Mast cell (MC) tumors are frequent neoplasms in dogs (Macy, 1985; London and Seguin, 2003; Misdorp, 2004). These tumors are often detected in the skin, but can also develop in and metastasize into other (visceral) organs (Macy, 1985; London and Seguin, 2003; Misdorp, 2004).
The biological behavior of canine mastocytomas varies considerably among patients (Rogers, 1996; Govier, 2003; London and Seguin, 2003; Cahalane et al., 2004; Misdorp, 2004). Metastatic and progressive MC tumors usually exhibit a poor response to conventional antineoplastic drugs and thus have an unfavorable prognosis (Rogers, 1996; Govier, 2003; London and Seguin, 2003; Cahalane et al., 2004; Misdorp, 2004). Recent data suggest that neoplastic canine MC frequently express a constitutively active KIT receptor, which is often associated with mutations, deletions or duplications in the c-kit proto-oncogene (London et al., 1999; Reguera et al., 2000; Jones et al., 2004; Riva et al., 2005). However, not all MC tumors exhibit c-kit mutations. In the present study, expression of the KIT protein in canine neoplastic MC was demonstrable by ICC, IHC and flow cytometry. Apart from KIT and its ligand, stem cell factor (SCF), other factors and cytokines (and their receptors) may be involved in the regulation of growth and survival of neoplastic MC (Folkman and Klagsbrun, 1987; Gruber et al., 1995; Blair et al., 1997; Hermes et al., 2001; Levi-Schaffer and Pe¿er, 2001; Norrby, 2002; Nienartowicz et al., 2006).
Vascular endothelial growth factor (VEGF) is a major regulator of angiogenesis and a potential autocrine growth factor for neoplastic cells in various malignancies (Siemeister et al., 1998; Neufeld et al., 1999; Veikkola et al., 2000; Giles, 2001; Gerber and Ferrara, 2003; Shinkaruk et al., 2003).
In the present study, expression of VEGF and VEGF receptors has been investigated in primary canine neoplastic mast cells (MC) and the canine mastocytoma cell line C2. As assessed by immunostaining of tissue sections and cytospin slides, primary neoplastic MC and C2 cells were found to express the VEGF protein. In Northern blot and RT-PCR experiments, C2 cells expressed VEGF mRNA in a constitutive manner. VEGF mRNA expression in C2 cells was counteracted by LY294002 and rapamycin, suggesting involvement of the PI3-kinase/mTOR pathway. Moreover, C2 cells were found to express VEGF receptor-1 (Flt-1) and VEGF receptor-2 (KDR). However, recombinant VEGF failed to promote 3H-thymidine uptake in C2 cells, and a neutralizing anti-VEGF antibody (bevacizumab) failed to downregulate spontaneous proliferation in these cells. In addition, rapamycin decreased the expression of VEGF in C2 cells at the mRNA and protein level without suppressing their proliferation. Together, canine mastocytoma cells express VEGF as well as VEGF receptors. However, despite co-expression of VEGF and its receptors, VEGF is not utilized as an autocrine growth regulator by canine mastocytoma cells.