Upregulation of the EMT marker vimentin is associated with poor clinical outcome in acute myeloid leukemia

Acute myeloid leukemia (AML) is a myeloid lineage malignancy characterized by invasion of the bone marrow and blood by poorly differentiated cells with enhanced proliferative capabilities. Deregulation of various kinases, transcription factors, apoptotic regulators, epigenetic modifiers, etc. and their roles in AML progression have been rigorously studied and are well understood [1]. The deregulation of cytoskeletal proteins particularly those involved in the epithelial to mesenchymal transition (EMT) is well investigated in solid cancers but not in hematological malignancies such as AML. Vimentin (encoded by VIM) a type III intermediate filament, forms the cytoskeletal network with microtubules and microfilaments. It is the predominant intermediate filament in mesenchymal cells and leukocytes [2, 3]. A highly conserved protein, vimentin participates in crucial cellular processes such as cell motility, migration and adhesion as well as promotes resistance to various cell stressors [2, 4, 5]. Furthermore, though vimentin is a cytoskeletal protein, it has been shown to be involved in apoptotic progression [4, 6‐8]. During apoptosis, vimentin is cleaved by caspase-3, -7 and -6 resulting in cytoskeletal collapse and is thought to be the basis of the morphological changes that occur in a cell undergoing apoptosis [7]. Direct phosphorylation of vimentin by AKT1, a known cancer promoting kinase, protects it from caspase mediated cleavage and blocks apoptotic progression [9, 10]. Given that vimentin regulates cell migration and adhesion, which are crucial processes in cancer progression, it has long been seen as a factor in cancer progression. Vimentin overexpression has been reported in cancers of the prostate, gastrointestinal tract, breast and many others [10‐12]. In solid cancers, vimentin has been shown to promote metastatic progression by participating in the cytoskeletal reorganization that occurs during EMT as well as regulate pro-EMT signaling pathways [10]. Vimentin has also been used as a marker for pre-metastatic cells undergoing EMT and therefore, high vimentin expression is associated with worse outcomes in patients with solid cancer [11].

The epithelial to mesenchymal transition (EMT) is an embryonic developmental process in which epithelial cells transition to cells with a mesenchymal phenotype defined by the loss of E-cadherin and gain of vimentin expression markers [18]. EMT has been shown to play oncogenic roles in cancer progression starting from the initiation of the primary tumor thorough metastasis and resistance to cancer treatment [18]. EMT has also been linked to cancer stem cell survival and maintenance. Importantly, the role of EMT in cancer is not limited to cancer of epithelial origin, but also extend to non-epithelial cancers such as glioma and hematological malignancies [19]. In hematological malignancies, several studies have reported on the role of the EMT transcription factors TWIST1 and TWIST2 in both myeloid and lymphoblastic leukemia [19]. Using an inducible MLL-AF9-driven AML mouse model, a recent study demonstrated that the expression of MLL-AF9 in hematopoietic stem cells (HSC) drives an invasive and chemoresistant AML expressing EMT markers (such as VIM among others) [20].

VIM is a cytoskeletal protein which overexpression is known to be associated with aggressive disease and worse outcomes in solid cancers such breast, gastrointestinal, and prostate cancers [9‐12]. VIM regulates EMT, in part, due to its role in regulating cell migration, cell adhesion, EMT signaling pathways and cytoskeletal reorganization [4]. In this study, we characterized the EMT marker VIM, in the context of AML. As leukemic cells do not undergo EMT, the molecular functions of VIM are less clear in AML. Here we demonstrated that upregulation of VIM is associated with poor overall and disease-free survival in older patients with CN-AML. The clinical outcome of AML in older patients who are unable to receive intensive chemotherapy without unacceptable side effects remains dismal. Less than 15% of patients older than 60 years of age survive for longer than 5 years; and the median survival of these patients is only 5–10 months [1]. Whether vimentin protein levels correlate with its mRNA levels and similarly with clinical outcome remains to be examined. The lack of association between high VIM and patient’s mutational status is particularly interesting, suggesting other mechanisms such as epigenetic modification may be involved in regulating VIM expression. Indeed, we found that patients with high VIM (Z ≥ 1) have significantly lower VIM methylation in comparison with low VIM expression. Consistent with our observation, treatment with 5-Aza-Deoxycytidine, a methylation inhibitor, resulted in several folds increase of vimentin mRNA expression in different colon cancer cell lines [21]. Vimentin expression was also shown to be transactivated by β-catenin/TCF, binding to a site upstream of vimentin promoter in breast cancer cells [22]. Whether similar mechanisms are present in AML is unknown, however the role of β-catenin signaling pathway in maintaining leukemia stem cell survival is well established in AML [23, 24].

In conclusion, our results demonstrate that VIM upregulation is associated with poor clinical outcome in AML. The characterization of VIM mRNA expression reveals it as valuable prognostic marker in AML and provides the rationale for further functional and mechanistic investigation into the role of VIM in this disease.