PTPROt Inactivates the Oncogenic Fusion Protein BCR/ABL and Suppresses Transformation of K562 Cells

Abstract

Chronic myelogenous leukemia is typified by constitutive activation of the c-abl kinase as a result of its fusion to the breakpoint cluster region (BCR). Because the truncated isoform of proteintyrosine phosphatase receptor-type O (PTPROt) is specifically expressed in hematopoietic cells, we tested the possibility that it could potentially dephosphorylate and inactivate the fusion protein bcr/abl. Ectopic expression of PTPROt in the chronic myelogenous leukemia cell line K562 indeed resulted in hypophosphorylation of bcr/abl and reduced phosphorylation of its downstream targets CrkL and Stat5, confirming that PTPROt could inactivate the function of bcr/abl. Furthermore, the expression of catalytically active PTPROt in K562 cells caused reduced proliferation, delayed transition from G 0 /G 1 to S phase, loss of anchorage independent growth, inhibition of ex vivo tumor growth, and increased their susceptibility to apoptosis, affirming that this tyrosine phosphatase can revert the transformation potential of bcr/abl. Additionally, the catalytically inactive PTPROt acted as a trapping mutant that was also able to inhibit anchorage independence and facilitate apoptosis of K562 cells. The inhibitory action of PTPROt on bcr/abl was also confirmed in a murine myeloid cell line overexpressing bcr/abl. PTPROt expression was suppressed in K562 cells and was relieved upon treatment of the cells with 5-azacytidine, an inhibitor of DNA methyltransferase, with concomitant hypomethylation of the PTPRO CpG island. These data demonstrate that suppression of PTPROt by promoter methylation could contribute to the augmented phosphorylation and constitutive activity of its substrate bcr/abl and provide a potentially significant molecular therapeutic target for bcr/abl-positive leukemia.

Aberrant PTPRO methylation in tumor tissues as a potential biomarker that predicts clinical outcomes in breast cancer patients

“…The rather high frequency of methylation suggests that PTPRO is a common target for epigenetic silencing in breast tumors and that it may contribute to the development of this tumor type. As reported previously, demethylation of the PTPRO promoter resulted in gene re-expression [31]. These observations demonstrate growth-suppressor characteristics of PTPRO that are typical of a classical tumor suppressor gene.…”

ABL Tyrosine Kinases: Evolution of Function, Regulation, and Specificity

“…BCR-ABL1 transforming properties are blocked by PTPN1 (118) and by a variant form of PTPRO (119), which suggests that some PTPs function as negative regulators by reversing ABL-mediated tyrosine phosphorylation. In response to ionizing radiation treatment, ABL1 phosphorylates PTPN6 (also known as SHP-1), which binds to the SH3 domain of ABL1, on Tyr 536 and Tyr 564 (120).…”

MicroRNA-122 Inhibits Tumorigenic Properties of Hepatocellular Carcinoma Cells and Sensitizes These Cells to Sorafenib

“…Ex Vivo Tumor Growth of SK-Hep-1 Cells-Ex vivo tumor growth of SK-Hep-1 cells was performed as described (34). To image tumor growth in mice first Hep3B cells expressing luciferase were generated by transfecting pCMV-Luciferase and selecting with G418.…”