Proteasome levels and activities increase in canine mammary tumors

In human tumors of distinct histology, changes in the surface expression and/or function of the MHC class I antigens are frequently found and may provide malignant cells with a mechanism to escape the control of immune system. This altered HLA class I phenotype can be caused by either structural alterations or dysregulation of genes encoding the subunits of HLA class I antigens and/or components of the MHC class I antigen-processing machinery (APM). So far abnormalities in the expression and/or function of components of APM have been investigated only in a limited number of surgically removed neoplastic lesions. Therefore we have analyzed proteasome expression and activities in canine mammary tumors to establish whether this tumor presents alterations of the MHC class I APM. MATERIALS AND METHODS: Expression leves of APM components in tissue extracts from surgically removed canine mammary tumors and healthy mammary tissues were assessed by western blot analysis; proteasome activities were measured by fluorimetric assays as described in (1). RESULTS: Analysis of proteasome activities in tissue extracts from ten different subjects shows an enhancement of the proteasomal chymotrypsin- and caspase-like activities in neoplastic lesions compared to the corresponding healthy tissue. Moreover, malignant lesions present an enhanced chimotrypsin- and caspase-like activities compared to the benign lesions. In accord with these data, western blot analysis of several components of the MHC class I APM show significant differences in the expression levels of these proteins in tumor lesions and healthy breast. Specifically, one catalytic subunit of constitutive proteasomes (Y) and one of immunoproteasomes (LMP7) are upregulated in tumors. Furthermore, also proteasome activators PA28α and β are highly over-expressed in neoplastic lesions, although at different extent in benign and malignant tumors. Altogether, these preliminary results clearly show that MHC class I APM is modified in canine mammary tumors and these alterations may potentially provide tumor cells with a mechanism to escape form T cell recognition and destruction. 1. Cascio P. et al. (2002) Properties of the hybrid form of the 26S proteasome contining both 19S and PA28 complexes. Embo J., 21, 2636-45