Carcinoid tumor is a type of tumor that originates from neuroendocrine cells, which are found in organs throughout the body. Most carcinoid tumors begin in the digestive tract (stomach, appendix, small intestine, colon, and rectum) and the lungs. They are also found in the liver, but the finding is normally considered as a manifestation caused by metastatic tumor occurring elsewhere in the body. Carcinoid tumors grow very slowly compared to most malignant tumors, and they always do not cause signs and symptoms until late in the disease. Hormones produced by carcinoid tumors could lead to diarrhea or skin flushing, other symptoms mainly depend on the location of the tumor. Patients with a carcinoid tumor in the appendix usually have pain in the abdomen, while symptoms such as nausea, vomiting, or diarrhea are caused by carcinoid tumors in other parts of the digestive tract. When the tumor occurs in the lungs, patients usually have symptoms such as breathing trouble, chest pain, wheezing, or coughing up blood. Though the incidence of gastric carcinoids is found to increase in achlorhydria, Hashimoto's thyroiditis, and pernicious anemia, the mechanism of carcinoid formation is still unclear.
Figure 1. Carcinoid Signaling Pathway
The intracellular serine/threonine kinase, mammalian target rapamycin (mTOR) is playing an important role in the regulation of cell survival, proliferation, and metabolism. When combined with several proteins, mTOR could form multiprotein complexes mTOR complex 1 (mTORC1) and 2 (mTORC2), which mediate the signal transduction of growth factors and nutrients and finally participate in the anabolic and catabolic cellular processes. On the activation of mTORC1 by growth factors and nutrients, mTORC1 phosphorylates the translational regulators eukaryotic initiation factor 4E (eIF4E) binding protein (4E-BP1) and S6 kinase 1 (S6K1). The phosphorylation would then lead to the transcription of target genes that regulate cell progression, angiogenesis, energy metabolism, and metastasis. Besides, mTORC1 can also promote the synthesis of lipids and suppress autophagy. Similarly, mTORC2 can also respond to growth factor signals and regulate cell survival and migration. Studies have proved the importance of the mTOR pathway in the pathogenesis of carcinoids, exome sequencing analysis showed mutations of genes encoding components of mTOR, and the upregulation of the mTOR pathway is observed in several studies.
It is well known that serotonin (5-HT) and metabolites from the 5-HT pathway are of great significance in the development of carcinoids. The substrate for the synthesis of 5-HT is essential amino acid tryptophan (TRP). TRP was first conversed to short-lived 5-HT (5-hydroxytryptophan, 5-HTP) by the enzyme L-tryptophan-5-hydroxylase (TPH) and subsequently decarboxylated to 5-HT under the catalysis of aromatic acid decarboxylase (ADCC). 5-HT is a neurotransmitter that mainly acts on the gastrointestinal tract and central nervous system (CNS). The enteric neurons and the enterochromaffin cell of the gastrointestinal tract release approximately 95% of the total 5-HT in the human body, and the remaining 5% was produced in the brain. Altered metabolism of TRP is found in almost all patients with carcinoid, with increased blood 5-HT concentration due to the increased activity of TPH.
The Notch signaling pathway has been studied for years to understand the mechanisms of cancer. It is now known that canonical Notch signaling act as tumor suppressors in some cancers and oncogenes in others, while the non-canonical signaling also participates in the signaling regulation through PI3K/Akt, mTOR, NF-κB, and β-catenin. These signaling pathways and Hes-1/ASCL-1, have been found to regulate tumorigenesis via Notch in carcinoid. The notch signaling pathway plays a central role in multiple cellular processes such as proliferation, stem cell maintenance, and differentiation. In canonical signaling, Notch is activated and released by intracellular cleavage of the receptor promoted by the ligand binding. The Notch intracellular domain (NICD) then translocates into the nucleus and binds to the transcription factor CSL, and finally activates the expression of downstream genes. Non-canonical Notch signaling generally functions independent of CSL and can be both ligand-dependent and independent. Although information regarding mechanisms of non-canonical Notch signaling is limited, it was found to occur in some types of cancers. The pathway is providing a new research approach and may reveal new targets for therapeutic intervention in the translational setting.
The PI3K-Akt signaling pathway plays an important role in cell proliferation, motility, and survival. The PI3K is a heterodimer lipid kinase composed of the p85 and p110 subunits, which are activated by receptor tyrosine kinases (RTKs). On the activation of the pathway, PI3K catalyzes the conversion of phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-triphosphate (PIP3), which in turn activates the serine/threonine protein kinase Akt. The activated Akt affects several downstream targets, such as glycogen synthase kinase-3β (GSK3β), nuclear transcription factor κB (NF-κB), and mammalian target of rapamycin (mTOR), and finally regulates the gene transcription. These genes are found to be involved in the progression of various cancers. For example, mutations in the p85 or p110 subunits can lead to the failure of the phosphatase and tensin homolog (PTEN) and cause unregulated activation of Akt. The Akt1 isoform is found to function in multiple cancers, including human pancreatic carcinoid tumors, which could be a target of interest for the treatment of neuroendocrine tumors (NET).
Carcinoid is difficult to detect clinically due to its fairly common symptoms in the early stage. For example, abdominal pain or diarrhea could present in gastrointestinal tract diseases, such as irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), and gastritis, while wheezing can be caused by asthma or other respiratory problems. Platelet-rich plasma 5-HT is considered the most sensitive marker for the diagnosis of carcinoid, which shows sensitivity in tumors that produce a low amount of 5-HT. However, the clinical detection of 5-HT was limited due to its complexity, low reproducibility, and large individual variability. The urinary metabolite of plasma 5-HT, U-5-HIAA, is the most frequently used biomarker for the diagnosis of carcinoid. However, its diagnostic accuracy shows large variability between different tumors. The accuracy is higher for midgut tumors due to the elevated 5-HT production compared to foregut and hindgut tumors. Plasma fasting 5-HIAA measurement is another potential tool for the diagnosis of carcinoid. Studies showed that the fasting 5-HIAA levels are correlated with U-5-HIAA, and the plasma measurement provides better reliability over the urinary collection, which would probably play a more important role in the future.
Chromogranin A (CgA) is considered to be the best commonly used protein biomarker for diagnosis and therapeutic evaluation, which is increased in 50-100% of patients with neuroendocrine tumors. Generally, CgA exits throughout the neuronal and neuroendocrine systems. Cells such as endocrine cells of the anterior pituitary, parafollicular C cells of the thyroid, chief cells of the parathyroid, islets cells of the pancreas, and chromaffin cells of the adrenal medulla all produce the protein. It is also found in the neuroendocrine system of the bronchial and gastrointestinal tract but lacks in cells that are not of neuroendocrine origin. Immunohistochemical detection of CgA is now widely used in clinical practice, both typical neuroendocrine neoplasms and neuroendocrine cell hyperplasia could show CgA positive immunostaining.
The mTOR is an intracellular protein that participates in the regulation of cell growth, protein synthesis, and autophagy. It can also affect the downstream RTKs such as IGF, VEGF, and ErbB. Mutations in this pathway have been found in various tumors including carcinoids, which may lead to apoptosis resistance, increased proliferation, and altered metabolism. Two mTOR inhibitors, everolimus (RAD001) and temsirolimus (CCI-779) have been explored in clinical trials of neuroendocrine tumors, more than 1,000 patients were treated in the trials. Most data were about studies with everolimus. A recently published study using temsirolimus in NET patients showed little activity, although a high disease control rate with 6 months of study. Even so, the potential activity of the drug is still believed to be underestimated. Another phase II clinical trial using temsirolimus in combination with bevacizumab is being evaluated in NET.
Table 1 Clinical trials of mTOR inhibitor Everolimus
| NCT ID | Status | Lead sponsor | Study first posted |
| NCT03950609 | Recruiting | M.D. Anderson Cancer Center | May, 2019 |
| NCT04665739 | Recruiting | National Cancer Institute (NCI) | December, 2020 |
| NCT00412061 | Completed | Novartis Pharmaceuticals | December, 2006 |
Carcinoid is a kind of vascular tumor that often accompanies desmoplastic reactions. Expression of vascular endothelial growth factor (VEGF) and VEGFR, TGF-α and epidermal growth factor receptor (EGFR), PDGF, and its receptor PDGFR were all found in carcinoid tumors. A phase II study of bevacizumab, a monoclonal antibody against VEGF-A, showed improved progression-free survival at week 18 in patients with carcinoid compared to those receiving interferon treatment. Another phase II clinical trial evaluating gefitinib was delivered in patients with progressive metastatic neuroendocrine tumors. However, the modest result (3% partial response in carcinoid and 6% in islet-cell carcinoma) indicates the limitation of EGFR inhibitor as a single agent for the treatment of carcinoid and islet-cell carcinoma.
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