The tyrosine phosphatase TC48 interacts with and inactivates the oncogenic fusion protein BCR-Abl but not cellular Abl

Abstract

The chimeric oncoprotein BCR-Abl exhibits deregulated protein tyrosine kinase activity and is responsible for the pathogenesis of certain human leukemias, such as chronic myelogenous leukemia. The activities of cellular Abl (c-Abl) and BCR-Abl are stringently regulated and the cellular mechanisms involved in their inactivation are poorly understood. Protein tyrosine phosphatases can negatively regulate Abl mediated signaling by dephosphorylating the kinase and/or its substrates. This study investigated the ability of the intracellular T cell protein tyrosine phosphatase (TCPTP/PTPN2) to dephosphorylate and regulate the functions of BCR-Abl and c-Abl. TCPTP is expressed as two alternately spliced isoforms - TC48 and TC45, which differ in their C-termini and localize to the cytoplasm and nucleus respectively. We show that TC48 dephosphorylates BCR-Abl but not c-Abl and inhibits its activity towards its substrate, CrkII. Y1127 and Y1294 residues whose phosphorylation corresponds with BCR-Abl activation status were the primary sites targeted by TC48. Co-localization and immunoprecipitation experiments showed that TC48 interacted with BCR-Abl but not with c-Abl, and BCR domain was sufficient for interaction. TC48 expression resulted in the stabilization of Bcr-Abl protein dependent on its phosphatase activity. Inactivation of cellular TC48 in K562 cells by stable expression of a dominant negative catalytically inactive mutant TC48, enhanced proliferation. TC48 expressing K562 clones showed reduced proliferation and enhanced sensitivity to STI571 compared to control clones suggesting that TC48 can repress the growth of CML cells. This study identifies a novel cellular regulator that specifically inhibits the activity of oncogenic BCR-Abl but not that of the cellular Abl kinase.

PTPRG and PTPRC modulate nilotinib response in chronic myeloid leukemia cells

“…Several PTPs previously reported to modulate the response to the first-generation inhibitor imatinib, including PTPN1 [ 11 ], PTPN2 [ 13 , 14 ], and PTPN22 [ 17 ] were not associated with response in our study. It is possible that the more efficient inhibition of BCR-ABL1-mediated transformation by nilotinib may override PTP-mediated regulation in these cases.…”

“…Higher mRNA levels for PTPRG ( RPTPγ ) and PTPRC ( CD45 ) (Figure 1 , Table 2 ) as well as for PTPN13 ( FAP1, PTP-BAS ), PTPRA ( RPTPα ), and PTPRM ( RPTPμ ) (Table 2 ) significantly supported MR 4 probabilities after 9 months of treatment. Notably, no significant associations were found for PTPN1 (PTP1B), PTPN2 (TC-PTP), PTPN6 (SHP-1; Figure 1 ), or PTPN22 (Lyp), PTPs whose expression had previously been reported to positively associate with imatinib response [ 11 – 14 , 17 ].…”

“…Therefore, it has been previously considered that the expression levels of specific PTPs may modify the response to BCR-ABL1 inhibitors. For example, for imatinib responses, promoting effects of PTPN1 (PTP1B) [ 11 ], PTPN6 (SHP-1) [ 12 ], and PTPN2 (TC-PTP) [ 13 , 14 ] were reported. However, for PTPN2 and PTPN6, findings were contradictory [ 15 , 16 ].…”

Current Views on the Interplay between Tyrosine Kinases and Phosphatases in Chronic Myeloid Leukemia

“…Contrarily, the TC48 cytoplasmic form acts directly on the BCR-ABL1 protein by binding with its BCR domain, thus producing dephosphorylation and reducing its activity. As in the nuclear context, expression of TC48 variant may reduce cell proliferation and increase the effects of IM [ 166 ].…”

Elucidation of protein-protein interactions necessary for maintenance of the BCR-ABL signaling complex

“…the pool of signaling intermediates associating directly with BCR-ABL is only beginning to emerge. Active BCR-ABL is phosphorylated on tyrosines 177, 1127 and 1294 (Mitra et al 2013), which serve as docking sites for binding of proteins containing SH3 and PTB domains. Several such proteins have been identified, including adapters Gab2, CrkL and SHC1, adapter/phosphatase SHP2, p85 subunit of the PI3-kinase, ubiquitin ligase cCbl, and others (Brehme et al 2009).…”