Understanding the Hedgehog Signaling Pathway in Acute Myeloid Leukemia Stem Cells: A Necessary Step toward a Cure

Signaling pathways governing the behaviors of leukemia stem cells

Leukemia is a malignancy in the blood that develops from the lymphatic system and bone marrow. Although various treatment options have been used for different types of leukemia, understanding the molecular pathways involved in the development and progression of leukemia is necessary. Recent studies showed that leukemia stem cells (LSCs) play essential roles in the pathogenesis of leukemia by targeting several signaling pathways, including Notch, Wnt, Hedgehog, and STAT3. LSCs are highly proliferative cells that stimulate tumor initiation, migration, EMT, and drug resistance. This review summarizes cellular pathways that stimulate and prevent LSCs' self-renewal, metastasis, and tumorigenesis.

Reactive oxygen species and its role in pathogenesis and resistance to therapy in acute myeloid leukemia

Relapse following a short clinical response to therapy is the major challenge for the management of acute myeloid leukemia (AML) patients. Leukemic stem cells (LSC), as the source of relapse, have been investigated for their metabolic preferences and their alterations at the time of relapse. As LSC rely on oxidative phosphorylation (OXPHOS) for energy requirement, reactive oxygen species (ROS), as by-products of OXPHOS, have been investigated for their role in the effectiveness of the standard AML therapy. Increased levels of non-mitochondrial ROS, generated by nicotinamide adenine dinucleotide phosphate oxidase, in a subgroup of AML patients add to the complexity of studying ROS. Although there are various studies presenting the contribution of ROS to AML pathogenesis, resistance, and its inhibition or activation as a target, a model that can clearly explain its role in AML has not been conceptualized. This is due to the heterogeneity of AML, the dynamics of ROS production, which is influenced by factors such as the type of treatment, cell differentiation state, mitochondrial activity, and also the heterogeneous generation of non-mitochondrial ROS and limited available data on their interaction with the microenvironment. This review summarizes these challenges and the recent progress in this field.

The role of bone marrow microenvironment (BMM) cells in acute myeloid leukemia (AML) progression: immune checkpoints, metabolic checkpoints, and signaling pathways

Acute myeloid leukemia (AML) comprises a multifarious and heterogeneous array of illnesses characterized by the anomalous proliferation of myeloid cells in the bone marrow microenvironment (BMM). The BMM plays a pivotal role in promoting AML progression, angiogenesis, and metastasis. The immune checkpoints (ICs) and metabolic processes are the key players in this process. In this review, we delineate the metabolic and immune checkpoint characteristics of the AML BMM, with a focus on the roles of BMM cells e.g. tumor-associated macrophages, natural killer cells, dendritic cells, metabolic profiles and related signaling pathways. We also discuss the signaling pathways stimulated in AML cells by BMM factors that lead to AML progression. We then delve into the roles of immune checkpoints in AML angiogenesis, metastasis, and cell proliferation, including co-stimulatory and inhibitory ICs. Lastly, we discuss the potential therapeutic approaches and future directions for AML treatment, emphasizing the potential of targeting metabolic and immune checkpoints in AML BMM as prognostic and therapeutic targets. In conclusion, the modulation of these processes through the use of directed drugs opens up new promising avenues in combating AML. Thereby, a comprehensive elucidation of the significance of these AML BMM cells' metabolic and immune checkpoints and signaling pathways on leukemic cells can be undertaken in the future investigations. Additionally, these checkpoints and cells should be considered plausible multi-targeted therapies for AML in combination with other conventional treatments in AML. Video Abstract.

High GLI-1 Expression is a Reliable Indicator of Bad Prognosis in Newly Diagnosed Acute Leukemia Patients

PURPOSE

To explore the expression and prognostic significance of Hedgehog signaling transcription factor GLI-1 in newly diagnosed acute myeloid leukemia (AML) patients.

METHODS

Clinical specimens were obtained from 46 recently diagnosed AML patients. Real-time qPCR was used to measure the GLI-1 mRNA expression in bone marrow mononuclear cells.Also, the relationship between GLI-1 mRNA levels and clinical variables and prognostic variables was assessed.

RESULTS

GLI-1 was overexpressed in the bone marrow samples of our patients. GLI-1mRNA expression did not differ significantly across different age groups, between both sexes, or between different FAB subtypes (P = 0.882, P = 0.246, and P = 0.890, respectively). GLI-1 expression varied significantly in different risk categories, with the greatest levels observed in 11 patients with poor risk (24.6 versus 22.7) compared to intermediate risk (5.2 versus 3.9; P = 0.006) and favorable risk (4.2 versus 3; P = 0.001). Comparing patients with the wild FLT3 allele to those with the mutant one, GLI-1 gene levels were considerably greater in those with the mutant allele of FLT3.Following induction chemotherapy, the levels of GLI-1 mRNA were significantly higher in 22 patients who did not experience complete remission (CR) diagnosed with de novo non-acute promyelocytic leukemia (APL) compared to 17 patients who did (P = 0.017). Significantly greater levels of expression were observed in each category of the patients with favorable risk; wild FLT3 allele (P = 0.033) and CR failure P = 0.005).

CONCLUSION

GLI-1 overexpression is a risk factor for poor prognosis and could be a novel therapeutic target for AML.

Understanding the Wnt Signaling Pathway in Acute Myeloid Leukemia Stem Cells: A Feasible Key against Relapses

Wnt signaling is a highly conserved pathway in evolution which controls important processes such as cell proliferation, differentiation and migration, both in the embryo and in the adult. Dysregulation of this pathway can favor the development of different types of cancer, such as acute myeloid leukemia and other hematological malignancies. Overactivation of this pathway may promote the transformation of pre-leukemic stem cells into acute myeloid leukemia stem cells, as well as the maintenance of their quiescent state, which confers them with self-renewal and chemoresistance capacity, favoring relapse of the disease. Although this pathway participates in the regulation of normal hematopoiesis, its requirements seem to be greater in the leukemic stem cell population. In this review, we explore the possible therapeutic targeting of Wnt to eradicate the LSCs of AML.

The Hedgehog Pathway as a Therapeutic Target in Chronic Myeloid Leukemia

Despite the development of therapeutic agents that selectively target cancer cells, relapse driven by acquired drug resistance and resulting treatment failure remains a significant issue. The highly conserved Hedgehog (HH) signaling pathway performs multiple roles in both development and tissue homeostasis, and its aberrant regulation is known to drive the pathogenesis of numerous human malignancies. However, the role of HH signaling in mediating disease progression and drug resistance remains unclear. This is especially true for myeloid malignancies. The HH pathway, and in particular the protein Smoothened (SMO), has been shown to be essential for regulating stem cell fate in chronic myeloid leukemia (CML). Evidence suggests that HH pathway activity is critical for maintaining the drug-resistant properties and survival of CML leukemic stem cells (LSCs), and that dual inhibition of BCR-ABL1 and SMO may comprise an effective therapeutic strategy for the eradication of these cells in patients. This review will explore the evolutionary origins of HH signaling, highlighting its roles in development and disease, which are mediated by canonical and non-canonical HH signaling. Development of small molecule inhibitors of HH signaling and clinical trials using these inhibitors as therapeutic agents in cancer and their potential resistance mechanisms, are also discussed, with a focus on CML.

MicroRNA mediated gene regulatory circuits leads to machine learning based preliminary detection of Acute Myeloid Leukemia

Acute Myeloid Leukemia (AML) can be detected based on morphology, cytochemistry, immunological markers, and cytogenetics. MicroRNAs (miRNAs) influence key biological pathways in multiple haematological malignancies including AML. In this work, we have analysed the miRNome and the transcriptome of normal and AML samples and have identified the significant set of miRNA-target mRNA pairs present within AML- Peripheral Blood and AML- Bone Marrow samples from both tissue and cell lines. The miRNA target genes are further filtered based on their functional significance in AML system. These filtered genes constitute the set of selected miRNA target features, which have been finally used for developing machine learning based prediction tool, 'TbAMLPred' for preliminary detection of AML. This model implements both unsupervised clustering and supervised classification algorithms that would increase the reliability of prediction. Our results show that the selected miRNA target-based features can separate the control and disease samples linearly. Overall, we put forward 'TbAMLPred' for a non-invasive mode of preliminary AML diagnosis in future. Github link for accessing TbAMLPred: https://github.com/zglabDIB/TbAMLPred.

Inhibition of GPR39 restores defects in endothelial cell-mediated neovascularization under the duress of chronic hyperglycemia: Evidence for regulatory roles of the sonic hedgehog signaling axis

Impaired endothelial cell (EC)-mediated angiogenesis contributes to critical limb ischemia in diabetic patients. The sonic hedgehog (SHH) pathway participates in angiogenesis but is repressed in hyperglycemia by obscure mechanisms. We investigated the orphan G protein-coupled receptor GPR39 on SHH pathway activation in ECs and ischemia-induced angiogenesis in animals with chronic hyperglycemia. Human aortic ECs from healthy and type 2 diabetic (T2D) donors were cultured in vitro. GPR39 mRNA expression was significantly elevated in T2D. The EC proliferation, migration, and tube formation were attenuated by adenovirus-mediated GPR39 overexpression (Ad-GPR39) or GPR39 agonist TC-G-1008 in vitro. The production of proangiogenic factors was reduced by Ad-GPR39. Conversely, human ECs transfected with GPR39 siRNA or the mouse aortic ECs isolated from GPR39 global knockout (GPR39^KO) mice displayed enhanced migration and proliferation compared with their respective controls. GPR39 suppressed the basal and ligand-dependent activation of the SHH effector GLI1, leading to attenuated EC migration. Coimmunoprecipitation revealed that the GPR39 direct binding of the suppressor of fused (SUFU), the SHH pathway endogenous inhibitor, may achieve this. Furthermore, in ECs with GPR39 knockdown, the robust GLI1 activation and EC migration were abolished by SUFU overexpression. In a chronic diabetic model of diet-induced obesity (DIO) and low-dose streptozotocin (STZ)-induced hyperglycemia, the GPR39^KO mice demonstrated a faster pace of revascularization from hind limb ischemia and lower incidence of tissue necrosis than GPR39 wild-type (GPR39^WT) counterparts. These findings have provided a conceptual framework for developing therapeutic tools that ablate or inhibit GPR39 for ischemic tissue repair under metabolic stress.

Precision Medicine in Therapy of Non-solid Cancer

The development and approval of the tyrosine kinase inhibitor imatinib in 2001 has heralded the advance of directed therapy options. Today, an armamentarium of targeted therapeutics is available and enables the use of precision medicine in non-solid cancer. Precision medicine is guided by the detection of tumor-specific and targetable characteristics. These include pathogenic fusions and/or mutations, dependency on specific signaling pathways, and the expression of certain cell surface markers. Within the first part, we review approved targeted therapies for the compound classes of small molecule inhibitors, antibody-based therapies and cellular therapies. Particular consideration is given to the underlying pathobiology and the respective mechanism of action. The second part emphasizes on how biomarkers, whether they are of diagnostic, prognostic, or predictive relevance, are indispensable tools to guide therapy choice and management in precision medicine. Finally, the examples of acute myeloid leukemia, chronic lymphocytic leukemia, and chronic myeloid leukemia illustrate how integration of these biomarkers helps to tailor therapy.

The Development and Clinical Applications of Oral Arsenic Trioxide for Acute Promyelocytic Leukaemia and Other Diseases

Appreciation of the properties of arsenic trioxide (ATO) has redefined the treatment landscape for acute promyelocytic leukaemia (APL) and offers promise as a treatment for numerous other diseases. The benefits of ATO in patients with APL is related to its ability to counteract the effects of PML::RARA, an oncoprotein that is invariably detected in the blood or bone marrow of affected individuals. The PML::RARA oncoprotein is degraded specifically by binding to ATO. Thus ATO, in combination with all-trans retinoic acid, has become the curative treatment for ATO. The multiple mechanisms of action of ATO has also paved the way for application in various condition encompassing autoimmune or inflammatory disorders, solid organ tumours, lymphomas and other subtypes of AML. The development of oral formulation of ATO (oral ATO) has reduced costs of treatment and improved treatment convenience allowing widespread applicability. In this review, we discuss the mechanisms of action of ATO, the development of oral ATO, and the applications of oral ATO in APL and other diseases.

Profile of Glasdegib for the Treatment of Newly Diagnosed Acute Myeloid Leukemia (AML): Evidence to Date

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy primarily affecting older adults. Historically, the highest rates of response have been achieved with intensive induction chemotherapy; however, a significant portion of older or unfit adults with AML are unable to tolerate intensive therapy or have chemotherapy-resistant disease, creating a large need for active and less intensive treatment strategies. Glasdegib, an oral inhibitor of the transmembrane protein Smoothened (SMO) involved in the Hedgehog (Hh) signaling pathway, was approved in 2018 for older or unfit adults with AML and attained a role in clinical practice after showing an overall survival (OS) advantage when combined with the established agent low-dose cytarabine (LDAC). Since that time, however, several other highly active lower intensity therapies such as venetoclax plus a hypomethylating agent (HMA) have garnered a dominant role in the treatment of this patient population. In this review, we summarize the role of glasdegib in the current treatment landscape of newly diagnosed AML and discuss ongoing investigations into its role in novel combination therapies.

Understanding the Notch Signaling Pathway in Acute Myeloid Leukemia Stem Cells: From Hematopoiesis to Neoplasia

The Notch signaling pathway is fundamental to early fetal development, but its role in acute myeloid leukemia is still unclear. It is important to elucidate the function that contains Notch, not only in acute myeloid leukemia, but in leukemic stem cells (LSCs). LSCs seem to be the principal cause of patient relapse. This population is in a quiescent state. Signaling pathways that govern this process must be understood to increase the chemosensitivity of this compartment. In this review, we focus on the conserved Notch signaling pathway, and its repercussions in hematopoiesis and hematological neoplasia. We found in the literature both visions regarding Notch activity in acute myeloid leukemia. On one hand, the activation of Notch leads to cell proliferation, on the other hand, the activation of Notch leads to cell cycle arrest. This dilemma requires further experiments to be answered, in order to understand the role of Notch not only in acute myeloid leukemia, but especially in LSCs.

Management of Acute Myeloid Leukemia: Current Treatment Options and Future Perspectives

Treatment of acute myeloid leukemia (AML) has improved in recent years and several new therapeutic options have been approved. Most of them include mutation-specific approaches (e.g., gilteritinib for AML patients with activating FLT3 mutations), or are restricted to such defined AML subgroups, such as AML-MRC (AML with myeloid-related changes) or therapy-related AML (CPX-351). With this review, we aim to present a comprehensive overview of current AML therapy according to the evolved spectrum of recently approved treatment strategies. We address several aspects of combined epigenetic therapy with the BCL-2 inhibitor venetoclax and provide insight into mechanisms of resistance towards venetoclax-based regimens, and how primary or secondary resistance might be circumvented. Furthermore, a detailed overview on the current status of AML immunotherapy, describing promising concepts, is provided. This review focuses on clinically important aspects of current and future concepts of AML treatment, but will also present the molecular background of distinct targeted therapies, to understand the development and challenges of clinical trials ongoing in AML patients.

Macrophages and cancer stem cells: a malevolent alliance

Myeloid cells infiltrating tumors are gaining ever growing attention in the last years because their pro-tumor and immunosuppressive functions are relevant for disease progression and therapeutic responses. The functional ambiguity of tumor-associated macrophages (TAMs), mostly promoting tumor evolution, is a challenging hurdle. This is even more evident in the case of cancer stem cells (CSCs); as active participants in the specialized environment of the cancer stem cell niche, TAMs initiate a reciprocal conversation with CSCs. TAMs contribute to protect CSCs from the hostile environment (exogenous insults, toxic compounds, attacks from the immune cells), and produce several biologically active mediators that modulate crucial developmental pathways that sustain cancer cell stemness. In this review, we have focused our attention on the interaction between TAMs and CSCs; we describe how TAMs impact on CSC biology and, in turn, how CSCs exploit the tissue trophic activity of macrophages to survive and progress. Since CSCs are responsible for therapy resistance and tumor recurrence, they are important therapeutic targets. In view of the recent success in oncology obtained by stimulating the immune system, we discuss some macrophage-targeted therapeutic strategies that may also affect the CSCs and interrupt their malevolent alliance.

Can the New and Old Drugs Exert an Immunomodulatory Effect in Acute Myeloid Leukemia?

Simple Summary

The advent of novel immunotherapeutic strategies has revealed the importance of immune dysregulation and of a tolerogenic microenvironment for acute myeloid leukemia (AML) fitness. We reviewed the “off-target” effects on the immune system of different drugs used in the treatment of AML to explore the advantages of this unexpected interaction.

Abstract

Acute myeloid leukemia (AML) is considered an immune-suppressive neoplasm capable of evading immune surveillance through cellular and environmental players. Increasing knowledge of the immune system (IS) status at diagnosis seems to suggest ever more attention of the crosstalk between the leukemic clone and its immunologic counterpart. During the last years, the advent of novel immunotherapeutic strategies has revealed the importance of immune dysregulation and suppression for leukemia fitness. Considering all these premises, we reviewed the “off-target” effects on the IS of different drugs used in the treatment of AML, focusing on the main advantages of this interaction. The data reported support the idea that a successful therapeutic strategy should consider tailored approaches for performing leukemia eradication by both direct blasts killing and the engagement of the IS.