Macropinocytosis Renders a Subset of Pancreatic Tumor Cells Resistant to mTOR Inhibition

Macropinocytosis at the crossroad between nutrient scavenging and metabolism in cancer

Macropinocytosis (MP), the actin-dependent bulk uptake of extracellular fluids, plays a central role in nutrient scavenging, allowing cancer cells to sustain their growth in the hypoxic and nutrient-deprived microenvironment often found in solid tumours. The lack of soluble nutrients and several oncogenic signalling pathways, with RAS being the most studied, push MP-dependent internalisation of extracellular proteins, which are then digested in the lysosomes, replenishing the intracellular nutrient pools. This review will highlight recent advances in understanding how MP is regulated in hypoxic cancers, how it impinges on chemoresistance, and how different MP cargos facilitate tumour growth. Finally, I will highlight the crosstalk between MP and extracellular matrix receptors.

Excitatory amino acid transporter supports inflammatory macrophage responses

Excitatory amino acid transporters (EAATs) are responsible for excitatory amino acid transportation and are associated with auto-immune diseases in the central nervous system and peripheral tissues. However, the subcellular location and function of EAAT2 in macrophages are still obscure. In this study, we demonstrated that LPS stimulation increases expression of EAAT2 (coded by Slc1a2) via NF-κB signaling. EAAT2 is necessary for inflammatory macrophage polarization through sustaining mTORC1 activation. Mechanistically, lysosomal EAAT2 mediates lysosomal glutamate and aspartate efflux to maintain V-ATPase activation, which sustains macropinocytosis and mTORC1. We also found that mice with myeloid depletion of Slc1a2 show alleviated inflammatory responses in LPS-induced systemic inflammation and high-fat diet induced obesity. Notably, patients with type II diabetes (T2D) have a higher level of expression of lysosomal EAAT2 and activation of mTORC1 in blood macrophages. Taken together, our study links the subcellular location of amino acid transporters with the fate decision of immune cells, which provides potential therapeutic targets for the treatment of inflammatory diseases.

A Mammalian Conserved Circular RNA CircLARP1B Regulates Hepatocellular Carcinoma Metastasis and Lipid Metabolism

Circular RNAs (circRNAs) have emerged as crucial regulators in physiology and human diseases. However, evolutionarily conserved circRNAs with potent functions in cancers are rarely reported. In this study, a mammalian conserved circRNA circLARP1B is identified to play critical roles in hepatocellular carcinoma (HCC). Patients with high circLARP1B levels have advanced prognostic stage and poor overall survival. CircLARP1B facilitates cellular metastatic properties and lipid accumulation through promoting fatty acid synthesis in HCC. CircLARP1B deficient mice exhibit reduced metastasis and less lipid accumulation in an induced HCC model. Multiple lines of evidence demonstrate that circLARP1B binds to heterogeneous nuclear ribonucleoprotein D (HNRNPD) in the cytoplasm, and thus affects the binding of HNRNPD to sensitive transcripts including liver kinase B1 (LKB1) mRNA. This regulation causes decreased LKB1 mRNA stability and lower LKB1 protein levels. Antisense oligodeoxynucleotide complementary to theHNRNPD binding sites in circLARP1B increases the HNRNPD binding to LKB1 mRNA. Through the HNRNPD-LKB1-AMPK pathway, circLARP1B promotes HCC metastasis and lipid accumulation. Results from AAV8-mediated hepatocyte-directed knockdown of circLARP1B or Lkb1 in mouse models also demonstrate critical roles of hepatocytic circLARP1B regulatory pathway in HCC metastasis and lipid accumulation, and indicate that circLARP1B may be potential target of HCC treatment.

Longitudinal evaluation of the biodistribution and cellular internalization of the bispecific CD3xTRP1 antibody in syngeneic mouse tumor models

BACKGROUND

CD3 bispecific antibodies (CD3-bsAbs) require binding of both a tumor-associated surface antigen and CD3 for their immunotherapeutic effect. Their efficacy is, therefore, influenced by the tumor uptake and the extracellular dose. To optimize their currently limited efficacy in solid tumors, increased understanding of their pharmacokinetics and in vivo internalization is needed.

METHODS

Here, were studied the pharmacokinetics and in vivo internalization of CD3xTRP1, a fully murine Fc-inert bsAb, in endogenous TRP1-expressing immunocompetent male C57BL/6J mice bearing TRP1-positive and negative tumors over time. Matching bsAbs lacking TRP1-binding or CD3-binding capacity served as controls. BsAbs were radiolabeled with 111In to investigate their pharmacokinetics, target binding, and biodistribution through SPECT/CT imaging and ex vivo biodistribution analyses. Co-injection of 111In- and 125I-labeled bsAb was performed to investigate the in vivo internalization by comparing tissue concentrations of cellular residing 111In versus effluxing 125I. Antitumor therapy effects were evaluated by monitoring tumor growth and immunohistochemistry.

RESULTS

SPECT/CT and biodistribution analyses showed that CD3xTRP1 specifically targeted TRP1-positive tumors and CD3-rich lymphoid organ and uptake peaked 24 hours pi (KPC3-TRP1: 37.7%ID/g±5.3%ID/g, spleen: 29.0%ID/g±3.9%ID/g). Studies with control bsAbs demonstrated that uptake of CD3xTRP1 in TRP1-positive tumors and CD3-rich tissues was primarily receptor-mediated. Together with CD3xTRP1 in the circulation being mainly unattached, this indicates that CD3+ T cells are generally not traffickers of CD3-bsAbs to the tumor. Additionally, target-mediated clearance by TRP1-expressing melanocytes was not observed. We further demonstrated rapid internalization of CD3xTRP1 in KPC3-TRP1 tumors (24 hours pi: 54.9%±2.3% internalized) and CD3-rich tissues (spleen, 24 hours pi: 79.7%±0.9% internalized). Therapeutic effects by CD3xTRP1 were observed for TRP1-positive tumors and consisted of high tumor influx of CD8+ T cells and neutrophils, which corresponded with increased necrosis and growth delay.

CONCLUSIONS

We show that CD3xTRP1 efficiently targets TRP1-positive tumors and CD3-rich tissues primarily through receptor-mediated targeting. We further demonstrate rapid receptor-mediated internalization of CD3xTRP1 in TRP1-positive tumors and CD3-rich tissues. Even though this significantly decreases the therapeutical available dose, CD3xTRP1 still induced effective antitumor T-cell responses and inhibited tumor growth. Together, our data on the pharmacokinetics and mechanism of action of CD3xTRP1 pave the way for further optimization of CD3-bsAb therapies.

Tumor microenvironmental nutrients, cellular responses, and cancer

Over the last two decades, the rapidly expanding field of tumor metabolism has enhanced our knowledge of the impact of nutrient availability on metabolic reprogramming in cancer. Apart from established roles in cancer cells themselves, various nutrients, metabolic enzymes, and stress responses are key to the activities of tumor microenvironmental immune, fibroblastic, endothelial, and other cell types that support malignant transformation. In this article, we review our current understanding of how nutrient availability affects metabolic pathways and responses in both cancer and "stromal" cells, by dissecting major examples and their regulation of cellular activity. Understanding the relationship of nutrient availability to cellular behaviors in the tumor ecosystem will broaden the horizon of exploiting novel therapeutic vulnerabilities in cancer.

Quantitation of macropinocytosis in glioblastoma based on high-content analysis

BACKGROUND

Macropinocytosis is a pathway utilized for the internalization of extracellular fluid, albumin and dissolved molecules. Assessing macropinocytosis has been challenging in the past because the combination of manual acquisition and visual evaluation of images is laborious, making this type of assessment difficult for high-throughput applications. Therefore, there is a need to develop sensitive and specific macropinocytosis evaluation methods.

METHODS

This paper proposed a quantitative and time-saving method for macropinocytosis detection based on high-content analysis (HCA). Additionally, cell proliferation was evaluated using CCK8 test.

RESULTS

The term "macropinosome index" was defined to estimate macropinocytosis and allow comparisons between different cell lines and treatments. Furthermore, we demonstrated that macropinocytosis can promote glioblastoma (GBM) cell survival under L-glutamine (L-Gln)-deficient conditions that resemble the tumour microenvironment.

CONCLUSIONS

HCA represents a novel, nonsubjective and high-throughput assay for macropinocytosis assessment. In addition, L-Gln deprivation increased the macropinosome index in GBM cells, suggesting that this process may be used to design GBM therapies.

AVAILABILITY OF DATA AND MATERIALS

The datasets supporting the conclusions of this article are included within the article and its supplementary materials.

Macropinocytosis: mechanisms and regulation

Macropinocytosis is defined as an actin-dependent but coat- and dynamin-independent endocytic uptake process, which generates large intracellular vesicles (macropinosomes) containing a non-selective sampling of extracellular fluid. Macropinocytosis provides an important mechanism of immune surveillance by dendritic cells and macrophages, but also serves as an essential nutrient uptake pathway for unicellular organisms and tumor cells. This review examines the cell biological mechanisms that drive macropinocytosis, as well as the complex signaling pathways - GTPases, lipid and protein kinases and phosphatases, and actin regulatory proteins - that regulate macropinosome formation, internalization, and disposition.

An emerging master inducer and regulator for epithelial-mesenchymal transition and tumor metastasis: extracellular and intracellular ATP and its molecular functions and therapeutic potential

Despite the rapid development of therapeutic strategies in cancer treatment, metastasis remains the major cause of cancer-related death and scientific challenge. Epithelial-Mesenchymal Transition (EMT) plays a crucial role in cancer invasion and progression, a process by which tumor cells lose cell-cell adhesion and acquire increased invasiveness and metastatic activity. Recent work has uncovered some crucial roles of extracellular adenosine 5'- triphosphate (eATP), a major component of the tumor microenvironment (TME), in promoting tumor growth and metastasis. Intratumoral extracellular ATP (eATP), at levels of 100-700 µM, is 10³-10⁴ times higher than in normal tissues. In the current literature, eATP's function in promoting metastasis has been relatively poorly understood as compared with intracellular ATP (iATP). Recent evidence has shown that cancer cells internalize eATP via macropinocytosis in vitro and in vivo, promoting cell growth and survival, drug resistance, and metastasis. Furthermore, ATP acts as a messenger molecule that activates P2 purinergic receptors expressed on both tumor and host cells, stimulating downstream signaling pathways to enhance the invasive and metastatic properties of tumor cells. Here, we review recent progress in understanding eATP's role in each step of the metastatic cascade, including initiating invasion, inducing EMT, overcoming anoikis, facilitating intravasation, circulation, and extravasation, and eventually establishing metastatic colonization. Collectively, these studies reveal eATP's important functions in many steps of metastasis and identify new opportunities for developing more effective therapeutic strategies to target ATP-associated processes in cancer.

The phospholipid flippase ATP9A enhances macropinocytosis to promote nutrient starvation tolerance in hepatocellular carcinoma

Macropinocytosis is an effective strategy to mitigate nutrient starvation. It can fuel cancer cell growth in nutrient-limited conditions. However, whether and how macropinocytosis contributes to the rapid proliferation of hepatocellular carcinoma cells, which frequently experience an inadequate nutrient supply, remains unclear. Here, we demonstrated that nutrient starvation strongly induced macropinocytosis in some hepatocellular carcinoma cells. It allowed the cells to acquire extracellular nutrients and supported their energy supply to maintain rapid proliferation. Furthermore, we found that the phospholipid flippase ATP9A was critical for regulating macropinocytosis in hepatocellular carcinoma cells and that high ATP9A levels predicted a poor outcome for patients with hepatocellular carcinoma. ATP9A interacted with ATP6V1A and facilitated its transport to the plasma membrane, which promoted plasma membrane cholesterol accumulation and drove RAC1-dependent macropinocytosis. Macropinocytosis inhibitors significantly suppressed the energy supply and proliferation of hepatocellular carcinoma cells characterised by high ATP9A expression under nutrient-limited conditions. These results have revealed a novel mechanism that overcomes nutrient starvation in hepatocellular carcinoma cells and have identified the key regulator of macropinocytosis in hepatocellular carcinoma. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Acetyl-Coenzyme A Synthetase 2 Potentiates Macropinocytosis and Muscle Wasting Through Metabolic Reprogramming in Pancreatic Cancer

BACKGROUND & AIMS

Rapid deconditioning, also called cachexia, and metabolic reprogramming are two hallmarks of pancreatic cancer. Acetyl-coenzyme A synthetase short-chain family member 2 (ACSS2) is an acetyl-enzyme A synthetase that contributes to lipid synthesis and epigenetic reprogramming. However, the role of ACSS2 on the nonselective macropinocytosis and cancer cachexia in pancreatic cancer remains elusive. In this study, we demonstrate that ACSS2 potentiates macropinocytosis and muscle wasting through metabolic reprogramming in pancreatic cancer.

METHODS

Clinical significance of ACSS2 was analyzed using samples from patients with pancreatic cancer. ACSS2-knockout cells were established using the clustered regularly interspaced short palindromic repeats-associated protein 9 system. Single-cell RNA sequencing data from genetically engineered mouse models was analyzed. The macropinocytotic index was evaluated by dextran uptake assay. Chromatin immunoprecipitation assay was performed to validate transcriptional activation. ACSS2-mediated tumor progression and muscle wasting were examined in orthotopic xenograft models.

RESULTS

Metabolic stress induced ACSS2 expression, which is associated with worse prognosis in pancreatic cancer. ACSS2 knockout significantly suppressed cell proliferation in 2-dimensional and 3-dimensional models. Macropinocytosis-associated genes are upregulated in tumor tissues and are correlated with worse prognosis. ACSS2 knockout inhibited macropinocytosis. We identified Zrt- and Irt-like protein 4 (ZIP4) as a downstream target of ACSS2, and knockdown of ZIP4 reversed ACSS2-induced macropinocytosis. ACSS2 upregulated ZIP4 through ETV4-mediated transcriptional activation. ZIP4 induces macropinocytosis through cyclic adenosine monophosphate response element-binding protein-activated syndecan 1 (SDC1) and dynamin 2 (DNM2). Meanwhile, ZIP4 drives muscle wasting and cachexia via glycogen synthase kinase-β (GSK3β)-mediated secretion of tumor necrosis factor superfamily member 10 (TRAIL or TNFSF10). ACSS2 knockout attenuated muscle wasting and extended survival in orthotopic mouse models.

CONCLUSIONS

ACSS2-mediated metabolic reprogramming activates the ZIP4 pathway, and promotes macropinocytosis via SDC1/DNM2 and drives muscle wasting through the GSK3β/TRAIL axis, which potentially provides additional nutrients for macropinocytosis in pancreatic cancer.

Molecular Docking and Intracellular Translocation of Extracellular Vesicles for Efficient Drug Delivery

Extracellular vesicles (EVs), including exosomes, mediate intercellular communication by delivering their contents, such as nucleic acids, proteins, and lipids, to distant target cells. EVs play a role in the progression of several diseases. In particular, programmed death-ligand 1 (PD-L1) levels in exosomes are associated with cancer progression. Furthermore, exosomes are being used for new drug-delivery systems by modifying their membrane peptides to promote their intracellular transduction via micropinocytosis. In this review, we aim to show that an efficient drug-delivery system and a useful therapeutic strategy can be established by controlling the molecular docking and intracellular translocation of exosomes. We summarise the mechanisms of molecular docking of exosomes, the biological effects of exosomes transmitted into target cells, and the current state of exosomes as drug delivery systems.

The Role and Therapeutic Potential of Macropinocytosis in Cancer

Macropinocytosis, a unique endocytosis pathway characterized by nonspecific internalization, has a vital role in the uptake of extracellular substances and antigen presentation. It is known to have dual effects on cancer cells, depending on cancer type and certain microenvironmental conditions. It helps cancer cells survive in nutrient-deficient environments, enhances resistance to anticancer drugs, and promotes invasion and metastasis. Conversely, overexpression of the RAS gene alongside drug treatment can lead to methuosis, a novel mode of cell death. The survival and proliferation of cancer cells is closely related to macropinocytosis in the tumor microenvironment (TME), but identifying how these cells interface with the TME is crucial for creating drugs that can limit cancer progression and metastasis. Substantial progress has been made in recent years on designing anticancer therapies that utilize the effects of macropinocytosis. Both the induction and inhibition of macropinocytosis are useful strategies for combating cancer cells. This article systematically reviews the general mechanisms of macropinocytosis, its specific functions in tumor cells, its occurrence in nontumor cells in the TME, and its application in tumor therapies. The aim is to elucidate the role and therapeutic potential of macropinocytosis in cancer treatment.

Targeting Membrane Trafficking as a Strategy for Cancer Treatment

Membrane trafficking is emerging as an attractive therapeutic strategy for cancer. Recent reports have found a connection between Wnt signaling, receptor-mediated endocytosis, V-ATPase, lysosomal activity, and macropinocytosis through the canonical Wnt pathway. In macropinocytic cells, a massive internalization of the plasma membrane can lead to the loss of cell-surface cadherins, integrins, and other antigens that mediate cell–cell adhesion, favoring an invasive phenotype. V-ATPase is a key regulator in maintaining proper membrane trafficking, homeostasis, and the earliest developmental decisions in the Xenopus vertebrate development model system. Here, we review how the interference of membrane trafficking with membrane trafficking inhibitors might be clinically relevant in humans.

Macropinocytosis and Cancer: From Tumor Stress to Signaling Pathways

Macropinocytosis is an evolutionarily conserved endocytic pathway that mediates the nonselective acquisition of extracellular material via large endocytic vesicles known as macropinosomes. In addition to other functions, this uptake pathway supports cancer cell metabolism through the uptake of nutrients. Cells harboring oncogene or tumor suppressor mutations are known to display heightened macropinocytosis, which confers to the cancer cells the ability to survive and proliferate despite the nutrient-scarce conditions of the tumor microenvironment. Thus, macropinocytosis is associated with cancer malignancy. Macropinocytic uptake can be induced in cancer cells by different stress stimuli, acting as an adaptive mechanism for the cells to resist stresses in the tumor milieu. Here, we review the cellular stresses that are known to promote macropinocytosis, as well as the underlying molecular mechanisms that drive this process.

KRAS Addiction Promotes Cancer Cell Adaptation in Harsh Microenvironment Through Macropinocytosis

KRAS is the most frequently mutated oncogene in cancer and despite intensive studies, attempts to develop effective therapies targeting KRAS or its downstream signaling have failed mostly due to the complexity of KRAS activation and function in cancer initiation and progression. Over the years, KRAS has been involved in several biological processes including cell survival, proliferation, and metabolism by promoting not only a favorable tumor environment but also a cell-microenvironment dialog to allow cancer cells to adapt to tumor microenvironment scarcity. One of the mechanisms involved in this adaption is KRAS-mediated macropinocytosis. Macropinocytosis is an evolutionarily conserved, large-scale, and nonselective form of endocytosis involving actin-driven cell membrane remodeling to engulf large amounts of extracellular fluids and proteins from the local environment. While macropinocytosis process has been known for decades, recent gain interest due to its regulation of KRAS-driven tumor growth in adverse microenvironments. By promoting extracellular protein and other macromolecules internalization, macropinocytosis provides a survival mechanism under nutrient scarce conditions and the potential for unrestricted tumor growth. Thus, a better understanding of macropinocytotic process is needed to develop alternative therapeutic strategies.

Hypoxia-induced macropinocytosis represents a metabolic route for liver cancer

Hepatocellular carcinoma (HCC) invariably exhibits inadequate O₂ (hypoxia) and nutrient supply. Hypoxia-inducible factor (HIF) mediates cascades of molecular events that enable cancer cells to adapt and propagate. Macropinocytosis is an endocytic process initiated by membrane ruffling, causing the engulfment of extracellular fluids (proteins), protein digestion and subsequent incorporation into the biomass. We show that macropinocytosis occurs universally in HCC under hypoxia. HIF-1 activates the transcription of a membrane ruffling protein, EH domain-containing protein 2 (EHD2), to initiate macropinocytosis. Knockout of HIF-1 or EHD2 represses hypoxia-induced macropinocytosis and prevents hypoxic HCC cells from scavenging protein that support cell growth. Germline or somatic deletion of Ehd2 suppresses macropinocytosis and HCC development in mice. Intriguingly, EHD2 is overexpressed in HCC. Consistently, HIF-1 or macropinocytosis inhibitor suppresses macropinocytosis and HCC development. Thus, we show that hypoxia induces macropinocytosis through the HIF/EHD2 pathway in HCC cells, harnessing extracellular protein as a nutrient to survive.

Cancer cells rely on macropinocytosis to scavenge extracellular proteins for growth. Here the authors show that macropinocytosis supports the survival of hypoxic hepatocellular carcinoma cells and this is dependent on HIF-1, which in turns activates the transcription of a membrane ruffling protein, EH domain-containing protein 2.

Revealing macropinocytosis using nanoparticles

Endocytosis mechanisms are one of the methods that cells use to interact with their environments. Endocytosis mechanisms vary from the clathrin-mediated endocytosis to the receptor independent macropinocytosis. Macropinocytosis is a niche of endocytosis that is quickly becoming more relevant in various fields of research since its discovery in the 1930s. Macropinocytosis has several distinguishing factors from other receptor-mediated forms of endocytosis, including: types of extracellular material for uptake, signaling cascade, and niche uses between cell types. Nanoparticles (NPs) are an important tool for various applications, including drug delivery and disease treatment. However, surface engineering of NPs could be tailored to target them inside the cells exploiting different endocytosis pathways, such as endocytosis versus macropinocytosis. Such surface engineering of NPs mainly, size, charge, shape and the core material will allow identification of new adapter molecules regulating different endocytosis process and provide further insight into how cells tweak these pathways to meet their physiological need. In this review, we focus on the description of macropinocytosis, a lesser studied endocytosis mechanism than the conventional receptor mediated endocytosis. Additionally, we will discuss nanoparticle endocytosis (including macropinocytosis), and how the physio-chemical properties of the NP (size, charge, and surface coating) affect their intracellular uptake and exploiting them as tools to identify new adapter molecules regulating these processes.

Dual blockade of macropinocytosis and asparagine bioavailability shows synergistic anti-tumor effects on KRAS-mutant colorectal cancer

Mutations of KRAS gene are found in various types of cancer, including colorectal cancer (CRC). Despite intense efforts, no pharmacological approaches are expected to be effective against KRAS-mutant cancers. Macropinocytosis is an evolutionarily conserved actin-dependent endocytic process that internalizes extracellular fluids into large vesicles called macropinosomes. Recent studies have revealed macropinocytosis's important role in metabolic adaptation to nutrient stress in cancer cells harboring KRAS mutations. Here we showed that KRAS-mutant CRC cells enhanced macropinocytosis for tumor growth under nutrient-depleted conditions. We also demonstrated that activation of Rac1 and phosphoinositide 3-kinase were involved in macropinocytosis of KRAS-mutant CRC cells. Furthermore, we found that macropinocytosis was closely correlated with asparagine metabolism. In KRAS-mutant CRC cells engineered with knockdown of asparagine synthetase, macropinocytosis was accelerated under glutamine-depleted condition, and albumin addition could restore the glutamine depletion-induced growth suppression by recovering the intracellular asparagine level. Finally, we discovered that the combination of macropinocytosis inhibition and asparagine depletion dramatically suppressed the tumor growth of KRAS-mutant CRC cells in vivo. These results indicate that dual blockade of macropinocytosis and asparagine bioavailability could be a novel therapeutic strategy for KRAS-mutant cancers.

Pancreatic cancer and adaptive metabolism in a nutrient-deficient environment

Despite tremendous progress in the treatment of many cancer types, leading to a significant increase in survival, pancreatic ductal adenocarcinoma (PDAC) is still burdened with high mortality rates (5-year survival rate < 9%) due to late diagnosis, aggressiveness, and a lack of more effective treatment methods. Early diagnosis and new therapeutic approaches based on the adaptive metabolism of the tumor in a nutrient-deficient environment are expected to improve the future treatment of PDAC patients. It was found that blocking selected metabolic pathways related to the local adaptive metabolic activity of pancreatic cancer cells, improving nutrient acquisition and metabolic crosstalk within the microenvironment to sustain proliferation, may inhibit cancer development, increase cancer cell death, and increase sensitivity to other forms of treatment (e.g., chemotherapy). The present review highlights selected metabolic signaling pathways and their regulators aimed at inhibiting the neoplastic process. Particular attention is paid to the adaptive metabolism of pancreatic cancer, including fatty acids, autophagy, macropinocytosis, and deregulated cell-surface glycoproteins, which promotes cancer cell development in an oxygen-deficient and nutrient-poor environment.

Supply and demand: Cellular nutrient uptake and exchange in cancer

Nutrient supply and demand delineate cell behavior in health and disease. Mammalian cells have developed multiple strategies to secure the necessary nutrients that fuel their metabolic needs. This is more evident upon disruption of homeostasis in conditions such as cancer, when cells display high proliferation rates in energetically challenging conditions where nutritional sources may be scarce. Here, we summarize the main routes of nutrient acquisition that fuel mammalian cells and their implications in tumorigenesis. We argue that the molecular mechanisms of nutrient acquisition not only tip the balance between nutrient supply and demand but also determine cell behavior upon nutrient limitation and energetic stress and contribute to nutrient partitioning and metabolic coordination between different cell types in inflamed or tumorigenic environments.

Unraveling and targeting RAS-driven metabolic signaling for therapeutic gain

RAS mutations are among the most frequent oncogenic drivers observed in human cancers. With a lack of available treatment options, RAS-mutant cancers account for many of the deadliest cancers in the United States. Recent studies established that altered metabolic requirements are a hallmark of cancer, and many of these alterations are driven by aberrant RAS signaling. Specifically, RAS-driven cancers are characterized by upregulated glycolysis, the differential channeling of glycolytic intermediates, upregulated nutrient scavenging pathways such as autophagy and macropinocytosis, and altered glutamine utilization and mitochondrial function. This unique metabolic landscape promotes tumorigenesis, proliferation, survival in nutrient deficient environments and confers resistance to conventional cytotoxic and targeted therapies. Emerging work demonstrates how these dependencies can be therapeutically exploited in vitro and in vivo with many metabolic inhibitors currently in clinical trials. This review aims to outline the unique metabolic requirements induced by aberrant RAS signaling and how these altered dependencies present opportunities for therapeutic intervention.

Transcriptional activation of macropinocytosis by the Hippo pathway following nutrient limitation

Cancer cells must adapt metabolism to thrive despite nutrient limitations in the tumor microenvironment. In this issue of Genes & Development, King and colleagues (pp. 1345-1358) report a role for transcriptional regulators of the Hippo pathway to facilitate protein scavenging and support proliferation under some nutrient-deprived conditions.

Macropinocytosis: mechanism and targeted therapy in cancers

Macropinocytosis is a form of endocytosis which provides an effective way for non-selective uptakes of extracellular proteins, liquids, and particles. The endocytic process is initiated by the activation of the growth factors signaling pathways. After activation of the biochemical signal, the cell starts internalizing extracellular solutes and nutrients into the irregular endocytic vesicles, known as macropinosomes that deliver them into the lysosomes for degradation. Macropinocytosis plays an important role in the nutritional supply of cancer cells. Due to the rapid expansion of cancer cells and the abnormal vascular microenvironment, cancer cells are usually deprived of oxygen and nutrients. Therefore, they must transform their metabolism to survive and grow in this harsh microenvironment. To satisfy their energy needs, cancer cells enhance the activity of macropinocytosis. Therefore, this metabolic adaptation that is used by cancer cells can be exploited to develop new targeted cancer therapies. In this review, we discuss the molecular mechanism that actuates the process of macropinocytosis in a variety of cancers, and the novel anti-cancer therapeutics in targeting macropinocytosis.

Macropinocytosis: Insights from immunology and cancer

Macropinocytosis is increasingly recognized for its versatile adaptations and functions as a highly conserved, ubiquitous pathway for the bulk uptake of fluid, particulate cargo, and membranes. Innate immune cells and transformed cancer cells share the capacity for both constitutive and induced macropinocytosis, which is used for immune surveillance, ingestion of pathogens, immune response shaping, and enhancement of scavenging for nutrients as fuel for cell survival and proliferation. Immunology and cancer biology are leading a resurgence of interest in defining the molecular and physiological regulation of macropinocytosis, partly in pursuit of ways to control macropinocytic uptake in disease settings. New approaches, including high-resolution live imaging, screening of cell surface molecular inventories, biophysics, and exploration of cell microenvironments, have converged to provide new insights into macropinosome induction, formation, and maturation. Recent studies reveal mechanisms for fluid control in and by macrophage macropinosomes that impinge on membrane trafficking and cell migration. EGFR, PTEN, V-ATPase, syndecan 1, and galectin-3 have roles variably in the metabolic regulation of Ras or PI3K signaling for Rac1-mediated macropinocytosis in cancer. These molecular pathways and mechanisms contribute to the impressive adaptability of macropinocytosis in many cells and tissues and in disease.