Atrial septal defects are a common form of congenital heart disease [85]. As discussed earlier, Pcsk6 KO mice exhibit a plethora of cardiac abnormalities, including common atrium, double-outlet right ventricle, ventricular septal defects, persistent truncus arteriosus, and dextrocardia [65]. Common atrium is a severe type of atrial septal defect, in which the entire atrial septum is missing. In mice, Nodal and Bmp4 are downstream effectors of Pcsk6 in heart formation [65,66]. However, the upstream molecular network that regulates PCSK6 expression in developing hearts is not well defined.

TBX5, NKX2-5, and GATA4 are major transcription factors in heart development. Deleterious TBX5, NKX2-5, and GATA4 mutations are found in individuals with congenital heart disease, including atrial septal defects [86,87]. Protein odd-skipped-related 1 (OSR1) is a zinc-finger transcription factor, essential for cardiac progenitor growth and atrial septum formation [88]. In mouse embryonic hearts, Tbx5 is an immediate Osr1 upstream gene in the second heart field for atrial septation [89]. Osr1 deletion results in common atrium and embryonic death between E11.5 and E12.5 days [90]. The similar phenotype of atrial septal defects in Tbx5-, Osr1-, and Pcsk6-deficient mice suggests a possible TBX5-OSR1-PCSK6 pathway in promoting TGFβ-like growth factor signaling in atrial septum formation.

In agreement with this hypothesis, gene profiling in Tbx5- and Osr1-deficient embryos identifies Pcsk6 as one of the major genes in atrial septation regulated by Tbx5 and Osr1 [91]. Human genome-wide linkage analysis also suggests a connection between PCSK6 and congenital heart disease [92]. Moreover, a PCSK6 variant is found in a Spanish family with atrial septal defects and interatrial septal aneurysm [91]. These findings indicate that TBX5 and OSR1 are possible regulators in PCSK6 expression during heart development, providing new insights into the genetic mechanisms underlying atrial septal defects.

Endothelial lipase, a member of the triglyceride lipase family, is a secreted protein, consisting of a signal peptide, a 40 kDa N-terminal catalytic domain, and a 28 kDa C-terminal domain [93]. Upon secretion, endothelial lipase binds to heparan sulfate proteoglycans on the cell surface, where it hydrolyzes triglycerides and phospholipids in plasma lipoproteins [94]. In addition to endothelial cells, endothelial lipase is expressed in hepatocytes, macrophages, and SMCs. Endothelial lipase modifies high-density lipoprotein (HDL) structure and metabolism, as indicated by high levels of plasma HDL cholesterol, bigger HDL particle sizes, and slower HDL clearance in endothelial lipase-deficient mice [95,96]. Conversely, plasma HDL cholesterol levels are decreased in endothelial lipase overexpressing mice [96,97]. In humans, deleterious variants in the LIPG gene, encoding endothelial lipase, are associated with increased plasma HDL cholesterol levels [98,99,100].

In membrane-bound serine proteases, ectodomain cleavage is a common mechanism in limiting protease activity on the cell surface [101,102]. A comparable mechanism exists in endothelial lipase inactivation on the cell surface. In the conditioned medium from human endothelial cells, a 40 kDa endothelial lipase fragment was detected [46,97]. A similar fragment was also found from human liver HepG2 cell culture [103]. As revealed by biochemical analyses, the fragment is derived by cleavage at a specific site, RNKR↓, which reduces the endothelial lipase activity [46,103]. Furin, PC5A, and PCSK6 are likely responsible for the cleavage [46,103]. Interestingly, lipoprotein lipase, another member of the triglyceride lipase family, contains an analogous site, RAKR↓, which is also cleaved by furin, PC5A, and PCSK6 in similar experiments [46]. These data indicate that PCSK-mediated endothelial lipase inactivation is a cellular mechanism in regulating lipoprotein metabolism.

Consistently, hepatic overexpression of pro-furin, an inhibitor of furin, PCSK5, and PCSK6 reduce endothelial lipase inactivation and lower plasma HDL cholesterol in mice [104]. In Lipg KO mice, such an effect of pro-furin on plasma HDL cholesterol is not observed, supporting a PCSK-endothelial lipase-dependent mechanism in HDL metabolism. Studies in those mice also validate a role of PCSK-mediated activation of an endogenous endothelial lipase inhibitor, angiopoietin-like 3 [104,105]. Further analyses in hepatocyte-specific Furin and Pcsk5 conditional KO and Pcsk6 global KO mice show that hepatic furin is primarily responsible for cleavage of endothelial lipase and angiopoietin-like 3 in vivo [106]. However, plasma levels of HDL cholesterol are only slightly reduced in the hepatocyte Furin conditional KO mice or not changed in the hepatocyte Pcsk5 conditional and Pcsk6 global KO mice, compared to that in WT mice [106]. These findings suggest functional redundancy among PCSKs in endothelial lipase inactivation, at least in mice.

Atrial and B-type natriuretic peptides (ANP and BNP, respectively) are hormones in the natriuretic peptide system that preserves body fluid balance and cardiovascular homeostasis [107]. Genetic studies in mice and humans establish ANP as a key factor in blood pressure regulation [108,109,110]. Upon binding to its receptor, natriuretic peptide receptor A (also called guanylate cyclase A), ANP enhances renal salt excretion and relaxes blood vessels, thereby lowering blood volume and pressure. Variants in the NPPA gene, encoding ANP, are associated with increased risks of cardiovascular disease, such as hypertension, stroke, and heart disease [111,112].

Corin is a membrane-bound protease, highly expressed in the heart [113], where it converts pro-ANP to ANP [114,115,116]. Like most proteases, corin is produced in a pro-form, which is activated at a specific site, RMNKR↓ [117]. The cleavage sequence with paired basic residues indicates that corin is likely activated by one of the PCSKs. Indeed, PCSK6 has been identified as the corin activator [53]. Both PCSK6 and corin are expressed in cardiomyocytes, where PCSK6 activates corin on the cell surface [53,118]. In cultured murine cardiomyocytes, blocking Pcsk6 expression prevents corin activation [53]. In Pcsk6-deficient mice, corin activation and pro-ANP processing in the heart are eliminated [53]. Like Corin KO mice, Pcsk6-deficient mice develop salt-sensitive hypertension [53,119], indicating that PCSK6 is the corin activator in vivo and that this function cannot be substituted by other PCSKs.

In line with these findings, genetic studies support a role of PCSK6 in corin activation and cardiovascular function in humans. For example, several CORIN variants identified in hypertensive patients are defective in PCSK6-mediated activation [53,120]. PCSK6 variants are associated with hypertension [53,120] and coronary artery stenosis [121]. Studies in humans and rat models also indicate an important PCSK6-corin-ANP pathway in regulating renal aquaporin 2 and β-epithelial sodium channel expression in response to a high-salt diet [122], consistent with salt-sensitive hypertension in Pcsk6, Corin, and Nppa KO mice [53,109,119,123]. Moreover, reduced cardiac and renal PCSK6 and corin expression correlates with worsening cardiac and renal function in a rat heart failure model [124]. These data highlight the importance of PCSK6 in corin activation and body fluid-electrolyte homeostasis.

Atherosclerosis is a major vascular disease, characterized by the formation of atherosclerotic plaques in the intima of medium- to large-sized arteries [125]. Depending on disease stages, the plaque usually contains lipid-packed macrophages, also called foam cells, and SMCs that are surrounded by accumulated extracellular matrix proteins and proteoglycans [125]. As the disease progresses, the macrophages and SMCs undergo apoptosis, creating a highly thrombotic necrotic core that is prone to rupture, thereby causing thrombosis formation [125]. To date, several lines of evidence point to a potential role of PCSK6 in regulating SMC migration, vascular remodeling, and atherosclerotic plaque formation.

In patients with aortic dissections in a Korean population, for example, genomic alternations are found in a locus where the PCSK6 gene is located [126]. Genome-wide expression analysis indicates elevated PCSK6 expression in atherosclerotic plaques [127,128]. In cultured human monocytes and endothelial cells, PCSK6 expression and activity are increased by pro-atherogenic lipid oxidation products [129]. In Pcsk6 KO mice, compromised vascular remodeling, as indicated by enlarged systolic and diastolic circumferences and reduced contractile SMC markers, is observed in carotid arteries exposed to increased blood flow [130]. Increased PCSK6 expression is also detected in smooth muscle α-actin (SMA) (an SMC marker) -positive cells in unstable carotid plaques, where inflammation and extracellular matrix degradation are active [131]. Moreover, PCSK6 expression in cultured human carotid SMCs is increased by proinflammatory factors, such as tumor necrosis factor and interferon-γ [131]. These findings suggest a connection between PCSK6 and SMC-derived cells in the vessel wall where inflammation and pathological remodeling occur.

Consistently, a recent human study links a PCSK6 variant with SMA-positive cell numbers in carotid stenosis lesions and artery wall thickness [132]. In human and rodent carotid arteries, increased PCSK6 expression correlates with SMC activation, intimal hyperplasia, and MMP2/MMP14 activation [132]. Conversely, decreased intimal hyperplasia and MMP14 activation are found in Pcsk6 KO mice with carotid artery ligation [132]. Moreover, aortic SMCs from Pcsk6 KO mice exhibit poor proliferation and migration induced by platelet-derived growth factor BB (PGDFBB), whereas in human SMCs overexpressing PCSK6, PDGFBB-stimulated cell proliferation and migration are increased [132]. PCSK6 is known to activate MMPs in cancers [133]. The latest findings suggest that PCSK6-mediated MMP activation may be important in SMC phenotypic changes and pathological vascular remodeling in atherosclerosis.

MI triggers a series of cellular events, including cell death, inflammatory cell infiltration, and gradual wound healing with myofibroblast proliferation and ultimate scar formation [134]. Tissue remodeling depends on the interplay among various cell types, including immune cells, cardiomyocytes, fibroblasts, and vascular cells. Both autocrine and paracrine mechanisms are involved in cell–cell interactions in infarcted hearts [134].

Many serine proteases have been implicated in cardiac structure and function [135]. In a recent study, PCSK6 was identified as one of the highly secreted proteins from hypoxic cardiomyocytes [136]. The finding is confirmed in mouse hearts undergoing coronary artery ligation [136]. The PCSK6 expression and secretion in hypoxic cardiomyocytes promote TGFβ secretion from the same cells and subsequent SMAD (small and mothers against decapentapletic) signaling in cardiac fibroblasts [136]. Moreover, high levels of collagen I production and fibrosis-related gene expression (e.g., Col1a1, Col3a1, and Mmp14) are observed in cardiac fibroblasts treated with the PCSK6-containing conditioned medium from hypoxic cardiomyocytes [136]. These findings suggest that upregulated PCSK6 in ischemic cardiomyocytes activates TGFβ, which, in turn, binds to its receptor on cardiac fibroblasts, thereby enhancing downstream SMAD signaling to promote collagen production and cardiac fibrosis [136].

Increased fibrosis is a hallmark of poor cardiac remodeling, which impairs cardiac function. Consistently, PCSK6 overexpression in cardiomyocytes increases cardiac hypertrophy and fibrosis and decreases cardiac function in a mouse MI model [136]. Moreover, increased serum PCSK6 levels are observed in patients with acute MI, which peaks on day 3 post incidence [136]. Previously, increased ventricular, but not atrial, Pcsk6 expression was noticed post MI in a rat model [137]. These data support a role of PCSK6 in a paracrine mechanism, underlying cardiac remodeling after MI. In another study [138], serum PCSK6 levels were associated with cardiovascular events in a subset of patients undergoing coronary angiography. Further studies will be important to evaluate if serum PCSK6 can be used as a biomarker to predict cardiac remodeling and function in patients with heart disease.

In aging hearts, altered protein expression and signaling often lead to deteriorating cardiac structure and function. In addition to apoptosis, senescence is a common feature in aging cardiomyocytes, as indicated by DNA damage, dysregulated gene expression, increased oxidative stress, mitochondrial dysfunction, and poor contractility [139,140]. Natriuretic peptide-mediated signaling is critical in cardiomyocyte homeostasis [111]. In humans, variants in the NPPA gene are associated with impaired cardiovascular responsiveness in the elderly [141]. In rodents, decreased ANP secretion is found in aging hearts and senescent cardiomyocytes in culture [142,143].

PCSK6 is necessary for corin activation and ANP generation in the heart [53]. A recent study indicates that PCSK6 deficiency may contribute to senescence in cardiomyocytes [144]. In aged mouse hearts and senescent cardiomyocytes, Pcsk6 expression is reduced. Moreover, Pcsk6 downregulation causes senescence in cultured cardiomyocytes, as indicated by increased advanced glycation end products, oxidative stress, and apoptosis [144]. Conversely, Pcsk6 overexpression prevents senescence and dysfunction in cultured cardiomyocytes under similar experimental conditions [144].

The function of PCSK6 in cardiomyocyte senescence appears mediated, at least in part, by pathways related to ER stress. In aging mouse hearts and Pcsk6 knockdown cardiomyocytes, high levels of DNA-damage inducible transcript 3 (Ddit3) are observed [144]. Ddit3, also called C/EBP homologous protein, is a pro-apoptotic transcription factor inducted by ER stress [145]. In cardiomyocytes subjected to ER stress, Ddit3 expression is suppressed by PCSK6 expression [144], suggesting that PCSK6 may regulate cardiomyocyte senescence by reducing ER stress via a DDIT3-related mechanism. Consistent with these findings, increased ER stress is reported in human prostate cancer cells, in which the PCSK6 gene is downregulated [24]. In a mouse model of heart failure, Ddit3 deletion prevents ER-stress-induced cell death and cardiac dysfunction [146]. As discussed earlier, premature ovarian senescence is observed in Pcsk6 KO mice [69]. It will be important to examine if similar premature aging exists in other major organs in Pcsk6 KO mice.