Keywords

In the long way of cancer research, many studies were carried out by the scientific-medical community. Day by day, new results of studies clarify many questions, but in turn new questions arise that need to be clarified to improve and direct new therapies [16]. The tumor microenvironment (and all cellular elements that compose it) is considered as determinant for cancer development and progression and has been exhaustively evaluated by many authors [1, 2, 8, 9, 13, 20, 26, 32, 35, 37, 39, 41, 44, 45]. Endothelial cells are involved with tumor development/progression, due to its close proximity to the primary constituent element of the tumor and serving as a pavement for the oxygen and biochemical transport. These cells also act as a barrier and stimulus for cellular migration, together with one or several circulating tumor cells, giving them the advantage to start a neovasculature directly inside the blood vessel.

In recent years, circulating endothelial cells (CECs) have materialized as markers of vascular damage. Although they are present in healthy individuals, they increase in cardiovascular diseases, vascular infections, vasculitis, and type 2 diabetes. Furthermore, these cells are predictive factors of a possible cardiovascular disease in patients with coronary cancer and in patients with chronic hemodialysis treatment. Other studies related endothelial damage in women with a history of pre-eclampsia (Tuzcu et al. 2015). These cells are also seen to be increased in patients with cancer, inflammatory, infectious, ischemic, and autoimmune processes such as systemic lupus erythematosus [7, 12, 22].

The development of new blood vessels, or neovascularization, is necessary for embryonic development and stimulation of injured tissues, but also promotes the growth of tumors and inflammatory diseases [11, 40]. Vascular and lymphatic endothelial cells are activated by pro-angiogenic growth factors such as vascular endothelial growth factor (VEGF), which stimulates the proliferation and migration of endothelial cells, promoting the formation of new vessels [17, 21, 31, 36].

In 1997, a research team described for the first time the bone marrow-derived circulating endothelial progenitor cells (EPCs) [4]. Subsequently, studies showed that endothelial cells would eventually ascend from cells derived from the bone marrow. They also demonstrated that endothelial cells derived from human bone marrow could infiltrate tumors and contribute to the angiogenesis [6, 33].

It has recognized that postnatal neovascularization is stimulated by proliferation and in situ migration of pre-existing endothelial cells (ECs). It is also evident that EPC would be housed in neovascularization sites and differentiate in EC in situ (vasculogenesis), well described for embryonic and postnatal neovascularization [5].

Studies have proposed that neovascular ECs are produced from bone marrow stem cells or tumors that express VEGF receptor 2 (VEGFR-2+) [3, 19, 23, 29, 34, 42, 43].

The amounts of CEC and EPC, kinetics, and viability can be measured by positive enrichment by immune beads and flow cytometry. However, since there is no one specific antigen for endothelial cells, a multiparametric analysis is necessary [7, 15, 46].

Nakajima and colleagues isolated endothelium from surgical specimens of pancreatic cancer and normal pancreas by magnetic selection. The primary culture of tumor CEs was confirmed by positive expression of endothelial markers, CD31 and ERG1. The cells showed short vessel formations and capillary network initiation, revealing little angiogenic vigor, in addition, peripheral blood lymphocytes exhibiting fewer adherences to the tumor CE [30]. Preclinical and clinical studies revealed that circulating endothelial progenitor cells (EPCs) are incorporated in centers of physiological or pathological neovascularization as in tumor vessels [5, 7], usually at low frequencies. They also suggested that EPCs are crucial in the vasculogenesis as well in the later stages of cancer. For this reason, anti-angiogenic drugs could, in principle, prevent the growth of cancer [7, 10, 28, 47].

Among the obstacles to the success of immunotherapy for the cure, there is the fact that cancer patients develop resistance to the immune response. Possibly this is due to phenomena such as the deployment of tumor-associated antigens or tumor secretions and/or the use of endothelium associated with tumors that could act as a guardian of the infiltration of immune cells in the tumor [30].

Circulating endothelial cell clusters would originate from the tumor vasculature, and it has hypothesized that the count of the clusters will decrease after tumor resection. To test this, a study collected samples and data from 17 patients with colorectal cancer before and after surgical resection of the tumor (n = 34 samples in total). The results indicated that tumor resection significantly decreased the number of these circulating endothelial cell clusters, supporting that these structures are derived from the tumor. Furthermore, it would indicate that the clusters of these cells are not produced from the peripheral circulation by the growth of single circulating endothelial cells, but that they would be released as groups of the tumor vasculature [14].

A study conducted with 42 patients with gastric cancer showed that the number of endothelial progenitor cells (EPCs) and endothelial cells (ECs) in patients with stage III was higher than in stages I and II. The number of EPC in patients in stage IV was reduced, while the number of EC increased significantly compared to those in patients in stages I, II, or III. In addition, the number of EPC decreased in patients with tumors that had not invaded the serosa or with distant metastases. In addition, the number of EPC and EC in patients with lymph node metastasis increased significantly compared to patients without metastases. This would indicate that EPC could be involved in lymph node metastasis in gastric cancer. This study hypothesizes that EPCs are involved in angiogenesis in stages I and II, CE and EPCs are linked in angiogenesis in stage III, and EC would be the main cell involved in angiogenesis in stage IV. Factors such as hypoxia, neovascularization, and cell adhesion molecules stimulate the recruitment of EPC [25]. EPCs have demonstrated their promising value as markers of tumor diagnosis in renal cell and lung adenocarcinoma [25, 27], breast cancer [18, 38], and colorectal cancer [24]. It has been found that adrenomedullin receptor antagonists achieve targeted therapy of pancreatic and renal tumors in mice by inhibiting the mobilization of tumor endothelial cells and EPC [25].

As demonstrated here, knowledge about EPC, CEC, associated angiogenic factors, inhibitory factors of endogenous angiogenesis, and synthetic inhibitors of exogenous angiogenesis may encompass angiogenic inhibition therapy and may be a promising anticancer treatment (Fig. 10.1). Studies are needed to investigate the factors that affect the mobilization, migration, and differentiation of EPC and CEC in different clinical stages.

Fig. 10.1
figure 1

Cytokines secreted by the tumor activate the bone marrow cells, resulting in the mobilization of subsets of EPCs from the bone marrow en route to the tumor bed in response to chemotaxis. Subsequently, EPCs enter the blood and interact with the wall of the blood vessels; this interaction activates integrins which mediate intercellular adhesion and facilitate transendothelial migration of the EPCs to the tumor. Both integrins and proteases are essential for tissue invasion. EPCs differentiate into mature endothelial cells in three steps: (i) integrin-mediated adhesion to the extracellular matrix, (ii) production of paracrine/juxtacrine factor, and (iii) expression of genes that regulate endothelial maturation. EPCs regulate the angiogenic process through the paracrine secretion of pro-angiogenic factors and provide a structural function to the new vessels

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