Expression of the JCV early protein T-antigen in transgenic mice leads to the development of cerebellar primitive neuroectodermal tumors (PNETs). In light of earlier reports on the association of JCV with PNETs in humans and the involvement of the Wnt signaling pathway in the development of cerebellar tumors, we investigated the interplay between T-antigen and bcatenin, the key protein of the Wnt pathway. Our results demonstrate the physical interaction of T-antigen with bcatenin through the central domain of T-antigen spanning residues 82-628, and the C-terminus of b-catenin located between amino acids 695 and 781. The association of Tantigen with b-catenin elevates the level of b-catenin in the cells due to increased in the stability of the protein. In the presence of T-antigen, b-catenin was found in the nuclei of cells, suggesting that the interaction of b-catenin with T-antigen facilitates its nuclear import. In cells expressing mutant T-antigen with no nuclear localization domain, b-catenin was found in the cytoplasm. Coexpression of T-antigen with b-catenin increased the transcription of the c-myc promoter, a known downstream target of b-catenin, and artificial promoter whose activity is bcatenin dependent. These observations ascribe a new oncogenic pathway for T-antigen, and offer an alternative mechanism for the deregulation of the Wnt pathway through stabilization of b-catenin upon its association with the viral oncoprotein.

By using the early genome of the human neurotropic polyomavirus, JCV, we have created transgenic animals that develop cerebellar primitive neuroectodermal tumors which model human medulloblastoma. Expression of T-antigen was found in some, but not all, tumor cells, and examination of the clonal cell lines derived from the tumor population showed enhanced tumorigenicity of cells expressing T-antigen in comparison to T-antigen negative cells. Considering the earlier notion on the potential involvement of beta-catenin with human medulloblastoma, we investigated various components of the Wnt signaling pathway including beta-catenin, its partner transcription factor, LEF-1, and their downstream target gene c-myc in these two cell populations. Immunohistochemical staining of the cells revealed enhanced nuclear appearance of beta-catenin in T-antigen positive cells. Results from Western blot showed higher levels of beta-catenin and LEF-1 in T-antigen positive cells in comparison to those in T-antigen negative cells. The enhanced level of LEF-1 expression correlated with the increase in DNA binding activity of this protein in nuclear extracts of T-antigen positive cells. Results from Northern and Western blot analyses revealed that the level of c-myc expression is augmented both at the RNA and protein levels in T-antigen positive cells. These observations corroborated results from transfection studies indicating the ability of JCV T-antigen to stimulate c-myc promoter activity. Further, co-transfection experiments revealed that the amount of c-myc and T-antigen protein in tumor cells may dictate the activity of JCV early promoter in these cells. These observations are interesting in light of recent discoveries on the association of JCV with human medulloblastoma and suggest that communication between JCV and the Wnt pathway may be an important event in the genesis of these tumors.

No abstract

No abstract