Showing papers on "Signal transduction published in 2001"
TL;DR: Recent studies have begun to shed light on the physiological functions of MAPK cascades in the control of gene expression, cell proliferation and programmed cell death.
Abstract: Mitogen-activated protein kinases (MAPKs) are important signal transducing enzymes, unique to eukaryotes, that are involved in many facets of cellular regulation. Initial research concentrated on defining the components and organization of MAPK signalling cascades, but recent studies have begun to shed light on the physiological functions of these cascades in the control of gene expression, cell proliferation and programmed cell death.
4,973 citations
TL;DR: Evidence is accumulating that the signaling pathways associated with each TLR are not identical and may, therefore, result in different biological responses.
Abstract: Recognition of pathogens is mediated by a set of germline-encoded receptors that are referred to as pattern-recognition receptors (PRRs). These receptors recognize conserved molecular patterns (pathogen-associated molecular patterns), which are shared by large groups of microorganisms. Toll-like receptors (TLRs) function as the PRRs in mammals and play an essential role in the recognition of microbial components. The TLRs may also recognize endogenous ligands induced during the inflammatory response. Similar cytoplasmic domains allow TLRs to use the same signaling molecules used by the interleukin 1 receptors (IL-1Rs): these include MyD88, IL-1R--associated protein kinase and tumor necrosis factor receptor--activated factor 6. However, evidence is accumulating that the signaling pathways associated with each TLR are not identical and may, therefore, result in different biological responses.
4,686 citations
TL;DR: This work has shown that activation of inflammatory and antimicrobial innate immune responses through recognition of Toll-like receptors expressed on dendritic cells triggers functional maturation of dendrites and leads to initiation of antigen-specific adaptive immune responses.
Abstract: Toll-like receptors have a crucial role in the detection of microbial infection in mammals and insects. In mammals, these receptors have evolved to recognize conserved products unique to microbial metabolism. This specificity allows the Toll proteins to detect the presence of infection and to induce activation of inflammatory and antimicrobial innate immune responses. Recognition of microbial products by Toll-like receptors expressed on dendritic cells triggers functional maturation of dendritic cells and leads to initiation of antigen-specific adaptive immune responses.
4,102 citations
TL;DR: Neurotrophins regulate development, maintenance, and function of vertebrate nervous systems, and control synaptic function and synaptic plasticity, while continuing to modulate neuronal survival.
Abstract: Neurotrophins regulate development, maintenance, and function of vertebrate nervous systems. Neurotrophins activate two different classes of receptors, the Trk family of receptor tyrosine kinases and p75NTR, a member of the TNF receptor superfamily. Through these, neurotrophins activate many signaling pathways, including those mediated by ras and members of the cdc-42/ras/rho G protein families, and the MAP kinase, PI-3 kinase, and Jun kinase cascades. During development, limiting amounts of neurotrophins function as survival factors to ensure a match between the number of surviving neurons and the requirement for appropriate target innervation. They also regulate cell fate decisions, axon growth, dendrite pruning, the patterning of innervation and the expression of proteins crucial for normal neuronal function, such as neurotransmitters and ion channels. These proteins also regulate many aspects of neural function. In the mature nervous system, they control synaptic function and synaptic plasticity, while continuing to modulate neuronal survival.
3,968 citations
TL;DR: This review focuses on the biochemical components and regulation of mammalian stress-regulated mitogen-activated protein kinase (MAPK) pathways, and the nuclear factor-kappa B pathway, a second stress signaling paradigm.
Abstract: The molecular details of mammalian stress-activated signal transduction pathways have only begun to be dissected. This, despite the fact that the impact of these pathways on the pathology of chroni...
3,338 citations
TL;DR: Tremendous progress has been made in understanding the molecular basis of the antiviral actions of interferons (IFNs), as well as strategies evolved by viruses to antagonize the actions of IFNs.
Abstract: Tremendous progress has been made in understanding the molecular basis of the antiviral actions of interferons (IFNs), as well as strategies evolved by viruses to antagonize the actions of IFNs. Furthermore, advances made while elucidating the IFN system have contributed significantly to our understanding in multiple areas of virology and molecular cell biology, ranging from pathways of signal transduction to the biochemical mechanisms of transcriptional and translational control to the molecular basis of viral pathogenesis. IFNs are approved therapeutics and have moved from the basic research laboratory to the clinic. Among the IFN-induced proteins important in the antiviral actions of IFNs are the RNA-dependent protein kinase (PKR), the 2',5'-oligoadenylate synthetase (OAS) and RNase L, and the Mx protein GTPases. Double-stranded RNA plays a central role in modulating protein phosphorylation and RNA degradation catalyzed by the IFN-inducible PKR kinase and the 2'-5'-oligoadenylate-dependent RNase L, respectively, and also in RNA editing by the IFN-inducible RNA-specific adenosine deaminase (ADAR1). IFN also induces a form of inducible nitric oxide synthase (iNOS2) and the major histocompatibility complex class I and II proteins, all of which play important roles in immune response to infections. Several additional genes whose expression profiles are altered in response to IFN treatment and virus infection have been identified by microarray analyses. The availability of cDNA and genomic clones for many of the components of the IFN system, including IFN-alpha, IFN-beta, and IFN-gamma, their receptors, Jak and Stat and IRF signal transduction components, and proteins such as PKR, 2',5'-OAS, Mx, and ADAR, whose expression is regulated by IFNs, has permitted the generation of mutant proteins, cells that overexpress different forms of the proteins, and animals in which their expression has been disrupted by targeted gene disruption. The use of these IFN system reagents, both in cell culture and in whole animals, continues to provide important contributions to our understanding of the virus-host interaction and cellular antiviral response.
2,621 citations
TL;DR: This work has elucidated how LPS is recognized by monocytes and macrophages of the innate immune system and activates a variety of transcription factors that include NF-kappaB (p50/p65) and AP-1 (c-Fos/c-Jun), which coordinate the induction of many genes encoding inflammatory mediators.
2,269 citations
Journal Article•
TL;DR: The focus of this review is the current and evolving understanding of the contribution of GRKs, beta-arrestins, and endocytosis to GPCR-specific patterns of desensitization and resensitized.
Abstract: G protein-coupled receptors (GPCRs) are seven transmembrane proteins that form the largest single family of integral membrane receptors. GPCRs transduce information provided by extracellular stimuli into intracellular second messengers via their coupling to heterotrimeric G proteins and the subsequent regulation of a diverse variety of effector systems. Agonist activation of GPCRs also initiates processes that are involved in the feedback desensitization of GPCR responsiveness, the internalization of GPCRs, and the coupling of GPCRs to heterotrimeric G protein-independent signal transduction pathways. GPCR desensitization occurs as a consequence of G protein uncoupling in response to phosphorylation by both second messenger-dependent protein kinases and G protein-coupled receptor kinases (GRKs). GRK-mediated receptor phosphorylation promotes the binding of beta-arrestins, which not only uncouple receptors from heterotrimeric G proteins but also target many GPCRs for internalization in clathrin-coated vesicles. beta-Arrestin-dependent endocytosis of GPCRs involves the direct interaction of the carboxyl-terminal tail domain of beta-arrestins with both beta-adaptin and clathrin. The focus of this review is the current and evolving understanding of the contribution of GRKs, beta-arrestins, and endocytosis to GPCR-specific patterns of desensitization and resensitization. In addition to their role as GPCR-specific endocytic adaptor proteins, beta-arrestins also serve as molecular scaffolds that foster the formation of alternative, heterotrimeric G protein-independent signal transduction complexes. Similar to what is observed for GPCR desensitization and resensitization, beta-arrestin-dependent GPCR internalization is involved in the intracellular compartmentalization of these protein complexes.
1,898 citations
TL;DR: Analysis of the multiple processes that modulate EGFR signal transduction has revealed new therapeutic opportunities and elucidated mechanisms contributing to the efficacy of existing anticancer treatments.
1,627 citations
TL;DR: This review focuses on proteins that transduce the signals generated at TNF receptors to nuclear targets such as AP-1 and NF-kappaB, which are likely to be used by other members of the TNF family.
1,619 citations
TL;DR: In this paper, the authors discuss cellular genes and disease states associated with activation of the NF-κB pathway and consider therapeutic strategies to prevent the prolonged activation of this pathway, such as glucocorticoids and aspirin.
Abstract: NF-κB comprises a family of inducible transcription factors that serve as important regulators of the host immune and inflammatory response. In addition, NF-κB is also involved in protecting cells from undergoing apoptosis in response to DNA damage or cytokine treatment. Stimulation of the NF-κB pathway is mediated by diverse signal transduction cascades. These signals activate the IκB kinases, IKKα and IKKβ, which phosphorylate inhibitory proteins known as IκB to result in their ubiquitination and degradation by the proteasome. The degradation of IκB results in the translocation of NF-κB from the cytoplasm to the nucleus where it activates the expression of specific cellular genes. As we better understand the regulation of the NF-κB pathway, the potential for inhibiting this pathway has received attention. Agents that inhibit this pathway, such as glucocorticoids and aspirin, can reduce the inflammatory response, while other agents such as dominant negative IκB proteins potentiate the effects of chemotherapy and radiation therapy in the treatment of cancer. Here, we discuss cellular genes and disease states associated with activation of the NF-κB pathway and consider therapeutic strategies to prevent the prolonged activation of the NF-κB pathway.
TL;DR: The variety of intracellular target proteins of S100 proteins and, in some cases, of a single S100 protein, and the cell specificity of expression of certain S100 members suggest that these proteins might have a role in the fine regulation of effector proteins and/or specific steps of signaling pathways/cellular functions.
TL;DR: Intriguingly, most organelle-specific death responses finally lead to either MMP or caspase activation, both of which might function as central integrators of the death pathway, thereby streamlining lysosome-, Golgi- or ER-elicited responses into a common pathway.
Abstract: Nuclear DNA damage and ligation of plasma-membrane death receptors have long been recognized as initial triggers of apoptosis that induce mitochondrial membrane permeabilization (MMP) and/or the direct activation of caspases. Accumulating evidence suggests that other organelles, including the endoplasmic reticulum (ER), lysosomes and the Golgi apparatus, are also major points of integration of pro-apoptotic signalling or damage sensing. Each organelle possesses sensors that detect specific alterations, locally activates signal transduction pathways and emits signals that ensure inter-organellar cross-talk. The ER senses local stress through chaperones, Ca2+-binding proteins and Ca2+ release channels, which might transmit ER Ca2+ responses to mitochondria. The ER also contains several Bcl-2-binding proteins, and Bcl-2 has been reported to exert part of its cytoprotective effect within the ER. Upon membrane destabilization, lysosomes release cathepsins that are endowed with the capacity of triggering MMP. The Golgi apparatus constitutes a privileged site for the generation of the pro-apoptotic mediator ganglioside GD3, facilitates local caspase-2 activation and might serve as a storage organelle for latent death receptors. Intriguingly, most organelle-specific death responses finally lead to either MMP or caspase activation, both of which might function as central integrators of the death pathway, thereby streamlining lysosome-, Golgi- or ER-elicited responses into a common pathway.
TL;DR: It is shown that IKKα is required for B cell maturation, formation of secondary lymphoid organs, increased expression of certain NF-κB target genes, and processing of the NF-σκB2 (p100) precursor.
Abstract: In mammals, the canonical nuclear factor kappaB (NF-kappaB) signaling pathway activated in response to infections is based on degradation of IkappaB inhibitors. This pathway depends on the IkappaB kinase (IKK), which contains two catalytic subunits, IKKalpha and IKKbeta. IKKbeta is essential for inducible IkappaB phosphorylation and degradation, whereas IKKalpha is not. Here we show that IKKalpha is required for B cell maturation, formation of secondary lymphoid organs, increased expression of certain NF-kappaB target genes, and processing of the NF-kappaB2 (p100) precursor. IKKalpha preferentially phosphorylates NF-kappaB2, and this activity requires its phosphorylation by upstream kinases, one of which may be NF-kappaB-inducing kinase (NIK). IKKalpha is therefore a pivotal component of a second NF-kappaB activation pathway based on regulated NF-kappaB2 processing rather than IkappaB degradation.
TL;DR: It is shown that bradykinin- or NGF-mediated potentiation of thermal sensitivity in vivo requires expression of VR1, a heat-activated ion channel on sensory neurons, and biochemical studies suggest that VR1 associates with this complex.
Abstract: Tissue injury generates endogenous factors that heighten our sense of pain by increasing the response of sensory nerve endings to noxious stimuli. Bradykinin and nerve growth factor (NGF) are two such pro-algesic agents that activate G-protein-coupled (BK2) and tyrosine kinase (TrkA) receptors, respectively, to stimulate phospholipase C (PLC) signalling pathways in primary afferent neurons. How these actions produce sensitization to physical or chemical stimuli has not been elucidated at the molecular level. Here, we show that bradykinin- or NGF-mediated potentiation of thermal sensitivity in vivo requires expression of VR1, a heat-activated ion channel on sensory neurons. Diminution of plasma membrane phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) levels through antibody sequestration or PLC-mediated hydrolysis mimics the potentiating effects of bradykinin or NGF at the cellular level. Moreover, recruitment of PLC-gamma to TrkA is essential for NGF-mediated potentiation of channel activity, and biochemical studies suggest that VR1 associates with this complex. These studies delineate a biochemical mechanism through which bradykinin and NGF produce hypersensitivity and might explain how the activation of PLC signalling systems regulates other members of the TRP channel family.
TL;DR: A protein is described, Mal (MyD88-adapter-like), which joins MyD88 as a cytoplasmic TIR-domain-containing protein in the human genome, which is therefore an adapter in TLR-4 signal transduction.
Abstract: The recognition of microbial pathogens by the innate immune system involves Toll-like receptors (TLRs), which recognize pathogen-associated molecular patterns. Different TLRs recognize different pathogen-associated molecular patterns, with TLR-4 mediating the response to lipopolysaccharide from Gram-negative bacteria. All TLRs have a Toll/IL-1 receptor (TIR) domain, which is responsible for signal transduction. MyD88 is one such protein that contains a TIR domain. It acts as an adapter, being involved in TLR-2, TLR-4 and TLR-9 signalling; however, our understanding of how TLR-4 signals is incomplete. Here we describe a protein, Mal (MyD88-adapter-like), which joins MyD88 as a cytoplasmic TIR-domain-containing protein in the human genome. Mal activates NF-kappaB, Jun amino-terminal kinase and extracellular signal-regulated kinase-1 and -2. Mal can form homodimers and can also form heterodimers with MyD88. Activation of NF-kappaB by Mal requires IRAK-2, but not IRAK, whereas MyD88 requires both IRAKs. Mal associates with IRAK-2 by means of its TIR domain. A dominant negative form of Mal inhibits NF-kappaB, which is activated by TLR-4 or lipopolysaccharide, but it does not inhibit NF-kappaB activation by IL-1RI or IL-18R. Mal associates with TLR-4. Mal is therefore an adapter in TLR-4 signal transduction.
TL;DR: Rec recombinant EDA, but not other recombinant fibronectin domains, activates human TLR4 expressed in a cell type (HEK 293 cells) that normally lacks this Toll-like receptor.
TL;DR: It is suggested that A2a adenosine receptors are a critical part of the physiological negative feedback mechanism for limitation and termination of both tissue-specific and systemic inflammatory responses.
Abstract: Inappropriate or prolonged inflammation is the main cause of many diseases; for this reason it is important to understand the physiological mechanisms that terminate inflammation in vivo. Agonists for several Gs-protein-coupled receptors, including cell-surface adenosine purinergic receptors, can increase levels of immunosuppressive cyclic AMP in immune cells; however, it was unknown whether any of these receptors regulates inflammation in vivo. Here we show that A2a adenosine receptors have a non-redundant role in the attenuation of inflammation and tissue damage in vivo. Sub-threshold doses of an inflammatory stimulus that caused minimal tissue damage in wild-type mice were sufficient to induce extensive tissue damage, more prolonged and higher levels of pro-inflammatory cytokines, and death of male animals deficient in the A2a adenosine receptor. Similar observations were made in studies of three different models of inflammation and liver damage as well as during bacterial endotoxin-induced septic shock. We suggest that A2a adenosine receptors are a critical part of the physiological negative feedback mechanism for limitation and termination of both tissue-specific and systemic inflammatory responses.
TL;DR: A novel mechanism by which fatty acids modulate signaling pathways and target gene expression is represented by both SFA-induced COX-2 expression and its inhibition by UFAs are mediated through a common signaling pathway derived from Tlr4.
TL;DR: It is shown that by deleting ASK1 in mice, TNF‐ and H2O2‐induced sustained activations of JNK and p38 are lost inASK1−/− embryonic fibroblasts, and that ASK 1−-/− cells are resistant to TNF- and H1N1‐induced apoptosis.
Abstract: Apoptosis signal‐regulating kinase (ASK) 1 is activated in response to various cytotoxic stresses including TNF, Fas and reactive oxygen species (ROS) such as H2O2, and activates c‐Jun NH2‐terminal kinase (JNK) and p38. However, the roles of JNK and p38 signaling pathways during apoptosis have been controversial. Here we show that by deleting ASK1 in mice, TNF‐ and H2O2‐induced sustained activations of JNK and p38 are lost in ASK1 −/− embryonic fibroblasts, and that ASK1 −/− cells are resistant to TNF‐ and H2O2‐induced apoptosis. TNF‐ but not Fas‐induced apoptosis requires ROS‐dependent activation of ASK1–JNK/p38 pathways. Thus, ASK1 is selectively required for TNF‐ and oxidative stress‐induced sustained activations of JNK/p38 and apoptosis.
TL;DR: This study shows that the Drosophila insulin receptor autonomously controls cell and organ size, and that overexpression of a gene encoding an insulin-like peptide is sufficient to increase body size.
TL;DR: The results indicate that eNOS activity is up-regulated in Cav-1 null animals, and this activity can be blunted by using a specific NOS inhibitor, nitro-l-arginine methyl ester.
TL;DR: The PI3K-Akt signaling pathway plays a critical role in mediating survival signals in a wide range of neuronal cell types and may also use metabolic pathways to regulate cell survival.
TL;DR: Evidence is provided that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin.
Abstract: Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the α-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.
TL;DR: This review uses transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated as a framework to build a profile of PI3k function within both the cell and the organism and focuses, in particular, on the role ofPI3K in cell regulation, immunity, and development.
Abstract: The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.
TL;DR: The characterization of the MyD88-independent pathway via TLR4 is reported, a MyD 88-dependent pathway that is critical to the induction of inflammatory cytokines and a Myd88/TNFR-associated factor 6- independent pathway that regulates induction of IP-10.
Abstract: Bacterial lipopolysaccharide (LPS) triggers innate immune responses through Toll-like receptor (TLR) 4, a member of the TLR family that participates in pathogen recognition. TLRs recruit a cytoplasmic protein, MyD88, upon pathogen recognition, mediating its function for immune responses. Two major pathways for LPS have been suggested in recent studies, which are referred to as MyD88-dependent and -independent pathways. We report in this study the characterization of the MyD88-independent pathway via TLR4. MyD88-deficient cells failed to produce inflammatory cytokines in response to LPS, whereas they responded to LPS by activating IFN-regulatory factor 3 as well as inducing the genes containing IFN-stimulated regulatory elements such as IP-10. In contrast, a lipopeptide that activates TLR2 had no ability to activate IFN-regulatory factor 3. The MyD88-independent pathway was also activated in cells lacking both MyD88 and TNFR-associated factor 6. Thus, TLR4 signaling is composed of at least two distinct pathways, a MyD88-dependent pathway that is critical to the induction of inflammatory cytokines and a MyD88/TNFR-associated factor 6-independent pathway that regulates induction of IP-10.
TL;DR: Three-dimensional structures of the Trk receptors, in one instance in association with a neurotrophin, have revealed the structural bases underlying specificity in neurotrophIn signaling, where key intermediates are localized to different membrane compartments.
TL;DR: The delicate intracellular interplay between oxidizing and reducing equivalents allows ROS to function as second messengers in the control of cell proliferation and differentiation.
Abstract: Reactive oxygen species (ROS) are generated following ligand-receptor interactions and function as specific second messengers in signaling cascades involved in cell proliferation and differentiation.
TL;DR: It is suggested that OPN delivers an antiapoptotic “ECM-like” signal via multiple ligand-receptor interactions to cells, both adherent and nonadherent.
Abstract: OPN is a multifunctional cytokine and adhesion protein that contains an integrin-binding RGD sequence and additional sequences that interact with CD44v6/7 or other adhesive receptors. Its expression is increased in response to early proinflammatory cytokines and to mechanical strain in bone. The function of the secreted protein may be altered by extracellular enzymes, including thrombin and kinases. The study of OPN-null mice has revealed roles for OPN in a broad range of homeostatic (bone remodeling, tissue debridement) and pathologic (cellular immunity, wound healing, cancer metastasis) processes. While these processes seem disparate, they are linked by several common themes, including enhanced expression of OPN in response to stress or tissue injury, and stimulation of cell motility and cell survival pathways via interactions of OPN with adhesive receptors.
OPN is chemotactic for various cell types, notably monocytes/macrophages, which are attracted to sites of infection and inflammation. It is essential for cell-mediated immunity and a normal Th1 cytokine response during granuloma formation. OPN serves both to attach bone cells to bone matrix and to generate intracellular signals essential for normal osteoclast motility on bone; it may mediate osteocyte recognition of bone strain. OPN activates intracellular signaling pathways and regulates gene expression as a consequence of its interactions with its various receptors. The best-characterized is the integrin-stimulated FAK-Src-Rho pathway, which alters gelsolin function and podosome formation in osteoclasts. Identification and dissection of the signal transduction pathways and their targets are complicated by the fact that OPN can engage more than one type of receptor on the cell. For this reason, it is important to ascertain which receptors are in play in any given experimental system.
There is compelling evidence that soluble OPN can in a variety of situations help cells survive an otherwise lethal insult. Remarkably, this survival signaling is mediated by receptors that are generally considered to be receptors for ECM components. We suggest that OPN delivers an antiapoptotic “ECM-like” signal via multiple ligand-receptor interactions to cells, both adherent and nonadherent.
TL;DR: The diverse biological roles of the LRP include functions in lipid metabolism, and also in the homeostasis of proteinases and proteinase inhibitors, cellular entry of viruses and toxins, activation of lysosomal enzymes, cellular signal transduction, and neurotransmission.
Abstract: The LDL receptor–related protein (LRP) is larger than but structurally similar to other members of the LDL receptor gene family, an ancient family of endocytic receptors (1–3). Whereas the LDL receptor, the founding member of this family, appears to act solely in lipoprotein metabolism, the LRP and other members of this family appear to have other distinct functions. In this article, we will focus on the diverse biological roles of the LRP, which include functions in lipid metabolism, and also in the homeostasis of proteinases and proteinase inhibitors, cellular entry of viruses and toxins, activation of lysosomal enzymes, cellular signal transduction, and neurotransmission.