Reduced expression of protein tyrosine phosphatase gamma in lung and ovarian tumors

Based on LOH studies protein tyrosine phosphataseγ (PTPγ) has been suggested as a candidate tumor suppressor gene involved in the oncogenesis of lung and renal cancers. In order to assess the involvement of PTPγ in tumor development we developed a PTPγ-specific monoclonal antibody (γTL1) (IgM isotype) by immunization with a synthetic peptide of 15 amino acids corresponding to the amino acid sequence nos. 1423–1438 just outside the phosphatase domain-II. In line with the fact that the antibody was raised to an intracellular domain of the PTPγ molecule the antibody labeled the cell membrane of fixed cells but did not stain the outside of the cell membrane in the immunofluorescence assay. The Mab γTL1 recognized a full-length baculovirus recombinant PTPγ protein of 185 kDa, in addition to putative cleavage products of 120 kDa, 114/110 kDa and 80 kDa, on Western blots of lysates of PTPγ-gene transfected Sf9 insect cells but not of tumor cell lysates. Based on immunoperoxidase and immunofluorescence assays on cryostat sections, however, PTPγ was expressed in more than 90% of both normal, human tissue samples and in the (non-) tumor cells of carcinoma samples. However, PTPγ was not found in 28% of the overall lung tumor samples, i.e. in 50% of the lung adenocarcinoma samples, while the expression was weak and heterogeneous in 71% of squamous lung cell carcinomas. PTPγ was not suppressed in the normal cells between the lung carcinoma cells. The presence of PTPγ, assayed by immunofluorescence in lung tumor cell lines (H69, H128, H82, C3) was confirmed by RT–PCR assay. Interestingly, the 90% expression score of PTPγ protein in normal ovarian tissue samples was reduced dramatically to 44 and 38% in both the non-tumorous and tumorous cells, respectively, in ovarian tumor samples. PTPγ was absent in the HT29 human colon carcinoma cell line both by immunofluorescence and RT–PCR assay. In summary, we have developed a PTPγ-specific monoclonal antibody, that demonstrated that the expression of PTPγ is severely reduced (>50%) in lung tumors and ovarian tumors.