Altered Expression of PTEN and Its Major Regulator MicroRNA-21 in Pulmonary Neuroendocrine Tumors

Role of microRNAs in regulating cell proliferation, metastasis and chemoresistance and their applications as cancer biomarkers in small cell lung cancer

Small cell lung cancer (SCLC), a smoking-related highly aggressive neuroendocrine cancer, is characterized by rapid cell proliferation, early metastatic dissemination, and early relapse due to chemoresistance to first-line platinum-doublet chemotherapy. Genomically, SCLC tumors show nearly universal loss of TP53 and RB1 tumor suppressor genes, while gene expression signature classifies them into 4 distinct subgroups based on the expression patterns of lineage transcription factors - ASCL1/ASH1, NEUROD1, YAP-1, and POU2F3. Due to the lack of targetable molecular alterations and clinically useful diagnostic, prognostic and predictive biomarker, there is insignificant progress in the therapeutic management of SCLC patients. Numerous studies have shown a significant involvement of non-coding RNAs in the regulation of cell proliferation, invasion and migration, apoptosis, metastasis, and chemoresistance in various human cancers. In this review, we comprehensively discuss the role of microRNAs (miRNAs) in regulating the aforementioned biological process in SCLC. For this, we searched the scientific literature and selected studies that have evaluated the role of miRNAs in the disease pathogenesis or as a cancer biomarker in SCLC. Our review suggests that several miRNAs are involved in the pathogenesis of SCLC mainly by regulating cell proliferation, metastasis, and chemoresistance. Few studies have also demonstrated the clinical utility of miRNAs in monitoring response to chemotherapy as well as in predicting survival outcomes. However, more in-depth mechanistic studies utilizing in vivo models and multicentric studies with larger patient cohorts are needed before the applications of miRNAs as therapeutic targets or as biomarkers are translated from the laboratory into clinics.

Prognostic and predictive role of the PI3K-AKT-mTOR pathway in neuroendocrine neoplasms

Neuroendocrine neoplasms (NENs) are considered a heterogeneous and rare entity. Its natural history is influenced by multiple clinicopathological characteristics, which guide the management of these patients. The development of molecular biology reveals that the PI3K-AKT-mTOR pathway plays a relevant role in tumorigenesis and progression of NENs. Mammalian target of rapamycin (mTOR) inhibitors, targeted agents that block this pathway, has improved outcomes in neuroendocrine tumors (NETs). Different therapeutic approaches, such as somatostatin analogs, chemotherapy, peptide receptor radionuclide therapy, and targeted agents, have shown benefits in the treatment of NETs. However, there are not any established prognostic or predictive biomarkers to select the best therapy option to individualize treatment. Although a relation between alterations in the PI3K-AKT-mTOR pathway and clinical outcomes has not been found, these anomalies are considered attractive biomarkers. Additional molecular analysis should be integrated in future clinical trials' design to identify potential predictive or prognostic biomarkers.

Expression of hsa-let-7b-5p, hsa-let-7f-5p, and hsa-miR-222-3p and their putative targets HMGA2 and CDKN1B in typical and atypical carcinoid tumors of the lung

Typical and atypical carcinoid tumors belong to the neuroendocrine lung tumors. They have low recurrence and proliferation rate, lymph node, and distant metastases. Nevertheless, these tumors have shown a more aggressive behavior. In the last years, microRNAs were screened as new tumor markers for their potential diagnostic and therapeutic relevance. The expression of hsa-let-7b-5p, hsa-let-7f-5p, hsa-miR-222-3p, and their targets HMGA2 (high-mobility group A2) and CDKN1B (cyclin-dependent kynase inhibitor 1B, p27^kip1) was evaluated in this rare small group of patients. We analyzed the clinical data of all typical and atypical carcinoid tumors of patients who underwent surgical operation at Marburg University Hospital (n = 18) from 2000. Quantitative reverse transcription polymerase chain reaction was performed in formalin-fixed paraffin-embedded tumor tissue versus four tumor-free lung tissue samples. HMGA2 was stable or downregulated; only one patient showed a significant overexpression. CDKN1B showed a significant overexpression or a stable level; it was downregulated in two samples only. Hsa-miR-222-3p resulted almost stable or overexpressed except for two samples (significantly downregulated). Hsa-let-7f-5p was stable or overexpressed in the majority of analyzed samples, whereas hsa-let-7b-5p was significantly downregulated. HMGA2 and CDKN1B are differently expressed between atypical and typical carcinoid tumors, thus representing valid biomarkers for the classification of the two tumor groups. Hsa-let-7f-5p and HMGA2 are inversely correlated. Hsa-miR-222-3p does not correlate with its predicted target CDKN1B.

The regulatory role of aberrant Phosphatase and Tensin Homologue and Liver Kinase B1 on AKT/mTOR/c-Myc axis in pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumor (pNET) is an uncommon type of pancreatic neoplasm. Low Phosphatase and Tensin Homologue (PTEN) expression and activation of the mechanistic target of rapamycin (mTOR) pathway have been noted in pNETs, and the former is associated with poor survival in pNET patients. Based on the results of the RADIANT-3 study, everolimus, an oral mTOR inhibitor, has been approved to treat advanced pNETs. However, the exact regulatory mechanism for the mTOR pathway in pNETs remains largely unknown. PTEN and liver kinase B1 (LKB1) are well-known for their regulatory role in the mTOR pathway. We evaluated the expression of PTEN and LKB1 in 21 pNET patients, and low PTEN and LKB1 expression levels were noted in 48% and 24% of the patients, respectively. Loss of PTEN and LKB1 synergistically promoted cell proliferation of pNET, attenuated the sensitivity of cells to mTOR inhibitors and enhanced c-Myc expression, which back-regulated PTEN, AKT, mTOR and its downstream effectors. For pNET cells with low expression levels of PTEN and LKB1, silencing the expression of c-Myc by shRNA reduced their proliferative rate, while adding either c-Myc inhibitor or AMP-activated protein kinase activator reversed their resistance to mTOR inhibitors in vitro and in vivo. Furthermore, high c-Myc expression was subsequently identified in 81% of pNETs, suggesting that up-regulation of c-Myc expression in pNETs may occur through PTEN/LKB1-dependent and PTEN/LKB1-independent regulation. The results delineated the regulation of PTEN and LKB1 on the AKT/mTOR/c-Myc axis and suggested that both c-Myc and mTOR are potential therapeutic targets for pNET.

Genetic and epigenetic drivers of neuroendocrine tumours (NET)

Neuroendocrine tumours (NET) of the gastrointestinal tract and the lung are a rare and heterogeneous group of tumours. The molecular characterization and the clinical classification of these tumours have been evolving slowly and show differences according to organs of origin. Novel technologies such as next-generation sequencing revealed new molecular aspects of NET over the last years. Notably, whole-exome/genome sequencing (WES/WGS) approaches underlined the very low mutation rate of well-differentiated NET of all organs compared to other malignancies, while the engagement of epigenetic changes in driving NET evolution is emerging. Indeed, mutations in genes encoding for proteins directly involved in chromatin remodelling, such as DAXX and ATRX are a frequent event in NET. Epigenetic changes are reversible and targetable; therefore, an attractive target for treatment. The discovery of the mechanisms underlying the epigenetic changes and the implication on gene and miRNA expression in the different subgroups of NET may represent a crucial change in the diagnosis of this disease, reveal new therapy targets and identify predictive markers. Molecular profiles derived from omics data including DNA mutation, methylation, gene and miRNA expression have already shown promising results in distinguishing clinically and molecularly different subtypes of NET. In this review, we recapitulate the major genetic and epigenetic characteristics of pancreatic, lung and small intestinal NET and the affected pathways. We also discuss potential epigenetic mechanisms leading to NET development.

Differential miRNA-Expression as an Adjunctive Diagnostic Tool in Neuroendocrine Tumors of the Lung

Pulmonary malignancies with neuroendocrine differentiation represent a rare subclass of lung carcinomas, which vary in the extent of differentiation and grade of biological aggressiveness. In particular, neuroendocrine tumors are classified into well differentiated typical and atypical carcinoids as well as poorly differentiated large cell neuroendocrine and small cell lung carcinomas. Tiny MicroRNAs have been identified as reliable classifiers in distinct cancer types and seem to play important roles in cellular processes like regulation of cell growth, differentiation and apoptosis. In the present study, two different microRNAs (miR-21 and miR-34a) were explored for their involvements in pathogenesis of subtypes and finally in differential diagnosis of pulmonary neuroendocrine tumors. miR-21 was upregulated in poorly differentiated neuroendocrine tumors (mean rank: 26.8; 28.75) as compared to carcinoids (mean rank: 12.33; 12.07) with a significance of 0.00033. High-expression levels of miR-34a were associated with atypical carcinoids (p = 0.010). A close association is implicated between the elevated miR-21 values in high-grade and miR-34a patterns in low-grade atypical neuroendocrine lung carcinomas, which could potentially be exploited as practical supportive markers for differential lung cancer diagnosis in routine. However, some additional extended research and validation studies are needed to utilize them as routine markers or potential molecular targets for personalized medicine.

Pulmonary atypical carcinoid in a patient with Cowden syndrome

Cowden syndrome is a phosphatase and tensin homologue on chromosome ten (PTEN) hamartoma tumour syndrome. The loss of function of the PTEN protein is associated with the risk of development and progression of several types of tumours, such as pulmonary neuroendocrine tumours and carcinoids. We present a 65-year old male non-smoker, who was diagnosed with Cowden syndrome and had a pulmonary tumour. He underwent wedge resection via video-assisted thoracoscopic surgery and was histopathologically diagnosed with an atypical carcinoid. To our knowledge, this is the first report on a pulmonary atypical carcinoid in a patient with Cowden syndrome. The present findings suggest a potential link between phosphoinositide-3 kinase/protein kinase B signalling in Cowden syndrome and the development of pulmonary neuroendocrine tumours, such as carcinoids.

Inhibition of NADPH oxidase protects against metastasis of human lung cancer by decreasing microRNA-21

The objective of this study was to detect the effect of NADPH oxidase (NOX) inhibition on metastasis of lung cancer. Primary human lung cancer cells were isolated from surgical tissues using the Cancer Cell Isolation Kit. Invasion was detected using the BD Biocoat Matrigel Invasion Chamber assay. Expressions of microRNA-21 (miR-21), PTEN, MMP9, and p47 were detected by qPCR. Groups of nude mice were challenged with A549 cells with or without DPI and detected for tumor metastasis and survival. NOX inhibition in human lung cancer cells significantly reduced their invasive potential in vitro. NOX inhibition in vivo led to decreased metastasis of human lung cancer and prolonged the survival time of tumor-bearing nude mice. Further, NOX inhibition resulted in decreased expression of miR-21 in human lung cancer cells. Increased expression of miR-21 abrogated the effect of NOX inhibitor on metastasis of human lung cancer in vitro and in vivo. Decreased expression of miR-21 facilitated the effect of NOX inhibitor on metastasis of human lung cancer in vitro and in vivo. Furthermore, increased expression of PTEN and decreased expression of MMP9 were observed in human lung cancer cells in response to NOX inhibition. Finally, close correlations of miR-21 expression levels with NADPH oxidase expression level and differentiation state of tumor cells were observed in lung cancer patients. Inhibition of NADPH oxidase protected against metastasis of human lung cancer cells by decreasing miR-21 expression, which could facilitate the understanding of lung cancer pathogenesis and provided clues for the development of novel therapeutics for lung cancer patients.

microRNA-21 promotes osteogenic differentiation of mesenchymal stem cells by the PI3K/β-catenin pathway

Osteogenesis of mesenchymal stem cells (MSCs) is essential for bone repair. Recently, microRNAs have been proven to play an important role in the regulation of MSC differentiation, including osteogenesis. Here, the function of microRNA-21 (miR-21) in the osteogenic differentiation of human umbilical cord mesenchymal stem cells (hUMSCs) was investigated. Briefly, the miR-21 mimics (m-miR-21) and the antisense miR-21 (as-miR-21) were transfected to hUMSCs, and the capacity of miR-21 for the osteogenic differentiation of hUMSCs was evaluated by the expression of osteogenic markers encoding alkaline phosphatase (ALP), runt-related gene-2 (RUNX-2) and osteocalcin (OCN), as well as by Alizarin red S staining. The results indicated that the overexpression of miR-21 elevated the expression level of the osteogenesis-related genes of hUMSCs. During this process, the PI3K-AKT signaling pathway activity had an increasing tendency responding to miR-21 up-regulation. This enhancement promoted the phosphorylation of GSK-3β, leading to the stabilization and high concentration accumulation of β-catenin in cytoplasm to activate the transcription of RUNX-2, and finally increased the osteogenesis of hUMSCs. This work demonstrated that miR-21 and its target PI3K-AKT-GSK3β pathway played an important role in the osteogenic differentiation of hUMSCs by stabilizing β-catenin.