PLZF/RARalpha binding partners and their influence on the induction of acute promyelocytic leukaemia
The acute promyelocytic leukaemia (APL) accounts for 10% of the adult AML patients. APL patients carry chromosomal translocations involving the rarα gene on chromosome 17. The most frequent translocations lead to the expression of the fusion proteins PML/RARα (98%; t(15;17)) and PLZF/RARα (1%; t(11;...
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|The acute promyelocytic leukaemia (APL) accounts for 10% of the adult AML patients. APL patients carry chromosomal translocations involving the rarα gene on chromosome 17. The most frequent translocations lead to the expression of the fusion proteins PML/RARα (98%; t(15;17)) and PLZF/RARα (1%; t(11;17)) (X-RARα). The APL is characterised by a differentiation block at the promyelocytic stage and leads to an increased amount of myeloid blasts in the bone marrow and in the peripheral blood. Both X-RARα fusion proteins build high molecular weight (HMW) complexes, which are essential to excert the leukaemic potential. The purpose of this work was to validate and analyse binding partners of the PLZF/RARα fusion protein for their leukaemic potential.
IT-based pathway analyses of 20 putative PLZF/RARα binding partners revealed the central proteins actin, c-Fos and Erk1/2. VASP, TFII-I and PI4KIIα were selected for further studies, due to their direct influence on the proteins mentioned above. The interaction studies validated the binding of PLZF/RARα with all three proteins: VASP, TFII-I and PI4KIIα. The binding occurs in all cases mainly through the RARα portion. The PLZF/RARα expression causes the dephosphorylation of the actin-associated phosphoprotein VASP, which leads to its cellular delocalisation. The hypophosphorylation of VASP is probably connected with the proteasomal degradation of the kinases PKA and/or PKG. Treatment with the standard APL therapeutic all-trans retinoic acid (ATRA) reestablished VASP phosphorylation in PLZF/RARα -expressing cells. The leukaemic fusion protein co-localised with the transcription factor TFII-I in the nucleus. Furthermore, the expression of both X-RARα fusion proteins causes the deregulation of TFII-I target genes. TFII-I is at least sequestered by PML/RARα, since ATRA treatment reverts in nearly all cases the aberrant expression of the TFII-I target genes. Additionally, the important TFII-I target gene c-fos, which is an protooncogene and part of the AP-1 transcription factor family was overexpressed in the presence of the X-RARα proteins. Moreover, PLZF/RARα caused the translocation of the lipid kinase PI4KIIα to the nucleus, leading to their co-localisation. It was shown before that PI4KIIα overexpression leads to an increased synthesis of PI(4)P and the subsequent induction of Erk1/2, while the expression of PLZF/RARα inhibits PI4KIIα activity. Further analyses showed that the sequestration of the kinase resulted in the inactivation of Erk1/2. ATRA treatment induced Erk1/2 activation in control and PML/RARα -expressing cells, but not in the ATRA-resistent PLZF/RARα -expressing cells. Differentiation experiments in murine haematopoietic progenitor cells showed that PI4KIIα expression is essential for the myeloid differentiation. However, the overexpression of PI4KIIα in PLZF/RARα -expressing cells alone could not overcome the differentiation block.
In summary, the PLZF/RARα fusion protein disturbs the normal activity and functionality of its interaction partners, which contributes to the development of APL. New APL marker proteins and pathways were identified and validated. These marker proteins represent putative therapeutic targets. Further examinations could lead to an improved treatment of patients with the PLZF/RARα-associated APL.