Clinicopathological significance of platelet-derived growth factor (PDGF)-B and vascular endothelial growth factor-A expression, PDGF receptor-β phosphorylation, and microvessel density in gastric cancer

Abstract

Background: Angiogenesis is important in the growth and metastasis of various kinds of solid tumors, including gastric cancers. The angiogenic process is triggered by several key growth factors, including vascular endothelial growth factor (VEGF)-A and platelet-derived growth factor (PDGF)-B, that are secreted by tumors. Our aim was to define: i) the expression pattern of VEGF-A and PDGF-B in tumor cells and the activation of PDGF receptor (PDGFR)-β tyrosine kinase in stromal cells of human gastric adenocarcinomas; and ii) the relationship between VEGF-A and PDGF-B expression and microvessel density (MVD), to determine if there is a rationale for a new therapeutic strategy.

Methods: A series of 109 gastric adenocarcinoma cases that had undergone surgical resection was examined immunohistochemically using antibodies against VEGF-A, PDGF-B, and CD34, followed by further examination of PDGFR-β phosphorylation by immunoblotting analysis.

Results: MVD was higher in diffuse-type than intestinal-type cancers (p < 0.001). VEGF-A overexpression correlated to PDGF-B overexpression in both the intestinal-type (p < 0.005) and diffuse-type (p < 0.0001) groups, indicating that VEGF-A and PDGF-B are secreted simultaneously in the same tumor, and may thus play important roles together in angiogenesis. However, several differences between intestinal-type and diffuse-type cancers were observed. In the diffuse-type cancer group, higher MVD was related to the PDGF-B proportion (p < 0.05) and VEGF-A overexpression (p < 0.05), but not to PDGF-B overexpression or the VEGF-A proportion. On the other hand, in the intestinal-type cancer group, higher MVD was correlated to overexpression (p < 0.005), intensity (p < 0.05), and proportion (p < 0.05) of PDGF-B, but not of VEGF-A. In addition, phosphorylation of PDGFR-β was correlated with depth of cancer invasion at statistically significant level.

Conclusions: Our results indicate that PDGF-B, which is involved in the maintenance of microvessels, plays a more important role in angiogenesis in intestinal-type gastric carcinomas than VEGF-A, which plays a key role mainly in the initiation of new blood vessel formation. In contrast, VEGF-A has a critical role for angiogenesis more in diffuse-type cancers, but less in those of intestinal type. Thus, a therapy targeting the PDGF-B signaling pathway could be effective for intestinal-type gastric carcinoma, whereas targeting VEGF-A or both VEGF-A and PDGF-B signaling pathways could be effective for diffuse-type gastric carcinomas.

Figure 1

Microvessel density staining for CD34 in intestinal type gastric cancer. A hot-spot with low angiogenesis (a) contains fewer positive staining (arrow) than those of hot spot with intermediate (b) or high (c) angiogenesis. The vascular grade of the tumor would be intermediate or grade 2, if the three hot-spots were from the same tumor.

Figure 2

Immunohistochemical staining of VEGF-A, PDGF-B, PDGFR-β and CD34 with hematoxylin and eosin staining (HE) in representative tissue sections obtained from human gastric carcinoma. According to the presence/absence in overexpression of VEGF and PDGF, as measured by a summation score of staining intensity and proportion of positive staining cells for VEGF-A and PDGF-B, cases were classified to 4 types: Type1, VEGF-A+/PDGF-B+; Type2, VEGF-A+PDGF-B-; Type3, VEGF-A-/PDGF-B+; Type4, VEGF-A-/PDGF-B-. Immunoreactivity for VEGF-A is detected predominantly in the cytoplasm of the carcinoma cells (arrow), whereas PDGF-B reactivity is strong in the nuclei and weak in the cytoplasm (arrow head). Staining for PDGFR-β was seen in many pericytes (outline arrow) of PDGF-B overexpressing carcinomas, implying that PDGF produced by cancer cells caused increased pericyte coverage around vessels.

Figure 3

Protein levels of PDGF-B, PDGFR-β, and p-PDGFR-β evaluated by immunoblotting analysis in representative two cases (G1, PDGF-B overexpression -/p-PDGFR-β-; G27, PDGF-B overexpression +/p-PDGFR-β +) of tumor and paired normal tissue. Porcine aortic endothelial cells expressing PDGFR-β (PAE-β), was stimulated by PDGF-B and used as positive control for phosphorylation of PDGFR-β. N, normal tissue; C, cancer tissue.