VASP Protein, a Binding Partner Of BCR-ABL and FAK, May Be Implicated In The Chronic Myeloid Leukemia Phenotype

VASP (vasodilator-stimulated phosphoprotein) and Zyxin are actin regulatory proteins that control cell-cell adhesion. Zyxin directs actin assembly by interacting and recruiting VASP to specific sites of adhesion. The phosphorylation of VASP modifies their activity in cell-cell junctions. PKA phosphorylates VASP at serine 157 regulating VASP cellular functions. VASP is a substrate of BCR-ABL oncoprotein and is tyrosine-phosphorylated in leukemic cells. However, the function of VASP and Zyxin in hematopoietic cells, in the BCR-ABL pathway and its participation in chronic myeloid leukemia (CML) remains unknown.

To analyze VASP and Zyxin expression in bone marrow cells from CML patients and healthy donors, as well the involvement of these proteins in hematopoietic cell differentiation and in the BCR-ABL signaling pathway.

VASP and Zyxin expression and phosphorylation were studied in bone marrow samples from 29 individuals (5 healthy donors, 5 CML patients at diagnosis, 16 CML patients responsive to treatment with tyrosine kinase inhibitors (ITK) and 3 CML patients resistant to ITK). One patient was analyzed at diagnosis and after ITK response. VASP or Zyxin silencing was performed by shRNA-lentiviral delivery in K562 cell line, an appropriated shControl was used. ShControl, shVASP and shZyxin K562 cells were induced to megakaryocytic differentiation with 20nM of PMA (phorbol myristate -13 -12 acetate) during 4 days and CD61 expression, a marker for maturing megakaryocytes, was verified by flow cytometry. During megakaryocytic differentiation, VASP and Zyxin gene expressions were evaluated by quantitative PCR; protein expression and activation were determined by Western blotting. Effector proteins of proliferation, apoptosis and adhesion in the BCR-ABL signaling pathway were analyzed in cells silenced for VASP or Zyxin. The interaction of VASP and BCR-ABL or FAK was evaluated by co-immunoprecipiation.

Healthy donors showed p-VASP ser157 expression, in contrast to CML patients at diagnosis who did not present phospho-VASP ser157. After Imatinib treatment CML patients restored VASP phosphorylation however resistant patients maintained this absence. Zyxin showed the same expression in patients and healthy donors. During Imatinib treatment of K562 cells, phospho-VASP ser157 expression was increased and its interaction with BCR-ABL protein was reduced. VASP and Zyxin gene expressions were upregulated during megakaryocyte differentiation of K562 cells (8.7-fold increase, P=0.0115, and 3.6-fold increase, P=0.015, respectively). VASP and Zyxin protein expressions were increased during megakaryocytic differentiation, including the active form of these proteins (p-VASP ser157 and p-Zyxin ser142). VASP silencing in K562 cells resulted in a 40% decrease of CD61 expression at the end of the megakaryocytic differentiation (P<0.05). In addition, VASP and Zyxin silencing resulted in a decrease of BCL-2 and BCL-XL proteins. VASP binds to FAK, an adhesion effector protein of the BCR-ABL pathway, and it´s silencing resulted in a decreased phosphorylation of FAK y925.

In BCR-ABL cells, VASP and Zyxin modulated anti-apoptotic proteins and megakaryocytic differentiation. Hence, the altered expression of VASP activity in CML patients may contribute to the pathogenesis of the disease, affecting cellular differentiation or leukemic cell adhesion.

No relevant conflicts of interest to declare.