HPA is also named HPA-1, which differs from HPA-2. The HPA gene, located on chromosome 4q21.2, is the sole known mammalian endoglycosidase that cleaves the heparan sulfate (HS) side chains of HS proteoglycans (HSPGs) intra- and extracellularly. In vivo, HPA exists within lysosomes as a precursor with a molecular weight of 65 kDa (42). The active form of HPA is generated by lysosomal cathepsin-induced cleavage of the pro-enzyme yielding 50- and 8-kDa fragments, both of which are needed for its activity. Active HPA has enzymatic and non-enzymatic activities that participate in multiple processes (42). HPA serves an important role in promoting pathological processes such as tumor growth, metastasis, angiogenesis, thrombosis, fibrosis, inflammation, autoimmunity and renal dysfunction (43-49). Upregulation of HPA expression is associated with tumor size and progression, enhanced metastasis and a poor prognosis (50).

When HPA is secreted outside the cell, it cleaves the HS side chain and contributes to the activation of integrin, epidermal growth factor receptor and other signaling pathways. HPA not only trims HS that is bound to glycocalyx proteoglycan core proteins, but also degrades proteoglycans that are attached to the extracellular matrix (ECM), thus remodeling the ECM. HS is a high-sulfur polysaccharide that is widely found in the ECM and plasma membrane, and within the cell (51). Therefore, it plays an important role in maintaining ECM integrity, barrier function and cell-ECM interactions (52). HPA is crucial for the normal turnover of HS (53). HPA affects the function of HSPGs by degrading HS. Several linear HS chains covalently bind to a core protein to form HSPGs, which participate in cell-cell and cell-matrix adhesion (54). HSPGs not only provide a repository for heparin-binding molecules, such as growth factors, chemokines and enzymes in the tissue microenvironment, but also regulate their accessibility, function and mode of action. Degradation of HS by HPA not only begins to remove physical barriers that prevent cell invasion, but also releases various proteins that bind to HS, promoting activation of cellular signaling pathways and responses (55). However, it has been reported that the exact role of HPA in inflammation is difficult to determine, as HPA may act in other ways, either enhancing or inhibiting the inflammatory response (47).

Intracellular HPA binds to autophagosomes to enable autophagy, binds to exosomes to promote their release from cells and enters the nucleus to regulate gene expression (56). In addition, HPA has an important role in the normal physiology of lysosomes (57). HPA regulates gene expression, activates innate immune cells, promotes the formation of exosomes and autophagosomes, and promotes signal transduction through enzymatic and non-enzymatic activities (58). HPA promotes the secretion of exosomes that interact with tumor and host cells, and drives their transition to an aggressive tumor phenotype (57). Exosomes are mediators of intercellular communication that initiate tumor progression by regulating tumor and host cell behavior locally and distally throughout the body in the tumor microenvironment (59). HPA enhances tumor growth and chemotherapy resistance by enhancing autophagy (60).

HPA promotes degradation of the endothelial glycocalyx in glomeruli and renal tubules during sepsis (Fig. 1). The glycocalyx covers the surface of the vascular endothelium and its degradation destroys the integrity and permeability of the vascular system. Lygizos et al (7) showed that glomerular HPA was activated during sepsis and contributed to the occurrence of S-AKI. HPA destroys the integrity and permeability of renal TECs by degrading HS, resulting in renal inflammation, and urinary HS primarily reflects the degradation of the renal glycocalyx (7). Schmidt et al (61) compared 30 patients with septic shock in the medical ICU with 25 patients with severe trauma in the surgical ICU that acted as controls. The study revealed that compared with that in the control group, the level of HS in the urine of septic patients was significantly higher. The degradation function of HPA on HS suggests that HPA may contribute to regulation of the renal glycocalyx during sepsis and lead to S-AKI.

HPA not only promotes the progression of S-AKI, but also plays an important role in other types of AKI, such as toxic AKI. I/R is a complication of AKI and HPA is part of the biological network triggered by I/R injury (62). Masola et al (62) demonstrated through in vivo and in vitro experiments that HPA can regulate macrophage polarization, and renal damage and repair following I/R, and that HPA inhibitors can partially restore renal function and reduce apoptosis. In addition, another study showed that HPA promotes the onset of I/R-induced AKI in an I/R mouse model (63). This previous study found that HPA is upregulated in I/R mouse models, particularly in HPA-overexpressing transgenic mice, whereas pretreatment with PG545, an HPA inhibitor, can eliminate I/R-induced renal dysfunction. Another study demonstrated that HPA is a key factor involved in the occurrence and development of I/R-induced epithelial-mesenchymal transition (EMT) (64). In a podocyte model of Adriamycin-induced toxic kidney injury, HPA overexpression was shown to preserve glomerular structure and renal function, and elevated HPA levels could promote cell protection against apoptosis (65), thus exhibiting a protective effect. This previous study also reported that exposure to toxic damage resulted in a significant increase in autophagic flux in the podocytes of HPA-overexpressing mice, which could be reversed by the HPA inhibitor, Roneparstat.

Therefore, previous studies have suggested that HPA plays an important role in pathological processes, such as tumor growth, migration/invasion of ECs, infiltration of immune cells, metastasis, angiogenesis, thrombosis, fibrosis, autoimmunity, autophagy, exosome release, promotion of pro-inflammatory cell adhesion, signal transduction and renal dysfunction (42-44,46,57,66-71).

HPA plays an important role in inflammation, sepsis and AKI (72-74). Furthermore, HPA has effects on the endothelial glycocalyx and ECM in the kidney. HPA is activated under inflammatory conditions and can promote the degradation and shedding of the glycocalyx, leading to increased cell permeability, thus causing vascular leakage and hypovolemia. HPA also mediates its enzymatic activity on the ECM, particularly the basement membrane, and promotes the release of pro-inflammatory factors (i.e., IL-2, IL-8, basic fibroblast growth factor and TGF-β) and heparin-binding molecules in the ECM. These inflammatory factors stimulate the recruitment, rolling process and extravasation of leukocytes by regulating the interaction between leukocytes and the ECM surface (75-77) (Fig. 1).

There is additional evidence to support the role of HPA in S-AKI through the immune response during sepsis. In S-AKI, HPA increases the presence of adhesion molecules, and enhances vascular permeability, tissue edema, leukocyte adhesion, platelet aggregation and vasodilation-related dysfunction by promoting glycocalyx degradation (78,79). The glycocalyx plays a vital role in maintaining the integrity and permeability of the vascular system, providing vascular tension and regulating leukocyte adhesion (80). HPA induces the apoptosis of tubular cells and induces the production of damage-associated molecular patterns. HS fragments released by HPA can also activate TLRs on macrophages and tubular cells (62). Tubular cells produce proinflammatory cytokines in response to direct hypoxic stimulation and TLR activation, resulting in the attraction and activation of macrophages. In addition, high levels of HPA can promote M1 polarization of infiltrating macrophages and aggravate parenchymal injury (62). HPA also induces a procoagulant effect and renal fibrosis by binding to cellular HS (48), and interacts with tissue factor pathway inhibitor (TFPI) on the surface of ECs, resulting in TFPI dissociation and cell coagulation activity promotion (81). In addition, during S-AKI, HPA leads to platelet aggregation and vasodilation-related dysfunction by promoting glycocalyx degradation. Furthermore, it can lead to increased microvascular dysfunction and endothelial injury, thus affecting microcirculatory blood flow and aggravating coagulation (74). HPA is also involved in hemostasis through non-enzymatic mechanisms (48). For example, HPA reduces unfractionated heparin to low-molecular weight heparin, which functions as a co-factor for factor Xa inhibition by antithrombase (48). Bayam et al (82) confirmed that elevated heparin levels may promote thrombosis. Therefore, HPA may also enhance procoagulant activity in S-AKI. In addition, HPA can promote renal fibrosis. Abassi et al (63) confirmed that PG545 suppressed renal dysfunction and the upregulation of HPA, as well as pro-inflammatory and pro-fibrotic factors that were induced by I/R in AKI. HPA promotes renal fibrosis by participating in fibroblast growth factor-2-dependent EMT of renal TECs (83).

In conclusion, HPA plays a major role in S-AKI, mainly through the degradation of the endothelial glycocalyx, the remodeling of the ECM, and the release of pro-inflammatory cytokines and heparin-binding molecules in the ECM, thus aggravating the immune response and inducing a procoagulant effect and renal fibrosis. However, HPA may also influence S-AKI through other mechanisms.

Autophagy is a highly conserved lysosome degradation pathway in mammals, which removes protein aggregates and damaged organelles to maintain cellular homeostasis (84,85). It is reported that autophagy occurs at low levels under physiological conditions to maintain the homeostasis of the intracellular environment, but is upregulated upon exposure to stress conditions, such as hunger, hypoxia, ischemia and oxidative stress (86). In healthy and diseased states, autophagy plays a key role in maintaining the morphology, activity and dynamic balance between various cell types in the kidney (87). It has been reported that autophagy induced by unilateral ureteral obstruction (UUO) has renoprotective effects, and treatment with the autophagy inhibitor, 3-methyladenine, enhances tubular cell apoptosis and tubulointerstitial fibrosis in the obstructed kidney following UUO (88). However, uncontrolled autophagy leads to excessive degradation of cellular proteins and organelles, eventually resulting in the death of autophagic cells (89).

A previous study has shown that HPA regulates autophagy (90). Shteingauz et al (60) and White (91) first described the role of lysosomal HPA in regulating autophagy, which presents intracellular proteins, lipids and other molecules or organelles for degradation and recycling. The PI3K/AKT/mTOR signaling pathway is known to inhibit autophagy (92). Certain studies (60,93) have reported that overexpression of HPA downregulates mTOR activity, and the inhibition of HPA by PG545 reverses this. mTOR is usually scattered throughout the cytoplasm of normal cells; however, when HPA expression increases, the majority of mTOR and HPA is concentrated around the nucleus, thus reducing the activity of mTOR and increasing autophagy (94). Therefore, it was speculated that HPA may cause renal autophagy through the PI3K/Akt/mTOR signaling pathway and promote the progression of S-AKI (Table I; the protective role in S-AKI). HPA may also cause renal autophagy through the AMPK/mTOR signaling pathway, which is another autophagy signaling pathway. Further in-depth investigations are needed to validate these hypotheses.

Exosomes are membranous vesicles with a diameter of 40-100 mm. All living cells secrete exosomes, which exist in the blood, urine, saliva, lymph and other body fluids (95,96). Exosomes do not contain nuclei and cannot self-replicate. Exosomes are, reportedly, relatively good drug delivery systems due to their stability. Recipient cells endocytose exosomes and release exosome contents, and thus play a crucial role in cell-to-cell communication and information transmission, as exosomes are involved in transporting proteins, lipids, microRNA (miRNA/miR), mRNA and other bioactive substances. Exosomes also participate in the regulation of the inflammatory response, immune response, tumorigenesis, infection and other pathophysiological processes (97,98). Additionally, exosomes have a significant role in sepsis. The overall contribution of exosomes to sepsis was previously studied using GW4869, which was shown to inhibit the production of exosomes. Essandoh et al (99) showed that GW4869 significantly improved the survival rate of mice injected with lipopolysaccharide (LPS) or the septic mouse model of CLP.

miRNAs are released into body fluids, such as serum and urine, and their high specificity and sensitivity render them suitable for use as potential biomarkers for monitoring the progression of AKI (100). Viñas et al (101) reported that the levels of miR-486-5p expressed in the proximal tubules and ECs were increased upon injection with miR-486-5p mimics into AKI mice. Furthermore, the levels of plasma creatinine, tissue damage, and neutrophil infiltration and apoptosis improved. Urinary exosomes are excreted by all nephron segments. Exosome miR-27b from bone marrow mesenchymal stem cells has been shown to inhibit the development of sepsis by downregulating Jumonji domain-containing protein-3 and inactivating the NF-κB signaling pathway (102). Zhang et al (103) showed that human umbilical cord mesenchymal stem cell-derived exosomes downregulated the expression of the miR-146b target gene, interleukin-1 receptor-associated kinase, by upregulating the expression of miR-146b. Furthermore, a previous study showed that exosome-mediated pyroptosis of miR-93-TXNIP-NLRP3 creates functional differences between M1 and M2 macrophages in S-AKI (104). miR-19b-3p, derived from LPS-induced AKI mouse TECs, has been reported to promote macrophage infiltration and tubulointerstitial inflammation by inhibiting the suppressor of cytokine signaling-1 in vitro, leading to the activation of NF-κB, and upregulation of monocyte chemoattractant protein-1 (MCP-1), IL-1β, IL-6, TNF-α and iNOS (105) (Fig. 2).

HPA-induced enhancement of exosome biogenesis was initially identified in human myeloma cells transfected with HPA cDNA (106). The formation of extracellular vesicles is dependent on syntenin and its interaction with syndecans. Roucourt et al (107) reported that HPA activated the syndecan-syntenin-ALIX exosome pathway, and that it also played a role in regulating this pathway. Syndecans, a family of membrane proteoglycans, are composed of chondroitin sulfate and HS chains linked to a 31-kDa integral membrane protein (108). Syndecans and syntenin are involved in the regulation of exosome biogenesis through their interaction with ALIX (109). However, HPA alters syndecan and syntenin composition, and biological function through promoting exosome secretion (106). In human cancer cells, exosome secretion is significantly increased when HPA expression is enhanced or when tumor cells are exposed to exogenous HPA. Furthermore, HPA activity is necessary to enhance exosome secretion (106). In conclusion, HPA is involved in exosome biogenesis and activates the syndecan-syntenin-ALIX pathway (Fig. 2).

HPA promotes exosome formation, which act on the kidneys through their biologically relevant cargo. It has been reported that the upregulation of miR-21-5p improves renal function through several mechanisms; it has been shown to reduce the pathological damage to renal tissue, reduce serum inflammatory response, reduce renal tissue apoptosis, reduce oxidative stress responses and regulate the expression of endothelial glycocalyx injury markers, such as syndecan-1 and HPA in CLP rats (8). HPA also enters the nucleus and regulates gene transcription through exosomes. The role of HPA in exosome activity has been determined by its regulation of HS cleavage (55). HPA localizes to the exosome surface, where it is activated and degrades HS within the ECM (110). HPA can regulate the transcription of various genes involved in neovascularization, such as matrix metalloproteinase 9 (MMP-9), coagulation and inflammatory responses. HPA has been shown to increase the expression of MMP-9 and to increase cleavage of syndecan-1. The released syndecan-1 can be transported into the nucleus, where the bound HS chain and HPA transported into the nucleus can affect a number of mechanisms, including promotion of mitotic spindle formation and subsequent chromosome stability, inhibition of DNA topoisomerase I activity and regulation of cell proliferation (55). In conclusion, HPA functions on S-AKI through exosomes and the release of bioactive substances, and certain miRNAs derived from exosomes can improve renal function in S-AKI (Table I; the protective roles of HPA in S-AKI).

In conclusion, HPA contributes to the pathogenesis of S-AKI by inducing the degradation of HS and the destruction of ECM, thus promoting inflammation, macrophage polarization, fibrosis, dysregulation of inflammatory response, excessive activation of autophagy and exosome biogenesis (Table I, the adverse roles of HPA in S-AKI.). However, the precise molecular mechanisms underlying the pathogenesis of S-AKI require further investigation.