Inhibition of the Axl pathway impairs breast and prostate cancer metastasis to the bones and bone remodeling

The Role of TAM Receptors in Bone

The TAM (TYRO3, MERTK, and AXL) family of receptor tyrosine kinases are pleiotropic regulators of adult tissue homeostasis maintaining organ integrity and self-renewal. Disruption of their homeostatic balance fosters pathological conditions like autoinflammatory or degenerative diseases including rheumatoid arthritis, lupus erythematodes, or liver fibrosis. Moreover, TAM receptors exhibit prominent cell-transforming properties, promoting tumor progression, metastasis, and therapy resistance in various cancer entities. Emerging evidence shows that TAM receptors are involved in bone homeostasis by regulating osteoblastic bone formation and osteoclastic bone resorption. Therefore, TAM receptors emerge as new key players of the regulatory cytokine network of osteoblasts and osteoclasts and represent accessible targets for pharmacologic therapy for a broad set of different bone diseases, including primary and metastatic bone tumors, rheumatoid arthritis, or osteoporosis.

Structure-based drug discovery of novel fused-pyrazolone carboxamide derivatives as potent and selective AXL inhibitors

As a novel and promising antitumor target, AXL plays an important role in tumor growth, metastasis, immunosuppression and drug resistance of various malignancies, which has attracted extensive research interest in recent years. In this study, by employing the structure-based drug design and bioisosterism strategies, we designed and synthesized in total 54 novel AXL inhibitors featuring a fused-pyrazolone carboxamide scaffold, of which up to 20 compounds exhibited excellent AXL kinase and BaF3/TEL-AXL cell viability inhibitions. Notably, compound 59 showed a desirable AXL kinase inhibitory activity (IC50: 3.5 nmol/L) as well as good kinase selectivity, and it effectively blocked the cellular AXL signaling. In turn, compound 59 could potently inhibit BaF3/TEL-AXL cell viability (IC50: 1.5 nmol/L) and significantly suppress GAS6/AXL-mediated cancer cell invasion, migration and wound healing at the nanomolar level. More importantly, compound 59 oral administration showed good pharmacokinetic profile and in vivo antitumor efficiency, in which we observed significant AXL phosphorylation suppression, and its antitumor efficacy at 20 mg/kg (qd) was comparable to that of BGB324 at 50 mg/kg (bid), the most advanced AXL inhibitor. Taken together, this work provided a valuable lead compound as a potential AXL inhibitor for the further antitumor drug development.

Inhibition of Mertk Signaling Enhances Bone Healing after Tooth Extraction

Regeneration of alveolar bone is an essential step in restoring healthy function following tooth extraction. Growth of new bone in the healing extraction socket can be variable and often unpredictable when systemic comorbidities are present, leading to the need for additional therapeutic targets to accelerate the regenerative process. One such target is the TAM family (Tyro3, Axl, Mertk) of receptor tyrosine kinases. These proteins have been shown to help resolve inflammation and maintain bone homeostasis and thus may have therapeutic benefits in bone regeneration following extraction. Treatment of mice with a pan-TAM inhibitor (RXDX-106) led to accelerated alveolar bone fill following first molar extraction in a mouse model without changing immune infiltrate. Treatment of human alveolar bone mesenchymal stem cells with RXDX-106 upregulated Wnt signaling and primed the cells for osteogenic differentiation. Differentiation of human alveolar bone mesenchymal stem cells with osteogenic media and TAM-targeted inhibitor RXDX-106 (pan-TAM), ASP-2215 (Axl specific), or MRX-2843 (Mertk specific) showed enhanced mineralization with pan-TAM or Mertk-specific inhibitors and no change with Axl-specific inhibitor. First molar extractions in Mertk-/- mice had increased alveolar bone regeneration in the extraction socket relative to wild type controls 7 d postextraction. Flow cytometry of 7-d extraction sockets showed no difference in immune cell numbers between Mertk-/- and wild type mice. RNAseq of day 7 extraction sockets showed increased innate immune-related pathways and genes associated with bone differentiation in Mertk-/- mice. Together, these results indicate that TAM receptor signaling, specifically through Mertk, can be targeted to enhance bone regeneration after injury.

Functional Interrogation of Ca2+ Signals in Human Cancer Cells In Vitro and Ex Vivo by Fluorescent Microscopy and Molecular Tools

Genetically encoded calcium indicators (GECIs) and high-resolution confocal microscopy enable dynamic visualization of calcium signals in cells and tissues. Two-dimensional and 3D biocompatible materials mimic the mechanical microenvironments of tumor and healthy tissues in a programmable manner. Cancer xenograft models and ex vivo functional imaging of tumor slices reveal physiologically relevant functions of calcium dynamics in tumors at different progression stages. Integration of these powerful techniques allows us to quantify, diagnose, model, and understand cancer pathobiology. Here, we describe detailed materials and methods used to establish this integrated interrogation platform, from generating transduced cancer cell lines that stably express CaViar (GCaMP5G + QuasAr2) to in vitro and ex vivo calcium imaging of the cells in 2D/3D hydrogels and tumor tissues. These tools open the possibility for detailed explorations of mechano-electro-chemical network dynamics in living systems.

Current methods for studying metastatic potential of tumor cells

Cell migration and invasiveness significantly contribute to desirable physiological processes, such as wound healing or embryogenesis, as well as to serious pathological processes such as the spread of cancer cells to form tumor metastasis. The availability of appropriate methods for studying these processes is essential for understanding the molecular basis of cancer metastasis and for identifying suitable therapeutic targets for anti-metastatic treatment. This review summarizes the current status of these methods: In vitro methods for studying cell migration involve two-dimensional (2D) assays (wound-healing/scratch assay), and methods based on chemotaxis (the Dunn chamber). The analysis of both cell migration and invasiveness in vitro require more complex systems based on the Boyden chamber principle (Transwell migration/invasive test, xCELLigence system), or microfluidic devices with three-dimensional (3D) microscopy visualization. 3D culture techniques are rapidly becoming routine and involve multicellular spheroid invasion assays or array chip-based, spherical approaches, multi-layer/multi-zone culture, or organoid non-spherical models, including multi-organ microfluidic chips. The in vivo methods are mostly based on mice, allowing genetically engineered mice models and transplant models (syngeneic mice, cell line-derived xenografts and patient-derived xenografts including humanized mice models). These methods currently represent a solid basis for the state-of-the art research that is focused on understanding metastatic fundamentals as well as the development of targeted anti-metastatic therapies, and stratified treatment in oncology.

Bone Marrow Endothelial Cells Increase Prostate Cancer Cell Apoptosis in 3D Triculture Model of Reactive Stroma

Simple Summary

Prostate cancer (PCa) metastasizes preferentially to the bone marrow where it becomes difficult to treat. PCa cells in the bone marrow may survive, dormant and undetected for many years before patients eventually relapse with metastatic disease. Bone marrow is a complex tissue that initially is hostile to the PCa cells, Understanding how cancer cells survive in the bone marrow and what changes to the bone microenvironment permit them to switch to an actively growing state could offer new therapeutic strategies to combat metastatic PCa. In this study, we describe a method to culture PCa cells with two other cell types from the bone marrow, stromal cells and endothelial cells, as a way to study the interactions among these cell types. We found that factors produced by bone marrow endothelial cells, but not endothelial cells from other tissues, trigger PCa cells to either die or enter a dormant state, similar to what has been observed in patients when PCa cells initially colonize the bone marrow. Further analysis of the cell interactions within the culture model described in this study will offer increased understanding of PCa interaction with the bone marrow environment.

Abstract

The bone marrow tumor microenvironment (BMTE) is a complex network of cells, extracellular matrix, and sequestered signaling factors that initially act as a hostile environment for disseminating tumor cells (DTCs) from the cancerous prostate. Three-dimensional (3D) culture systems offer an opportunity to better model these complex interactions in reactive stroma, providing contextual behaviors for cancer cells, stromal cells, and endothelial cells. Using a new system designed for the triculture of osteoblastic prostate cancer (PCa) cells, stromal cells, and microvascular endothelial cells, we uncovered a context-specific pro-apoptotic effect of endothelial cells of the bone marrow different from those derived from the lung or dermis. The paracrine nature of this effect was demonstrated by observations that conditioned medium from bone marrow endothelial cells, but not from dermal or lung endothelial cells, led to PCa cell death in microtumors grown in 3D BMTE-simulating hydrogels. Analysis of the phosphoproteome by reverse phase protein analysis (RPPA) of PCa cells treated with conditioned media from different endothelial cells identified the differential regulation of pathways involved in proliferation, cell cycle regulation, and apoptosis. The findings from the RPPA were validated by western blotting for representative signaling factors identified, including forkhead box M1 (FOXM1; proliferation factor), pRb (cell cycle regulator), and Smac/DIABLO (pro-apoptosis) among treatment conditions. The 3D model presented here thus presents an accurate model to study the influence of the reactive BMTE, including stromal and endothelial cells, on the adaptive behaviors of cancer cells modeling DTCs at sites of bone metastasis. These findings in 3D culture systems can lead to a better understanding of the real-time interactions among cells present in reactive stroma than is possible using animal models.

Towards an integrative understanding of cancer mechanobiology: calcium, YAP, and microRNA under biophysical forces

An increasing number of studies have demonstrated the significant roles of the interplay between microenvironmental mechanics in tissues and biochemical-genetic activities in resident tumor cells at different stages of tumor progression. Mediated by molecular mechano-sensors or -transducers, biomechanical cues in tissue microenvironments are transmitted into the tumor cells and regulate biochemical responses and gene expression through mechanotransduction processes. However, the molecular interplay between the mechanotransduction processes and intracellular biochemical signaling pathways remains elusive. This paper reviews the recent advances in understanding the crosstalk between biomechanical cues and three critical biochemical effectors during tumor progression: calcium ions (Ca²⁺), yes-associated protein (YAP), and microRNAs (miRNAs). We address the molecular mechanisms underpinning the interplay between the mechanotransduction pathways and each of the three effectors. Furthermore, we discuss the functional interactions among the three effectors in the context of soft matter and mechanobiology. We conclude by proposing future directions on studying the tumor mechanobiology that can employ Ca²⁺, YAP, and miRNAs as novel strategies for cancer mechanotheraputics. This framework has the potential to bring insights into the development of novel next-generation cancer therapies to suppress and treat tumors.

Therapeutic Targeting of the Gas6/Axl Signaling Pathway in Cancer

Many signaling pathways are dysregulated in cancer cells and the host tumor microenvironment. Aberrant receptor tyrosine kinase (RTK) pathways promote cancer development, progression, and metastasis. Hence, numerous therapeutic interventions targeting RTKs have been actively pursued. Axl is an RTK that belongs to the Tyro3, Axl, MerTK (TAM) subfamily. Axl binds to a high affinity ligand growth arrest specific 6 (Gas6) that belongs to the vitamin K-dependent family of proteins. The Gas6/Axl signaling pathway has been implicated to promote progression, metastasis, immune evasion, and therapeutic resistance in many cancer types. Therapeutic agents targeting Gas6 and Axl have been developed, and promising results have been observed in both preclinical and clinical settings when such agents are used alone or in combination therapy. This review examines the current state of therapeutics targeting the Gas6/Axl pathway in cancer and discusses Gas6- and Axl-targeting agents that have been evaluated preclinically and clinically.