Protein Tyrosine Phosphatase Receptor Type O (PTPRO) Exhibits Tumor Suppressor Properties in K562 Cells by Dephosphorylating Bcr-Abl.

Regulation of protein phosphorylation by concerted action of protein kinases and phosphatases is important for normal physiological processes. Altered function or expression of one or more components of these regulatory molecules leads to many pathological conditions including cancer. We have previously shown that the truncated form of the receptor-type protein tyrosine phosphatase PTPROt predominantly expressed in haematopoietic cells is suppressed in Chronic Lymphocytic Leukemia (B-CLL). A direct correlation of CpG island methylation and reduced expression of the gene was observed in primary CLL samples and in several leukemia cell lines. To assess the functional significance of loss of PTPROt function in leukemia, we selected K562 cells as a model system, as PTPROt expression is silenced in these cells and is reactivated upon treatment with DNA hypomethylating agents. Ectopic expression of the catalytically active PTPROt inhibited growth of K562 cells and their clonogenic survival in soft agar (a characteristic of cancer cells). Further, cells expressing PTPROt exhibited delayed entry into S-phase from G0/G1 phase. Induction of apoptosis increased significantly in K562 cells expressing functional phosphatase upon serum withdrawal or exposure to the apoptogenic agent camptothecin. Tumorigenic potential of K562 cells in athymic nude mice was also significantly reduced upon ectopic expression of PTPROt. Finally, we demonstrate that the Bcr-Abl fusion protein, product of abnormal chromosomal translocation [t(9;22)] in chronic myelogenous leukemia, is a substrate of PTPROt. Tyrosine phosphorylation of this potent kinase was markedly reduced in K562 cells expressing the catalytically active PTPROt. Enhanced dephosphorylation of Bcr-Abl by PTPROt both in vivo and in vitro explains the observed phenotypes of the PTPROt expressing K562 cells. These data taken together delineate the molecular mechanism of tumor suppressor function of PTPROt in leukemic cells characterized by Philadelphia chromosome.

(This work was supported by a grant CA101956 from the National Institutes of Health).