CD90+ mesothelial-like cells in peritoneal fluid promote peritoneal metastasis by forming a tumor permissive microenvironment

Metastasis-associated fibroblasts in peritoneal surface malignancies

Over decades, peritoneal surface malignancies (PSMs) have been associated with limited treatment options and poor prognosis. However, advancements in perioperative systemic chemotherapy, cytoreductive surgery (CRS), and hyperthermic intraperitoneal chemotherapy (HIPEC) have significantly improved clinical outcomes. PSMs predominantly result from the spread of intra-abdominal neoplasia, which then form secondary peritoneal metastases. Colorectal, ovarian, and gastric cancers are the most common contributors. Despite diverse primary origins, the uniqueness of the peritoneum microenvironment shapes the common features of PSMs. Peritoneal metastization involves complex interactions between tumour cells and the peritoneal microenvironment. Fibroblasts play a crucial role, contributing to tumour development, progression, and therapy resistance. Peritoneal metastasis-associated fibroblasts (MAFs) in PSMs exhibit high heterogeneity. Single-cell RNA sequencing technology has revealed that immune-regulatory cancer-associated fibroblasts (iCAFs) seem to be the most prevalent subtype in PSMs. In addition, other major subtypes as myofibroblastic CAFs (myCAFs) and matrix CAFs (mCAFs) were frequently observed across PSMs studies. Peritoneal MAFs are suggested to originate from mesothelial cells, submesothelial fibroblasts, pericytes, endothelial cells, and omental-resident cells. This plasticity and heterogeneity of CAFs contribute to the complex microenvironment in PSMs, impacting treatment responses. Understanding these interactions is crucial for developing targeted and local therapies to improve PSMs patient outcomes.

Mesothelial cells with mesenchymal features enhance peritoneal dissemination by forming a protumorigenic microenvironment

Malignant ascites accompanied by peritoneal dissemination contain various factors and cell populations as well as cancer cells; however, how the tumor microenvironment is shaped in ascites remains unclear. Single-cell proteomic profiling and a comprehensive proteomic analysis are conducted to comprehensively characterize malignant ascites. Here, we find defects in immune effectors along with immunosuppressive cell accumulation in ascites of patients with gastric cancer (GC) and identify five distinct subpopulations of CD45(-)/EpCAM(-) cells. Mesothelial cells with mesenchymal features in CD45(-)/EpCAM(-) cells are the predominant source of chemokines involved in immunosuppressive myeloid cell (IMC) recruitment. Moreover, mesothelial-mesenchymal transition (MMT)-induced mesothelial cells strongly express extracellular matrix (ECM)-related genes, including tenascin-C (TNC), enhancing metastatic colonization. These findings highlight the definite roles of the mesenchymal cell population in the development of a protumorigenic microenvironment to promote peritoneal dissemination.

Cytokine- and chemokine-induced inflammatory colorectal tumor microenvironment: Emerging avenue for targeted therapy

Colorectal cancer (CRC) is a predominant life-threatening cancer, with liver and peritoneal metastases as the primary causes of death. Intestinal inflammation, a known CRC risk factor, nurtures a local inflammatory environment enriched with tumor cells, endothelial cells, immune cells, cancer-associated fibroblasts, immunosuppressive cells, and secretory growth factors. The complex interactions of aberrantly expressed cytokines, chemokines, growth factors, and matrix-remodeling enzymes promote CRC pathogenesis and evoke systemic responses that affect disease outcomes. Mounting evidence suggests that these cytokines and chemokines play a role in the progression of CRC through immunosuppression and modulation of the tumor microenvironment, which is partly achieved by the recruitment of immunosuppressive cells. These cells impart features such as cancer stem cell-like properties, drug resistance, invasion, and formation of the premetastatic niche in distant organs, promoting metastasis and aggressive CRC growth. A deeper understanding of the cytokine- and chemokine-mediated signaling networks that link tumor progression and metastasis will provide insights into the mechanistic details of disease aggressiveness and facilitate the development of novel therapeutics for CRC. Here, we summarized the current knowledge of cytokine- and chemokine-mediated crosstalk in the inflammatory tumor microenvironment, which drives immunosuppression, resistance to therapeutics, and metastasis during CRC progression. We also outlined the potential of this crosstalk as a novel therapeutic target for CRC. The major cytokine/chemokine pathways involved in cancer immunotherapy are also discussed in this review.

Intraperitoneal Glucose Transport to Micrometastasis: A Multimodal In Vivo Imaging Investigation in a Mouse Lymphoma Model

Bc-DLFL.1 is a novel spontaneous, high-grade transplantable mouse B-cell lymphoma model for selective serosal propagation. These cells attach to the omentum and mesentery and show dissemination in mesenteric lymph nodes. We aimed to investigate its early stage spread at one day post-intraperitoneal inoculation of lymphoma cells (n = 18 mice), and its advanced stage at seven days post-inoculation with in vivo [18F]FDG-PET and [18F]PET/MRI, and ex vivo by autoradiography and Cherenkov luminescence imaging (CLI). Of the early stage group, nine animals received intraperitoneal injections, and nine received intravenous [18F]FDG injections. The advanced stage group (n = 3) received intravenous FDG injections. In the early stage, using autoradiography we observed a marked accumulation in the mesentery after intraperitoneal FDG injection. Using other imaging methods and autoradiography, following the intravenous injection of FDG no accumulations were detected. At the advanced stage, tracer accumulation was clearly detected in mesenteric lymph nodes and in the peritoneum after intravenous administration using PET. We confirmed the results with immunohistochemistry. Our results in this model highlight the importance of local FDG administration during diagnostic imaging to precisely assess early peritoneal manifestations of other malignancies (colon, stomach, ovary). These findings also support the importance of applying topical therapies, in addition to systemic treatments in peritoneal cancer spread.

Role of LpL (Lipoprotein Lipase) in Macrophage Polarization In Vitro and In Vivo

OBJECTIVE

Fatty acid uptake and oxidation characterize the metabolism of alternatively activated macrophage polarization in vitro, but the in vivo biology is less clear. We assessed the roles of LpL (lipoprotein lipase)-mediated lipid uptake in macrophage polarization in vitro and in several important tissues in vivo. Approach and Results: We created mice with both global and myeloid-cell specific LpL deficiency. LpL deficiency in the presence of VLDL (very low-density lipoproteins) altered gene expression of bone marrow-derived macrophages and led to reduced lipid uptake but an increase in some anti- and some proinflammatory markers. However, LpL deficiency did not alter lipid accumulation or gene expression in circulating monocytes nor did it change the ratio of Ly6Chigh/Ly6Clow. In adipose tissue, less macrophage lipid accumulation was found with global but not myeloid-specific LpL deficiency. Neither deletion affected the expression of inflammatory genes. Global LpL deficiency also reduced the numbers of elicited peritoneal macrophages. Finally, we assessed gene expression in macrophages from atherosclerotic lesions during regression; LpL deficiency did not affect the polarity of plaque macrophages.

CONCLUSIONS

The phenotypic changes observed in macrophages upon deletion of Lpl in vitro is not mimicked in tissue macrophages.

Mechanisms of Metastasis in Colorectal Cancer and Metastatic Organotropism: Hematogenous versus Peritoneal Spread

Metastasis is the major cause of death in patients with colorectal carcinoma (CRC). The most common sites of metastasis are the liver and the peritoneum. Peritoneal carcinomatosis is often considered the end stage of the disease after the tumor has spread to the liver. However, almost half of CRC patients with peritoneal carcinomatosis do not present with liver metastasis. This brings up the question of whether peritoneal spread can still be considered as the end stage of a metastasized CRC or whether it should just be interpreted as a site of metastasis alternative to the liver. This review tries to discuss this question and summarize the current status of literature on potential characteristics in tumor biology in the primary tumor, i.e., factors (transcription factors and direct and indirect E-cadherin repressors) and pathways (WNT, TGF-β, and RAS) modulating EMT, regulation of EMT on a posttranscriptional and posttranslational level (miRNAs), and angiogenesis. In addition to tumor-specific characteristics, factors in the tumor microenvironment, immunological markers, ways of transport of tumor cells, and adhesion molecules appear to differ between hematogenous and peritoneal spread. Factors such as integrins and exosomal integrins, cancer stem cell phenotype, and miRNA expression appear to contribute in determining the metastatic route. We went through each step of the metastasis process comparing hematogenous to peritoneal spread. We identified differences with respect to organotropism, epithelial-mesenchymal transition, angiogenesis and inflammation, and tumor microenvironment which will be further elucidated in this review. A better understanding of the underlying mechanisms and contributing factors of metastasis development in CRC has huge relevance as it is the foundation to help find specific targets for treatment of CRC.

Prognostic impact of initial tumor load and intraperitoneal disease dissemination patterns in patients with advanced ovarian cancer undergoing complete cytoreductive surgery

INTRODUCTION

Complete removal of disease is the most important prognostic factor for patients with advanced epithelial ovarian carcinoma. However, the influence of carcinomatosis distribution on prognosis is unknown and the prognostic impact of implant size according to their location is poorly studied. Our objective was to assess the impact of peritoneal carcinomatosis quantitative and qualitative localizations on progression free survival (PFS) in patients with advanced epithelial ovarian carcinoma (AEOC) after complete cytoreductive surgery.

METHODS

We conducted a monocentric cohort study, retrospective from October 2001 to July 2014. Inclusion criteria were high-grade AEOC patients without residual disease (CC0) after primary debulking surgery (PDS) or after interval debulking surgery (IDS) following neoadjuvant chemotherapy (NACT). Peritoneal carcinomatosis was assessed according to qualitative criteria and quantitative criteria.

RESULTS

One hundred and one patients were included. Median PFS was 21·2 months and median OS was 62·2 months. On the whole population, involvement of adipocytes-enriched areas tended to be associated with a decreased PFS and was significantly associated with a decreased OS. Any localization was associated with PFS or OS in the "IDS" subgroup. In the "PDS" subgroup, PCI score and involvement of the right mesocolic area were associated with a decreased PFS.

CONCLUSION

Initial tumor load has not been found associated with PFS after complete surgery. Adipocytes-enriched areas and right mesocolic areas involvement were associated with poor prognosis in patients receiving primary debulking surgery. Larger-scale studies are needed to assess whether initial tumor load has a prognostic impact even after complete cytoreductive surgery is achieved.

Different Features of Tumor-Associated NK Cells in Patients With Low-Grade or High-Grade Peritoneal Carcinomatosis

Peritoneal carcinomatosis (PC) is a rare disease defined as diffused implantation of neoplastic cells in the peritoneal cavity. This clinical picture occurs during the evolution of peritoneal tumors, and it is the main cause of morbidity and mortality of patients affected by these pathologies, though cytoreductive surgery with heated intra-peritoneal chemotherapy (CRS/HIPEC) is yielding promising results. In the present study, we evaluated whether the tumor microenvironment of low-grade and high-grade PC could affect the phenotypic and functional features and thus the anti-tumor potential of NK cells. We show that while in the peritoneal fluid (PF) of low-grade PC most CD56dim NK cells show a relatively immature phenotype (NKG2A+KIR-CD57-CD16dim), in the PF of high-grade PC NK cells are, in large majority, mature (CD56dimKIR+CD57+CD16bright). Furthermore, in low-grade PC, PF-NK cells are characterized by a sharp down-regulation of some activating receptors, primarily NKp30 and DNAM-1, while, in high-grade PC, PF-NK cells display a higher expression of the PD-1 inhibitory checkpoint. The compromised phenotype observed in low-grade PC patients corresponds to a functional impairment. On the other hand, in the high-grade PC patients PF-NK cells show much more important defects that only partially reflect the compromised phenotype detected. These data suggest that the PC microenvironment may contribute to tumor escape from immune surveillance by inducing different NK cell impaired features leading to altered anti-tumor activity. Notably, after CRS/HIPEC treatment, the altered NK cell phenotype of a patient with a low-grade disease and favorable prognosis was reverted to a normal one. Our present data offer a clue for the development of new immunotherapeutic strategies capable of restoring the NK-mediated anti-tumor responses in association with the CRS/HIPEC treatment to increase the effectiveness of the current therapy.

Effects of mesenchymal stem cells on solid tumor metastasis in experimental cancer models: a systematic review and meta-analysis

Background

It has been reported mesenchymal stem cells (MSCs) are recruited to and become integral parts of the tumor microenvironment. MSCs might have an active role in solid tumor progression, especially cancer metastasis. However, the contribution of MSCs in the process of cancer metastasis is still controversial. In this review, we performed a meta-analysis on the effects of MSCs administration on cancer metastasis based on published preclinical studies.

Methods

The PRISMA guidelines were used. A total of 42 publications met the inclusion criteria. Outcome data on the incidence and the number of cancer metastasis as well as study characteristics were extracted. Quality of the studies was assessed according to SYRCLE Risk of Bias tool. Random-effects meta-analysis was used to pool estimates.

Results

Of the 42 studies included, 32 reported that MSCs administration promoted outcome events (numbers or incidences of cancer metastasis), and 39 reported data suitable for meta-analysis. The median effect size (RR) was 2.04 for the incidence of cancer metastasis (95% CI 1.57–2.65, I² = 21%), and the median effect size (SMD) was 1.23 for the number of cancer metastasis (95% CI 0.43–2.03, I² = 89%). Heterogeneity was observed, with the greater impact based on study length and different ways of metastasis measurement and MSCs administration.

Conclusion

Our results suggested MSCs administration increased the number and the incidence of cancer metastasis in experimental cancer models. High heterogeneity and poor reported risk of bias limit the quality of these findings. Further preclinical studies with better design and adequate reporting are still needed.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1484-9) contains supplementary material, which is available to authorized users.

Targeting tumour microenvironment by tyrosine kinase inhibitor

Tumour microenvironment (TME) is a key determinant of tumour growth and metastasis. TME could be very different for each type and location of tumour and TME may change constantly during tumour growth. Multiple counterparts in surrounding microenvironment including mesenchymal-, hematopoietic-originated cells as well as non-cellular components affect TME. Thus, therapeutics that can disrupt the tumour-favouring microenvironment should be further explored for cancer therapy. Previous efforts in unravelling the dysregulated mechanisms of TME components has identified numerous protein tyrosine kinases, while its corresponding inhibitors have demonstrated potent modulatory effect on TME. Recent works have demonstrated that beyond the direct action on cancer cells, tyrosine kinase inhibitors (TKIs) have been implicated in inactivation or normalization of dysregulated TME components leading to cancer regression. Either through re-sensitizing the tumour cells or reversing the immunological tolerance microenvironment, the emergence of these TME modulatory mechanism of TKIs supports the combinatory use of TKIs with current chemotherapy or immunotherapy for cancer therapy. Therefore, an appropriate understanding on TME modulation by TKIs may offer another mode of action of TKIs for cancer treatment. This review highlights mode of kinase activation or paracrine ligand production from TME components and summarises the findings on the potential use of various TKIs on regulating TME components. At last, the combination use of current TKIs with immunotherapy in the perspectives of efficacy and safety are discussed.

Thy-1+/- fibroblast subsets in the human peritoneum

Fibrotic thickening of the peritoneum develops in patients receiving peritoneal dialysis (PD) for renal failure. For unknown reasons, however, in some patients it progresses to extensive fibrosis that compromises dialysis capacity of the peritoneum. It is increasingly clear that fibroblasts display large heterogeneity not only between but also within tissues. Differential surface expression of thymocyte differentiation antigen 1 (Thy-1) has been shown to identify functionally distinct fibroblast subsets in several organs. Here, we isolated Thy-1^+/- subsets of human peritoneal fibroblasts (HPFB) and analyzed them in terms of profibrotic myofibroblast features. In healthy individuals, Thy-1⁺ cells constituted ~45% of the HPFB population found in the greater omentum but were not detected in the parietal peritoneum. When propagated in culture and compared with Thy-1^- cells, omentum-derived Thy-1⁺ HPFB consistently displayed an increased expression of α-smooth muscle actin, collagen I, and transforming growth factor-β1. They also showed greater proliferation capacity and enhanced contractile properties. The number of Thy-1⁺ HPFB increased significantly in PD patients and made up more than 70 and 95% of all HPFB found in the omentum and parietal peritoneum, respectively. These data indicate that the expansion of Thy-1⁺ fibroblasts may contribute to fibrotic thickening of the peritoneal membrane during PD.

Regional immunotherapy for liver and peritoneal metastases

Pancreatic adenocarcinoma is a biologically aggressive disease, with liver and peritoneal metastases being a frequent cause of death. We examine how the pancreatic carcinoma microenvironment and immunosuppressive landscape favor tumor progression. Immunotherapy has shown promise in select solid tumors, yet challenges remain in applying these gains to stage IV pancreatic adenocarcinoma. We discuss how regional therapy strategies may be leveraged to open new avenues for treating pancreatic carcinoma metastases with immunotherapy.

Blocking TGF-β1 by P17 peptides attenuates gastric cancer cell induced peritoneal fibrosis and prevents peritoneal dissemination in vitro and in vivo

Our previous study demonstrated that the peritoneal stroma environment favors proliferation of tumor cells by serving as a rich source of growth factors and chemokines known to be involved in tumor metastasis. In this study, we investigated the interaction between gastric cancer cells and peritoneal mesothelial cells, and determined the effects of TGF-β1 in this processing. Human peritoneal tissues and peritoneal wash fluid were obtained, which examined by hematoxylin and eosin staining or ELISA for measurements of TGF-β1 levels. The peritoneal mesothelial cells were co-incubated with the supernatants of gastric cancer, the expression of TGF-β1, collagen and fibronectin was observed by ELISA and western blot. We then investigated the effects of serum-free conditioned media from HSC-39 gastric cancer cells on the peritoneum of nude mice, and the effects of peritoneal fibrosis on the development of peritoneal metastasis in vivo. The peritoneum from gastric patients were thickened and contained extensive fibrosis. After co-culture both gastric tumor cells and mesothelial cells, we found that TGF-β1 expression was greatly increased in the co-culture system compared to individual culture condition. Serum-free Conditioned Media from HSC-39 was able to induce extracellular matrix expression in vitro and in vivo, and tumorigenicity in mice with peritoneal fibrosis was greater than in mice with normal peritoneum, while blocking TGF-β1 by peptide P17 can partially inhibit these effects. In conclusion, these results indicated that the interaction of gastric cancer with peritoneal fibrosis and determined that TGF-β1 plays a key role in induction of peritoneal fibrosis, which in turn affected dissemination of gastric cancer.

Cancer-associated peritoneal mesothelial cells lead the formation of pancreatic cancer peritoneal dissemination

The interaction between the cancer cells and the peritoneal mesothelial cells (PMCs) plays an important role in the peritoneal dissemination in several types of cancer. However, the role of PMCs in the peritoneal dissemination of pancreatic cancer remains unclear. In the present study, we investigated the interaction between the pancreatic cancer cells (PCCs) and the PMCs in the formation of peritoneal dissemination in vitro and in vivo. The tumor-stromal interaction of PCCs and PMCs significantly enhanced their mobility and invasiveness and enhanced the proliferation and anoikis resistance of PCCs. In a 3D organotypic culture model of peritoneal dissemination, co-culture of PCCs and PMCs significantly increased the cells invading into the collagen gel layer compared with mono-culture of PCCs. PMCs pre-invaded into the collagen gel, remodeled collagen fibers, and increased parallel fiber orientation along the direction of cell invasion. In the tissues of peritoneal dissemination of the KPC (LSL-KrasG12D/+; LSL-Trp53R172H/+;Pdx-1-Cre) transgenic mouse, the monolayer of PMCs was preserved in tumor-free areas, whereas PMCs around the invasive front of peritoneal dissemination proliferated and invaded into the muscle layer. In vivo, intraperitoneal injection of PCCs with PMCs significantly promoted peritoneal dissemination compared with PCCs alone. The present data suggest that the cancer-associated PMCs have important promoting roles in the peritoneal dissemination of PCCs. Therapy targeting cancer-associated PMCs may improve the prognosis of patients with pancreatic cancer.

Robot-assisted ureteral reconstruction using a tubularized peritoneal flap: a novel technique in a chronic porcine model

OBJECTIVE

To evaluate the feasibility and functional outcomes in porcine models of a novel robotic surgical technique for the treatment of complex ureteral injuries and strictures.

MATERIALS AND METHODS

Six pigs underwent robotic ureteral reconstruction using a long tabularized peritoneal flap and followed for 6-9 weeks after the surgery. Ureteral flap vascularity, intra-renal pressure, patency of the conduct, endoscopic aspect of the flap, renal function and histopathology were evaluated.

RESULTS

All animals successfully underwent ureteral reconstruction using a tubularized peritoneal flap. Median operative time was 223 min (162-360). Flap tubularization suture took 31 min (19-47), and proximal anastomosis took 20 min (15-38). Bladder mobilization with psoas hitch and distal anastomosis took 9 min (7-12) and 23 min (13-46), respectively. On follow-up, significant shrinkage of the ureteral flap in both length and width was observed. Antegrade pyelograms confirmed dilation and tortuosity of the proximal ureter, dilation of the renal pelvis, and major and minor calyxes without any definitive strictures. Microscopically, focal urothelial lining was seen in the neoureter. Creatinine level was significantly higher at the end of the follow-up period (p = 0.003).

CONCLUSIONS

Robot-assisted ureteral reconstruction using a tubularized peritoneum flap is technically feasible and reproducible. The flap sustained abundant vascular supply after different intervals of follow-up and the peritoneal mesenchymal cells differentiated into urothelium and myofibroblasts. Further studies are needed to address the issue of functional obstruction to improve long-term renal function outcomes.

Intraperitoneal Mesenchymal Cells Promote the Development of Peritoneal Metastasis Partly by Supporting Long Migration of Disseminated Tumor Cells

The human peritoneal cavity contains a small number of free cells of mesenchymal cell lineage. Intraperitoneal mesenchymal cells (PMC) play supportive roles in metastasis formation on the peritoneum. In this study, we found that PMC, when co-cultuerd with human gastric cancer cells, MKN45, enhanced the proliferation of MKN45 when cultured at low, but not high, cellular density. Also, PMC suppressed apoptotic cell death of MKN45 only under low density culture conditions. Time-lapse videoanalysis clearly demonstrated that PMC randomly migrated more vigorously than did MKN45, and strongly enhanced the migration behavior of co-cultured MKN45. In fact, the majority of MKN45 migrated together in direct physical contact with PMC, and the sum of migration lengths from original position of co-cultured MKN45 for 48 hours was approximately 10 times longer than that of MKN45 cultured alone. Our data suggest that enhanced migration can increase the chance of direct contact or positional proximity among sparcely distributed MKN45, which may bring survival advantages to tumor cells. This may be one of the important mechanisms of peritoneal metastasis, since only a small number of tumor cells are considered to be disseminated in the early step of metastasis formation on the peritoneum.

Integrated Molecular Profiling of Human Gastric Cancer Identifies DDR2 as a Potential Regulator of Peritoneal Dissemination

Peritoneal dissemination is the most frequent, incurable metastasis occurring in patients with advanced gastric cancer (GC). However, molecular mechanisms driving peritoneal dissemination still remain poorly understood. Here, we aimed to provide novel insights into the molecular mechanisms that drive the peritoneal dissemination of GC. We performed combined expression analysis with in vivo-selected metastatic cell lines and samples from 200 GC patients to identify driver genes of peritoneal dissemination. The driver-gene functions associated with GC dissemination were examined using a mouse xenograft model. We identified a peritoneal dissemination-associated expression signature, whose profile correlated with those of genes related to development, focal adhesion, and the extracellular matrix. Among the genes comprising the expression signature, we identified that discoidin-domain receptor 2 (DDR2) as a potential regulator of peritoneal dissemination. The DDR2 was upregulated by the loss of DNA methylation and that DDR2 knockdown reduced peritoneal metastasis in a xenograft model. Dasatinib, an inhibitor of the DDR2 signaling pathway, effectively suppressed peritoneal dissemination. DDR2 was identified as a driver gene for GC dissemination from the combined expression signature and can potentially serve as a novel therapeutic target for inhibiting GC peritoneal dissemination.

New developments in peritoneal fibroblast biology: implications for inflammation and fibrosis in peritoneal dialysis

Uraemia and long-term peritoneal dialysis (PD) can lead to fibrotic thickening of the peritoneal membrane, which may limit its dialytic function. Peritoneal fibrosis is associated with the appearance of myofibroblasts and expansion of extracellular matrix. The extent of contribution of resident peritoneal fibroblasts to these changes is a matter of debate. Recent studies point to a significant heterogeneity and complexity of the peritoneal fibroblast population. Here, we review recent developments in peritoneal fibroblast biology and summarize the current knowledge on the involvement of peritoneal fibroblasts in peritoneal inflammation and fibrosis.

Brief Report: Human Mesenchymal Stem-Like Cells Facilitate Floating Tumorigenic Cell Growth via Glutamine-Ammonium Cycle

How mesenchymal stem cells (MSCs) promote tumor growth remains incompletely understood. Here, we show that mesenchymal stem-like cells (MSLCs) are commonly present in malignant pleural effusion or ascites of cancer patients, where they directly interact with tumor cells. Chemokines and chemokine receptors, especially the CCL2/CCR2 pathway, are involved in this interaction. As a result, MSLCs exert tumor-promoting effects by enhancing the proliferation and colony formation of tumor-repopulating cells. The underlying molecular basis involves MSLC release of glutamine to tumorigenic cells. Inhibition of glutamine uptake impedes MSC-mediated tumor-promoting effects. More intriguingly, MSLCs take up tumor cell-released ammonium that, in turn, favors MSLC growth. Thus, glutamine and ammonium form a vicious cycle between MSLCs and tumorigenic cells. These findings suggest a potential clinical application by targeting MSLCs in patients with malignant pleural effusions or ascites.