Kidney stone disease, also known as nephrolithiasis or urolithiasis, is one of the oldest diseases known to medicine. It is estimated that 1-15% individuals suffer from kidney stone formation at some point during their lifetime, and the prevalence and incidence of kidney stone is reported to be increasing worldwide (1,2). A recent study concluded that the prevalence of kidney stones was 5.8% among Chinese adults (6.5% in men and 5.1% in women), with about 1 in 17 adults currently affected (3). Without proper treatment, kidney stones can cause the blockage of the ureter, blood in the urine, frequent urinary tract infections, vomiting or painful urination, culminating in the permanent functional damage of the kidneys (4). The worldwide prevalence of urolithiasis has increased over the past decades. Urolithiasis is often a recurrent and lifelong disease with a recurrence rate of 50% within 5-10 years and 75% within 20 years (5). Some studies have indicated that an increase in kidney stone occurrence is expected, due to multiple environmental factors, including changes in lifestyle and dietary habits, as well as global warming (1,4,6). However, precise factors responsible for the upward prevalence and recurrence of urolithiasis have not been identified yet. Due to its high prevalence in adults of working age, kidney stone disease has a substantial impact on the individual and society, and has become a public health issue, particularly in populations residing in regions with a hot and dry climate (7,8).

There are mainly five types of kidney stones according to the mineralogical composition, including calcium oxalate (CaOx; 65.9%), carbapatite (15.6%), urate (12.4%), struvite [(magnesium ammonium phosphate), 2.7%], brushite (1.7%) (9,10). Kidney stones can be broadly categorized into calcareous (calcium containing) stones and non-calcareous stones. The most common types of human kidney stones are CaOx and calcium phosphate (CaP), either alone or combined, which are calcareous and radio-opaque stones (9,11). Kidney stones form at a foundation of CaP termed Randall's plaques (RPs), which begins at the basement membranes of thin limbs of the loop of Henle on the renal papillary surface (12). CaOx and urate stones exhibit a higher occurrence in males, whereas higher percentages of carbapatite and struvite stones are observed in females than in males (10,13). However, the role of sex differences in the pathophysiological mechanisms of urinary stone disease are not yet fully understood.

Regardless of the type, kidney stone formation is a complex and multistep process that includes urinary supersaturation, crystal nucleation, growth and aggregation (11,14). Kidney stone formation is associated with systemic disorders, including diabetes (15), obesity, cardiovascular diseases, hypertension and metabolic syndrome (16,17). Conversely, nephrolithiasis patients [also known as kidney stone formers (KSF)] are at a risk of developing hypertension (18), chronic kidney disease (CKD) (19) and progression to end-stage renal disease (ESRD) (20,21). Multiple promoting factors and inhibitors have been reported to play critical roles in kidney stone formation. For example, hyperoxaluria, hyperuricosuria and phosphaturia are common promoting factors linked to kidney stone formation (22,23); inter-α-inhibitor (IαI), a member of the protease inhibitor family, has been shown to inhibit CaOx crystallization in vitro (24).

Although details of human stone formation have accumulated, kidney stone formation and growth mechanisms are far from being clarified. The present review provides an update on the mechanisms of kidney stone formation, in order to improve the understanding of kidney stones for urologists, nephrologists and primary care givers.

Urinary supersaturation and crystallization are the driving force for intrarenal crystal precipitation and is mainly caused by inherited or acquired diseases associated with renal function impairment. Additionally, urinary supersaturation and crystallization are influenced by urine pH and specific concentrations of substance excess, including CaOx, CaP, uric acids and urates, struvite, amino acids (cysteine), purines (2,8-dihydroxyadenine and xanthine) and drugs (e.g., atazanavir, sulfamethoxazole, amoxicillin, ceftriaxone) (25,26). Additionally, crystal formation and development are influenced by multiple modulator molecules, which are known as receptors, promoters and inhibitors.

RPs, first proposed by Alexander Randall in 1937 (55), are regions of subepithelial mineralized tissue at the papillary tip, surrounding the openings of the ducts of Bellini containing CaP (56). Scanning electron microscopy (SEM) examination has shown that RP are made of a mixing of tubules with calcified walls and of tubules obstructed by CaP plugs (57). RP consists of CaP crystals mixed with an organic matrix that is rich in various proteins and lipids, and includes membrane-bound vesicles or exosomes, collagen fibers, as well as other components of the extracellular matrix (58). An increasing number of studies have suggested that RPs are the origin of renal stones (57-60). Winfree et al (61) clarified that kidney stones develop as an overgrowth on RP, which contains unique organic composition (fibrillar collagen) that can be differentiated from the stone overgrowth by specific autofluorescence signatures. Of note, a previous study using a murine mode of RP revealed that vitamin D supplementation and calcium intake could notably accelerate RP formation (60). However, the precise mechanisms of RP formation remain unclear.

Recently, studies indicated that long non-coding RNAs (lcnRNAs) H19 and MALAT1 mediated osteogenic differentiation of human renal interstitial fibroblasts (hRIFs) and participated in RP formation (62-64). lcnRNA H19 has been shown to be significantly upregulated in RP, which can promote the osteogenic differentiation of hRIFs by activating Wnt/β-catenin signaling (63). lcnRNA H19 can also serve as a facilitator in the process of CaOx nephrocalcinosis-induced oxidative stress and renal tubular epithelial cell injury through the interaction with miR-216b and exerts its effect via the HMGB1/TLR4/NF-κB signaling pathway (64). lcnRNA MALAT1 can function as a competing endogenous RNA (ceRNA) that sponges miR-320a-5p, upregulates Runx2 expression and thus promotes the osteogenic phenotype of hRIFs (62).

These studies provide novel insight into the pathogenesis of RP-mediated kidney stone disease, while further studies are urgently anticipated to explore the mechanisms of RP formation, as well as additional roles of RP in the context of stone formation.

Statistical analyses have revealed that males have a higher incidence of CaOx nephrolithiasis than females at a ratio of 2-3:1 (4,65); however, the exact mechanism remain unclear. Previous studies have indicated that androgens increase and estrogens decrease urinary oxalate excretion, plasma oxalate concentration and kidney CaOx crystal deposition. Additionally, enhanced androgen signaling may be responsible for the association between sex and kidney stone formation (65-68). Androgen receptor (AR) signaling can directly upregulate hepatic glycolate oxidase (69) and kidney epithelial nicotinamide adenine dinucleotide phosphate oxidase (NAPDH), subunit p22-PHOX at the transcriptional level, so as to increase oxalate biosynthesis, ultimately leading to kidney stone formation (70). Peng et al (71) reported that testosterone contributes to nephrolithiasis development through the induction of renal tubular epithelial cells apoptosis and necrosis through HIF-1α/BNIP3 pathway. Changtong et al (72) revealed that testosterone could promote kidney stone disease via the enhanced COM crystal-cell adhesion by the increased surface α-enolase. Zhu et al (73) demonstrated that AR can inhibit the recruitment of macrophages and suppress the COM crystals phagocytic ability of macrophages via the decrease of the colony-stimulating factor 1 (CSF-1) signals, through miR-185-5p upregulation. These findings suggest that androgen receptor signaling may be a key player in the development of nephrolithiasis (Fig. 2).

Theoretically, AR may be a new potential target and can be evaluated for novel therapeutics for the suppression of kidney stone formation. The 5α-reductase inhibitor, finasteride, has been reported to abolish the promoting effect of testosterone on COM crystallization (74). Another newly developed AR degradation enhancer, dimethylcurcumin (ASC-J9), has been reported to suppress oxalate crystal formation via the modulation of oxalate biosynthesis and reactive oxygen species (ROS)-induced kidney tubular epithelial cell injury in a rat model (73). Reversely, estrogen may serve as a protective factor against kidney stone formation. An in vitro study demonstrated that estrogen led to changes in the cellular proteome of [Madin darby canine kidney (MDCK)] renal tubular cells that led to the decreased CaOx crystal receptor surface expression (annexin A1 and α-enolase), reduced intracellular ATP, and enhanced cell proliferation and renal tubular cell tissue healing (75). There is evidence to suggest that estrogen receptor β (ERβ) can suppress oxalate-induced oxidative stress via transcriptional suppression of the NADPH oxidase subunit 2 (NOX2) through the direct binding to the estrogen response elements (EREs) on the NOX2 5′ promoter (76), which exerts protective effects on renal CaOx crystal deposition.

All these findings may partly explain why a higher incidence of nephrolithiasis is encounter in males than in females. Targeting AR may be developed as a potential therapy for CaOx crystal-related kidney stone disease. However, these studies were performed in vitro and in vivo, using only cell lines or animal models. Further validation and clinical studies are required. Finasteride and ASC-J9 have been demonstrated to suppress a number of AR-mediated diseases, including prostate cancer (77,78), liver cancer and spinal and bulbar muscular atrophy neuron disease (79). However, additional future studies are necessary before the clinical application of finasteride and ASC-J9 in kidney stone prevention, considering the side-effects, including sexual dysfunction (80).

Emerging evidence has indicated that microorganisms belonging to the human microbiome, including microorganisms of the kidney and urinary tract, are likely to have a profound effect on urological health, both positive and negative, due to their metabolic output and other contributions (81).

Macrophage accumulation and macrophage-related inflammation or anti-inflammation is the main immune response alteration observed in kidney stone disease, which has been widely reported to play a crucial role in renal CaOx crystal formation (110).

Firstly, the recruited macrophages could promote the development of COM crystals via the interaction of CD44 with OPN and fibronectin (FN) (111), which are upregulated in renal tubular cells induced by crystals. Secondly, macrophages have been evidenced to secrete various mediators via classical secretory pathways that cause renal interstitial inflammation (112,113), particularly macrophage inhibitory protein-1, monocyte chemoattractant protein-1 and interleukin-8 (IL-8) (112). These chemokines consequently enhance the recruitment of various immune cells, including monocytes, macrophages, neutrophils, dendritic cells and T-cells into the inflammatory locale (114,115). Several studies have demonstrated that macrophage-derived exosomes following COM exposure are involved in kidney stone pathogenesis (112,113,116). A set of proteins in COM-treated macrophage exosomes were previously identified as proteins involved mainly in immune processes, including T-cell activation and homeostasis, Fcγ receptor-mediated phagocytosis, interferon-γ (IFN-γ) regulation and cell migration (112). Additionally, infiltrated monocytes could differentiate into different macrophage subtypes with a wide range of clinical manifestations, presentations and histological phenotypes (110,117), display protective or pathogenic activities in kidney stone development (110).

Increasing evidence has revealed that M1/M2-macrophage differentiation plays an important role in renal CaOx crystal formation (111,115,118-120). However, whether M1 macrophage-mediated inflammation that contributes to stone formation will initiate stone promoters and reduce stone inhibitors remains controversial. Khan et al (58) demonstrated that M1 macrophages could cause acute tissue injury, which was associated with crystal deposition and RP formation. Conversely, Taguchi et al (121) concluded that there was no association between renal dysfunction and increased crystal deposition, based on their observation that no changes were observed in urinary variables in lipopolysaccharide (LPS)-induced M1 macrophage-mediated acute renal injury. M2 anti-inflammatory macrophages can phagocytize and degrade CaOx kidney stone fragments through a clathrin-dependent mechanism (110,113,115,120,121) (Fig. 6).

Given the critical role of immune-response in CaOx crystal formation and development, the immunotherapy approach has been proposed to prevent stone recurrences in certain individuals through the modulation of the immune response, in order to degrade CaOx crystals and thus prevent stones from developing (122). However, investigations into immunotherapeutic targets for kidney stone disease are urgently required.

In the present review article, emerging concepts of mechanisms contributing to stone formation were summarized, by reviewing novel insight into kidney stone disease related-metabolic risk factors, receptors, promoters and inhibitors, through the examination of the roles of immune-response, microbiome and sex hormones in stone formation and development. The pathophysiology of kidney stone disease cannot be completely explained by crystallization processes alone. However, due to current limitations in research, there are still some research areas in kidney stone formation that remain poorly understood, and were not been discussed herein. Future comprehensive studies are mandatory to further elucidate the mechanisms of the microbiome and immune response in kidney stone formation, in order to develop novel prophylactic and therapeutic approaches.