Identification of Proteinase 3 (PR3) Gene Overexpresison and Protein Delocalization in Core Binding Factor Leukemias as a Mechanism Leading to Increased Chemosensitivity.

Proteinase 3 (PR3) gene codes for a serine protease with a broad spectrum of proteolytic activity. PR3 is involved in the control of proliferation of myeloid leukemia cells. When abnormally expressed it confers factor-independent growth to hematopoietic cells. The aim of this study was to investigate the role of PR3 gene in leukemic haematopoiesis. We analyzed the expression levels of PR3 by RQ-PCR in 113 BM samples collected from AML patients at diagnosis. The FAB distribution was as follows: M0=5, M1=12, M2=38, M3=12, M4=37, M5=5, M6=4. 19 patients were characterized by t(8;21) and 16 by inv(16). PR3 expression level was also analyzed in 57 BM and 42 PB samples from 88 MDS patients (44 RA, 32 RAEB and 12 secondary-AML) and in 15 BM and 40 PB samples from healthy volunteers. PR3 protein was analyzed by western blot (WB) and its localization determined by immunofluorescence assay using specific antibodies. The transcription factor C/EBPα, which negatively regulates PR3 expression was studied in parallel at the RNA and protein level by RQ-PCR and WB. The DNA binding activity of C/EBPα was investigated by EMSA assay. Gain and loss of function experiments were performed by transfecting COS and 293T cell lines with a plasmid containing the full length PR3 sequence and HL60, Me-1, and Kasumi cell lines with specific shRNA. We found that PR3 is significantly overexpressed in AML samples. The median value of 2^{−Δ ΔCt is 740, (range 15-5043).} Interestingly, patients affected by Core Binding Factor leukemias showed significantly higher PR3 values compared to patients with normal karyotypes (NK) (p<0,0002 for t(8;21), p<0,001 for inv16) and lower C/EPBα levels. EMSA assay demonstrated the absence of C/EBPα DNA binding activity in CBF AML cells but not in NK AML. In addition, PR3 overexpression was detected in 60% of RA patients (mean value: 10, range 3–268), and in all the cases of RAEB (mean value 201: range:128–803) and secondary AML (mean value 589, range 207–7131). WB demonstrated the correlation between the mRNA and protein amount. Interestingly, immunofluorescence demonstrated the de-localization of the protein within the nucleous in CBF AML but it is completely cytoplasmatic in leukemic cells with normal karyotype and in MDS. Transfection experiments with PR3 plasmid demonstrated that PR3 overexpression results into a significantly increased proliferation and reduced apoptosis. By contrast transfection with shRNA triggers apoptosis and cell growth inhibition. In addition, WB demonstrated that nuclear PR3 is able to cleavage the p65subunit of NF-kB into a p56 isoform which lacks any transcriptional activity as confirmed by EMSA. In conclusion, PR3 gene expression and protein are significantly increased in AML and MDS, particularly in CBF leukemias in which the protein is not only increased but also completely delocalized within the nucleous. PR3 overepression My be due to a significant downmodulation of C/EBPα. Ectopic expression of PR3 induces increased proliferation and apoptosis arrest. The abnormal nuclear localization of PR3 in CBF leukemias results into the loss of function of NF-kB thus representing one mechanism of chemo sensitivity in this group of patients.