The molecular pathogenesis and management of bronchial carcinoids

Tropomyosin receptor kinase: a novel target in screened neuroendocrine tumors

Tropomyosin receptor kinase (Trk) inhibitors are investigated as a novel targeted therapy in various cancers. We investigated the in vitro effects of the pan-Trk inhibitor GNF-5837 in human neuroendocrine tumor (NET) cells. The human neuroendocrine pancreatic BON1, bronchopulmonary NCI-H727 and ileal GOT1 cell lines were treated with GNF-5837 alone and in combination with everolimus. Cell viability decreased in a time- and dose-dependent manner in GOT1 cells in response to GNF-5837 treatment, while treatment in BON1 and NCI-H727 cells showed no effect on cellular viability. Trk receptor expression determined GNF-5837 sensitivity. GNF-5837 caused downregulation of PI3K-Akt-mTOR signaling, Ras-Raf-MEK-ERK signaling, the cell cycle and increased apoptotic cell death. The combinational treatment of GNF-5837 with everolimus showed a significant enhancement in inhibition of cell viability vs single substance treatments, due to a cooperative PI3K-Akt-mTOR and Ras-Raf-MEK-ERK pathway downregulation, as well as an enhanced cell cycle component downregulation. Immunohistochemical staining for Trk receptors were performed using a tissue microarray containing 107 tumor samples of gastroenteropancreatic NETs. Immunohistochemical staining with TrkA receptor and pan-Trk receptor antibodies revealed a positive staining in pancreatic NETs in 24.2% (8/33) and 33.3% (11/33), respectively. We demonstrated that the pan-Trk inhibitor GNF-5837 has promising anti-tumoral properties in human NET cell lines expressing the TrkA receptor. Immunohistochemical or molecular screening for Trk expression particularly in pancreatic NETs might serve as predictive marker for molecular targeted therapy with Trk inhibitors.

Targeted therapy of gastroenteropancreatic neuroendocrine tumours: preclinical strategies and future targets

Molecular targeted therapy of advanced neuroendocrine tumours (NETs) of the gastroenteropancreatic (GEP) system currently encompasses approved therapy with the mammalian target of rapamycin (mTOR) inhibitor everolimus and the multi-tyrosinkinase inhibitor sunitinib. However, clinical efficacy of these treatment strategies is limited by low objective response rates and limited progression-free survival due to tumour resistance. Further novel strategies for molecular targeted therapy of NETs of the GEP system are needed. This paper reviews preclinical research models and signalling pathways in NETs of the GEP system. Preclinical and early clinical data on putative novel targets for molecular targeted therapy of NETs of the GEP system are discussed, including PI3K, Akt, mTORC1/mTORC2, GSK3, c-Met, Ras-Raf-MEK-ERK, embryogenic pathways (Hedgehog, Notch, Wnt/beta-catenin, TGF-beta signalling and SMAD proteins), tumour suppressors and cell cycle regulators (p53, cyclin-dependent kinases (CDKs) CDK4/6, CDK inhibitor p27, retinoblastoma protein (Rb)), heat shock protein HSP90, Aurora kinase, Src kinase family, focal adhesion kinase and epigenetic modulation by histone deacetylase inhibitors.

Molecular strategies in the management of bronchopulmonary and thymic neuroendocrine neoplasms

Thoracic NETs [bronchopulmonary NETs (BPNETs) and thymic NETs (TNET)] share a common anatomic primary location, likely a common cell of origin, the "Kulchitsky cell" and presumably, a common etiopathogenesis. Although they are similarly grouped into well-differentiated [typical carcinoids (TC) and atypical carcinoids (AC)] and poorly differentiated neoplasms and both express somatostatin receptors, they exhibit a wide variation in clinical behavior. TNETs are more aggressive, are frequently metastatic, and have a lower 5-year survival rate (~50% vs. ~80%) than BPNETs. They are typically symptomatic, most often secreting ACTH (40% of tumors) but both tumor groups share secretion of common biomarkers including chromogranin A and 5-HIAA. Consistently effective and accurate circulating biomarkers are, however, currently unavailable. Surgery is the primary therapeutic tool for both BPNET and TNETs but there remains little consensus about later interventions e.g., targeted therapy, or how these can be monitored. Genetic analyses have identified different topographies (e.g., significant alterations in chromatin and epigenetic remodeling in BPNETs versus frequent chromosomal abnormalities in TNETs) but there is an absence of clinically actionable mutations in both tumor groups. Liquid biopsies, tools that can measure neoplastic signatures in peripheral blood, can potentially be leveraged to detect disease early i.e., recurrence, predict tumors that may respond to specific therapies and serve as real-time monitors for treatment responses. Recent studies have identified that mRNA transcript analysis in blood effectively identifies both BPNET and TNETs. The clinical utility of this gene expression assay includes use as a diagnostic, confirmation of completeness of surgical resection and use as a molecular management tool to monitor efficacy of PRRT and other therapeutic strategies.

The selective PI3Kα inhibitor BYL719 as a novel therapeutic option for neuroendocrine tumors: Results from multiple cell line models

Background/Aims

The therapeutic options for metastatic neuroendocrine tumors (NETs) are limited. As PI3K signaling is often activated in NETs, we have assessed the effects of selective PI3Kp110α inhibition by the novel agent BYL719 on cell viability, colony formation, apoptosis, cell cycle, signaling pathways, differentiation and secretion in pancreatic (BON-1, QGP-1) and pulmonary (H727) NET cell lines.

Methods

Cell viability was investigated by WST-1 assay, colony formation by clonogenic assay, apoptosis by caspase3/7 assay, the cell cycle by FACS, cell signaling by Western blot analysis, expression of chromogranin A and somatostatin receptors 1/2/5 by RT-qPCR, and chromogranin A secretion by ELISA.

Results

BYL719 dose-dependently decreased cell viability and colony formation with the highest sensitivity in BON-1, followed by H727, and lowest sensitivity in QGP-1 cells. BYL719 induced apoptosis and G0/G1 cell cycle arrest associated with increased p27 expression. Western blots showed inhibition of PI3K downstream targets to a varying degree in the different cell lines, but IGF1R activation. The most sensitive BON-1 cells displayed a significant, and H727 cells a non-significant, GSK3 inhibition after BYL719 treatment, but these effects do not appear to be mediated through the IGF1R. In contrast, the most resistant QGP-1 cells showed no GSK3 inhibition, but a modest activation, which would partially counteract the other anti-proliferative effects. Accordingly, BYL719 enhanced neuroendocrine differentiation with the strongest effect in BON-1, followed by H727 cells indicated by induction of chromogranin A and somatostatin receptor 1/2 mRNA-synthesis, but not in QGP-1 cells. In BON-1 and QGP-1 cells, the BYL719/everolimus combination was synergistic through simultaneous AKT/mTORC1 inhibition, and significantly increased somatostatin receptor 2 transcription compared to each drug separately.

Conclusion

Our results suggest that the agent BYL719 could be a novel therapeutic approach to the treatment of NETs that may sensitize NET cells to somatostatin analogs, and that if there is resistance to its action this may be overcome by combination with everolimus.

The MTH1 inhibitor TH588 demonstrates anti-tumoral effects alone and in combination with everolimus, 5-FU and gamma-irradiation in neuroendocrine tumor cells

Modulation of the redox system in cancer cells has been considered a promising target for anti-cancer therapy. The novel MTH1 inhibitor TH588 proved tremendous potential in terms of cancer cell eradication, yet its specificity has been questioned by recent reports, indicating that TH588 may also induce cancer cell death by alternative mechanisms than MTH1 inhibition. Here we used a panel of heterogeneous neuroendocrine tumor cells in order to assess cellular mechanisms and molecular signaling pathways implicated in the effects of TH588 alone as well as dual-targeting approaches combining TH588 with everolimus, cytotoxic 5-fluorouracil or γ-irradiation. Our results reflect that TH588 alone efficiently decreased the survival of neuroendocrine cancer cells by PI3K-Akt-mTOR axis downregulation, increased apoptosis and oxidative stress. However, in the dual-targeting approaches cell survival was further decreased due to an even stronger downregulation of the PI3K-Akt-mTOR axis and augmentation of apoptosis but not oxidative stress. Furthermore, we could attribute TH588 chemo- and radio-sensitizing properties. Collectively our data not only provide insights into how TH588 exactly kills cancer cells but also depict novel perspectives for combinatorial treatment approaches encompassing TH588.

Cabozantinib and Tivantinib, but Not INC280, Induce Antiproliferative and Antimigratory Effects in Human Neuroendocrine Tumor Cells in vitro: Evidence for 'Off-Target' Effects Not Mediated by c-Met Inhibition

BACKGROUND/AIMS

The hepatocyte growth factor/transmembrane tyrosine kinase receptor c-Met has been defined as a potential target in antitumoral treatment of various carcinomas. We aimed to investigate the direct effect of c-Met inhibition on neuroendocrine tumor cells in vitro.

METHODS

The effects of the multi-tyrosine kinase inhibitors cabozantinib and tivantinib and of the highly specific c-Met inhibitor INC280 were investigated in human pancreatic neuroendocrine BON1, bronchopulmonary NCI-H727 and midgut GOT1 cells in vitro.

RESULTS

INC280, cabozantinib and tivantinib inhibited c-Met phosphorylation, respectively. However, while equimolar concentrations (10 μM) of cabozantinib and tivantinib inhibited cell viability and cell migration, INC280 had no inhibitory effect. Knockdown experiments with c-Met siRNA also did not demonstrate effects on cell viability. Cabozantinib and tivantinib caused a G2 arrest in neuroendocrine tumor cells.

CONCLUSIONS

Our in vitro data suggest that c-Met inhibition alone is not sufficient to exert direct antitumoral or antimigratory effects in neuroendocrine tumor cells. The multi-tyrosine kinase inhibitors cabozantinib and tivantinib show promising antitumoral and antimigratory effects in neuroendocrine tumor cells, which are most probably 'off-target' effects, not mediated by c-Met.

Additive Anti-Tumor Effects of Lovastatin and Everolimus In Vitro through Simultaneous Inhibition of Signaling Pathways

Background

The mTORC1-inhibitor everolimus shows limited efficacy in treating patients with gastro-entero-pancreatic or pulmonary neuroendocrine tumors (NETs), and poor outcome in patients with malignant pheochromocytoma or hepatic carcinoma. We speculated that any effect may be enhanced by antogonising other signaling pathways.

Methods

Therefore, we tested the effect of lovastatin—known to inhibit both ERK and AKT signaling—and everolimus, separately and in combination, on cell viability and signaling pathways in human midgut (GOT), pancreatic (BON1), and pulmonary (H727) NET, hepatocellular carcinoma (HepG2, Huh7), and mouse pheochromocytoma (MPC, MTT) cell lines.

Results

Lovastatin and everolimus separately significantly reduced cell viability in H727, HepG2, Huh7, MPC and MTT cells at clinically relevant doses (P ≤ 0.05). However, high doses of lovastatin were necessary to affect GOT or BON1 cell viability. Clinically relevant doses of both drugs showed additive anti-tumor effects in H727, HepG2, Huh7, MPC and MTT cells (P ≤ 0.05), but not in BON1 or GOT cells. In all cell lines investigated, lovastatin inhibited EGFR and AKT signaling. Subsequently, combination treatment more strongly inhibited EGFR and AKT signaling than everolimus alone, or at least attenuated everolimus-induced EGFR or AKT activation. Vice versa, everolimus constantly decreased pp70S6K and combination treatment more strongly decreased pp70S6K than lovastatin alone, or attenuated lovastatin-induced p70S6K activation: in BON1 cells lovastatin-induced EGFR inhibition was least pronounced, possibly explaining the low efficacy and consequent absent additive effect.

Conclusion

In summary, clinically relevant doses of lovastatin and everolimus were effective separately and showed additive effects in 5 out of 7 cell lines. Our findings emphasize the importance of targeting several interacting signaling pathways simultaneously when attempting to attenuate tumor growth. However, the variable reactions of the different cell lines highlight the necessity to understand the unique molecular aberrations in any tumor. Nevertheless, this combination seems worthy of being tested in vivo.

Unraveling tumor grading and genomic landscape in lung neuroendocrine tumors

Currently, grading in lung neuroendocrine tumors (NETs) is inherently defined by the histological classification based on cell features, mitosis count, and necrosis, for which typical carcinoids (TC) are low-grade malignant tumors with long life expectation, atypical carcinoids (AC) intermediate-grade malignant tumors with more aggressive clinical behavior, and large cell NE carcinomas (LCNEC) and small cell lung carcinomas (SCLC) high-grade malignant tumors with dismal prognosis. While Ki-67 antigen labeling index, highlighting the proportion of proliferating tumor cells, has largely been used in digestive NETs for assessing prognosis and assisting therapy decisions, the same marker does not play an established role in the diagnosis, grading, and prognosis of lung NETs. Next generation sequencing techniques (NGS), thanks to their astonishing ability to process in a shorter timeframe up to billions of DNA strands, are radically revolutionizing our approach to diagnosis and therapy of tumors, including lung cancer. When applied to single genes, panels of genes, exome, or the whole genome by using either frozen or paraffin tissues, NGS techniques increase our understanding of cancer, thus realizing the bases of precision medicine. Data are emerging that TC and AC are mainly altered in chromatin remodeling genes, whereas LCNEC and SCLC are also mutated in cell cycle checkpoint and cell differentiation regulators. A common denominator to all lung NETs is a deregulation of cell proliferation, which represents a biological rationale for morphologic (mitoses and necrosis) and molecular (Ki-67 antigen) parameters to successfully serve as predictors of tumor behavior (i.e., identification of pathological entities with clinical correlation). It is envisaged that a novel grading system in lung NETs based on the combined assessment of mitoses, necrosis, and Ki-67 LI may offer a better stratification of prognostic classes, realizing a bridge between molecular alterations, morphological features, and clinical behavior.

Potent antitumor activity of the novel HSP90 inhibitors AUY922 and HSP990 in neuroendocrine carcinoid cells

The heat shock protein (HSP) 90 chaperone machine involved in numerous oncogenic signaling pathways is over-expressed in cancer cells and is currently being evaluated for anticancer therapy. Neuroendocrine tumors (NETs) of the gastroenteropancreatic (GEP) system comprise a heterogeneous group of tumors with increasing incidence and poor prognosis. Here, we report the antiproliferative effects of the HSP90 inhibitors AUY922 and HSP990 in neuroendocrine tumor cells. Treatment of human pancreatic BON1, bronchopulmonary NCI-H727 and midgut carcinoid GOT1 neuroendocrine tumor cells with increasing concentrations of AUY922 and HSP990 dose-dependently suppressed cell viability. Significant effects on neuroendocrine cell viability were observed with inhibitor concentrations as low as 5 nM. Inhibition of cell viability was associated with the induction of apoptosis as demonstrated by an increase in sub-G1 events and PARP cleavage. HSP90 inhibition was associated with decreased neuroendocrine ErbB and IGF-I receptor expression, decreased Erk and Akt phosphorylation and the induction of HSP70 expression. These findings provide evidence that targeted inhibition of upregulated HSP90 activity could be useful for the treatment of aggressive neuroendocrine tumors resistant to conventional therapy.

mTOR, p70S6K, AKT, and ERK1/2 levels predict sensitivity to mTOR and PI3K/mTOR inhibitors in human bronchial carcinoids

Bronchial carcinoids (BCs) are rare neuroendocrine tumors that are still orphans of medical treatment. Human BC primary cultures may display resistance to everolimus, an inhibitor of the mammalian target of rapamycin (mTOR), in terms of cell viability reduction. Our aim was to assess whether the novel dual phosphatidylinositol 3-kinase (PI3K)/mTOR inhibitor NVP-BEZ235 is effective in everolimus-resistant human BC tissues and cell lines. In addition, we searched for possible markers of the efficacy of mTOR inhibitors that may help in identifying the patients who may benefit from treatment with mTOR inhibitors, sparing them from ineffective therapy. We found that NVP-BEZ235 is twice as potent as everolimus in reducing cell viability and activating apoptosis in human BC tissues that display sensitivity to mTOR inhibitors, but is not effective in everolimus-resistant BC tissues and cell lines that bypass cyclin D1 downregulation and escape G0/G1 blockade. Rebound AKT activation was not observed in response to treatment with either mTOR inhibitor in the 'resistant' BC cells. In addition to total mTOR levels, putative markers of the sensitivity of BCs to mTOR inhibitors are represented by AKT, p70S6K (RPS6KB2), and ERK1/2 (MAPK3/1) protein levels. Finally, we validated these markers in an independent BC group. These data indicate that the dual PI3K/mTOR inhibitor NVP-BEZ235 is more potent than everolimus in reducing the proliferation of human BC cells. 'Resistant' cells display lower levels of mTOR, p70S6K, AKT, and ERK1/2, indicating that these proteins may be useful as predictive markers of resistance to mTOR and PI3K/mTOR inhibitors in human BCs.