FLT3/ ITD regulates leukaemia cell adhesion through α4β1 integrin and Pyk2 signalling

Clinical and biological advances of critical complications in acute myeloid leukemia

Managing acute myeloid leukemia (AML) and its critical complications requires understanding the complex interplay between disease biology, treatment strategies, and patient characteristics. Complications like sepsis, acute respiratory failure (ARF), hyperleukocytosis, coagulopathy, tumor lysis syndrome (TLS) and central nervous system (CNS) involvement present unique challenges needing precise evaluation and tailored interventions. Venetoclax-induced TLS and differentiation syndrome (DS) from IDH1/IDH2 or menin inhibitors highlight the need for ongoing research and innovative approaches. As the microbiological landscape evolves and new therapeutic agents emerge, adapting strategies to mitigate harmful pharmacological interactions is crucial. Advances in understanding the genetic profiles of patients with hyperleukocytosis contribute to better-targeted therapeutic strategies. Effective AML management relies on collaborative efforts from hematologists, specialized services, and intensive care units (ICUs). This review analyzes recent data on critical AML complications, identifies areas for further investigation, and proposes ways to advance clinical research and enhance patient care strategies.

Understanding mechanisms of resistance to FLT3 inhibitors in adult FLT3-mutated acute myeloid leukemia to guide treatment strategy

The presence of FLT3 mutations, including the most common FLT3-ITD (internal tandem duplications) and FLT3-TKD (tyrosine kinase domain), is associated with an unfavorable prognosis in patients affected by acute myeloid leukemia (AML). In this setting, in recent years, new FLT3 inhibitors have demonstrated efficacy in improving survival and treatment response. Nevertheless, the development of primary and secondary mechanisms of resistance poses a significant obstacle to their efficacy. Understanding these mechanisms is crucial for developing novel therapeutic approaches to overcome resistance and improve the outcomes of patients. In this context, the use of novel FLT3 inhibitors and the combination of different targeted therapies have been studied. This review provides an update on the molecular alterations involved in the resistance to FLT3 inhibitors, and describes how the molecular monitoring may be used to guide treatment strategy in FLT3-mutated AML.

Protein tyrosine kinase 2b inhibition reverts niche-associated resistance to tyrosine kinase inhibitors in AML

FLT3 tyrosine kinase inhibitor (TKI) therapy evolved into a standard therapy in FLT3-mutated AML. TKI resistance, however, develops frequently with poor outcomes. We analyzed acquired TKI resistance in AML cell lines by multilayered proteome analyses. Leupaxin (LPXN), a regulator of cell migration and adhesion, was induced during early resistance development, alongside the tyrosine kinase PTK2B which phosphorylated LPXN. Resistant cells differed in cell adhesion and migration, indicating altered niche interactions. PTK2B and LPXN were highly expressed in leukemic stem cells in FLT3-ITD patients. PTK2B/FAK inhibition abrogated resistance-associated phenotypes, such as enhanced cell migration. Altered pathways in resistant cells, assessed by nascent proteomics, were largely reverted upon PTK2B/FAK inhibition. PTK2B/FAK inhibitors PF-431396 and defactinib synergized with different TKIs or daunorubicin in FLT3-mutated AML. Midostaurin-resistant and AML cells co-cultured with mesenchymal stroma cells responded particularly well to PTK2B/FAK inhibitor addition. Xenograft mouse models showed significant longer time to leukemia symptom-related endpoint upon gilteritinib/defactinib combination treatment in comparison to treatment with either drug alone. Our data suggest that the leupaxin-PTK2B axis plays an important role in acquired TKI resistance in AML. PTK2B/FAK inhibitors act synergistically with currently used therapeutics and may overcome emerging TKI resistance in FLT3-mutated AML at an early timepoint.

Integrins, anchors and signal transducers of hematopoietic stem cells during development and in adulthood

Hematopoietic stem cells (HSCs), the apex of the hierarchically organized blood cell production system, are generated in the yolk sac, aorta-gonad-mesonephros region and placenta of the developing embryo. To maintain life-long hematopoiesis, HSCs emigrate from their site of origin and seed in distinct microenvironments, called niches, of fetal liver and bone marrow where they receive supportive signals for self-renewal, expansion and production of hematopoietic progenitor cells (HPCs), which in turn orchestrate the production of the hematopoietic effector cells. The interactions of hematopoietic stem and progenitor cells (HSPCs) with niche components are to a large part mediated by the integrin superfamily of adhesion molecules. Here, we summarize the current knowledge regarding the functional properties of integrins and their activators, Talin-1 and Kindlin-3, for HSPC generation, function and fate decisions during development and in adulthood. In addition, we discuss integrin-mediated mechanosensing for HSC-niche interactions, ex vivo protocols aimed at expanding HSCs for therapeutic use, and recent approaches targeting the integrin-mediated adhesion in leukemia-inducing HSCs in their protecting, malignant niches.

CD44 engagement enhances acute myeloid leukemia cell adhesion to the bone marrow microenvironment by increasing VLA-4 avidity

Adhesive properties of leukemia cells shape the degree of organ infiltration and the extent of leukocytosis. CD44 and the integrin VLA-4, a CD49d/CD29 heterodimer, are important factors in progenitor cell adhesion in bone marrow. Here, we report their cooperation in acute myeloid leukemia (AML) by a novel non-classical CD44-mediated way of inside-out VLA-4 activation. In primary AML bone marrow samples from patients and the OCI-AML3 cell line, CD44 engagement by hyaluronan induced inside-out activation of VLA-4 resulting in enhanced leukemia cell adhesion on VCAM-1. This was independent of VLA-4 affinity regulation but based on ligand-induced integrin clustering on the cell surface. CD44-induced VLA-4 activation could be inhibited by the Src family kinase inhibitor PP2 and the multikinase inhibitor midostaurin. As a further consequence, the increased adhesion on VCAM-1 allowed AML cells to bind stromal cells strongly. Thereby, the VLA-4/VCAM-1 interaction promoted activation of Akt, MAPK, NF-kB and mTOR signaling and decreased AML cell apoptosis. Collectively, our investigations provide a mechanistic description of an unusual CD44 function in regulating VLA-4 avidity in AML, enhancing AML cell retention in the supportive bone marrow microenvironment.

Reversible switching of leukemic cells to a drug-resistant, stem-like subset via IL-4 mediated cross-talk with mesenchymal stroma

Chemoresistance of leukemic cells has largely been attributed to clonal evolution secondary to accumulating mutations. Here, we show that a subset of leukemic blasts in contact with the mesenchymal stroma undergo cellular conversion into a distinct cell type that exhibits a stem cell-like phenotype and chemoresistance. These stroma-induced changes occur in a reversible and stochastic manner driven by cross-talk, whereby stromal contact induces interleukin-4 in leukemic cells that in turn targets the mesenchymal stroma to facilitate the development of new subset. This mechanism was dependent on interleukin-4-mediated upregulation of vascular cell adhesion molecule- 1 in mesenchymal stroma, causing tight adherence of leukemic cells to mesenchymal progenitors for generation of new subsets. Together, our study reveals another class of chemoresistance in leukemic blasts via functional evolution through stromal cross-talk, and demonstrates dynamic switching of leukemic cell fates that could cause a non-homologous response to chemotherapy in concert with the patient-specific microenvironment.

Combination of a mitogen-activated protein kinase inhibitor with the tyrosine kinase inhibitor pacritinib combats cell adhesion-based residual disease and prevents re-expansion of FLT3-ITD acute myeloid leukaemia

Minimal residual disease (MRD) in acute myeloid leukaemia (AML) poses a major challenge due to drug insensitivity and high risk of relapse. Intensification of chemotherapy and stem cell transplantation are often pivoted on MRD status. Relapse rates are high even with the integration of first-generation FMS-like tyrosine kinase 3 (FLT3) inhibitors in pre- and post-transplant regimes and as maintenance in FLT3-mutated AML. Pre-clinical progress is hampered by the lack of suitable modelling of residual disease and post-therapy relapse. In the present study, we investigated the nature of pro-survival signalling in primary residual tyrosine kinase inhibitor (TKI)-treated AML cells adherent to stroma and further determined their drug sensitivity in order to inform rational future therapy combinations. Using a primary human leukaemia-human stroma model to mimic the cell-cell interactions occurring in patients, the ability of several TKIs in clinical use, to abrogate stroma-driven leukaemic signalling was determined, and a synergistic combination with a mitogen-activated protein kinase (MEK) inhibitor identified for potential therapeutic application in the MRD setting. The findings reveal a common mechanism of stroma-mediated resistance that may be independent of mutational status but can be targeted through rational drug design, to eradicate MRD and reduce treatment-related toxicity.

Normal and leukemic stem cell niche interactions

PURPOSE OF REVIEW

Normal hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs) interact with the stem cell niche bone marrow in different ways. Understanding the potentially unique microenvironmental regulation of LSCs is key to understanding in-vivo leukemogenic mechanisms and developing novel antileukemic therapies.

RECENT FINDINGS

When leukemic cells are engrafted in the stem cell niche, the cellular nature of the niche - including mesenchymal stromal cells - is reprogramed. Altered mesenchymal cells selectively support leukemic cells and reinforce the pro-leukemic environment. As the niche plays an active role in leukemogenesis, its remodeling may significantly influence the leukemogenic pattern, and cause differences in clinical prognosis. Notably, niche cells could be stimulated to revert to a pronormal/antileukemic state, creating potential for niche-based antileukemic therapy.

SUMMARY

Bone marrow microenvironments are under dynamic regulation for normal and leukemic cells, and there is bi-directional control of leukemic cells in the niche. Leukemic cells are both protected by stroma and able to reprogram stromal cells to transform the niche to a state, which reinforces leukemogenesis. Because of its dynamic nature, the niche could be converted to an environment with antileukemic properties, making it an attractive target for therapy.

Integrin expression and adhesivity to fibronectin in primary acute myeloid leukemia cells: Impact of NPM1 and FLT3 mutations

OBJECTIVES

Interaction of leukemia cells with the bone marrow extracellular matrix promotes cell survival and resistance to chemotherapy. In this work, we analyzed integrin expression and adhesivity to fibronectin in primary cells from patients with acute myeloid leukemia.

METHODS

Surface expression of integrins β1 and αVβ3 on primary leukemia cells (N = 46) was correlated with the stem cell marker CD34, as well as with cell adhesivity to fibronectin. The results were analyzed with regard to the mutational status of NPM1 and FLT3 genes.

RESULTS

The integrin β1 was omnipresent, whereas αVβ3 was often more expressed on CD34-positive cells. In particular, higher αVβ3 expression on CD34+ cells was associated with NPM1 mutation (P = .0018). Monocytic leukemias had significantly higher αVβ3 expression compared to less maturated cases (P = .0008). Cells from patients with internal tandem duplications in FLT3 (FLT3-ITD) had lower adhesivity to fibronectin compared to cells with wild-type FLT3 (P = .031), specifically in less differentiated myeloblasts. Inhibition of a putative FLT3-ITD target, EZH2, increased cell adhesivity in MV4-11 cell line (P = .024).

CONCLUSIONS

The integrin αVβ3 is expressed in particular on CD34+ cells with NPM1 mutation and might have a prognostic value in patients with mutated NPM1. FLT3-ITD is associated with lower cell adhesivity, especially in patients with less differentiated leukemias.

Oncogenic Deregulation of Cell Adhesion Molecules in Leukemia

Numerous cell⁻cell and cell⁻matrix interactions within the bone marrow microenvironment enable the controlled lifelong self-renewal and progeny of hematopoietic stem and progenitor cells (HSPCs). On the cellular level, this highly mutual interaction is granted by cell adhesion molecules (CAMs) integrating differentiation, proliferation, and pro-survival signals from the surrounding microenvironment to the inner cell. However, cell⁻cell and cell⁻matrix interactions are also critically involved during malignant transformation of hematopoietic stem/progenitor cells. It has become increasingly apparent that leukemia-associated gene products, such as activated tyrosine kinases and fusion proteins resulting from chromosomal translocations, directly regulate the activation status of adhesion molecules, thereby directing the leukemic phenotype. These observations imply that interference with adhesion molecule function represents a promising treatment strategy to target pre-leukemic and leukemic lesions within the bone marrow niche. Focusing on myeloid leukemia, we provide a current overview of the mechanisms by which leukemogenic gene products hijack control of cellular adhesion to subsequently disturb normal hematopoiesis and promote leukemia development.

Role of Pyk2 in Human Cancers

Proline-rich tyrosine kinase 2 (Pyk2) plays essential roles in tumorigenesis and tumor progression. Pyk2 serves as a non-receptor tyrosine kinase regulating tumor cell survival, proliferation, migration, invasion, metastasis, and chemo-resistance, and is associated with poor prognosis and shortened survival in various cancer types. Thus, Pyk2 has been traditionally regarded as an oncogene and potential therapeutic target for cancers. However, a few studies have also demonstrated that Pyk2 exerts tumor-suppressive effects in some cancers, and anti-cancer treatment of Pyk2 inhibitors may only achieve marginal benefits in these cancers. Therefore, more detailed knowledge of the contradictory functions of Pyk2 is needed. In this review, we summarized the tissue distribution, expression, interactive molecules of Pyk2 in the signaling pathway, and roles of Pyk2 in cancers, and focused on regulation of the interconnectivity between Pyk2 and its downstream targets. The potential use of inhibitors of Pyk2 and its related pathways in cancer therapy is also discussed.

Heterocellular molecular contacts in the mammalian stem cell niche

Adult tissue homeostasis and repair relies on prompt and appropriate intervention by tissue-specific adult stem cells (SCs). SCs have the ability to self-renew; upon appropriate stimulation, they proliferate and give rise to specialized cells. An array of environmental signals is important for maintenance of the SC pool and SC survival, behavior, and fate. Within this special microenvironment, commonly known as the stem cell niche (SCN), SC behavior and fate are regulated by soluble molecules and direct molecular contacts via adhesion molecules providing connections to local supporting cells and the extracellular matrix. Besides the extensively discussed array of soluble molecules, the expression of adhesion molecules and molecular contacts is another fundamental mechanism regulating niche occupancy and SC mobilization upon activation. Some adhesion molecules are differentially expressed and have tissue-specific consequences, likely reflecting the structural differences in niche composition and design, especially the presence or absence of a stromal counterpart. However, the distribution and identity of intercellular molecular contacts for adhesion and adhesion-mediated signaling within stromal and non-stromal SCN have not been thoroughly studied. This review highlights common details or significant differences in cell-to-cell contacts within representative stromal and non-stromal niches that could unveil new standpoints for stem cell biology and therapy.

Structural Basis for the Interaction between Pyk2-FAT Domain and Leupaxin LD Repeats

Proline-rich tyrosine kinase 2 (Pyk2) is a nonreceptor tyrosine kinase and belongs to the focal adhesion kinase (FAK) family. Like FAK, the C-terminal focal adhesion-targeting (FAT) domain of Pyk2 binds to paxillin, a scaffold protein in focal adhesions; however, the interaction between the FAT domain of Pyk2 and paxillin is dynamic and unstable. Leupaxin is another member in the paxillin family and was suggested to be the native binding partner of Pyk2; Pyk2 gene expression is strongly correlated with that of leupaxin in many tissues including primary breast cancer. Here, we report that leupaxin interacts with Pyk2-FAT. Leupaxin has four leucine–aspartate (LD) motifs. The first and third LD motifs of leupaxin preferably target the two LD-binding sites on the Pyk2-FAT domain, respectively. Moreover, the full-length leupaxin binds to Pyk2-FAT as a stable one-to-one complex. Together, we propose that there is an underlying selectivity between leupaxin and paxillin for Pyk2, which may influence the differing behavior of the two proteins at focal adhesion sites.

Structural and mechanistic insights into the interaction between Pyk2 and paxillin LD motifs

Proline-rich tyrosine kinase 2 (Pyk2) is a member of the focal adhesion kinase (FAK) subfamily of cytoplasmic tyrosine kinases. The C-terminal Pyk2-focal adhesion targeting (FAT) domain binds to paxillin, an adhesion molecule. Paxillin has five leucine-aspartate (LD) motifs (LD1-LD5). Here, we show that the second LD motif of paxillin, LD2, interacts with Pyk2-FAT, similar to the known Pyk2-FAT/LD4 interaction. Both LD motifs can target two ligand binding sites on Pyk2-FAT. Interestingly, they also share similar binding affinity for Pyk2-FAT with preferential association to one site relative to the other. Nevertheless, the LD2-LD4 region of paxillin (paxillin(133-290)) binds to Pyk2-FAT as a 1:1 complex. However, our data suggest that the Pyk2-FAT and paxillin complex is dynamic and it appears to be a mixture of two distinct conformations of paxillin that almost equally compete for Pyk2-FAT binding. These studies provide insight into the underlying selectivity of paxillin for Pyk2 and FAK that may influence the differing behavior of these two closely related kinases in focal adhesion sites.

SYK is a critical regulator of FLT3 in acute myeloid leukemia

Cooperative dependencies between mutant oncoproteins and wild-type proteins are critical in cancer pathogenesis and therapy resistance. Although spleen tyrosine kinase (SYK) has been implicated in hematologic malignancies, it is rarely mutated. We used kinase activity profiling to identify collaborators of SYK in acute myeloid leukemia (AML) and determined that FMS-like tyrosine kinase 3 (FLT3) is transactivated by SYK via direct binding. Highly activated SYK is predominantly found in FLT3-ITD positive AML and cooperates with FLT3-ITD to activate MYC transcriptional programs. FLT3-ITD AML cells are more vulnerable to SYK suppression than FLT3 wild-type counterparts. In a FLT3-ITD in vivo model, SYK is indispensable for myeloproliferative disease (MPD) development, and SYK overexpression promotes overt transformation to AML and resistance to FLT3-ITD-targeted therapy.