Urokinase-type plasminogen activator and plasminogen mediate activation of macrophage phagocytosis during liver repair in vivo

Macrophage fate: to kill or not to kill?

Macrophages are dynamic innate immune cells that either reside in tissue, serving as sentinels, or recruited as monocytes from bone marrow into inflamed and infected tissue. In response to cues in the tissue microenvironment (TME), macrophages polarize on a continuum toward M1 or M2 with diverse roles in progression and resolution of disease. M1-like macrophages exhibit proinflammatory functions with antimicrobial and anti-tumorigenic activities, while M2-like macrophages have anti-inflammatory functions that generally resolve inflammatory responses and orchestrate a tissue healing process. Given these opposite phenotypes, proper spatiotemporal coordination of macrophage polarization in response to cues within the TME is critical to effectively resolve infectious disease and regulate wound healing. However, if this spatiotemporal coordination becomes disrupted due to persistent infection or dysregulated coagulation, macrophages' inappropriate response to these cues will result in the development of diseases with clinically unfavorable outcomes. Since plasticity and heterogeneity are hallmarks of macrophages, they are attractive targets for therapies to reprogram toward specific phenotypes that could resolve disease and favor clinical prognosis. In this review, we discuss how basic science studies have elucidated macrophage polarization mechanisms in TMEs during infections and inflammation, particularly coagulation. Therefore, understanding the dynamics of macrophage polarization within TMEs in diseases is important in further development of targeted therapies.

Efferocytosis in liver disease

The process of dead cell clearance by phagocytic cells, called efferocytosis, prevents inflammatory cell necrosis and promotes resolution and repair. Defective efferocytosis contributes to the progression of numerous diseases in which cell death is prominent, including liver disease. Many gaps remain in our understanding of how hepatic macrophages carry out efferocytosis and how this process goes awry in various types of liver diseases. Thus far, studies have suggested that, upon liver injury, liver-resident Kupffer cells and infiltrating monocyte-derived macrophages clear dead cells, limit inflammation, and, through macrophage reprogramming, repair liver damage. However, in unusual settings, efferocytosis can promote liver disease. In this review, we will focus on efferocytosis in various types of acute and chronic liver diseases, including metabolic dysfunction-associated steatohepatitis. Understanding the mechanisms and consequences of efferocytosis by hepatic macrophages has the potential to shed new light on liver disease pathophysiology and to guide new treatment strategies to prevent disease progression.

Targeting innate immune responses to attenuate acetaminophen-induced hepatotoxicity

Acetaminophen (APAP) hepatotoxicity is an important cause of acute liver failure, resulting in massive deaths in many developed countries. Currently, the metabolic process of APAP in the body has been well studied. However, the underlying mechanism of APAP-induced liver injury remains elusive. Increasing clinical and experimental evidences indicate that the innate immune responses are involved in the pathogenesis of APAP-induced acute liver injury (AILI), in which immune cells have dual roles of inducing inflammation to exacerbate hepatotoxicity and removing dead cells and debris to help liver regeneration. In this review, we summarize the latest findings of innate immune cells involved in AILI, particularly emphasizing the activation of innate immune cells and their different roles during the injury and repair phases. Moreover, current available treatments are discussed according to the different roles of innate immune cells in the development of AILI. This review aims to update the knowledge about innate immune responses in the pathogenesis of AILI, and provide potential therapeutic interventions for AILI.

Exploring the Pro-Phagocytic and Anti-Inflammatory Functions of PACAP and VIP in Microglia: Implications for Multiple Sclerosis

Multiple sclerosis (MS) is a chronic neuroinflammatory and demyelinating disease of the central nervous system (CNS), characterised by the infiltration of peripheral immune cells, multifocal white-matter lesions, and neurodegeneration. In recent years, microglia have emerged as key contributors to MS pathology, acting as scavengers of toxic myelin/cell debris and modulating the inflammatory microenvironment to promote myelin repair. In this review, we explore the role of two neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP), as important regulators of microglial functioning during demyelination, myelin phagocytosis, and remyelination, emphasising the potential of these neuropeptides as therapeutic targets for the treatment of MS.

Plasmin and Plasminogen System in the Tumor Microenvironment: Implications for Cancer Diagnosis, Prognosis, and Therapy

Simple Summary

In this review, we present a detailed discussion of how the plasminogen-activation system is utilized by tumor cells in their unrelenting attack on the tissues surrounding them. Plasmin is an enzyme which is responsible for digesting several proteins that hold the tissues surrounding solid tumors together. In this process tumor cells utilize the activity of plasmin to digest tissue barriers in order to leave the tumour site and spread to other parts of the body. We specifically focus on the role of plasminogen receptor—p11 which is an important regulatory protein that facilitates the conversion of plasminogen to plasmin and by this means promotes the attack by the tumour cells on their surrounding tissues.

Abstract

The tumor microenvironment (TME) is now being widely accepted as the key contributor to a range of processes involved in cancer progression from tumor growth to metastasis and chemoresistance. The extracellular matrix (ECM) and the proteases that mediate the remodeling of the ECM form an integral part of the TME. Plasmin is a broad-spectrum, highly potent, serine protease whose activation from its precursor plasminogen is tightly regulated by the activators (uPA, uPAR, and tPA), the inhibitors (PAI-1, PAI-2), and plasminogen receptors. Collectively, this system is called the plasminogen activation system. The expression of the components of the plasminogen activation system by malignant cells and the surrounding stromal cells modulates the TME resulting in sustained cancer progression signals. In this review, we provide a detailed discussion of the roles of plasminogen activation system in tumor growth, invasion, metastasis, and chemoresistance with specific emphasis on their role in the TME. We particularly review the recent highlights of the plasminogen receptor S100A10 (p11), which is a pivotal component of the plasminogen activation system.

Reduced Expression of Urokinase Plasminogen Activator in Brown Adipose Tissue of Obese Mouse Models

In recent decades, the obesity epidemic has resulted in morbidity and mortality rates increasing globally. In this study, using obese mouse models, we investigated the relationship among urokinase plasminogen activator (uPA), metabolic disorders, glomerular filtration rate, and adipose tissues. Two groups, each comprised of C57BL/6J and BALB/c male mice, were fed a chow diet (CD) and a high fat diet (HFD), respectively. Within the two HFD groups, half of each group were euthanized at 8 weeks (W8) or 16 weeks (W16). Blood, urine and adipose tissues were collected and harvested for evaluation of the effects of obesity. In both mouse models, triglyceride with insulin resistance and body weight increased with duration when fed a HFD in comparison to those in the groups on a CD. In both C57BL/6J and BALB/c HFD mice, levels of serum uPA initially increased significantly in the W8 group, and then the increment decreased in the W16 group. The glomerular filtration rate declined in both HFD groups. The expression of uPA significantly decreased in brown adipose tissue (BAT), but not in white adipose tissue, when compared with that in the CD group. The results suggest a decline in the expression of uPA in BAT in obese m models as the serum uPA increases. There is possibly an association with BAT fibrosis and dysfunction, which may need further study.

Regulation of macrophage activation in the liver after acute injury: Role of the fibrinolytic system

The liver functions, in part, to prevent exposure of the body to potentially harmful substances ingested in the diet. While it is highly efficient at accomplishing this, it is frequently prone to liver injury due to the biotransformation of xenobiotics into toxic metabolites. To counter this injury, the liver has evolved a unique capacity to rapidly and efficiently repair itself. Successful resolution of acute liver injury relies on hepatic macrophage populations that orchestrate the reparative response. After injury, Kupffer cells, the resident macrophages of the liver, become activated and secrete proinflammatory cytokines. These cytokines recruit other immune cells, including monocyte-derived macrophages, to the liver where they contribute to the repair process. Monocyte-derived macrophages traffic into the necrotic foci where they rapidly phagocytose dead cell debris. Simultaneous with this process, these cells change phenotype from a proinflammatory macrophage to a pro-restorative macrophage that produce pro-mitogenic growth factors and anti-inflammatory cytokines. Ultimately this process triggers resolution of inflammation, and along with proliferation of other hepatic cells, restores the liver architecture and function. While the mechanisms regulating specific macrophage functions during repair remain to be elucidated, recent studies indicate a key role for the fibrinolytic system in coordinating macrophage function during repair. In this review, we will highlight the function and role of hepatic macrophages in repair after acute liver injury, and will discuss the role of the fibrinolytic enzyme, plasmin, in regulation of these various processes.

Plasminogen is a critical regulator of cutaneous wound healing

Wound healing is a complicated biological process that consist of partially overlapping inflammatory, proliferation and tissue remodelling phases. A successful wound healing depends on a proper activation and subsequent termination of the inflammatory phase. The failure to terminate the inflammation halts the completion of wound healing and is a known reason for formation of chronic wounds. Previous studies have shown that wound closure is delayed in plasminogendeficient mice, and a role for plasminogen in dissection of extracellular matrix was suggested. However, our finding that plasminogen is transported to the wound by inflammatory cells early during the healing process, where it potentiates inflammation, indicates that plasminogen may also have other roles in the wound healing process. Here we report that plasminogen-deficient mice have extensive fibrin and neutrophil depositions in the wounded area long after re-epithelialisation, indicating inefficient debridement and chronic inflammation. Delayed formation of granulation tissue suggests that fibroblast function is impaired in the absence of plasminogen. Therefore, in addition to its role in the activation of inflammation, plasminogen is also crucial for subsequent steps, including resolution of inflammation and activation of the proliferation phase. Importantly, supplementation of plasminogen-deficient mice with human plasminogen leads to a restored healing process that is comparable to that in wild-type mice. Besides of being an activator of the inflammatory phase during wound healing, plasminogen is also required for the subsequent termination of inflammation. Based on these results, we propose that plasminogen may be an important future therapeutic agent for wound treatment.

Supplementary Material to this article is available online at www.thrombosis-online.com.

Effects of pharmacological inhibition of plasminogen binding on liver regeneration in rats

The fibrinolysis system is thought to play an important role in liver regeneration. We previously found that plasminogen (Plg) is localized to the cell surface of regenerating liver tissue as well as proliferating hepatocytes in vitro. Here, we investigated the significance of Plg binding to the cell surface during liver regeneration. Pre-administration of tranexamic acid (TXA), which is a competitive inhibitor of Plg binding, to hepatectomized rats mildly delayed restoration of liver weight in vivo. Although binding of Plg to the cell membrane decreased following TXA administration, TXA showed little effect on hepatocyte proliferation in rats. We also discovered that Plg treatment did not stimulate proliferation of primary rat hepatocytes in vitro. These results suggest that Plg/plasmin potentiates liver regeneration via a pathway distinct from those through which hepatocyte proliferation is stimulated.

The Tissue Fibrinolytic System Contributes to the Induction of Macrophage Function and CCL3 during Bone Repair in Mice

Macrophages play crucial roles in repair process of various tissues. However, the details in the role of macrophages during bone repair still remains unknown. Herein, we examined the contribution of the tissue fibrinolytic system to the macrophage functions in bone repair after femoral bone defect by using male mice deficient in plasminogen (Plg^–/–), urokinase-type plasminogen activator (uPA^–/–) or tissue-type plasminogen activator (tPA^–/–) genes and their wild-type littermates. Bone repair of the femur was delayed in uPA^–/– mice until day 6, compared with wild-type (uPA^+/+) mice. Number of Osterix-positive cells and vessel formation were decreased in uPA^–/– mice at the bone injury site on day 4, compared with those in uPA^+/+ mice. Number of macrophages and their phagocytosis at the bone injury site were reduced in uPA^–/– and Plg^–/–, but not in tPA^–/– mice on day 4. Although uPA or plasminogen deficiency did not affect the levels of cytokines, including TNF-α, IL-1β, IL-6, IL-4 and IFN-γ mRNA in the damaged femur, the elevation in CCL3 mRNA levels was suppressed in uPA^–/– and Plg^–/–, but not in tPA^–/– mice. Neutralization of CCL3 antagonized macrophage recruitment to the site of bone injury and delayed bone repair in uPA^+/+, but not in uPA^–/– mice. Our results provide novel evidence that the tissue fibrinolytic system contributes to the induction of macrophage recruitment and CCL3 at the bone injury site, thereby, leading to the enhancement of the repair process.

Tissue-type plasminogen activator deficiency delays bone repair: roles of osteoblastic proliferation and vascular endothelial growth factor

Further development in research of bone regeneration is necessary to meet the clinical demand for bone reconstruction. Recently, we reported that plasminogen is crucial for bone repair through enhancement of vessel formation. However, the details of the role of tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA) in the bone repair process still remain unknown. Herein, we examined the effects of plasminogen activators on bone repair after a femoral bone defect using tPA-deficient (tPA(-/-)) and uPA-deficient (uPA(-/-)) mice. Bone repair of the femur was delayed in tPA(-/-) mice, unlike that in wild-type (tPA(+/+)) mice. Conversely, the bone repair was comparable between wild-type (uPA(+/+)) and uPA(-/-) mice. The number of proliferative osteoblasts was decreased at the site of bone damage in tPA(-/-) mice. Moreover, the proliferation of primary calvarial osteoblasts was reduced in tPA(-/-) mice. Recombinant tPA facilitated the proliferation of mouse osteoblastic MC3T3-E1 cells. The proliferation enhanced by tPA was antagonized by the inhibition of endogenous annexin 2 by siRNA and by the inhibition of extracellular signal-regulated kinase (ERK)1/2 phosphorylation in MC3T3-E1 cells. Vessel formation as well as the levels of vascular endothelial growth factor (VEGF) and hypoxia-inducible factor-1α (HIF-1α) were decreased at the damaged site in tPA(-/-) mice. Our results provide novel evidence that tPA is crucial for bone repair through the facilitation of osteoblast proliferation related to annexin 2 and ERK1/2 as well as enhancement of vessel formation related to VEGF and HIF-1α at the site of bone damage.

Plasminogen promotes macrophage phagocytosis in mice

The phagocytic function of macrophages plays a pivotal role in eliminating apoptotic cells and invading pathogens. Evidence implicating plasminogen (Plg), the zymogen of plasmin, in phagocytosis is extremely limited with the most recent in vitro study showing that plasmin acts on prey cells rather than on macrophages. Here, we use apoptotic thymocytes and immunoglobulin opsonized bodies to show that Plg exerts a profound effect on macrophage-mediated phagocytosis in vitro and in vivo. Plg enhanced the uptake of these prey by J774A.1 macrophage-like cells by 3.5- to fivefold Plg receptors and plasmin proteolytic activity were required for phagocytosis of both preys. Compared with Plg(+/+) mice, Plg(-/-) mice exhibited a 60% delay in clearance of apoptotic thymocytes by spleen and an 85% reduction in uptake by peritoneal macrophages. Phagocytosis of antibody-mediated erythrocyte clearance by liver Kupffer cells was reduced by 90% in Plg(-/-) mice compared with Plg(+/+) mice. A gene array of splenic and hepatic tissues from Plg(-/-) and Plg(+/+) mice showed downregulation of numerous genes in Plg(-/-) mice involved in phagocytosis and regulation of phagocytic gene expression was confirmed in macrophage-like cells. Thus, Plg may play an important role in innate immunity by changing expression of genes that contribute to phagocytosis.

Plasminogen plays a crucial role in bone repair

The further development in research of bone regeneration is necessary to meet the clinical demand for bone reconstruction. Plasminogen is a critical factor of the tissue fibrinolytic system, which mediates tissue repair in the skin and liver. However, the role of the fibrinolytic system in bone regeneration remains unknown. Herein, we investigated bone repair and ectopic bone formation using plasminogen-deficient (Plg⁻/⁻) mice. Bone repair of the femur is delayed in Plg⁻/⁻ mice, unlike that in the wild-type (Plg⁺/⁺) mice. The deposition of cartilage matrix and osteoblast formation were both decreased in Plg⁻/⁻ mice. Vessel formation, macrophage accumulation, and the levels of vascular endothelial growth factor (VEGF) and transforming growth factor-β (TGF-β) were decreased at the site of bone damage in Plg⁻/⁻ mice. Conversely, heterotopic ossification was not significantly different between Plg⁺/⁺ and Plg⁻/⁻ mice. Moreover, angiogenesis, macrophage accumulation, and the levels of VEGF and TGF-β were comparable between Plg⁺/⁺ and Plg⁻/⁻ mice in heterotopic ossification. Our data provide novel evidence that plasminogen is essential for bone repair. The present study indicates that plasminogen contributes to angiogenesis related to macrophage accumulation, TGF-β, and VEGF, thereby leading to the enhancement of bone repair.