Genetic or therapeutic neutralization of ALK1 reduces LDL transcytosis and atherosclerosis in mice

IL-1β Induces LDL Transcytosis by a Novel Pathway Involving LDLR and Rab27a

BACKGROUND

In early atherosclerosis, circulating LDLs (low-density lipoproteins) traverse individual endothelial cells by an active process termed transcytosis. The CANTOS trial (Canakinumab Antiinflammatory Thrombosis Outcome Study) treated advanced atherosclerosis using a blocking antibody for IL-1β (interleukin-1β); this significantly reduced cardiovascular events. However, whether IL-1β regulates early disease, particularly LDL transcytosis, remains unknown.

METHODS

We used total internal reflection fluorescence microscopy to quantify transcytosis by human coronary artery endothelial cells exposed to IL-1β. To investigate transcytosis in vivo, we injected wild-type and knockout mice with IL-1β and LDL to visualize acute LDL deposition in the aortic arch.

RESULTS

Exposure to picomolar concentrations of IL-1β induced transcytosis of LDL but not of albumin by human coronary artery endothelial cells. Surprisingly, expression of the 2 known receptors for LDL transcytosis, ALK-1 (activin receptor-like kinase-1) and SR-BI (scavenger receptor BI), was unchanged or decreased. Instead, IL-1β increased the expression of the LDLR (LDL receptor); this was unexpected because LDLR is not required for LDL transcytosis. Overexpression of LDLR had no effect on basal LDL transcytosis. However, knockdown of LDLR abrogated the effect of IL-1β on transcytosis rates while the depletion of Cav-1 (caveolin-1) did not. Since LDLR was necessary but overexpression had no effect, we reasoned that another player must be involved. Using public RNA sequencing data to curate a list of Rab (Ras-associated binding) GTPases affected by IL-1β, we identified Rab27a. Overexpression of Rab27a alone had no effect on basal transcytosis, but its knockdown prevented induction by IL-1β. This was phenocopied by depletion of the Rab27a effector JFC1 (synaptotagmin-like protein 1). In vivo, IL-1β increased LDL transcytosis in the aortic arch of wild-type but not Ldlr-/- or Rab27a-deficient mice. The JFC1 inhibitor nexinhib20 also blocked IL-1β-induced LDL accumulation in the aorta.

CONCLUSIONS

IL-1β induces LDL transcytosis by a distinct pathway requiring LDLR and Rab27a; this route differs from basal transcytosis. We speculate that induction of transcytosis by IL-1β may contribute to the acceleration of early disease.

In the Beginning, Lipoproteins Cross the Endothelial Barrier

Atherosclerosis begins with the infiltration of cholesterol-containing lipoproteins into the arterial wall. White blood cell (WBC)-associated inflammation follows. Despite decades of research using genetic and pharmacologic methods to alter WBC function, in humans, the most effective method to prevent the initiation and progression of disease remains low-density lipoprotein (LDL) reduction. However, additional approaches to reducing cardiovascular disease would be useful as residual risk of events continues even with currently effective LDL-reducing treatments. Some of this residual risk may be due to vascular toxicity of triglyceride-rich lipoproteins (TRLs). Another option is that LDL transcytosis continues, albeit at reduced rates due to lower circulating levels of this lipoprotein. This review will address these two topics. The evidence that TRLs promote atherosclerosis and the processes that allow LDL and TRLs to be taken up by endothelial cells leading to their accumulation with the subendothelial space.

Mutations causing premature termination codons discriminate and generate cellular and clinical variability in HHT

ABSTRACT

For monogenic diseases caused by pathogenic loss-of-function DNA variants, attention focuses on dysregulated gene-specific pathways, usually considering molecular subtypes together within causal genes. To better understand phenotypic variability in hereditary hemorrhagic telangiectasia (HHT), we subcategorized pathogenic DNA variants in ENG/endoglin, ACVRL1/ALK1, and SMAD4 if they generated premature termination codons (PTCs) subject to nonsense-mediated decay. In 3 patient cohorts, a PTC-based classification system explained some previously puzzling hemorrhage variability. In blood outgrowth endothelial cells (BOECs) derived from patients with ACVRL1+/PTC, ENG+/PTC, and SMAD4+/PTC genotypes, PTC-containing RNA transcripts persisted at low levels (8%-23% expected, varying between replicate cultures); genes differentially expressed to Bonferroni P < .05 in HHT+/PTC BOECs clustered significantly only to generic protein terms (isopeptide-bond/ubiquitin-like conjugation) and pulse-chase experiments detected subtle protein maturation differences but no evidence for PTC-truncated protein. BOECs displaying highest PTC persistence were discriminated in unsupervised hierarchical clustering of near-invariant housekeeper genes, with patterns compatible with higher cellular stress in BOECs with >11% PTC persistence. To test directionality, we used a HeLa reporter system to detect induction of activating transcription factor 4 (ATF4), which controls expression of stress-adaptive genes, and showed that ENG Q436X but not ENG R93X directly induced ATF4. AlphaFold accurately modeled relevant ENG domains, with AlphaMissense suggesting that readthrough substitutions would be benign for ENG R93X and other less rare ENG nonsense variants but more damaging for Q436X. We conclude that PTCs should be distinguished from other loss-of-function variants, PTC transcript levels increase in stressed cells, and readthrough proteins and mechanisms provide promising research avenues.

Sphingosine-1-phosphate receptor 3 regulates the transendothelial transport of high-density lipoproteins and low-density lipoproteins in opposite ways

AIMS

The entry of lipoproteins from blood into the arterial wall is a rate-limiting step in atherosclerosis. It is controversial whether this happens by filtration or regulated transendothelial transport.Because sphingosine-1-phosphate (S1P) preserves the endothelial barrier, we investigated in vivo and in vitro, whether S1P and its cognate S1P-receptor 3 (S1P3) regulate the transendothelial transport of lipoproteins.

METHODS AND RESULTS

Compared to apoE-haploinsufficient mice (CTRL), apoE-haploinsufficient mice with additional endothelium-specific knock-in of S1P3 (S1P3-iECKI) showed decreased transport of LDL and Evan's Blue but increased transport of HDL from blood into the peritoneal cave. After 30 weeks of high-fat diet feeding, S1P3-iECKI mice had lower levels of non-HDL-cholesterol and less atherosclerosis than CTRL mice. In vitro stimulation with an S1P3 agonist increased the transport of 125I-HDL but decreased the transport of 125I-LDL through human aortic endothelial cells (HAECs). Conversely, inhibition or knock-down of S1P3 decreased the transport of 125I-HDL but increased the transport of 125I-LDL. Silencing of SCARB1 encoding scavenger receptor B1 (SR-BI) abrogated the stimulation of 125I-HDL transport by the S1P3 agonist. The transendothelial transport of 125I-LDL was decreased by silencing of SCARB1 or ACVLR1 encoding activin-like kinase 1 but not by interference with LDLR. None of the three knock-downs prevented the stimulatory effect of S1P3 inhibition on transendothelial 125I-LDL transport.

CONCLUSION

S1P3 regulates the transendothelial transport of HDL and LDL oppositely by SR-BI-dependent and SR-BI-independent mechanisms, respectively. This divergence supports a contention that lipoproteins pass the endothelial barrier by specifically regulated mechanisms rather than passive filtration.

Apolipoprotein A1 and high-density lipoprotein limit low-density lipoprotein transcytosis by binding SR-B1

Atherosclerosis results from the deposition and oxidation of LDL and immune cell infiltration in the sub-arterial space leading to arterial occlusion. Studies have shown that transcytosis transports circulating LDL across endothelial cells lining blood vessels. LDL transcytosis is initiated by binding to either scavenger receptor B1 (SR-B1) or activin A receptor-like kinase 1 on the apical side of endothelial cells leading to its transit and release on the basolateral side. HDL is thought to partly protect individuals from atherosclerosis due to its ability to remove excess cholesterol and act as an antioxidant. Apolipoprotein A1 (APOA1), an HDL constituent, can bind to SR-B1, raising the possibility that APOA1/HDL can compete with LDL for SR-B1 binding, thereby limiting LDL deposition in the sub-arterial space. To examine this possibility, we used in vitro approaches to quantify the internalization and transcytosis of fluorescent LDL in coronary endothelial cells. Using microscale thermophoresis and affinity capture, we find that SR-B1 and APOA1 interact and that binding is enhanced when using the cardioprotective variant of APOA1 termed Milano (APOA1-Milano). In male mice, transiently increasing the levels of HDL reduced the acute deposition of fluorescently labeled LDL in the atheroprone inner curvature of the aorta. Reduced LDL deposition was also observed when increasing circulating wild-type APOA1 or the APOA1-Milano variant, with a more robust inhibition from the APOA1-Milano. The results suggest that HDL may limit SR-B1-mediated LDL transcytosis and deposition, adding to the mechanisms by which it can act as an atheroprotective particle.

Pathogenic Variant Frequencies in Hereditary Haemorrhagic Telangiectasia Support Clinical Evidence of Protection from Myocardial Infarction

Hereditary haemorrhagic telangiectasia (HHT) is a vascular dysplasia inherited as an autosomal dominant trait, due to a single heterozygous loss-of-function variant, usually in ACVRL1 (encoding activin receptor-like kinase 1 [ALK1]), ENG (encoding endoglin [CD105]), or SMAD4. In a consecutive single-centre series of 37 positive clinical genetic tests performed in 2021-2023, a skewed distribution pattern was noted, with 30 of 32 variants reported only once, but ACVRL1 c.1231C>T (p.Arg411Trp) identified as the disease-causal gene in five different HHT families. In the same centre's non-overlapping 1992-2020 series where 110/134 (82.1%) HHT-causal variants were reported only once, ACVRL1 c.1231C>T (p.Arg411Trp) was identified in nine further families. In a 14-country, four-continent HHT Mutation Database where 181/250 (72.4%) HHT-causal variants were reported only once, ACVRL1 c.1231C>T (p.Arg411Trp) was reported by 12 different laboratories, the adjacent ACVRL1 c.1232G>A (p.Arg411Gln) by 14, and ACVRL1 c.1120C>T (p.Arg374Trp) by 18. Unlike the majority of HHT-causal ACVRL1 variants, these encode ALK1 protein that reaches the endothelial cell surface but fails to signal. Six variants of this type were present in the three series and were reported 6.8-25.5 (mean 8.9) times more frequently than the other ACVRL1 missense variants (all p-values < 0.0039). Noting lower rates of myocardial infarction reported in HHT, we explore potential mechanisms, including a selective paradigm relevant to ALK1's role in the initiating event of atherosclerosis, where a plausible dominant negative effect of these specific variants can be proposed. In conclusion, there is an ~9-fold excess of kinase-inactive, cell surface-expressed ACVRL1/ALK1 pathogenic missense variants in HHT. The findings support further examination of differential clinical and cellular phenotypes by HHT causal gene molecular subtypes.

LDL Transcytosis by the Arterial Endothelium-Atherosclerosis by a Thousand Cuts?

PURPOSE OF REVIEW

The accumulation of LDL in the arterial intima is an initiating event in atherosclerosis. After decades of controversy, it is now clear that transcytosis of LDL across an intact endothelial monolayer contributes to its intimal deposition. We review recent observations in this field and address the question of whether LDL transcytosis can be manipulated therapeutically.

RECENT FINDINGS

The development of a live-cell imaging method for studying transcytosis using total internal reflection fluorescence (TIRF) microscopy has catalyzed recent discoveries. LDL transcytosis is mediated by SR-BI and ALK1. Estrogen down-regulates SR-BI and inhibits LDL transcytosis, while the nuclear structural protein HMGB1 promotes LDL transcytosis. LDL transcytosis by ALK1 is independent of the receptor's kinase activity and is antagonized by BMP9, ALK1's canonical ligand. Inflammation stimulates LDL transcytosis. Identifying the function and mechanisms of LDL transcytosis may ultimately permit its therapeutic manipulation.