Proteasomes are not a target of doxorubicin in feline-injection site sarcoma

Doxorubicin is a potent anti-cancer agent of the anthracycline family widely used in the chemotherapeutic treatment of different tumors. The exact mechanism by which Doxorubicin induces death of neoplastic cells remains to be fully elucidated. Although anthracycline can exert cytotoxic action by directly liberating free oxygen radicals, Doxorubicin was also shown to bind and inhibit proteasomes. Notably, proteasomal inhibition is known to induce apoptosis of rapidly proliferating cells and, consistent with this notion, proteasome inhibitors are emerging as powerful tools against many tumors, especially plasma cell malignancies. Since recent work from our lab demonstrated an enhanced expression and activities of immunoproteasomes in feline injection-site sarcoma (FISS)1, a spontaneously occurring tumor of cats that is an informative model for the study of tumour biology in other species, including humans, we undertook this study to assess if clinical Doxorubicin treatment induces modulation of proteasomes level and functions in vivo. To this end, we measured proteasomal subunits expression levels and catalytic activities in tissue extracts from primary fibrosarcoma lesions and related healthy subcutis of nine cats affected by FISS who received two Doxorubicin treatments 31 and 10 days before surgery. By this approach we demonstrated that the enhanced immunoproteasomal expression and enzymatic activity characteristic of FISS is not at all affected by standard Doxorubicin administration. This unexpected finding might account for the reported low clinical effectiveness of such a treatment in FISS and provides the rationale for developing new therapeutic protocols aimed at achieving a better proteasomal inhibition in this and others poorly Doxorubicin-sensitive tumors.