Monocytes as Endothelial Progenitor Cells (EPCs), Another Brick in the Wall to Disentangle Tumor Angiogenesis

Abstract

Bone marrow contains endothelial progenitor cells (EPCs) that, upon pro-angiogenic stimuli, migrate and differentiate into endothelial cells (ECs) and contribute to re-endothelialization and neo-vascularization. There are currently no reliable markers to characterize EPCs, leading to their inaccurate identification. In the past, we showed that, in a panel of tumors, some cells on the vessel wall co-expressed CD14 (monocytic marker) and CD31 (EC marker), indicating a putative differentiation route of monocytes into ECs. Herein, we disclosed monocytes as potential EPCs, using in vitro and in vivo models, and also addressed the cancer context. Monocytes acquired the capacity to express ECs markers and were able to be incorporated into blood vessels, contributing to cancer progression, by being incorporated in tumor neo-vasculature. Reactive oxygen species (ROS) push monocytes to EC differentiation, and this phenotype is reverted by cysteine (a scavenger and precursor of glutathione), which indicates that angiogenesis is controlled by the interplay between the oxidative stress and the scavenging capacity of the tumor microenvironment.Over the last decade, different studies reported EPCs as essential in restoring injured vessels. EPCs belong to a subset of cells, arising from hematopoietic stem cells in bone marrow; upon pro-angiogenic stimuli, they proliferate, migrate, and differentiate into endothelial cells (ECs) [4][5][6]. Some reports addressing EPCs and disease, such as systemic sclerosis, showed contradictory and discrepant results about EPCs mobilization and differentiation; in part, because there is a lack of a precise panel of cell surface markers used for the characterization of this subset of cells [4][5][6][7][8][9][10]. In mouse embryonic vascular endothelium, erythro-myeloid progenitors (EMPs) can differentiate into ECs [11] and in a mouse model of carotid injury, monocytes (CD14 + cells) are capable of improving re-endothelialization [12]. In vivo and in vitro targeting of Tie2-monocytes decreases angiogenesis by abrogating EC proliferation [13][14][15] and an in vivo CCR2 (chemokine (C-C motif) receptor 2) knockout impairs monocytes recruitment and VEGFA (also named VEGF, vascular endothelial growth factor) expression, accompanied by a reduction in the angiogenesis rate [16]. The release of cytokines and pro-angiogenic factors (e.g., VEGFA, VEGFC, and VEGFD, TNFα (tumor necrosis factor α), IL-8 (interleukin-8), and FGF-2 (fibroblast growth factor-2), and extracellular matrix (ECM) modifying proteins (e.g., metalloproteinase-9 (MMP-9)) by macrophages enhances the tissue's ability to support capillary sprouting and vascular density [17,18]. The precise mechanism by which monocytes influence angiogenesis in tissue development, homeostasis, and diseases is not fully understood. However, different studies, have shown that under in vitro pro-angiogenic pressure, blood mononuclear cells can acquire endothelial markers and morphology [19][20][21]. In addition, in a previous study, we showed that some ECs sim...

Anti-Angiogenic Therapy: Current Challenges and Future Perspectives

“…VEGF-dependent mechanisms promote the generation of an immunosuppressive tumor microenvironment (TME), favoring cancer immune escape and cancer progression [ 133 ]. VEGF favors monocyte and macrophage recruitment to the TME [ 134 , 135 ], but it inhibits the differentiation and proliferation of CD8 + cytotoxic T-cells [ 136 , 137 ]. VEGFR-2 activation by VEGF on CD8 + cells induces the upregulation of immune checkpoint molecules, such as PD-1, TIM-3 (T-cell immunoglobulin mucin receptor 3), and CTLA-4 (cytotoxic T lymphocyte antigen 4), leading to cytotoxic T-cell exhaustion [ 138 ].…”

The Activation of Endothelial Cells Relies on a Ferroptosis-Like Mechanism: Novel Perspectives in Management of Angiogenesis and Cancer Therapy

“…Briefly, monocytes cultured for 4 days in colonyforming unit (CFU) medium (130-091-277, MACS Technology) and for 1 day in complete EBM-2 were incubated with von Willebrand factor (vWF; 1:500 in 0.5% BSA-0.1% saponin-PBS; A0082, Dako), for 60 min at 4°C with gentle shaking, followed by the incubation with Alexa Fluor 488 anti-rabbit, for 30 min at 4°L C in the dark, with gentle shaking. H 2 O 2 (15 mM) was used as a promoter of monocytes differentiation into ECs (35). vWF expression was detected by flow cytometry in a FACScalibur-Becton and data were analyzed using the FlowJo X v10.0.7 software (https://www.flowjo.com/).…”

Progressive Reinvention or Destination Lost? Half a Century of Cardiovascular Tissue Engineering

“…Yet, macrophages hold center-stage in this field, having been shown to transition into fibroblasts ( 85 ), myofiboblasts ( 86 , 87 ), and vascular smooth muscle cells ( 88 – 91 ). The boundary between stem cell / progenitor cell differentiation and post lineage trans-differentiation is still blurred as the role of circulating monocytes as endothelial progenitor cells underscores ( 92 ). The acknowledgment of trans-differentiation went hand in hand with that of the role of biomechanics ( 93 ).…”

“…At the same time, the issue of the cell source also remained a challenge for two-stage culture-based approaches. Attempts to optimize the ease of harvest and the yield followed again in the footsteps of the previous era by either trying allogenic cells ( 276 ) or different autologous cell sources like bone marrow cells ( 275 ), mesenchymal stem cells ( 277 ), umbilical cord blood derived EC progenitor cells ( 278 ), prenatally harvested progenitor cells ( 279 ), human induced pluripotent stem cells (hiPSCs) ( 280 ) endothelial progenitor cells (EPC) ( 92 ) including reprogrammed and reconditioned cells ( 281 ). Different from previous eras, however, significantly more resources and internationally attracted manpower had characterized the “Boston era.” Eventually, fellows of John Mayer's group carried the program to other institutions in the early 2000s: Simon Hoerstrup to Zurich ( 282 ) [later on also collaborating with Frank Baaijens' group in Eindhoven ( 283 )], Toshiharu Shin'oka to Tokyo ( 156 , 157 ) and Christopher Breuer to Yale (later Ohio State) ( 160 , 284 ).…”