Significance of Ubiquitin Ligase Subunit SKP-2 Proto-Oncogene and Proliferative Marker Ki67 Expression in Diffuse Large B-Cell Lymphoma.

S-phase kinase-associated protein 2 (SKP-2) is a proto-oncogene that has been shown to be expressed in a number of tumors. A number of studies have shown that SKP-2 plays a role in the degradation of tumor suppressor genes by increased proteosome-dependent degradation. SKP-2 overexpression is highly representative of intrinsic biological aggressiveness of certain cancers including breast, non-small cell lung cancer and gastric carcinoma, how ever its role in hematological malignancies have not yet been explored. Therefore, in this study we examined 100 clinical samples of diffuse large B-cell lymphoma (DLBCL) to study the expression pattern of ubiquitin ligase subunit SKP-2 proto-oncogenes and its relation to proliferative index marker protein Ki67 to correlate tumor aggressiveness. The expression of SKP-2 and Ki67 were examined by immunohistochemistry using specific antibodies on formalin-embedded tissue sections of DLBCL patients. Our data showed that SKP-2 was over expressed in majority of DLBCL and was associated with expression pattern of the proliferating index marker Ki67 protein. Since increased proteosome-dependent degradation of tumor suppressor genes play a critical role in the etiology of various tumors and proteosome inhibition is a novel approach for treating malignancies and has been approved for clinical use, we sought to determine whether inhibition of proteosome by MG132, a specific proteosome inhibitor induces apoptosis in a panel of DLBCL cell lines. Our data showed that treatment of DLBCL cell lines by MG132 induced apoptosis in a dose dependent manner. Inhibition of proteosome also decreased the expression of SKP-2 leading to subsequent disruption of mitochondrial membrane potential causing release of cytochrome c into the cytosol. Release of cytochrome c resulted in activation of caspase-3 and cleavage of PARP ultimately leading to apoptosis. These data suggests that SKP-2 expression plays a major role in the oncogenesis of DLBCL and overexpression of SKP-2 can be used as useful prognostic marker. Furthermore, proteosome inhibitors can be used as future therapeutic modality in treating DLBCL.