kip1 and activation of Akt between benign and malignant insulinomas. The low expression of Akt seen in insulinomas might explain the usual benign behavior of this type of pNET. Cytoplasmic p27^kip1 in both insulinomas and normal islet cells could reflect the low rate of replication of beta cells, while nuclear p27^kip1 would seem to indicate stabilization and nuclear anchoring of the cyclin D-Cdk4 complex. Our data seem to suggest that the Akt pathway is not involved in human insulinoma tumorigenesis.

Loss of p27 expression is associated with MEN1 gene mutations in sporadic parathyroid adenomas

MEN1 is the main gene responsible for tumorigenesis of syndromic and sporadic primary hyperparathyroidism (PHPT). Germline mutations of the CDKN1B/p27^Kip gene have been associated with multiple endocrine tumors in rats and humans. To evaluate the involvement of the CDKN1B gene and its relationship with MEN1 in sporadic PHPT, we carried out sequencing and loss of heterozygosity analyses of the CDKN1B gene in 147 sporadic parathyroid adenomas. p27 immunohistochemistry and genetic screening of the MEN1 gene were performed in 50 cases. Three germline CDKN1B variants (c.-80C>T, c.-29_-26delAGAG, c.397C>A) were identified in 3/147 patients. Reduction of CDKN1B gene transcription rate was demonstrated in vitro for the novel c.-80C>T and the c.-29_-26delAGAG variants. Loss of p27 expression was detected in the tumor carrying the c.-29_-26delAGAG variant. Two tumors carrying the CDKN1B variants also harbored a MEN1 mutation. Fifty-four percent of 50 CDKN1B mutation-negative tumors had a reduction of p27 nuclear staining. Somatic MEN1 mutations, identified in 15/50 samples, significantly segregated in tumors negative for nuclear and cytoplasmic p27 staining. The germline nature of the CDKN1B mutations suggests that they might predispose to PHPT. The lack of somatic CDKN1B mutations in our samples points to a rare involvement in parathyroid adenomas, despite the frequent loss of nuclear p27 expression. MEN1 biallelic inactivation seems to be directly related to down-regulation of p27 expression through the inhibition of CDKN1B gene transcription.

Phosphorylated Ribosomal Protein S6 Is Required for Akt-Driven Hyperplasia and Malignant Transformation, but Not for Hypertrophy, Aneuploidy and Hyperfunction of Pancreatic β-Cells

Constitutive expression of active Akt (Akt^tg) drives hyperplasia and hypertrophy of pancreatic β-cells, concomitantly with increased insulin secretion and improved glucose tolerance, and at a later stage the development of insulinoma. To determine which functions of Akt are mediated by ribosomal protein S6 (rpS6), an Akt effector, we generated mice that express constitutive Akt in β-cells in the background of unphosphorylatable ribosomal protein S6 (rpS6^P-/-). rpS6 phosphorylation deficiency failed to block Akt^tg-induced hypertrophy and aneuploidy in β-cells, as well as the improved glucose homeostasis, indicating that Akt carries out these functions independently of rpS6 phosphorylation. In contrast, rpS6 phosphorylation deficiency efficiently restrained the reduction in nuclear localization of the cell cycle inhibitor p27, as well as the development of Akt^tg-driven hyperplasia and tumor formation in β-cells. In vitro experiments with Akt^tg and rpS6^P-/-;Akt^tg fibroblasts demonstrated that rpS6 phosphorylation deficiency leads to reduced translation fidelity, which might underlie its anti-tumorigenic effect in the pancreas. However, the role of translation infidelity in tumor suppression cannot simply be inferred from this heterologous experimental model, as rpS6 phosphorylation deficiency unexpectedly elevated the resistance of Akt^tg fibroblasts to proteotoxic, genotoxic as well as autophagic stresses. In contrast, rpS6^P-/- fibroblasts exhibited a higher sensitivity to these stresses upon constitutive expression of oncogenic Kras. The latter result provides a possible mechanistic explanation for the ability of rpS6 phosphorylation deficiency to enhance DNA damage and protect mice from Kras-induced neoplastic transformation in the exocrine pancreas. We propose that Akt1 and Kras exert their oncogenic properties through distinct mechanisms, even though both show addiction to rpS6 phosphorylation.

DNA methylation profiles distinguish different subtypes of gastroenteropancreatic neuroendocrine tumors

AIM

Most studies have considered gastroenteropancreatic neuroendocrine tumors (GEP-NETs) as a homogenous group of samples or distinguish only gastrointestinal from pancreatic endocrine tumors. This article investigates if DNA methylation patterns could distinguish subtypes of GEP-NETs.

MATERIALS & METHODS

The DNA methylation level of 807 cancer-related genes was investigated in insulinomas, gastrinomas, non-functioning pancreatic endocrine tumors and small intestine endocrine tumors.

RESULTS

DNA methylation patterns were found to be tumor type specific for each of the pancreatic tumor subtypes and identified two distinct methylation-based groups in small intestine endocrine tumors. Differences of DNA methylation levels were validated by pyrosequencing for 20 candidate genes and correlated with differences at the transcriptional level for four candidate genes.

CONCLUSION

The heterogeneity of DNA methylation patterns in the different subtypes of gastroenteropancreatic neuroendocrine tumors suggests different underlying pathways and, therefore, these tumors should be considered as distinct entities in molecular and clinical studies.

Characterisation of prostate cancer lesions in heterozygous Men1 mutant mice

Background

Mutations of the MEN1 gene predispose to multiple endocrine neoplasia type 1 (MEN1) syndrome. Our group and others have shown that Men1 disruption in mice recapitulates MEN1 pathology. Intriguingly, rare lesions in hormone-dependent tissues, such as prostate and mammary glands, were also observed in the Men1 mutant mice.

Methods

To study the occurrence of prostate lesions, we followed a male mouse cohort of 47 Men1^+/- mice and 23 age-matched control littermates, starting at 18 months of age, and analysed the prostate glands from the cohort.

Results

Six Men1^+/- mice (12.8%) developed prostate cancer, including two adenocarcinomas and four in situ carcinomas, while none of the control mice developed cancerous lesions. The expression of menin encoded by the Men1 gene was found to be drastically reduced in all carcinomas, and partial LOH of the wild-type Men1 allele was detected in three of the five analysed lesions. Using immunostaining for the androgen receptor and p63, a basal epithelial cell marker, we demonstrated that the menin-negative prostate cancer cells did not display p63 expression and that the androgen receptor was expressed but more heterogeneous in these lesions. Furthermore, our data showed that the expression of the cyclin-dependent kinase inhibitor CDKN1B (p27), a Men1 target gene known to be inactivated during prostate cell tumorigenesis, was notably decreased in the prostate cancers that developed in the mutant mice.

Conclusion

Our work suggests the possible involvement of Men1 inactivation in the tumorigenesis of the prostate gland.

Unusual clinical and pathological presentation of a neuroendocrine tumor in a patient with multiple endocrine neoplasia type 1

Neuroendocrine tumors develop in various organs in patients with multiple endocrine neoplasia type 1 (MEN1). Among those, tumors developed in upper gastrointestinal tract, thymus and bronchus have historically been called "carcinoid tumor". Occurrence of "carcinoid tumor" in other region is very rare and molecular pathogenesis of such tumors is unknown. We have experienced a patient with MEN1 who have developed an "ectopic" retroperitoneal neuroendocrine tumor. Genetic analysis of the MEN1 gene in tumor cells revealed a somatic mutation in exon 9 as well as a germline mutation in exon 10. Allele-specific amplification followed by sequence analysis revealed these two mutations exist on the different allele, indicating both alleles are functionally inactivated. Immunohistochemical staining with an anti-menin antibody revealed that wild-type menin is not expressed in tumor cells. Expression of p27(Kip1) protein is not observed in tumor cells, either. These results confirmed the inactivation of the MEN1 gene as a genetic cause of an ectopically developed neuroendocrine tumor in a patient with MEN1.

The Cdk inhibitor p27 in human cancer: prognostic potential and relevance to anticancer therapy

The cyclin-dependent kinase (Cdk) inhibitor p27 (also known as KIP1) regulates cell proliferation, cell motility and apoptosis. Interestingly, the protein can exert both positive and negative functions on these processes. Diverse post-translational modifications determine the physiological role of p27. Phosphorylation regulates p27 binding to and inhibition of cyclin-Cdk complexes, its localization and its ubiquitin-mediated proteolysis. In cancers, p27 is inactivated through impaired synthesis, accelerated degradation and by mislocalization. Moreover, studies in several tumour types indicate that p27 expression levels have both prognostic and therapeutic implications.

Evaluation of CDKN2C/p18, CDKN1B/p27 and CDKN2B/p15 mRNA expression, and CpG methylation status in sporadic and MEN1-associated pancreatic endocrine tumours

OBJECTIVE

Menin, encoded by the multiple endocrine neoplasia type 1 (MEN1) gene at 11q13, enhances transcription of the cyclin-dependent kinase inhibitors (CDIs), CDKN2C (p18) and CDKN1B (p27) in mouse pancreatic islets, and inactivation of menin reduced CDKN2B (p15) expression in this mouse model. Here, we have compared the relative mRNA expression level and CpG methylation status of p18, p27 and p15 in 18 pancreatic endocrine tumours (PETs) with or without MEN1 gene mutations.

DESIGN

Real-time quantitative PCR, DNA sequencing and pyrosequencing methylation analysis were employed.

RESULTS

The p18 gene was expressed in 15 out of the 18 analysed PETs. The expression level was within the range of the normal pancreatic tissues or higher. Of the three remaining tumours with no expression, two displayed loss of heterozygocity (LOH) at 11q13, one derived from a MEN1 patient. The p27 gene was expressed in all PETs at a level higher than the normal pancreatic tissues, except for one tumour. Promoter methylation was not detected for p18 and p27. p15 expression was undetectable in 8/18 (44%) of the PETs, and no general relations to tumour syndrome, malignancy or MEN1 gene mutations were evident. This was not due to homozygous gene deletions, but the p15 promoter was hypermethylated in two insulinomas. No mutations were found in the p15 gene.

CONCLUSIONS

Expression of p15, p18 and p27 was not generally related to the MEN1 gene mutational status of the investigated 18 PETs. The p15 gene was silenced by promoter hypermethylation in two tumours. Dysregulation of menin and the CDIs are important in PET tumorigenesis, and their interrelations remain to be elucidated.

Reconstituted expression of menin in Men1-deficient mouse Leydig tumour cells induces cell cycle arrest and apoptosis

Multiple endocrine neoplasia type 1 (MEN1) is a hereditary syndrome caused by the inactivation of the responsible gene, MEN1. To date, the lack of MEN1-deficient cell lines derived directly from MEN1 tumours has hampered the detailed study of the MEN1 gene. We have established several stable Men1-deficient Leydig cell tumour (LCT) lines derived from a Leydig cell tumour developed in a male heterozygous Men1 mutant mouse. Our data show that these cell lines maintain the basic characteristics of Leydig cells in terms of both androgen synthesis and gene expression. Interestingly, reconstituted menin expression in one of Men1-deficient LCT cell lines resulted in cell growth inhibition, suggesting that the function of cell growth suppression of the menin pathway, apart from menin itself, is essentially preserved in these cells. Furthermore, we show that menin re-expression in these Men1-deficient cells leads to a block in the transition from G0/G1 to S phase of the cell cycle and an increase in apoptosis, accompanied by a marked increase of p18INK4C and p27Kip1 expression. The current study therefore highlights the importance of menin expression in cell cycle and cell survival control in endocrine cells, and may provide insights into the mechanisms of tumour suppression by menin in related endocrine tumours.

Intrinsic regulators of pancreatic beta-cell proliferation

Once thought incapable of significant proliferation, the pancreatic beta-cell has recently been shown to harbor immense powers of self-renewal. Pancreatic beta-cells, the sole source of insulin in vertebrate animals, can grow facultatively to a degree unmatched by other organs in experimental animals. beta-cell growth matches changes in systemic insulin demand, which increase during common physiologic states such as aging, obesity, and pregnancy. Compensatory changes in beta-cell mass are controlled by beta-cell proliferation. Here we review recent advances in our understanding of the intrinsic factors and mechanisms that control beta-cell cycle progression. Dysregulation of beta-cell proliferation is emerging as a fundamental feature in the pathogenesis of human disease states such as cancer and diabetes mellitus. New experimental observations and studies of these diseases suggest that beta-cell fate and expansion are coordinately regulated. We speculate on how these advances may accelerate the discovery of new strategies for the treatment of diseases characterized by a deficiency or excess of beta-cells.

Recognizing genes differentially regulated in vitro by the multiple endocrine neoplasia type 1 (MEN1) gene, using RNA interference and oligonucleotide microarrays

BACKGROUND

Data on downstream effects of MEN1 gene inactivation is scarce. In an effort to identify genes regulated by MEN1, we designed a silencing experiment in a human endocrine pancreatic tumor cell line (BON1).

METHODS

By using RNA interference, MEN1 mRNA expression was knocked-down by >85%. Gene expression was assessed by oligonucleotide microarrays and compared to expression in nonsilenced controls. We also investigated if genes were differentially expressed in 6 malignant endocrine pancreatic tumors (EPTs) with homozygous MEN1 inactivation compared to 2 without MEN1 gene alterations.

RESULTS

Using a cut-off of > or =2 times, 66 genes were found to be upregulated, and 22 were downregulated in the MEN1-silenced clones. We corroborated the microarray findings by performing quantitative-PCR on the RNA from the silencing experiments for 7 of the 88 differentially regulated genes. Genes involved in endocrine cell fate determination, as well as genes known to be involved in NFkappaB, Notch, and Wnt signaling pathways, were among genes verified as differentially regulated in vitro.

CONCLUSIONS

The demonstration of pathways affected by silencing of MEN1 in vitro provides novel insight into neoplastic processes of potential importance in vivo, which warrants further study.