VC is a complex process, and numerous ex vivo and in vivo studies have confirmed that VC is very similar to bone formation in terms of pathogenesis and progression (Figure 1) (Neven et al., 2011; Andrews et al., 2018; Durham et al., 2018). In the past few decades, the inducing factors of vascular calcification have been continuously proposed (Table 1). The interference of internal and external environments, such as aging, inflammation, diabetes, chronic kidney disease, oxidative stress, and mitochondrial dysfunction, can lead to VC. It has been reported that inflammation-related tumor necrosis factor (TNF-α) and interleukin-1 beta (IL-1β) can induce endothelial-mesenchymal transition (EndMT) by regulating BMPRs. These then induce osteogenic differentiation through the BMPR-JNK signaling axis, promoting vascular calcification (Sanchez-Duffhues et al., 2019). In diabetes, hyperglycemia and an imbalance in mineral ion homeostasis could lead to endothelial cell injury. Calcified vascular cells (CVCs) perceive extracellular damage signaling and induce CVCs to differentiate into osteoblast-like cells by upregulating osteogenic factors and activating Wnt signaling, promoting vascular calcification (Bartoli-Leonard et al., 2018). In chronic kidney disease, calcium and phosphorus imbalance can lead to mitochondrial dysfunction, increase the release of reactive oxygen species, trigger oxidative stress and inflammatory responses, and induce the reverse differentiation of VSMCs into osteoblast-like cells, leading to vascular calcification (Zhu et al., 2020; Phadwal et al., 2021). At the cellular level, when the structure and function of vascular endothelial cells are abnormal, the expression of proinflammatory cytokines TNF-α and IL-1β can induce EndMT via BMP signaling, thereby inducing osteogenic differentiation and promoting vascular calcification (Sanchez-Duffhues et al., 2019). After being subjected to biological stress or injury, VSMCs can regulate the level of contractile proteins and reconstruct the extracellular matrix (ECM) or differentiate into osteoblast/chondrocyte-like cells to induce vascular calcification (Durham et al., 2018; Pescatore et al., 2019; Lu et al., 2020).

The BMP signaling pathway is vital for osteogenesis, EndMT, and VSMC calcification. Several treatment strategies for vascular calcification related to the BMP signaling pathway have been proposed. For example, BMP antagonists (LDN-193189, which targets the BMP type I receptor; or BMPRIA-Fc, which targets the BMP ligands) inhibit the BMP signaling pathway, which can reduce vascular calcification and improve the survival rate of matrix Gla protein-knockout mice (Malhotra et al., 2015). Furthermore, overexpression of the MGP gene inhibits the BMP signaling pathway and reduces BMPR I receptor expression, thereby reducing the formation of vascular calcification in apolipoprotein E-knockout (ApoE^−/−) mice (Nakagawa et al., 2010; Yao et al., 2010; Ducy et al., 2015; Malhotra et al., 2015). The inhibition of BMP signaling by a high-fat diet reduces intimal calcification in low-density lipoprotein receptor (LDLR)-deficient mice (Derwall et al., 2012). Dorsmorphin homologous 1 inhibits the phosphate-induced differentiation of VSMCs to osteoblast-like cells by inhibiting BMP-2 (Lin et al., 2017). Gla Rich Protein (GRP) interacts with BMP-2 and the BMP-SMAD signaling pathway, inhibiting the BMP-2 signaling pathway and SMAD1/5/8 phosphorylation via noggin and dorsmorphin, respectively, to reduce VSMC calcification and bone/cartilage gene expression (Willems et al., 2018).

Depending on the mechanism of formation and location, VC can be classified as intimal or medial (Yang et al., 2020). Intimal calcification occurs in the inner layer of the blood vessel wall, in a process very similar to endochondral osteogenesis. It mainly occurs during atherosclerosis, which is caused by lipid accumulation, macrophage invasion, smooth muscle cell migration, and the proliferation or transdifferentiation of osteoblast-like cells. Medial calcification (also known as “Monckeberg’s medial sclerosis”) occurs in the medial layer, and it is commonly observed in aging patients or those with diabetes, CKD, or dyslipidemia. It is also related to mineral metabolism disorders, similar to intramembranous bone formation (Villa-Bellosta and O'Neill, 2018; Chen et al., 2020). In general, the process of these two types of calcification is similar to the transformation of smooth muscle cells into bone and cartilage-like cells through calcium deposition in VSMCs (Lee et al., 2020). Reduced vascular compliance is a common characteristic of both types of calcification. Whether plaque is formed via atherosclerosis, calcification, or medial mineralized crystal deposition, the common molecular signaling pathways for both processes include the regulatory protein BMP2 and the transcription factor Runx2, which drive the bone formation process (Thompson and Towler, 2012; Lee et al., 2020).

BMP signaling is essential for the development of organisms and the stability of the internal environment. It was initially identified by its ability to induce bone and cartilage formation but was later found to be highly expressed during vascular calcification (Cai et al., 2012; Morrell et al., 2016). BMPs are members of the transforming growth factor beta (TGF-β) family of proteins and are expressed in various cells. They bind to receptors via ligands to form receptor complexes composed of type I receptors (ALK2, ALK3, and ALK6) and type II receptors (BMPR2, ACVR2A, and ACVR2B) and are widely involved in signal transduction and molecular regulatory processes. BMPs are usually produced by endothelial cells, osteoblasts, and chondrocytes in the bone. However, some studies have found that there are some BMPs in serum samples, indicating that they may circulate in the blood to act at the systemic level (Cheng et al., 2004; Simic et al., 2006). BMP signaling cannot be separated from the interaction between ligands and receptors. Upon the binding of BMP ligands to their respective receptors, type II receptors phosphorylate and activate type I receptors, which in turn phosphorylate intracellular signaling molecules, activating SMAD-dependent and SMAD-independent signaling pathways. The canonical BMP-SMAD-dependent signaling pathway activates R-SMADs (R-SMAD1, R-SMAD5, R-SMAD 8) via type I and type II receptors, which then bind to SMAD4 to form a complex that translocates to the nucleus, where it binds to DNA and interacts with transcription factors to regulate transcription of downstream target genes (Miyazono et al., 2005; Sieber et al., 2009; Luo et al., 2010; Cai et al., 2012). BMP-SMAD-independent signaling pathways include the ERK, JNK, and p38 MAPK signaling pathways, whose main roles include regulating apoptosis, epithelial-mesenchymal transition, cell migration, cell proliferation and differentiation, and the extracellular matrix (Yu, et al., 2002). In addition, type I BMP receptors are widely expressed in various cell types, and the affinity of BMPs for type I receptors is higher than that for type II receptors (Lavery et al., 2008). BMP2 was initially found to be an inducer of ectopic bone and can be detected in both the cartilage and bone (Urist, 1965; Nomura and Yamamoto, 2000). BMP2 can stimulate bone formation through the BMP receptor, which preferentially interacts with BMPR1A compared to other receptors (Kai et al., 2009; Mang et al., 2020).

BMPR1A encodes type I TGF-β family receptors in the BMP2 and BMP4 signaling pathways (Mishina et al., 1995). Likewise, BMPR1A is also a serine/threonine kinase transmembrane protein type I receptor expressed in many tissues and plays a critical role in angiogenesis and the regulation of vascular homeostasis (Shi and Massagué, 2003; Lee et al., 2017; Sanchez-de-Diego et al., 2019). BMPR1A is the main type I receptor that transduces BMP signaling in preosteoblasts and has a higher ability to transduce BMP-SMAD signaling than BMPR1B or ACVR1 (Pan et al., 2017). In the absence of BMPR1A, the phosphorylation of Smad decreases while the activation of p38 and Erk increases, changing the balance of the BMP signaling cascade. Studies have shown that BMPR1A deficiency can lead to premature osteoblast differentiation and intramembranous ossification, which is necessary for cartilage formation and osteogenesis (Shi et al., 2017; Mang et al., 2020). Additionally, the inactivation of BMPR1A can lead to the excessive activation of downstream signaling by other BMPI receptors (T. Maruyama et al., 2021). According to a previous study, a lack of BMPR1A will impair the self-renewal, clonal expansion, and osteogenic ability of suture stem cells and plays an important role in inhibiting cell differentiation or promoting asymmetric cell division (T. Maruyama et al., 2021). Another study indicated that increased BMPR1A expression enhances the adipogenic differentiation of mesenchymal stem cells, activating the BMP-pSmad1/5/8 signaling pathway, and contributing to abnormal fat metaplasia and new bone formation in patients with ankylosing spondylitis (Liu et al., 2019). Similarly, BMPR1A is also essential for BMP-induced phosphorylation of Smad1/5/8 in heart valve progenitors (Lockhart et al., 2014). During aortic valve calcification, deletion of BMPR1A or inhibition of its activity can lead to absence of the BMP signaling pathway, resulting in the loss of osteochondrogenic features and blocked calcified nodule formation, thereby preventing aortic valve calcification (Gomez-Stallons et al., 2016). Thus, BMPR1A signaling plays an essential role in restricting preosteoblast proliferation and promoting osteoblast activity.

The bone is not only a site of mineral storage but also the main site for stem cell maintenance and hematopoiesis. It plays a dual role in maintaining the vascular integrity of endothelial progenitor cells and aggravating VC via inflammatory cells (Kim et al., 2020). Numerous studies have shown that vascular calcification is highly similar to bone formation. The minerals deposited in the vascular wall are mainly alkaline calcium phosphate in the form of apatite, including hydroxyapatite [Ca₁₀ (PO₄)₆ (OH) ₂ ] crystals, consistent with the composition of bone (Reynolds et al., 2004). Some secreted proteins that connect the bone and blood vessels play important roles in vascular calcification, such as receptor activator for nuclear factor kB (RANK)/RANK ligand (RANKL)/osteoprotegerin (OPG), BMPs, Runx2, osterix (OSX), fetuin-A, and MGP (Pereira and Frazao, 2020). Some studies have also shown that there is an independent negative correlation between bone mineral density and atherosclerosis and that patients with osteoporosis are more prone to aortic and carotid calcification (Hofbauer et al., 2007; Anagnostis et al., 2009; Zhang et al., 2016). In addition, disorders of bone volume and bone turnover in patients with uremia may increase the risk of VC and eventually lead to an extremely high risk of cardiovascular death (Adragao et al., 2009). Some clinical experiments have confirmed that bisphosphonates can inhibit vascular calcification while preventing fracture risk during the treatment of osteoporosis (Kranenburg et al., 2016). Recent studies have shown that BMP-induced miRNAs can regulate the proliferation and migration of VSMCs and induce their phenotypic conversion and that their abnormal regulation can lead to various vascular diseases (Park and Kang, 2020). Therefore, this evidence suggests that crosstalk exists between the bone and the vascular system.

ECs are an essential component of blood vessels and one of the key factors for maintaining normal cardiovascular function. They are highly active cell monolayers that can rapidly adapt to and respond to endogenous and exogenous signaling. Endothelial dysfunction has been observed in many cardiovascular diseases, including atherosclerosis (Widlansky et al., 2003). Currently, many researchers have studied VC as an outcome of atherosclerosis (Bjorklund et al., 2020). Inflammatory factors (such as TNF-α and IL-1β) and TGF-β family ligands (including BMPs) are jointly involved in the occurrence and development of calcified aortic plaques (Dhore et al., 2001; Gistera and Hansson, 2017). Furthermore, some studies have suggested that ECs can function as a special source of osteogenic progenitors during vascular calcification (Chen et al., 2015; Sanchez-Duffhues et al., 2015; Evrard et al., 2016).

Endothelial cells can undergo a process called EndMT under inflammation, characterized by the loss of endothelial characteristics and the acquisition of fibroblast-like phenotypes, which eventually re-differentiate into osteogenic potential cells under the guidance of various stimuli such as BMPs (Figure 1A) (Pavićević et al., 2016; Sanchez-Duffhues et al., 2019). The BMP-Smad signaling pathway plays an important role in EndMT (Figure 2). Studies have confirmed that most BMPs promote EndMT through the canonical Smad-dependent pathway and that IL-1β can activate the BMP signaling pathway by upregulating the expression of BMP2 (Shanmugam et al., 2015; Evrard et al., 2016; Hong et al., 2020; Yuan et al., 2021). Under conditions such as hyperglycemia, inflammation, and aging, endothelial cells are stimulated to release extracellular vesicles (EVs) or extracellular matrix protein particles (EMPs), which are enriched in Ca²⁺ and BMP2, and act as nucleation sites for calcification to induce mineralization (Yuan et al., 2021). Previous studies have shown that TNF-α can increase the expression of BMP2 and the release of EMPs in ECs, thus promoting osteogenesis and the calcification of VSMCs (Csiszar et al., 2005; Buendia et al., 2015). It is well known that the BMP2 signaling cascade is initiated by activating BMPR1A (ALK3), BMPR1B (ALK6), and BMPRII (Medici et al., 2010). BMPR1A is widely expressed in endothelial cells, and its overexpression is closely related to the early development of vascular calcification (Vinuesa et al., 2015; Wei et al., 2018). Hong et al. (2020) showed that BMPR1A and BMP type II receptors (BMPR-II, Act-RIIB) could activate IL-1β-induced EndMT, thereby promoting the expression of Runx2 and OSX. Huang et al. (2018) also demonstrated that IL-6 could promote the cell surface translocation of BMPR1A and enhance the BMP-2-induced osteogenic differentiation of mesenchymal cells by amplifying BMP-Smad signaling.

An increasing number of studies indicate that ECs play a significant role in vascular calcification via EndMT, the autocrine/paracrine pathway, EC-derived extracellular vesicles, angiogenesis, and mechanotransduction. These findings further confirmed that BMPR1A could induce osteogenic gene expression during vascular endothelial cell mesenchymal transformation and promote calcification plaque production, further promoting vascular calcification.

VSMCs have multiple phenotypes, including osteogenic, contractile, and synthetic, which can be changed from one phenotype to another under certain conditions (Bardeesi et al., 2017). Vascular calcification is an active, complex, and tightly regulated biological process that is mainly related to the transformation of the contractile phenotype of VSMCs to osteoblast-like cells (McCarty and DiNicolantonio, 2014; Evrard et al., 2015). This phenotypic transformation of VSMCs is characterized by the upregulation of osteogenic genes, such as BMP2, Runx2, OSX, ALP, and osteopontin (O’Neill, 2017). VSMCs constitute the majority of the vascular wall, maintaining vascular structure and regulating blood pressure (Waldo et al., 2005). Similar to osteoblasts, VSMCs are derived from mesenchymal precursor cells and express BMP2 (Sun et al., 2017). Apoptosis, the release of extracellular vesicles, the absence of calcification inhibitors (e.g., MGP), senescence-associated DNA damage, and osteo/chondrogenic differentiation are considered the main mechanisms of vascular smooth muscle cells undergoing vascular calcification (Neven et al., 2007; Liu et al., 2013; Schurgers et al., 2013).

VSMCs are the predominant cell type involved in vascular calcification, which can transdifferentiate into the chondrocyte, osteoblast, and osteocyte phenotypes in a calcified environment (Figure 1B) (Shroff et al., 2008; Zhu et al., 2011). The BMP-Smad signaling pathway plays an essential role in the transdifferentiation of VSMCs into osteoblasts and chondrocytes (Figure 2). Previous studies have reported that BMPR1A and BMPRII are highly expressed in VSMCs and mediate the response of SMCs to BMP2. BMPR1A and BMPRII are the dominant complexes mediating Smad1/5/8 phosphorylation in response to BMP2 (Upton et al., 2008; Goumans et al., 2018). Evidence suggests that the BMP2-Smad1/5/8 signaling pathway is involved in the osteogenic differentiation of VSMCs (Broege et al., 2013; Kee et al., 2014). When VSMCs receive stimulation signaling, phosphorylated Smad1/5/8 can accelerate translocation from the cytoplasm to the nucleus and initiate the osteogenic differentiation of VSMCs, which eventually leads to the increased expression of OSX, Runx2, and alkaline phosphatase (ALP) (Wang et al., 2018; Willems et al., 2018). One study showed that BMP2 accelerated the atherosclerotic intimal calcification in transgenic ApoE^−/− mice (Nakagawa et al., 2010). Another study indicated that BMP2 could affect SMC marker (SM22a and SM a-actin) loss and the expression of genes associated with the osteoblast phenotype, such as msh homeobox-2, ALP, and osteopontin (OPN) (Bardeesi et al., 2017). Furthermore, BMP2 plays a vital role in promoting VSMC apoptosis, Reactive oxygen species (ROS) production, and inflammatory responses (Agharazii et al., 2015; Voelkl et al., 2019). The inhibition of BMP2 expression in a CKD mouse model significantly reduced the osteogenic differentiation of VSMCs, thereby reducing BMP2-induced mineralization (Nguyen-Yamamoto et al., 2019). Sun et al. (2017) showed that endogenous BMP2 is involved in the osteogenic differentiation of VSMCs and subsequent calcification under IL-6 induction. BMPR1A, a high-affinity receptor of BMP2, also acts as a bridge to promote vascular calcification and plays a crucial role in the transdifferentiation of VSMCs into osteoblasts or chondrocyte-like cells.