MDM-2 oncoprotein overexpression, p53 gene mutation, and VEGF up-regulation in angiosarcomas

Abstract

The endothelium is one of the largest cellular compartments of the human body and has a high proliferative potential. However, angiosarcomas are among the rarest malignancies. Despite this interesting contradiction, data on growth and angiogenesis control mechanisms of angiosarcomas are scarce. In this study of 19 angiosarcomas and 10 benign vascular control lesions we investigated the sequence and expression of the p53 tumor suppressor gene and the expression of the mdm-2 proto-oncogene, which is a negative regulator of p53 activity and of the vascular endothelial growth factor (VEGF), whose expression, among other factors, is regulated by the p53/MDM-2 pathway. Ten sarcomas (53%) exhibited clear nuclear p53 protein accumulation. Two of these cases revealed mutations in the sequence-specific DNA binding domain of the p53 gene. Thirteen angiosarcomas (68%) showed an increased amount of MDM-2 protein. Elevated expression of p53 and MDM-2 protein correlated with increased VEGF expression, which was found in nearly 80% of the angiosarcoma cases. Negative or clearly lower immunostaining was obtained in cases from the benign control collective. Only one case of a juvenile hemangioma reached the cutoff value of p53 positivity coincidentally with high VEGF expression. Our data suggest that the p53/ MDM-2 pathway is impaired in about two-thirds (14/ 19) of the angiosarcomas. This may be a key event in the pathogenesis of human angiosarcomas. The increased VEGF expression observed supports this hypothesis.

Figure 1.

Morphological pattern of angiosarcomas. A: Angiosarcoma (case 18). H&E Magnification, ×265. B: Angiosarcoma (case 8). H&E Magnification, ×265. C: Angiosarcoma (case 12). H&E Magnification, ×265. D: Angiosarcoma (case 10). H&E Magnification, ×165

Figure 2.

Immunohistochemical expression pattern of p53, MDM-2 protein and VEGF. A: Angiosarcoma (case 1), immunolocalization of p53. Magnification, ×180. B: Angiosarcoma (case 17), immunolocalization of p53. Magnification, ×310. C: Angiosarcoma (case 10), nuclear immunolocalization of MDM-2. Magnification, ×180. D: Angiosarcoma (case 16), nuclear and cytoplasmic staining of MDM-2. Magnification, ×180. E: Angiosarcoma (case 16), immunolocalization of VEGF. Magnification, ×325. F: Angiosarcoma (case 1). Immunolocalization of VEGF, Magnification, ×260. G: Control (case C1), negative immunostaining for p53. Magnification, ×180. H: Control (case C7), negative immunostaining for MDM-2. Magnification, ×180.

Figure 3.

The hypothetical role of the p53/MDM-2 pathway in normal endothelium and angiosarcoma development. Expression of wt-p53 is linked to the control of angiogenesis. wt-p53 positively regulates genes involved in intercellular signaling such as thrombospondin-1, a negative regulator of angiogenesis, and it exerts negative influence on VEGF gene expression. Abrogation of wt-p53 in endothelial cells either by functional inactivation through overexpression of mdm-2 protein or by p53 mutation obviously reflects the loss of an important cellular growth control and angiosuppressive pathway in angiosarcomas. Discontinuation of wt-p53 function leads to a decrease in Thrombospondin-1 production and a decline in the negative transcriptional control of VEGF. This shifts the net balance of positive and negative angiogenic factors to neovascularisation.