Gas 6, weighing 75 kDa, is a relatively large member of the VKDP family. The concentration of plasma Gas 6 ranges from approximately 2.5 to 18.8 µg/l in healthy adults (74). Gas 6 is highly homologous with Protein S and carries an N-terminal Gla domain after vitamin K carboxylation. Gas 6 is widely expressed in brain, heart, lung, kidney and other tissues, with the exception of the liver (43). In 1995, Gas 6 was reported as the endogenous ligand for the TAM family for the first time (75). TAM is the acronym for three receptors: Tyro3, Axl and Mer. Among these, Axl has the highest affinity to Gas 6 (76). It has been reported that the laminin-like globular domain of Gas 6 at C-terminus appears to be the binding site of TAM receptors (75). However, after warfarin inhibits vitamin K-dependent carboxylation, inactivated Gas6, not only completely inhibits the autophosphorylation of the Axl receptor, but also fails to bind to the Axl receptor in vitro (77,78). Therefore, vitamin K-dependent carboxylation is the key to the interaction between Gas 6 and TAM receptor. In light of numerous previous studies, the binding of Gas 6 and its receptors activated downstream signaling, such as of phosphatidylinositol 3-kinase (PI3K), extracellular signal regulated kinase (ERK) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathways, to adjust the processes of apoptosis, survival, proliferation, migration and adhesion (48,79-82).

There is an inseparable relationship between Gas 6 and the cardiovascular system. The binding of Gas 6 and Axl limits the apoptosis of VSMCs by activating Akt and PI3K (80). It is worth mentioning that vitamin K₂ can inhibit VSMC calcification and apoptosis by restoring Gas6 expression and activating downstream signaling by Axl, Akt and Bcl2 (49,50). Endothelial progenitor cells (EPCs) are involved in the saving response to ischemic tissue through forming new blood vessels or proliferation of pre-existing vasculature. Autologous EPC transplantation therapy has been indicated as safe and practical in chronic myocardial ischemia (51). It has been identified that Gas6 has the ability to stimulate EPC proliferation and migration in vitro by activating the Akt signaling pathway (48). The finding provides a basis for the further therapy of vascular re-endothelialization. Vascular aging, a risk factor of CVDs, is characterized by vascular stiffness, vascular remodeling and endothelial dysfunction (83). Aging vessels provide a good environment for CVDs. Gas6/Axl can delay cell cycle arrest, which is a key cause in the development of VSMC senescence and promotes their transition from the G1 to the S phase. The PI3K/Akt/Forkhead box O (FoxO) signaling pathway is considered the major target of Gas6/Axl signaling in VSMC senescence, with FoxO being the key factor (84). Furthermore, clinical investigation has demonstrated that Gas 6 plasma levels at admission reflect the existence of potential cardiovascular risks and can prognosticate cardiovascular events (52).

Accumulating evidence has indicated that Gas 6 is significantly secreted by VSMCs in human atherosclerotic plaques, but there is no secretion in healthy blood vessels. The anti-inflammatory cytokine transforming growth factor β (TGF-β) induces the secretion of Gas 6 in VSMCs, and then, stimulated by Gas 6, the VSMCs suppress the expression of inflammatory factors, such as tumor necrosis factor (TNF) α and intracellular adhesion molecule (ICAM)-1 (85). Thus, Gas 6 acts as a protective factor in human atherosclerosis. Of note, Gas6 levels inversely related to complexity and stability in patients with carotid atherosclerotic plaques (85,86). In particular, it should be noted that Gas6-deficient mice show more stable atherosclerotic lesions than normal mice, and inhibition of Gas 6 is considered to be beneficial to plaque stabilization (87). The contradictory feature between humans and mice is associated with obvious species physiological differences (88).

Of note, overexpression of Gas 6 has a detrimental effect on some pathological processes. The renin-angiotensin-aldosterone system is closely connected with cardiovascular and renal inflammation and fibrosis. It has been emphasized that Gas 6 deficiency prevents the damage of aldosterone on target organs (89). In addition, cardiomyocyte-specific Gas 6 overexpression hastens the deterioration of pathological cardiac hypertrophy, mainly due to the activation of mitogen-activated protein kinase (MAPK) kinase 1/2-ERK 1/2 signaling (90).

The contribution of Gas 6 to acute kidney injury is closely related to its biological functions, such as anti-inflammation and immunoregulation (91,92). The kidney, despite being a rich blood supplying organ, is susceptible to hypoxic injury due to the complex balance of renal blood flow, glomerular filtration rate, oxygen consumption and arteriovenous oxygen shunting (93). Previous findings suggested that Gas 6 protected against renal ischemia-reperfusion injury in a mouse model (94). To be specific, with the assistance of Gas 6 treatment, creatinine and blood urea nitrogen decreased by 29 and 27%, respectively. Cell apoptosis was significantly decreased, attributable to Gas 6 enhancing macrophages to uptake apoptotic cells (95). Furthermore, the expression of pro-inflammatory cytokines, such as interleukin (IL)-1β and TNF-α, was markedly reduced by another Gas 6 function, dampening the inflammatory responses (11,91,94). Similarly, concentration of Gas 6 rose in sepsis-induced acute kidney injury mice, and improved the survival rate by reducing serum urea nitrogen, creatinine and renal tissue apoptosis (53). In addition, several reports demonstrated that Gas6 levels were significantly increased in CKD patients and chronic hemodialysis patients (96,97). Opinions regarding the potential mechanisms vary. Researchers tend to associate the elevation with endothelial function (Gas6 is expressed by endothelial cells) and inflammation because pro-inflammatory cytokines are abundant in the blood of these patients (96,98). It is reported that endothelial cells in CKD are subjected to specific stress overtime which leads to accelerated cardiovascular disease and high mortality (99). Disruption and inflammation of glomerular capillaries influence the evolution of CKD, and, consequently, elevated Gas 6 levels (100). It is worth noting that Gas 6 is upregulated in many forms of inflammatory nephropathy, for example, lupus nephritis and IgA nephropathy (101,102).

Diabetic nephropathy is a common complication of diabetes that can further develop into end-stage renal disease. There are opposing conclusions on the tendency of plasma Gas 6 in diabetes and diabetic nephropathy. Nagai et al first reported that the expression of both Gas6 and Axl was distinctly increased in diabetic rats and proved Gas 6 can induce mesangial cell hypertrophy, which further leads to glomerular hypertrophy in the early stage of diabetic nephropathy (103). Furthermore, a reliable mechanism was proposed in which high glucose stimulates mesangial cells, followed by activating Gas6/Axl and the Akt/mTOR pathway, which results in mesangial and glomerular hypertrophy (104). By contrast, Hung et al indicated that plasma Gas6 levels in impaired glucose tolerance patients and type 2 diabetes were significantly decreased (105). A study based on individuals with different degrees of albuminuria offers some insight into this controversy, and showed the blood level of Gas 6 decreased with the deterioration of proteinuria (106). Silaghi et al formulated a hypothesis that the interaction between molecular charge and weight may participate in glomerular filtration of Gas 6 (100). More specifically, Gas 6 and albumin (approximately 66 kDa) have a similar molecular weight and a net negative charge repelled the glomerular membrane. Complex interactions eventually lead Gas 6 to filter through the glomerular membrane and be excreted from the body (100). Therefore, the concentration of plasma Gas 6 changes in different stages of diabetes.

The contribution of Gas6 to cancer has been reported for a large number of cancer types. For example, Gas 6 is upregulated in breast cancer, melanoma and ovarian cancer (107-109). Tumor cells lack the competence to produce Gas 6, but can educate infiltrating macrophages to promote the production of Gas6 by producing IL-10 and macrophage colony-stimulating factor (M-CSF) (110). Previous findings have shown the pro-tumor effects of Gas6/TAM signaling. In the case of Gas 6 overexpression, the survival of myeloma cells was significantly increased in vitro and, conversely, the deficiency of Gas 6 led to rapid cell death of myeloma (111). In addition, the autocrine Gas 6 assists the resistance of myeloma cells to bortezomib (111). Recently, the pro-tumor effects of Gas 6 was also reported in lung cancer cells (54). In addition, blocking Gas 6/Mer signaling with Mer receptor inhibitors significantly limits the proliferation and growth of lung cancer cells (54). Interestingly, a high level expression of Axl and its ligand Gas 6 were recognized in non-small cell lung cancer patients, who acquired resistance with epidermal growth factor receptor tyrosine kinase inhibitors (112). It has been reported that Gas 6 negatively regulates the proliferation and interferon-γ production of natural killer cells to inhibit tumor immunity through binding with Casitas B cell lymphoma-b/TAM receptors (113). In addition, Gas 6 prolongs VSMC survival in the tumor microenvironment, which is requisite to tumor angiogenesis (79). Several investigations have indicated the roles of Gas 6 in predicting the prognostic risk of cancer. Gas 6 protein as an independent predictor always indicates a poor prognosis (107,109) (Fig. 1).

As its name suggests, GRP, which was first identified in sturgeon cartilage, has abundant Gla residues (15 Gla residues in human) (114,115). With unusually high capacity to bind calcium through Gla resdues, GRP accumulates in bone, cartilage and ectopic calcification, such as blood vessels and skin (112). During physiological conditions, GRP participates in the stabilization of cartilage matrix, chondrogenesis and inhibition of osteogenesis (116-118). Recently, GRP has attracted attention due to its crucial performance in combating ectopic calcification.

The growth of long bones is inseparable from the process of endochondral ossification. First, chondrocytes participate through a combination of proliferation, extracellular matrix secretion and hypertrophy. Then, hydroxyapatite crystals are deposited in the extracellular matrix surrounding late hypertrophic chondrocytes, known as mineralization. Next, chondrocyte death, matrix degradation and contents invasion occur. Finally, the growth plates close and the bones mature (119). Surmann-Schmitt et al reported GRP in the upper zone of the growth plate, termed unique cartilage matrix-associated protein, which exhibits a negative correlation with osteogenic differentiation (116). Both GRP knockdown zebrafish and warfarin-exposed zebrafish show irreversible growth retardation and altered skeletal development; therefore Gla residues are necessary for the function of GRP (117). It is worth mentioning that a similar feature is found in human warfarin embryopathy, which results in pregnant women from warfarin therapy (120,121). Surprisingly, GRP is not essential for mouse skeletal development (55). However, the fact that GRP is still expressed in adult mouse cartilage indicates that GRP may contribute to skeletal homeostasis and other calcification-associated pathological processes after infancy.

Osteoarthritis (OA), a painful joint disease, is characterized by articular cartilage degradation, bone remodeling, tissue inflammation and abnormal extracellular matrix mineralization. In fact, GRP plays a dual role in OA. GRP prevents articular cartilage degradation in two practical ways. On the one hand, GRP blocks the aggrecanase activity of A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4 and ADAMTS-5 by physical interaction (56,57). Aggrecanolysis is considered the main process of cartilage degradation, thus GRP protects cartilage by increasing its resistance to aggrecan cleavage in OA. By contrast, enhanced chondrocyte apoptosis accelerates the cartilage damage in OA. It is reported that chondrocyte cell death is markedly increased in GRP-deficient mice; thus, GRP protects articular cartilage by reducing chondrocyte apoptosis (56). However, GRP has also been implicated in bone remodeling, which is mediated with the altered function and metabolism of osteoblasts and osteoclasts in OA (122). Previous findings have shown osteoblasts contribute to phenotypic changes and osteoclasts are associated with cartilage destruction in OA (56,123). Additionally, GRP, as a downstream gene of runt-related transcription factor 2 and Osterix, stimulates osteoblast differentiation in OA (60). Similar results have been found in mice, in which osteoblasts and osteoclasts decreased during experimental OA in GRP-deficient mice, while there was no fluctuation in normal mice (56). The obvious conflict regarding the effect of GRP on osteoblastic differentiation may be explained by post-translational modification of GRP (56,60,116). Moreover, certain data have indicated GRP promotes osteophyte formation in OA and the effect occurs via bone remodeling rather than cartilage maturation (57,60). In addition, inflammation presents before the joint structure changes in OA joints (124). It has been demonstrated that GRP has a similar inhibitory effect on calcification and inflammation processes (58). Furthermore, synovial fluid GRP levels in OA patients exhibit a positive correlation with radiographic findings and symptomatic severity of OA (125). However, it is noteworthy that studies have shown that vitamin K deficiency is a potential risk factor for knee OA (126). Due to the lack of effective treatment and prevention methods currently, fully carboxylated GRP by vitamin K supplementation is a convenient and inexpensive candidate for the treatment of OA.

Vascular calcification is a pathological process characterized by the deposition of calcium phosphate crystals in vessel walls (127,128). According to the location of calcification, it can be classified into intimal calcification (related to plaque burden and luminal narrowing) and medial calcification (associated with vessel stiffness and vascular compliance decline) (128). VSMCs are a contractile phenotype in the physiological state that can regulate vascular tension. However, they lose expression of contractility-related genes when vascular injury exists and are further transformed into osteoblast-like cells (129). In addition, bone matrix regulatory proteins, such as BMP-2, BMP-4, osteopontin, MGP and OC, are expressed in calcifying vessels (130). In response to the high level of extracellular calcium and the lack of calcification inhibitors, VSMCs release extracellular vesicles (EVs) into circulation with good mineralization capability and form a nucleation site for hydroxyapatite (131,132). Fetuin-A, a 48 kDa protein, is synthesized in the liver and secreted into circulation as a powerful calcification inhibitor. Interestingly, fetuin-A is too large to enter the collagen fibril where the mineral grows (133). It is reported that the mineral grows only inside the fibril when fetuin-A exists, whereas it grows beyond the fibril without fetuin-A (134). Therefore, fetuin-A is the key factor in determining the location of mineral growth. Moreover, inflammatory activity takes part in early calcification. Many studies have indicated a synergistic interaction between macrophage and VSMC calcification. Activated macrophages produce a large number of proteases to enhance the degradation of elastin and collagen (124,135). Macrophages markedly increased BMP-2 expression in VSMCs and also released EVs with calcification capacity (136,137). In addition, many other factors influence the process of vascular calcification, for instance, VSMC apoptosis, oxidative stress and endothelial dysfunction (138,139).

GRP, a VKDP, has been identified as a powerful inhibitor of vascular calcification. GRP, MGP and fetuin-A form a large complex that is loaded in noncalcifying EVs but distinctly lowered in high calcium-loaded vesicles, thus recommending GRP as an important mineralization inhibitor (59,61). Furthermore, calciprotein particle (CPP), a fetuin-mineral complex, principally contains mineral, fetuin-A, MGP and GRP, and contributes greatly to the stabilization of minerals. Research has demonstrated that CKD patients possess CPPs with a lower content of fetuin-A and GRP compared with healthy individuals (138). Fetuin-A is predominant in healthy CPPs and retards the deterioration toward calcifying CPPs through collaboration with GRP (140,141). Moreover, GRP shows the ability to counteract inflammation and is found in macrophage-derived EVs (142). In vitro studies found calcification in both GRP-deficient and normal VSMCs in response to osteogenic medium after 6 days, yet GRP-deficient VSMCs calcified about twice as much as normal VSMCs 9 days later (143). Of note, there is an apparent increase in the expression of BMP-2 and its downstream marker (small mother against decapentaplegic, SMAD) and, finally, after comparing GRP with two different carboxylation states, the direct interaction between the carboxylated GRP and BMP-2 was confirmed (143). Therefore, GRP disturbs the BMP-2-SMAD signaling in calcifying VSMCs, playing a central role in VSMC calcification (Fig. 2).

Microcalcification, a small deposit of calcium with a diameter less than 1 mm in mammographic images, is vital for the diagnosis and prognosis of breast cancer (61). Ductal carcinoma in situ can be as high as 20-25% in women with asymptomatic breast cancer (144). Furthermore, 70% of ductal carcinoma in situ can be diagnosed only by microcalcification in mammography (145). Recent findings have shown that linear branching microcalcifications in mammography indicate the aggressive of tumor tissue (146). A differential accumulation pattern of carboxylated GRP (cGRP) and under-carboxylated GRP (ucGRP) by vitamin K has been recently emphasized in human breast cancer (147). In healthy mammary gland tissues, cGRP was predominant, while ucGRP was found to be either co-localized or undetectable. By contrast, ucGRP was widely detected in tumor cytoplasm, while cGRP was only intermittently found in certain tumor cells. There are many explanations for the large quantity of ucGRP in tumors. It has been observed that the decreased level of vitamin K in tumor areas is in contrast to non-tumorous areas (148). Patients with tumor complications, such as venous thromboembolism, have received long-term therapy with vitamin K antagonists and the potential detrimental effects to GRP should be noted (149). In addition, prolonged subclinical vitamin K deficiency has been identified in cancer patients. Furthermore, vitamin K preferentially supports the coagulation factor synthetic process in the liver, and only after the vitamin K supply has met the liver's need is the excess vitamin K transported to extra-hepatic tissues (177,150). Thus, ucGRP is widespread in tumor tissues. Furthermore, the formation mechanism of microcalcification in breast tumor tissue is similar to physiological bone mineralization and pathological vascular mineralization (151). Both cGRP and ucGRP showed an advanced affinity to calcium mineral deposits in breast cancer tissue. Thus, with the capability of resisting ectopic calcification, GRP is considered a novel effective antagonist against cancer. It is worth noting that triple-negative breast cancer is a subtype with low expression of estrogen receptor, progesterone receptor and human epidermal growth factor receptor 2 receptor (62). Therefore, there is a lack of effective targeted therapy drugs for triple-negative breast cancer. However, recent research may be useful in resolving this issue. GRP inhibits the growth, migration and invasion of triple-negative breast cancer tissues in vitro and in vivo (62). Moreover, according to survival analysis in the open database, the relapse-free survival rate of patients with triple-negative breast cancer was significantly correlated with high GRP expression (62).

Periostin, initially known as osteoblast-specific factor 2, was first cloned from a cDNA library of the mouse osteoblastic cell line MC3T3-E1 in Japan (152). Over a decade later, the Gla-containing protein, periostin, was determined to require vitamin K-dependent carboxylation and became the 13th member of the VKDP family (153). Characterized by fasciclin domains, periostin is particularly expressed in connective tissues submitted to constant mechanical stresses (153). For example, periosteum, the periodontal ligament, heart valves and skin. Periostin has also been implicated in fibrosis, inflammation, tumor metastasis and the fracture healing process (67,154-156).

Fractures are one of the most common traumatic injuries to humans. Most fractures can be repaired to their pre-injury state through a process similar to embryonic skeletal development. According to the characteristics of fracture healing, the process is divided into four partially overlapping phases: The inflammation phase, the soft callus phase, the hard callus phase and the remodeling phase (157). The inflammation phase is marked by acute inflammation, hematoma formation and skeletal stem cell recruitment. During the soft callus phase, cartilaginous callus and nascent blood vessels form. During the hard callus phase, the most active phase of osteogenesis, the cartilage is reabsorbed and bone is deposited by osteoblasts (158). Angiogenesis also continues during this phase. During the last phase, primary bone is eventually replaced by lamellar bone, which supports normal skeletal functions, and vascular remodeling is finally completed (158,159). There is a vital association between periosteum and fracture repair. In a mouse model in which graft femoral bone was segmentally transplanted, the periosteum showed positive osteogenic and angiogenic activity, leading to superior healing and repair of live isografts (160). However, absence of the periosteum led to poor cartilaginous callus formation, and even fracture non-union (160,161). The periosteum is anatomically comprised of an outer fibrous layer and an inner cambium layer. The fibrous layer contains fibroblasts, collagen, and elastin fibers, along with a nerve and microvascular network (162). The cambium layer is directly closed to the bone surface and contains high-quality mesenchymal progenitor cells, osteoblasts, fibroblasts, microvessels and sympathetic nerves (162,163). In human bones, periostin is highly expressed in the cambium layer, where it is highly active during bone remodeling (164). In a mouse model of fracture, rapid periostin gene expression occurred during the inchoate phase of fracture healing (155). In the first 1-2 weeks after fracture, human serum periostin is decreased initially, prior to a progressive elevation that peaks at 8 weeks, and is present for about 26 weeks (165).

Periostin participates in almost all phases of fracture healing. In the early inflammation phase, as a result of the inflammatory response or paracrine effects of the periosteum, periostin is present at a low level in serum (165). Transplantation of the periosteum of periostin-deficient mice to the fracture site of wild-type mice induced negative fracture repair, indicating that periostin regulates periosteum activation (66). Skeletal stem cells (SSCs), with local osteogenic potential, are recruited in the early stage of bone regeneration and periosteum and is considered one of its major sources (166). As an extracellular matrix protein, periostin promotes the migration of SSCs by binding integrin receptors on the cell surface (162,167). Notably, periosteal cells, another form of convened cells that shares a common embryonic origin with SSCs, have been revealed to have greater regenerative potential than SSCs (63). Moreover, periosteal cell functions are impaired in mice lacking periostin, suggesting that periostin contributes to periosteal cell activation (63). During the callus phases, induced by BMP-2, periostin is upregulated in soft callus and osteoblasts (168). Accumulating evidence has indicated that abundant periostin facilitates the proliferation, differentiation and adhesion of osteoblasts in bone formation (64,65). In addition, periostin may interfere with osteoclasts in a similar way (65). It is reported that periostin markedly increases arterioles in a calvarial defects model, proving periostin promotes angiogenesis (66). Periostin has a crucial mission in the last phase of fracture healing, that is, to recover the periosteum niche of periosteal cells. In a periosteum transplantation model, periosteal cells may still be re-activated to contribute to cartilage within the callus after three injury cycles (63). By contrast, when periostin-deficient grafts were transplanted into wild-type hosts, the contribution of periosteal cells to repairing of the second fracture injury disappeared, leading to defective callus formation and fibrosis, and furthermore this was not due to deficient proliferation (63). Therefore, periostin plays a crucial role in maintaining periosteal cell niche and supporting bone remodeling.

Long-term anticoagulant therapy with vitamin K antagonists, such as warfarin, reduces bone density and increases the risk of osteoporosis (169,170). Previous findings have shown that warfarin significantly inhibits osteoblastic differentiation (171). Warfarin interferes in the carboxylation of periostin by antagonizing the function of vitamin K, and the decrease of carboxylated periostin is one of the main causes of bone density reduction (172). By contrast, vitamin K₂ promotes mineralization of osteoblasts (173). In recent years, periostin has been recommended as a potential predictive marker of bone events. Osteoporotic fracture is a major cause of disability in the elderly, while the ability of current predicting methods is limited. In a cohort of 607 postmenopausal women from France that were followed up for 7 years, a positive correlation between serum periostin and fracture risk was observed (174). Furthermore, the association was independent of bone mineral density and prior fractures, indicating that periostin is an independent predictive marker of fracture risk. This hypothesis was confirmed in another case control study of Korean postmenopausal women (175). Interestingly, high plasma periostin levels prefer non-vertebral fractures to vertebral fractures, such as limb fractures (175). These clinical outcomes seem contrary to the popular view of periostin. The specific mechanisms for these conclusions need further study as they may be related to the carboxylation state of periostin or to the distribution of periostin in the body. Specifically, periostin in bone are induced to circulation. Notably, it has been demonstrated that lower serum periostin concentrations were related to prevalence of knee OA in women (176). This provides a new idea for the application of periostin in bone event prediction.

During embryogenesis, periostin supports normal valve leaflet morphogenesis and cardiac skeleton maturity (177). Periostin is implicated in CVDs, such as myocardial infarction, atherosclerosis and cardiac fibrosis-related diseases (67). Cardiac fibrosis is a prominent feature of cardiac remodeling that can further lead to heart failure and impaired cardiac function. Fibroblasts, the most abundant cell population in the heart except cardiomyocytes, rapidly differentiate into myofibroblasts in the cardiac fibrosis process (67). Abundant differentiated myofibroblasts found in hearts suffering failure also support the transformation (179). Emerging evidence suggests that the myofibroblast phenotype still has latent reversibility in end-stage heart failure (67,178). Of note, periostin, as the most specific product, is expressed in essentially all myofibroblasts (67,68). Certain data have indicated targeted ablated periostin-expressing myofibroblasts led to a diminished fibrotic area and improved the ejection fraction in hearts in AngII-induced fibrosis mice (68). In addition, not only was cardiac fibrosis reduced, but treatment also did not affect scar stability in myocardial infarction mice (68). Moreover, periostin antibody treatment visibly restricted cell viability of myofibroblasts in vitro (69). Therefore, periostin is a novel central factor contributing to the function of myofibroblasts during cardiac fibrosis. Research has demonstrated that ginsenoside Rb1, the bioactive component of ginseng, reduced the expression levels of periostin and protected rats against myocardial fibrosis (179).

According to whether exons 17 and 21 exist or not, periostin can be divided into four isoforms, i.e., Pn-1 to Pn-4. In detail, Pn-1 is a full-length form, Pn-2 is short of exon 17, Pn-3 is short of exon 21, and Pn-4 is short of exons 17 and 21 (69). Using an antibody that specifically inhibits exon 17, the dispute regarding the functions of different periostin isoforms has been settled. It is not surprising that the expression of periostin increased in the border zone on day 5 after myocardial infarction (69). However, total infarction and fibrosis size were notably reduced in an adult mouse model by selectively neutralizing an antibody against exon 17 (69). In addition, cardiac dysfunctions were improved. Moreover, Pn-2 contributes to angiogenesis in in vitro experiments, while Pn-1 does not (69). Low expression of TGF-β, a fibrosis-related gene, was associated with the inhibition of fibrosis. Thus, Pn-1 contributes to fibrosis and heart remodeling after myocardial infarction, and there is potential to improve the prognosis of myocardial infarction via selectively inhibited Pn-1 treatment. Nevertheless, neonatal mice were capable of regenerating myocardium after myocardial infarction. On day 21 after myocardial infarction, the infarcted areas of neonatal mice almost disappeared (180). However, myocardial regeneration was inhibited in periostin-deficient neonatal mice, presenting with a larger infarcted area, which was attributed to the inhibition of PI3K/glycogen synthase kinase 3β/cyclin D1 signaling pathway (180). Therefore, periostin is pivotal for myocardial regeneration at the early stage of myocardial infarction, and is involved in fibrillogenesis and scar generation in the later chronic stage.

Previous findings have shown that periostin is abundantly expressed in patients with atherosclerosis (181-183). In the 'Pathobiological Determinants of Atherosclerosis in Youth' study, the variant encoding periostin gene was connected with atherosclerotic lesion traits (181). Matrix metalloproteinases, enzymes implicated in atherosclerosis and vascular remodeling, which were induced by periostin, led to valve thickening in mice fed high-fat diets (182). Additionally, periostin stimulates angiogenesis both in vitro and ex vivo (179). In response to injury, periostin was markedly upregulated in neointimal SMCs and adventitial myofibroblasts, and promoted cell migration (183). By contrast, the plaques of periostin-deleted mice, not only had a smaller necrotic core and fibrous cap, but also possessed more cholesterol clefts (184). The deficiency of periostin also reduced the infiltration of macrophages into the plaque (184). Thus, periostin plays a considerable role in atherosclerosis, and targeted periostin treatment may delay progression of diseases associated with atherosclerosis (Fig. 3).

In the last decade, the role of periostin in airway development and diseases has been widely emphasized. For instance, periostin was reduced in tracheal aspirate fluid of bronchopulmonary dysplasia during the window period (185). Then, TGF-β upregulated the expression of periostin in the interstitial fibrosis region (185,186). Thus, periostin is recognized as a potential biomarker that predicts the risk of bronchopulmonary dysplasia and the need for preventative therapies in preterm infants. Periostin has been involved in many respiratory disorders, such as idiopathic pulmonary fibrosis (IPF), asthma, chronic rhinosinusitis, idiopathic eosinophilic pneumonia and allergic bronchopulmonary aspergillosis (156,187-190). The most notable of these are IPF and asthma.

IPF, a common pulmonary fibrotic conditions, is a chronic progressive parenchymal lung disease of unclear cause that is limited to the lungs (191,192). Patients are predominantly older individuals and typically have progressive worsening lung function, leading to a grave prognosis (193). It has been indicated that periostin was elevated in IPF patients' circulation (190). Furthermore, more periostin was found in the lungs of IPF patients and concentrated in areas of active fibrosis (187). Interestingly, the exon 21 of the periostin gene is more likely to be spliced out in IPF lung samples than in the control (194). Injury factors activate alveolar epithelial cells disrupting the homeostatic balance between epithelial and mesenchymal cells, thus fibrotic response is driven. As an extracellular matrix protein, periostin and TGF-β regulate each other in fibroblasts (195); specifically, TGF-β increases the expression of periostin. In return, periostin significantly upregulates the production of TGF-β in fibroblasts and increases type I collagen production (70,195). However, periostin activates fibroblasts to produce type I collagen via β1 integrin, rather than the TGF-β signal (195). Similar to heart fibrosis, periostin promotes differentiation of fibroblasts to myofibroblasts. By mediating epithelial-mesenchymal transformation, periostin induces alveolar epithelial cells to take on the characteristics of mesenchymal cells, which leads to the aggravation of fibrosis (70). Emerging evidence suggests that periostin silencing drives the fibroblasts into G1 arrest of the cell cycle and retards the proliferation in IPF (196). Thus, periostin plays a pivotal role in lung fibroblast proliferation. Currently, early lung transplantation is a beneficial therapeutic option for IPF patients, and another two available drugs (Pirfenidone and Nintedanib) are able to limit IPF progress (197). Recently, a compound known as CP4715 was found to prohibit the interaction between TGF-β and periostin (197). CP4715, not only lessened bleomycin-induced pulmonary fibrosis, but also disturbed TGF-β signals in fibroblasts from IPF (195). Therefore, CP4715 may become a latent drug therapy to provide more therapeutic possibilities for IPF. It is worth mentioning that vitamin K antagonists are related to the rising mortality of IPF (198). The carboxylation status of periostin in IPF patients deserves further study. Some scholars have proposed that the use of vitamin K instead of vitamin K antagonists may help reduce the progression of IPF, but this idea needs further verification (198).

Asthma, as a heterogeneous disease, has been defined as several phenotypes according to different clinical features and physiological indexes. Nevertheless, type-2 airway inflammation is one of the main causes of asthma, which is supported by activity of type 2 cytokines, such as IL-4 and IL-13. As a result of chronic airflow limitation, airway remodeling develops in chronic severe asthma. Many studies have shown periostin is deeply involved in the process of asthma, from airway inflammation to remodeling. The periostin gene is highly induced in asthmatic airway epithelial cells with a 4.4-fold increase compared to healthy controls (199). In a cohort of asthmatics from Sweden, a negative correlation between serum periostin and lung function was observed (71). Type-2 inflammation attracts large numbers of immune cells to release cytokines, such as IL-4, IL-13 and TGF-β. These cytokines stimulate the production of periostin from fibroblasts, epithelial cells and endothelial cells, which are known as the main sources of periostin in asthma (200), and some researchers have hypothesized that eosinophils also secrete periostin (201). As an integrin ligand, periostin binds to integrin αMβ2 and α4β1 on eosinophil, guiding recruitment of eosinophils and increasing eosinophil adhesion to fibronectin (156,202). In addition, through its fibrogenic function, periostin participates in the process of subepithelial fibrosis, which is feature of airway remodeling in asthma (185). Periostin, secreted from airway epithelial cells, activates TGF-β and upregulates type I collagen via autocrine effects (70,203). Similarly, periostin activates TGF-β-mediated fibroblasts to increase the production of type I collagen (70,203). Clinical studies from Japan have reported that vitamin K2 therapy has an effective rate of up to 90.9% in patients with mild asthma (204). The effective rate was 86.7 and 72.7% in moderate and severe patients, respectively (204). In addition, vitamin K2 has a powerful ability to inhibit the release of inflammatory cytokines (205). It has also been shown that vitamin D, also a fat-soluble vitamin, can regulate inflammatory chemokines in asthma and significantly inhibit airway smooth muscle cell proliferation (205). Therefore, whether vitamin K2 can regulate the release of inflammatory factors in asthma and thus inhibit the production of large amounts of periostin remains to be further studied. Additionally, periostin increases gel elasticity formed by type 1 collagen, thus mediating the biomechanical capabilities of the airway and leading to airway remodeling (206). Accumulated evidence has indicated that high serum periostin concentrations were implicated in certain characteristics of asthma. It is reported that serum periostin concentrations were not combined with atopic status or treatment status of asthma, while high level serum periostin was related to older patients at the onset of asthma, aspirin intolerance or nasal disorders (207-209). As serum biomarkers are more convenient than lung function tests in some special cases of asthma, periostin has become one of the practical biomarkers of asthma. For instance, periostin rises significantly in severe asthma and acute asthma exacerbation of children, which is an important serum biomarker in assessing the severity of asthma (206). Of note, periostin is a helpful biomarker to detect long-term bronchial obstruction in severe asthmatic patients, as well as the sensitivity of sputum periostin beyond the serum periostin (210).