Serum concentrations of VEGF-A, VEGF-C, ANGPT1 and ANGPT2 are increased in patients with mastocytosis compared to healthy controls. Some of these mediators such as VEGF-A [21, 31-34], VEGF-C [21, 29], and ANGPT1 and ANGPT2 [35] are expressed by human primary and neoplastic (e.g., LAD2, HMC-1) mast cells. There is a clinical correlation between the severity of mastocytosis and the plasma levels of these mast cell-derived mediators. In fact, circulating levels of VEGF-A, ANGPT2 and VEGF-C are increased in symptomatic, but not asymptomatic mastocytosis patients. Interestingly, the serum concentration of ANGPT1, which is mainly produced by pericytes and inhibits endothelial cell permeability [63], is increased in all mastocytosis patients.
The angiopoietin (ANGPT) family is an important group of factors, specific for vascular endothelium, whose functions are mediated through two tyrosine kinase receptors, Tie1 and Tie2 [43, 63]. The ANGPT-Tie ligand-receptor system exerts a key role in regulating vascular integrity [68, 69]. Beside their roles in the regulation of angiogenesis [62, 70] and lymphangiogenesis [71, 72], ANGPTs also modulate inflammation in several disorders [39, 48, 69, 73]. ANGPT1, produced by peri-endothelial mural cells (pericytes) [74] and immune cells [35, 75], is a potent agonist of Tie2 receptor on endothelial cells [44, 70]. ANGPT1 is an anti-inflammatory molecule that maintains vascular integrity [68, 76, 77]. ANGPT2, stored in Weibel–Palade bodies in endothelial cells [78] is considered a pro-inflammatory molecule [61, 79]. ANGPT2 inhibits ANGPT1/Tie2 interaction [62, 63], resulting in vascular instability and leakage [61].
It has been demonstrated that ANGPT1 inhibits the in vitro activation of the mouse mastocytoma cell line P815 and experimental anaphylactic shock in mice [80]. Anaphylaxis and anaphylactoid reactions are more frequent in mastocytosis patients compared to the general population [81]. It is possible to speculate that the increase in circulating ANGPT1 in all patients with mastocytosis might represent a protective factor in counterbalancing the vasopermeability effect of VEGF-A [41, 42, 82] and ANGPT2 [61].
Tryptase is a serine protease highly expressed by human mast cells and to a minor extent by basophils [83, 84]. Measurements of tryptase levels in serum have been used to assess mast cell load in systemic mastocytosis [64, 85-87]. In this study, we found that serum concentrations of tryptase are increased in indolent and advanced mastocytosis. Tryptase concentrations are positively correlated to the circulating levels of ANGPT2 and negatively correlated to VEGF-C. The latter observation is difficult to reconcile because there is evidence that activated human mast cells release both tryptase and VEGF-C [21, 88]. Moreover, tryptase serum concentrations are not correlated to the levels of VEGF-A and ANGPT1. Again, these results are rather unexpected because VEGF-A [28-34] and ANGPT1 [35] are expressed by human mast cells. However, many other immune and non-immune cells can produce and release VEGF-A [89-91] and ANGPT1 [63].
This study also examined the differential expression of several mediators in indolent and advanced mastocytosis. There is compelling evidence that mastocytosis is a heterogeneous condition with strikingly different prognostic profiles [2, 92]. Serum concentrations of tryptase, VEGF-A and ANGPT1 are increased in indolent and advanced mastocytosis compared to healthy controls. However, the lack of correlation between tryptase and both VEGF-A and ANGPT1 might indicate that alternative sources of the two latter mediators are involved in mastocytosis. This observation suggests that there are complex cellular and biochemical alterations in mastocytosis, in addition to the proliferation of mast cells.
ANGPT2, which is released mainly by endothelial cells [78], and ANGPT1/ANGPT2 ratio, an index of vascular permeability [53], are increased only in advanced mastocytosis. We found that serum concentrations of ANGPT2 are correlated to those of tryptase. The latter correlation might indicate that in patients with advanced mastocytosis these mediators are mainly derived from activated mast cells. These results are in line with those of our previous work in which we demonstrated that an endothelial dysfunction is detectable in patients with mastocytosis and is more severe in patients with high tryptase levels and advanced disease. Endothelial function appears to be negatively influenced by MC proliferation rather than by the severity of mediator-related symptoms [93].
VEGF-C and VEGF-D are the most important modulators of inflammatory and tumor lymphangiogenesis [94, 95] acting on VEGF receptor 3 (VEGFR-3) on LECs [38, 96]. These factors can be detected [66] and can be produced by activated human mast cells [21, 39]. Our results indicate that the serum concentrations of VEGF-C but not VEGF-D are markedly increased in patients with mastocytosis compared to healthy controls. The differential alterations of VEGF-C and VEGF-D in these patients is intriguing but not surprising. Recent evidence indicates that VEGF-C and VEGF-D can differently modulate the immune system [94]. The possible role of VEGF-C in mastocytosis deserves further investigations.
Human mast cells constitutively express VEGF receptors and Tie receptors for ANGPTs [28, 29, 35]. These receptors are functionally active because VEGFs [29] and ANGPT1 exert a chemotactic effect on human mast cells [35]. In this scenario, one may envisage a novel autocrine-loop involving angiogenic factors (i.e., VEGFs, ANGPTs) and their receptors on mast cells. In fact, VEGFs and ANGPT1 released by activated mast cells might attract progenitors of these cells to sites of neoplastic growth through the engagement of VEGFs and Tie2 receptors, respectively.
There is compelling evidence that human mast cells are a major source of several canonical (VEGF-A, ANGPTs) [21, 31-35] and non-canonical angiogenic factors (LTC4, LTD4, tryptase) [97, 98]. Valent and collaborators demonstrated that bone marrow microvessel density (MVD) is increased in patients with mastocytosis [99]. Moreover, BM MVD was significantly higher in systemic mastocytosis compared to cutaneous mastocytosis and healthy controls. Immunohistochemical staining revealed expression of VEGF-A in mast cell infiltrates. The same group of investigators extended the previous observation to canine mastocytosis by demonstrating the presence of VEGF in primary dog mastocytomas by immunohistochemistry and VEGF mRNA by PCR [100].
We have previously shown by immunohistochemistry that HLMCs contain VEGF-A, VEGF-C, and VEGF-D [66]. In this study, we confirm that immunoreactive VEGF-A is present in HLMCs and can be spontaneously released. We also examined the content and spontaneous release of several angiogenic factors in ROSA mast cell lines with (ROSAKIT D816V) and without KIT mutation (ROSAKIT WT) [59]. Both ROSAKIT WT and ROSAKIT D816V contained and spontaneously released VEGF-A, VEGF-C, and ANGPT1. These findings agree with previous observations that the histamine content and FceRI expression did not differ between both ROSAKIT WT and ROSAKIT D816V cell lines [59].
MicroRNA (miRNAs) are a large class of single-stranded RNA molecules that regulate a wide spectrum of cellular functions [101, 102]. Several miRNAs modulate different genes during mast cell activation [101, 103, 104] and the expression of pro- and anti-angiogenic factors [102, 105]. In particular, miR-221 and miR-222, highly expressed in endothelial cells, are upregulated during mast cell activation [103] and modulate the secretion of cytokines [106] and angiogenesis [105]. Therefore, it would be of interest to evaluate the expression of miR-221/222 in patients with different variants of mastocytosis. We cannot exclude the possibility that complex interplay between miR-221/222 and angiogenic/lymphangiogenic factors could contribute to the neoplastic progression of mastocytosis.
This study has a limitation that should be pointed out. Although more than 90% of patients with systemic mastocytosis have a mutation in codon 816 of KIT (KIT D816V) [5, 58], alternative KIT mutation in codon 816 (e.g., D816A/F/H/I/N/T/Y) have been described. In addition, to the tyrosine kinase domain (exons 17 and 18; e,g, D820G or N8221/K), at least 30 different KIT mutations have been identified in the extracellular (exon 8-9), transmembrane (exon 19; e.g., F522C) and juxtramembrane domains (exon 11; e.g., V560 G/I) in a small percentage of mastocytosis patients [107-110]. In this study, the identification of KIT D816V was not performed in all patients examined. In addition, other less common mutations were not investigated.
Mastocytosis is a heterogeneous group of neoplastic disorders characterized by complex pathology, distinct subtypes, and highly variable clinical courses [2, 8, 9]. Our findings indicate that VEGF and ANGPT concentrations are increased in patients with mastocytosis compared to controls. Several studies have shown that in addition to activating KIT mutations, additional mutations in other genes may occur in mastocytosis [107-109]. The contribution of KIT and other mutations to the altered production of VEGFs and ANGPTs in patients with different forms of mastocytosis remains to be investigated. In addition, further studies on larger cohorts of patients with different variants of mastocytosis could highlight the theragnostic significance of VEGF and ANGPT assays in these patients. Finally, classical and novel inhibitors of angiogenesis and/or lymphangiogenesis alone or in combination with other anti-neoplastic drugs, are used in the treatment of cancer [63]. The current treatment options for patients with advanced mastocytosis need to be improved [2, 92]. Perhaps, the use of angiogenic/lymphangiogenic inhibitors could be considered for the treatment of selected patients with severe mastocytosis and high levels of circulating angiogenic factors.