Connective tissue growth factor expression in endocrine tumors is associated with high stromal expression of alpha-smooth muscle actin

Mesenteric Fibrosis in Neuroendocrine Neoplasms: a Systematic Review of New Thoughts on Causation and Potential Treatments

PURPOSE OF REVIEW

Mesenteric fibrosis (MF) is a hallmark of small intestinal neuroendocrine neoplasms (SI-NEN) and is frequently associated with significant morbidity due to related complications such as intestinal obstruction, ischemia, and cachexia.

RECENT FINDINGS

Herein we performed a systematic review to discuss the development of MF in SI-NEN. The pathophysiological mechanisms acknowledged as causative for the development of MF include the major components of the tumor microenvironment, such as fibroblasts, endothelial and immune cells and the extracellular matrix, which are involved in a complex interplay activating several signaling pathways that promote profibrotic factors and induce both a desmoplastic reaction and tumor proliferation. Surgery remains the mainstay of treatment, while several medical management options of MF complicating SI-NEN available present rather limited efficacy. MF is a frequent characteristic of SI-NEN that requires particular attention and targeted management to avoid complications.

Relationship Between Absorbed Dose and Response in Neuroendocrine Tumors Treated with [177Lu]Lu-DOTATATE

Peptide receptor radionuclide therapy presents the possibility of tracing and quantifying the uptake of the drug in the body and performing dosimetry, potentially allowing individualization of treatment schemes. However, the details of how neuroendocrine tumors (NETs) respond to different absorbed doses are insufficiently known. Here, we investigated the relationship between tumor-absorbed dose and tumor response in a cohort of patients with NETs treated with [177Lu]Lu-DOTATATE. Methods: This was a retrospective study based on 69 tumors in 32 patients treated within a clinical trial. Dosimetry was performed at each cycle of [177Lu]Lu-DOTATATE, rendering 366 individual absorbed dose assessments. Hybrid planar-SPECT/CT imaging using [177Lu]Lu-DOTATATE was used, including quantitative SPECT reconstruction, voxel-based absorbed dose rate calculation, semiautomatic image segmentation, and partial-volume correction. Changes in tumor volume were used to determine tumor response. The volume for each tumor was manually delineated on consecutive CT scans, giving a total of 712 individual tumor volume assessments. Tumors were stratified according to grade. The relationship between absorbed dose and response was investigated using mixed-effects models and logistic regression. Tumors smaller than 4 cm3 were excluded. Results: In grade 2 NETs, a clear relationship between absorbed dose and volume reduction was observed. Our observations suggest a 90% probability of partial tumor response for an accumulated tumor-absorbed dose of at least 135 Gy. Conclusion: Our findings are in accordance with previous observations regarding the relationship between tumor shrinkage and absorbed dose. Moreover, our data suggest an absorbed dose threshold for partial response in grade 2 NETs. These observations provide valuable insights for the design of dosimetry-guided peptide receptor radionuclide therapy schemes.

Novel therapeutic approaches in GEP-NETs based on genetic and epigenetic alterations

Gastroenteropancreatic neuroendocrine tumors (GEP-NETs) are heterogeneous malignancies with distinct prognosis based on primary tumor localization, grade, stage and functionality. Surgery remains the only curative option in localized tumors, but systemic therapy is the mainstay of treatment for patients with advanced disease. For decades, the therapeutic landscape of GEP-NETs was limited to chemotherapy regimens with low response rates. The arrival of novel agents such as somatostatin analogues, peptide receptor radionuclide therapy, tyrosine kinase inhibitors or mTOR-targeted drugs, has changed the therapeutic paradigm of GEP-NETs. However, the efficacy of these agents is limited in time and there is scarce knowledge of optimal treatment sequencing. In recent years, massive parallel sequencing techniques have started to unravel the genomic intricacies of these tumors, allowing us to better understand the mechanisms of resistance to current treatments and to develop new targeted agents that will hopefully start an era for personalized treatment in NETs. In this review we aim to summarize the most relevant genomic aberrations and signaling pathways underlying GEP-NET tumorigenesis and potential therapeutic strategies derived from them.

Immuno-histochemical correlation of fibrosis-related markers with the desmoplastic reaction of the mesentery in small intestine neuroendocrine neoplasms

INTRODUCTION

Small intestine neuroendocrine neoplasms (siNENs) will attain more importance due to their increasing incidence. Moreover, siNENs might lead to a desmoplastic reaction (DR) of the mesentery causing severe complications and deteriorating prognosis. The expression of fibrosis-related proteins appears to be the key mechanisms for the development of this desmoplastic reaction. Therefore, this study aimed to investigate the association of the desmoplastic mesentery with specific fibrosis-related protein expression levels.

MATERIALS AND METHODS

By immunohistochemistry, the protein expression levels of four fibrosis-related markers (APLP2, BNIP3L, CD59, DKK3) were investigated in primary tumors of 128 siNENs. The expression levels were correlated with the presence of a desmoplastic reaction and clinico-pathological parameters.

RESULTS

In the primary tumor, APLP2, BNIP3L, CD59 and DKK3 were highly expressed in 29.7% (n = 38), 64.9% (n = 83), 92.2% (n = 118) and 80.5% (n = 103), respectively. There was no significant correlation of a single marker or the complete marker panel to the manifestation of a desmoplastic mesentery. The desmoplastic mesentery was significantly associated with clinical symptoms, such as flushing and diarrhea. However, neither the fibrosis-related marker panel nor single marker expressions were associated with clinical symptoms.

DISCUSSION

The expression rates of four fibrosis-related markers in the primary tumor display a distinct pattern. However, the expression patterns are not associated with desmoplastic altered mesenteric lymph node metastases and the expression patterns did not correlate with prognosis. These findings suggest alternative mechanisms being responsible for the desmoplastic reaction.

What Causes Desmoplastic Reaction in Small Intestinal Neuroendocrine Neoplasms?

Purpose of Review

Mesenteric desmoplasia in small intestinal neuroendocrine neoplasms (SINENs) is associated with increased morbidity and mortality. In this paper, we discuss the development of desmoplasia in SINENs.

Recent Findings

The fibrotic reactions associated with these tumours could be limited to the loco-regional environment of the tumour and/or at distant sites. Mesenteric fibrotic mass forms around a local lymph node. Formation of desmoplasia is mediated by interactions between the neoplastic cells and its microenvironment via number of profibrotic mediators and signalling pathways. Profibrotic molecules that are mainly involved in the desmoplastic reaction include serotonin, TGFβ (transforming growth factor β) and CTGF (connective tissue growth factor), although there is some evidence to suggest that there are a number of other molecules involved in this process.

Summary

Desmoplasia is a result of autocrine and paracrine effects of multiple molecules and signalling pathways. However, more research is needed to understand these mechanisms and to develop targeted therapy to minimise desmoplasia.

Perspectives on the diagnostic, predictive and prognostic markers of neuroendocrine neoplasms (Review)

Neuroendocrine neoplasms (NENs) are a heterogeneous group of rare tumors with different types of physiology and prognosis. Therefore, prognostic information, including morphological differentiation, grade, tumor stage and primary location, are invaluable and contribute to the formulation of treatment decisions. Biomarkers that are currently used, including chromogranin A (CgA), serotonin and neuron-specific enolase, are singular parameters that cannot be used to accurately predict variables associated with tumor growth, including proliferation, metabolic rate and metastatic potential. In addition, site-specific biomarkers, such as insulin and gastrin, cannot be applied to all types of NENs. The clinical application of broad-spectrum markers, as it is the case for CgA, remains controversial despite being widely used. Due to limitations of the currently available mono-analyte biomarkers, recent studies were conducted to explore novel parameters for NEN diagnosis, prognosis, therapy stratification and evaluation of treatment response. Identification of prognostic factors for predicting NEN outcome is a critical requirement for the planning of adequate clinical management. Advances in ‘liquid’ biopsies and genomic analysis techniques, including microRNA, circulating tumor DNA or circulating tumor cells and sophisticated biomathematical analysis techniques, such as NETest or molecular image-based biomarkers, are currently under investigation as potentially novel tools for the management of NENs in the future. Despite these recent findings yielding promising observations, further research is necessary. The present review therefore summarizes the existing knowledge and recent advancements in the exploration of biochemical markers for NENs, with focus on gastroenteropancreatic-neuroendocrine tumors.

Peptide Receptor Radionuclide Therapy (PRRT) with 177Lu-DOTATATE; Differences in Tumor Dosimetry, Vascularity and Lesion Metrics in Pancreatic and Small Intestinal Neuroendocrine Neoplasms

Simple Summary

Patients suffering from disseminated, progressive, neuroendocrine neoplasms with a sufficient amount of somatostatin receptors and good kidney function can be treated with radioactive hormone-like molecules to prolong their life. In this study, the radioactivity in one tumor per patient at each treatment cycle was calculated and compared between 23 patients with pancreatic and 25 patients with small intestinal neuroendocrine neoplasia. Both types of tumors absorb a larger amount of radioactivity during early cycles that subsequently decline in the later cycles. This finding was more pronounced in the pancreatic tumors, which also expressed higher blood perfusion in the early cycles, known to facilitate the effect of radiation. This could be part of the reason why the pancreatic tumors shrunk more rapidly than the small intestinal ones. Our results also imply that increased administered activity in the early therapy cycles may be beneficial, at least in pancreatic neuroendocrine tumor patients.

Abstract

Dosimetry during peptide receptor radionuclide therapy (PRRT) has mainly focused on normal organs and less on the tumors. The absorbed dose in one target tumor per patient and several response related factors were assessed in 23 pancreatic neuroendocrine neoplasms (P-NENs) and 25 small-intestinal NEN (SI-NENs) during PRRT with ¹⁷⁷Lu-DOTATATE. The total administered activity per patient was (mean ± standard error of mean (SEM) 31.8 ± 1.9 GBq for P-NENs and 36 ± 1.94 GBq for SI-NENs. The absorbed tumor dose was 143.5 ± 2 Gy in P-NENs, 168.2 ± 2 Gy in SI-NENs. For both NEN types, a dose–response relationship was found between the absorbed dose and tumor shrinkage, which was more pronounced in P-NENs. A significant drop in the absorbed dose per cycle was shown during the course of PRRT. Tumor vascularization was higher in P-NENs than in SI-NENs at baseline but equal post-PRRT. The time to progression (RECIST 1.1) was similar for patients with P-NEN (mean ± SEM 30 ± 1 months) and SI-NEN (33 ± 1 months). In conclusion, a dose response relationship was established for both P-NENs and SI-NENs and a significant drop in the absorbed dose per cycle was shown during the course of PRRT, which warrants further investigation to understand the factors impacting PRRT to improve personalized treatment protocol design.

Connective Tissue Growth Factor in Digestive System Cancers: A Review and Meta-Analysis

Aim

A meta-analysis was conducted to estimate the impact of connective tissue growth factor (CTGF) on outcomes in patients with digestive system cancers.

Methods

A systemic literature survey was performed by searching the Cochrane Library and PubMed databases for articles that evaluated the impact of CTGF on outcomes in patients with digestive system cancers. Hazard ratios and 95% confidence intervals were calculated for prognostic factors, overall and recurrence-free survival using RevMan 5.3 software.

Results

This meta-analysis was conducted to evaluate a total of 11 studies that included 1730 patients. The results showed that elevated CTGF expression was significantly correlated with advanced age, larger tumor size, multiple tumors, and vascular invasion. Subgroup analysis by cancer type revealed increased risk for lymph node metastasis and advanced tumor node metastasis (TNM) stage in gastric cancer, compared with colorectal cancer. An unfavorable effect of elevated CTGF levels on overall survival was found in patients with hepatocellular carcinoma and patients with gastric cancer, while survival was improved in colorectal cancer patients with high CTGF expression, compared to those with normal levels of CTGF.

Conclusions

Elevated CTGF expression may be a novel biomarker for disease status and predicted survival outcomes in patients with specific digestive system cancers.

Management of Locally Advanced and Unresectable Small Bowel Neuroendocrine Tumours

Three subtypes of small bowel neuroendocrine tumours (SBNETs) have been described: Type A: SBNET with resectable mesenteric disease that does not involve the mesenteric root; Type B: "Borderline resectable" SBNET presenting with mesenteric nodal metastases and fibrosis adjacent but not encasing the main trunk of the superior mesenteric artery (SMA) and superior mesenteric vein (SMV); and Type C: "Locally advanced or irresectable" SBNET where tumour deposits and fibrosis encase the SMA and SMV. Type C SBNETs are rare and constitute around 5% of patients in reported series, although this may underestimate the prevalence. In these patients, almost all will present with symptoms of intestinal ischemia or obstruction and symptom management should be a primary main focus of treatment. All patients should be carefully staged with cross-sectional imaging and 68 Ga-dotate positron emission tomography, and discussed at a dedicated neuroendocrine tumour multidisciplinary meeting. Expert surgical review should always be sought as experienced centers have a high rate of successful resection of primary tumours and mesenteric disease. If resection is not feasible, surgical bypass should be considered in patients with a discrete and symptomatic point of obstruction. Non-operative management should emphasize symptomatic treatment with somatostatin analogs, nutritional advice and support and palliative care. Successful neoadjuvant approaches utilizing peptide radionucleide receptor therapy and systemic chemotherapy with everolimus or temazolamide/capecitabine have not been reported.

Pathogenesis and Clinical Management of Mesenteric Fibrosis in Small Intestinal Neuroendocine Neoplasms: A Systematic Review

Mesenteric fibrosis (MF) constitutes an underrecognized sequela in patients with small intestinal neuroendocrine neoplasms (SI-NENs), often complicating the disease clinical course. The aim of the present systematic review, carried out by Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology, is to provide an update in evolving aspects of MF pathogenesis and its clinical management in SI-NENs. Complex and dynamic interactions are present in the microenvironment of tumor deposits in the mesentery. Serotonin, as well as the signaling pathways of certain growth factors play a pivotal, yet not fully elucidated role in the pathogenesis of MF. Clinically, MF often results in significant morbidity by causing either acute complications, such as intestinal obstruction and/or acute ischemia or more chronic conditions involving abdominal pain, venous stasis, malabsorption and malnutrition. Surgical resection in patients with locoregional disease only or symptomatic distant stage disease, as well as palliative minimally invasive interventions in advanced inoperable cases seem clinically meaningful, whereas currently available systemic and/or targeted treatments do not unequivocally affect the development of MF in SI-NENs. Increased awareness and improved understanding of the molecular pathogenesis of MF in SI-NENs may provide better diagnostic and predictive tools for its timely recognition and intervention and also facilitates the development of agents targeting MF.

177Lu-DOTATATE Peptide Receptor Radionuclide Therapy: Dose Response in Small Intestinal Neuroendocrine Tumors

INTRODUCTION

Peptide receptor radionuclide therapy (PRRT) has during the last few years been frequently used in patients with progressive, disseminating, well-differentiated neuroendocrine tumors (NETs).

OBJECTIVE

To study whether the absorbed dose in small intestinal NET (SI-NET) metastases from PRRT with 177Lu-DOTATATE is related to tumor shrinkage.

MATERIALS AND METHODS

Dosimetry for 1 tumor was performed in each of 25 SI-NET patients based on sequential SPECT/CT 1, 4, and 7 days after 177Lu-DOTATATE infusion. The SPECT data were corrected for the partial volume effect based on previous phantom measurements, and the unit density sphere model from OLINDA was used for absorbed dose calculations. Morphological therapy response was assessed by CT/MRI regarding tumor diameter, tumor volume, total liver tumor volume, liver volume, and overall tumor response according to RECIST 1.1. Plasma chromogranin A and urinary 5-hydroxy-indole-acetic-acid were measured during PRRT and follow-up to assess biochemical response.

RESULTS

At the time of best response with respect to tumor diameter and volume shrinkage, the median absorbed dose was 128.6 Gy (range 28.4-326.9) and 140 Gy (range 50.9-487.4), respectively. All metrics regarding tumor shrinkage and biochemical response were unrelated to the absorbed dose. A correlation was, however, found between the administered radioactivity and the tumor volume shrinkage (p = 0.01) and between the administered radioactivity and RECIST 1.1 response (p = 0.01).

CONCLUSIONS

It was not possible to demonstrate a tumor dose-response relationship in SI-NET metastases with the applied dosimetry method, contrary to what was previously shown for pancreatic NETs.

The Tumor Microenvironment in Neuroendocrine Tumors: Biology and Therapeutic Implications

Neuroendocrine tumors (NETs) include a heterogeneous group of malignancies arising in the diffuse neuroendocrine system and characterized by indolent growth. Complex interactions take place among the cellular components of the microenvironment of these tumors, and the recognition of the molecular mediators of their interplay and cross talk is crucial to discover novel therapeutic targets. NET cells overexpress a plethora of proangiogenic molecules including vascular endothelial growth factor, platelet-derived growth factor, fibroblast growth factor, semaphorins, and angiopoietins that promote both recruitment and proliferation of endothelial cell precursors, thus resulting among the most vascularized cancers with a microvessel density 10-fold higher than epithelial tumors. Also, NETs operate multifaceted interactions with stromal cells, both at local and distant sites, and whether their paracrine secretion of serotonin, connective tissue growth factor, and transforming growth factor β primarily drives the fibroblast activation to enhance the tumor proliferation, on the other side NET-derived profibrotic factors accelerate the extracellular matrix remodeling and contribute to heart valves and/or mesenteric fibrosis development, namely, major complications of functioning NETs. However, at present, little is known on the immune landscape of NETs, but accumulating evidence shows that tumor-infiltrating neutrophils, mast cells, and/or macrophages concur to promote the neoangiogenic switch of these tumors by either direct or indirect mechanisms. On the other hand, immune checkpoint molecules are heterogeneously expressed in NETs' surrounding cells, and it is unclear whether or not tumor-infiltrating lymphocytes are antitumor armed within the microenvironment, given their low mutational load. Here, we review the current knowledge on both gastroenteropancreatic and pulmonary NETs' microenvironment as well as both established and innovative treatments aimed at targeting the tumor-host interplay.

Role of the tumor microenvironment in digestive neuroendocrine tumors

Gastroenteropancreatic neuroendocrine tumors (GEP-NETs) represent a group of heterogeneous tumors whose incidence increased over the past few years. Around half of patients already present with metastatic disease at the initial diagnosis. Despite extensive efforts, cytotoxic and targeted therapies have provided only limited efficacy for patients with metastatic GEP-NETs, mainly due to the development of a certain state of resistance. One factor contributing to both the failure of systemic therapies and the emergence of an aggressive tumor phenotype may be the tumor microenvironment (TME), comprising dynamic and adaptative assortment of extracellular matrix components and non-neoplastic cells, which surround the tumor niche. Accumulating evidence shows that the TME can simultaneously support both tumor growth and metastasis and contribute to a certain state of resistance to treatment. In this review, we summarize the current knowledge of the TME of GEP-NETs and discuss the current therapeutic agents that target GEP-NETs and those that could be of interest in the (near) future.

Small intestinal neuroendocrine tumours and fibrosis: an entangled conundrum

Small intestinal neuroendocrine tumours (SI-NETs) are neoplasms characterized by their ability to secrete biogenic amines and peptides. These cause distinct clinical pathology including carcinoid syndrome, marked by diarrhoea and flushing, as well as fibrosis, notably mesenteric fibrosis. Mesenteric fibrosis often results in significant morbidity by causing intestinal obstruction, oedema and ischaemia. Although advancements have been made to alleviate symptoms of carcinoid syndrome and prolong the survival of patients with SI-NETs, therapeutic options for patients with mesenteric fibrosis are still limited. As improved insight in the complex pathogenesis of mesenteric fibrosis is key to the development of new therapies, we evaluated the literature for known and putative mediators of fibrosis in SI-NETs. In this review, we discuss the tumour microenvironment, growth factors and signalling pathways involved in the complex process of fibrosis development and tumour progression in SI-NETs, in order to elucidate potential new avenues for scientific research and therapies to improve the management of patients suffering from the complications of mesenteric fibrosis.

A potential role for CCN2/CTGF in aggressive colorectal cancer

CCN2, also known as connective tissue growth factor (CTGF) is a transcriptional target of TGF-β signaling. Unlike its original name (“CTGF”) suggested, CCN2 is not an actual growth factor but a matricellular protein that plays an important role in fibrosis, inflammation and connective tissue remodeling in a variety of diseases, including cancer. In pancreatic ductal adenocarcinoma, CCN2 signaling induces stromal infiltration and facilitates a strong tumor-stromal interaction. In many types of cancer, CCN2 overexpression has been associated with poor outcome. CMS4 (Consensus Molecular Subtype 4) is a recently identified aggressive colorectal cancer subtype, that is characterized by up-regulation of genes involved in epithelial-to-mesenchymal transition, TGF-β signaling, angiogenesis, complement activation, and extracellular matrix remodeling. In addition, a high influx of stromal fibroblasts contributes to the mesenchymal-like gene expression profile of this subtype. Furthermore, compared with the other three CMS groups, CMS4 tumors have the worst prognosis. Based on these observations, we postulated that CCN2 might contribute to colorectal cancer progression, especially in the CMS4 subtype. This review discusses the available literature on the role of CCN2 in colorectal cancer, with a focus on the ‘fibrotic subtype’ CMS4.

Epithelial derived CTGF promotes breast tumor progression via inducing EMT and collagen I fibers deposition

Interactions among tumor cells, stromal cells, and extracellular matrix compositions are mediated through cytokines during tumor progression. Our analysis of 132 known cytokines and growth factors in published clinical breast cohorts and our 84 patient-derived xenograft models revealed that the elevated connective tissue growth factor (CTGF) in tumor epithelial cells significantly correlated with poor clinical prognosis and outcomes. CTGF was able to induce tumor cell epithelial-mesenchymal transition (EMT), and promote stroma deposition of collagen I fibers to stimulate tumor growth and metastasis. This process was mediated through CTGF-tumor necrosis factor receptor I (TNFR1)-IκB autocrine signaling. Drug treatments targeting CTGF, TNFR1, and IκB signaling each prohibited the EMT and tumor progression.

Increased CCN2, substance P and tissue fibrosis are associated with sensorimotor declines in a rat model of repetitive overuse injury

Key clinical features of cumulative trauma disorders include pain, muscle weakness, and tissue fibrosis, although the etiology is still under investigation. Here, we characterized the temporal pattern of altered sensorimotor behaviors and inflammatory and fibrogenic processes occurring in forearm muscles and serum of young adult, female rats performing an operant, high repetition high force (HRHF) reaching and grasping task for 6, 12, or 18 weeks. Palmar mechanical sensitivity, cold temperature avoidance and spontaneous behavioral changes increased, while grip strength declined, in 18-week HRHF rats, compared to controls. Flexor digitorum muscles had increased MCP-1 levels after training and increased TNFalpha in 6-week HRHF rats. Serum had increased IL-1beta, IL-10 and IP-10 after training. Yet both muscle and serum inflammation resolved by week 18. In contrast, IFNγ increased at week 18 in both muscle and serum. Given the anti-fibrotic role of IFNγ, and to identify a mechanism for the continued grip strength losses and behavioral sensitivities, we evaluated the fibrogenic proteins CCN2, collagen type I and TGFB1, as well as the nociceptive/fibrogenic peptide substance P. Each increased in and around flexor digitorum muscles and extracellular matrix in the mid-forearm, and in nerves of the forepaw at 18 weeks. CCN2 was also increased in serum at week 18. At a time when inflammation had subsided, increases in fibrogenic proteins correlated with sensorimotor declines. Thus, muscle and nerve fibrosis may be critical components of chronic work-related musculoskeletal disorders. CCN2 and substance P may serve as potential targets for therapeutic intervention, and CCN2 as a serum biomarker of fibrosis progression.

Input of microenvironmental regulation on colorectal cancer: Role of the CCN family

Colorectal cancer (CRC) is a major health problem causing significant morbidity and mortality. Previous results from various studies indicate that CRC tumorigenicity encompasses tumor microenvironment, emphasizing the complex interacting network between cancer cells and nearby host cells, which triggers diverse signaling pathways to promote the growth and spread of cancer cells. The CCN family proteins share a uniform modular structure, mediating a variety of physiological functions, including proliferation, apoptosis, migration, adhesion, differentiation, and survival. Furthermore, CCN proteins are also involved in CRC initiation and development. Many studies have shown that CCN members, such as CCN1, CCN2, CCN3, Wnt-induced secreted protein (WISP)-1, WISP-2, and WISP-3, are dysregulated in CRC, which implies potential diagnostic markers or therapeutic targets clinically. In this review, we summarize the research findings on the role of CCN family proteins in CRC initiation, development, and progression, highlighting their potential for diagnosis, prognosis, and therapeutic application.

Deregulated expression of connective tissue growth factor (CTGF/CCN2) is linked to poor outcome in human cancer

Connective tissue growth factor (CTGF/CCN2) has long been associated with human cancers. The role it plays in these neoplasms is diverse and tumour specific. Recurring patterns in clinical outcome, histological desmoplasia and mechanisms of action have been found. When CTGF is overexpressed compared to low-expressing normal tissue or is underexpressed compared to high-expressing normal tissue, the functional outcome favours tumour survival and disease progression. CTGF acts by altering proliferation, drug resistance, angiogenesis, adhesion and migration contributing to metastasis. The pattern of CTGF expression and tumour response helps to clarify the role of this matricellular protein across a multitude of human cancers.

EGFR/TGFα and TGFβ/CTGF Signaling in Neuroendocrine Neoplasia: Theoretical Therapeutic Targets

Neuroendocrine neoplasms (NENs) are a heterogeneous family of malignancies whose proliferation is partially dependent on growth factors secreted by the microenvironment and the tumor itself. Growth factors which were demonstrated to be important in experimental models of NENs include EGF (epidermal growth factor), TGF (transforming growth factor) α, TGFβ and CTGF (connective tissue growth factor). EGF and TGFα bind to the EGF receptor to stimulate an intact RAS/RAF/MAPK pathway, leading to the transcription of genes associated with cell proliferation, invasion and metastasis. Theoretically, TGFα stimulation can be inhibited at several points of the MAPK pathway, but success is limited to NEN models and is not evident in the clinical setting. TGFβ1 stimulates TGFβ receptors (TGFβRI and TGFβRII) resulting in inhibition of neuroendocrine cell growth through SMAD-mediated activation of the growth inhibitor P21(WAF1/CIP1). Although some NENs are inhibited by TGFβ1, paradoxical growth is seen in experimental models of gastric and small intestinal (SI) NENs. Therapeutic targeting of TGFβ1 in NENs is therefore complicated by uncertainty of the effect of TGFβ1 secretion on the direction of proliferative regulation. CTGF expression is associated with more malignant clinical phenotypes in a variety of cancers, including NENs. CTGF promotes growth in gastric and SI-NEN models, and is implicated as a mediator of local and distant fibrosis caused by NENs of enterochromaffin cell origin. CTGF inhibitors are available, but their anti-proliferative effect has not been tested in NENs. In summary, growth factors are essential for NEN proliferation, and although interventions targeting these proteins are effective in experimental models, only limited clinical efficacy has been identified.

Connective tissue growth factor in tumor pathogenesis

Key roles for connective tissue growth factor (CTGF/CCN2) are demonstrated in the wound repair process where it promotes myofibroblast differentiation and angiogenesis. Similar mechanisms are active in tumor-reactive stroma where CTGF is expressed. Other potential roles include prevention of hypoxia-induced apoptosis and promoting epithelial-mesenchymal transistion (EMT). CTGF expression in tumors has been associated to both tumor suppression and progression. For example, CTGF expression in acute lymphoblastic leukemia, breast, pancreas and gastric cancer correlates to worse prognosis whereas the opposite is true for colorectal, lung and ovarian cancer. This discrepancy is not yet understood. High expression of CTGF is a hallmark of ileal carcinoids, which are well-differentiated endocrine carcinomas with serotonin production originating from the small intestine and proximal colon. These tumors maintain a high grade of differentiation and low proliferation. Despite this, they are malignant and most patients have metastatic disease at diagnosis. These tumors demonstrate several phenotypes potentially related to CTGF function namely: cell migration, absent tumor cell apoptosis, as well as, reactive and well vascularised myofibroblast rich stroma and fibrosis development locally and in distal organs. The presence of CTGF in other endocrine tumors indicates a role in the progression of well-differentiated tumors.

The biological hallmarks of ileal carcinoids

Endocrine tumours derived from the small intestine, ileal carcinoids, produce and secrete the hormones tachykinins and serotonin, which induces the specific symptoms related to the tumour. Because of their low proliferation rate, they are often discovered at late stages when metastases have occurred. The biology that characterizes these tumours differs in many ways from what is generally recognized for other malignancies. In this overview, the current knowledge on the development and progression of ileal carcinoids is described.