Clonal Proliferation Within Smooth Muscle Cells in Unstable Human Atherosclerotic Lesions

BACKGROUND

Studies in humans and mice using the expression of an X-linked gene or lineage tracing, respectively, have suggested that clones of smooth muscle cells (SMCs) exist in human atherosclerotic lesions but are limited by either spatial resolution or translatability of the model.

METHODS

Phenotypic clonality can be detected by X-chromosome inactivation patterns. We investigated whether clones of SMCs exist in unstable human atheroma using RNA in situ hybridization (BaseScope) to identify a naturally occurring 24-nucleotide deletion in the 3'UTR of the X-linked BGN (biglycan) gene, a proteoglycan highly expressed by SMCs. BGN-specific BaseScope probes were designed to target the wild-type or deletion mRNA. Three different coronary artery plaque types (erosion, rupture, and adaptive intimal thickening) were selected from heterozygous females for the deletion BGN. Hybridization of target RNA-specific probes was used to visualize the spatial distribution of mutants. A clonality index was calculated from the percentage of each probe in each region of interest. Spatial transcriptomics were used to identify differentially expressed transcripts within clonal and nonclonal regions.

RESULTS

Less than one-half of regions of interest in the intimal plaque were considered clonal with the mean percent regions of interest with clonality higher in the intimal plaque than in the media. This was consistent for all plaque types. The relationship of the dominant clone in the intimal plaque and media showed significant concordance. In comparison with the nonclonal lesions, the regions with SMC clonality had lower expression of genes encoding cell growth suppressors such as CD74, SERF-2 (small EDRK-rich factor 2), CTSB (cathepsin B), and HLA-DPA1 (major histocompatibility complex, class II, DP alpha 1), among others.

CONCLUSIONS

Our novel approach to examine clonality suggests atherosclerosis is primarily a disease of polyclonally and to a lesser extent clonally expanded SMCs and may have implications for the development of antiatherosclerotic therapies.

Local, Downstream, and Systemic Evaluation after Femoral Artery Angioplasty with Kanshas Drug-Coated Balloons In Vitro and in a Healthy Swine Model

PURPOSE

To evaluate the incidence of distal embolism and local vascular responses after treatment with the Kanshas drug-coated balloon (DCB) in a preclinical model.

MATERIALS AND METHODS

A total of 90 femoral arteries from 35 healthy swine were treated with a single-dose (×1) or triple-dose (×3) Kanshas DCB that applies the Unicoat technology with 3.2 μg/mm2 of paclitaxel. An uncoated Kanshas balloon was used as a control. The arterial wall, downstream skeletal muscle, and nontarget organs (kidneys, lungs, lymph nodes, liver, spleen, and heart) were histologically evaluated. For pharmacokinetic evaluation, a total of 40 healthy swine were treated with ×1 Kanshas DCB, and treated vessels were evaluated ex vivo with high-performance liquid chromatography-mass spectrometry.

RESULTS

Arteries treated with the Kanshas DCB showed mild proteoglycan deposition accompanied by the loss of smooth muscle cells (SMCs). These changes increased in a dose-dependent manner (medial SMC loss at 28 days in the ×1 vs ×3 groups, in depth, 1 (0.75-1.38) vs 2 (1.63-2.44); P = .0008; in circumference, 0.83 (0.67-1) vs 1.5 (1.19-1.81); P = .0071). No evidence of distal embolization in skeletal muscles (0 of 210 histological sections) and nontarget organs (0 of 345 sections) was observed. The pharmacokinetic evaluation showed high paclitaxel concentration in the treated artery (912 ng/mg, peaking at 3 minutes), which remained detectable at up to 180 days (0.04 ng/mg).

CONCLUSIONS

The Kanshas DCB showed a local drug effect in treated arteries up to 180 days with a high concentration of paclitaxel and no histological evidence of distal embolization.

Histologic Assessment of Thromboemboli Due to Plaque Rupture, Plaque Erosion, or COVID-19 Microthrombi

Plaque rupture, plaque erosion, and COVID-19 infection can cause acute coronary syndromes (ACS). We illustrate case examples demonstrating the distinctive and characteristic pathologic findings underlying each of these various causes of acute myocardial infarction. A deeper understanding of the pathophysiology of ACS is necessary for the development of newer agents and techniques to improve outcomes after ACS. (Level of Difficulty: Advanced.).

CD163+ macrophages restrain vascular calcification, promoting the development of high-risk plaque

Vascular calcification (VC) is concomitant with atherosclerosis, yet it remains uncertain why rupture-prone high-risk plaques do not typically show extensive calcification. Intraplaque hemorrhage (IPH) deposits erythrocyte-derived cholesterol, enlarging the necrotic core and promoting high-risk plaque development. Pro-atherogenic CD163+ alternative macrophages engulf hemoglobin:haptoglobin (HH) complexes at IPH sites. However, their role in VC has never been examined to our knowledge. Here we show, in human arteries, the distribution of CD163+ macrophages correlated inversely with VC. In vitro experiments using vascular smooth muscle cells (VSMCs) cultured with HH-exposed human macrophage - M(Hb) - supernatant reduced calcification, while arteries from ApoE-/- CD163-/- mice showed greater VC. M(Hb) supernatant-exposed VSMCs showed activated NF-κB, while blocking NF-κB attenuated the anticalcific effect of M(Hb) on VSMCs. CD163+ macrophages altered VC through NF-κB-induced transcription of hyaluronan synthase (HAS), an enzyme that catalyzes the formation of the extracellular matrix glycosaminoglycan, hyaluronan, within VSMCs. M(Hb) supernatants enhanced HAS production in VSMCs, while knocking down HAS attenuated its anticalcific effect. NF-κB blockade in ApoE-/- mice reduced hyaluronan and increased VC. In human arteries, hyaluronan and HAS were increased in areas of CD163+ macrophage presence. Our findings highlight an important mechanism by which CD163+ macrophages inhibit VC through NF-κB-induced HAS augmentation and thus promote the high-risk plaque development.

The histological analysis of the coronary medial thickness: Implications for percutaneous coronary intervention

BACKGROUND

A deeper understanding of coronary medial thickness is important for coronary intervention because media thickness can limit the safety and effectiveness of interventional techniques. However, there is a paucity of detailed data on human coronary medial thickness so far.

MATERIALS AND METHODS

We investigated the thickness of the media by histologic analysis. A total of 230 sections from 10 individuals from the CVPath autopsy registry who died from non-coronary deaths were evaluated. We performed pathological analysis on 13 segments of the following primary vessels from coronary arteries: the left main trunk, proximal left anterior descending artery (LAD), mid LAD, distal LAD, proximal left circumflex artery (LCX), mid LCX, distal LCX, proximal right coronary artery (RCA), mid RCA, and the distal RCA. The following side branches were also evaluated: diagonal, obtuse margin, and posterior descending artery branches.

RESULTS

The average age of the studied individuals was 60.4±12.3 years. The median medial thickness for all sections was 0.202 (0.149-0.263) mm. The median medial thickness of the main branches was significantly higher compared to that of the side branches (p<0.001). Although the medial thicknesses of the main branch of LAD and LCX were significantly decreased from proximal to distal segments (p = 0.010, p = 0.006, respectively), the medial thickness of the main branch of RCA was not significantly decreased from proximal to distal (p = 0.170). The thickness of the media was positively correlated with vessel diameter, while it was negatively correlated with luminal narrowing (p<0.001 and p<0.001, respectively).

CONCLUSIONS

The human coronary arteries demonstrate variation in medial thickness which tends to vary depending upon an epicardial coronary artery itself, as well as its segments and branches. Understanding these variations in medial thickness can be useful for both the interventionalists and interventional product development teams.

Understanding the role of alternative macrophage phenotypes in human atherosclerosis

INTRODUCTION

Atherosclerosis-based ischemic heart disease is still the primary cause of death throughout the world. Over the past decades there has been no significant changes in the therapeutic approaches to atherosclerosis, which are mainly based on lipid lowering therapies and management of comorbid conditions such as diabetes and hypertension. The involvement of macrophages in atherosclerosis has been recognized for decades. More recently, a more detailed and sophisticated understanding of their various phenotypes and roles in the atherosclerotic process has been recognized. This new data is revealing how specific subtypes of macrophage-induced inflammation may have distinct effects on atherosclerosis progression and may provide new approaches for treatment, based upon targeting of specific macrophage subtypes.

AREAS COVERED

We will comprehensively review the spectrum of macrophage phenotypes and how they contribute to atherosclerotic plaque development and progression.

EXPERT OPINION

Various signals derived from atherosclerotic lesions drive macrophages into complex subsets with different gene expression profiles, phenotypes, and functions, not all of which are understood. Macrophage phenotypes include those that enhance, heal, and regress the atherosclerotic lesions though various mechanisms. Targeting of specific macrophage phenotypes may provide a promising and novel approach to prevent atherosclerosis progression.

Myeloid-associated lipin-1 transcriptional co-regulatory activity is atheroprotective

BACKGROUND AND AIMS

Atherosclerosis is the most prominent underlying cause of cardiovascular disease (CVD). It is initiated by cholesterol deposition in the arterial intima, which causes macrophage recruitment and proinflammatory responses that promote plaque growth, necrotic core formation, and plaque rupture. Lipin-1 is a phosphatidic acid phosphohydrolase for glycerolipid synthesis. We have shown that lipin-1 phosphatase activity promotes macrophage pro-inflammatory responses when stimulated with modified low-density lipoprotein (modLDL) and accelerates atherosclerosis. Lipin-1 also independently acts as a transcriptional co-regulator where it enhances the expression of genes involved in β-oxidation. In hepatocytes and adipocytes, lipin-1 augments the activity of transcription factors such as peroxisome proliferator-activated receptor (PPARs). PPARs control the expression of anti-inflammatory genes in macrophages and slow or reduce atherosclerotic progression. Therefore, we hypothesize myeloid-derived lipin-1 transcriptional co-regulatory activity reduces atherosclerosis.

METHODS

We used myeloid-derived lipin-1 knockout (lipin-1mKO) and littermate control mice and AAV8-PCSK9 along with high-fat diet to elicit atherosclerosis.

RESULTS

Lipin-1mKO mice had larger aortic root plaques than littermate control mice after 8 and 12 weeks of a high-fat diet. Lipin-1mKO mice also had increased serum proinflammatory cytokine concentrations, reduced apoptosis in plaques, and larger necrotic cores in the plaques compared to control mice.

CONCLUSIONS

Combined, the data suggest lipin-1 transcriptional co-regulatory activity in myeloid cells is atheroprotective.

Lipin-1 transcriptional co-regulatory activity enhances anti-inflammatory responses in lipid-loaded macrophages and induces atheroprotection

Metabolic disorders, like atherosclerosis, increase the lipid burden in macrophages leading to chronic inflammation. How macrophages sense and respond to excess lipid is not well understood. Lipin-1 is a phosphatidic acid phosphatase that also functions as a transcriptional co-regulator and is critical to regulating lipid burden in cells. We have shown that lipin-1 enzymatic activity promotes macrophage pro-inflammatory responses and accelerates atherosclerosis. Lipin-1 transcriptional co-regulatory activity binds to and augments transcription factors such as PPARγ. PPARγ increases macrophage anti-inflammatory responses. The objective of this study was to determine if after increased lipid burden in macrophages, lipin-1 transcriptional co-regulatory activity tailors PPARγ to promote anti-inflammatory responses by macrophages and reduce atherosclerosis. Upon RNA analysis in lipid stimulated macrophages, we demonstrated lipin-1 tailors PPAR-associated gene expression and promotes anti-inflammatory responses. Using mice that lack just lipin-1 enzymatic activity or all lipin-1 in myeloid cells, we examined the contribution of lipin-1 transcriptional co-regulatory activity to atherosclerosis progression. Our mice lacking both myeloid-associated lipin-1 activities had larger, more necrotic plaques. Using Nanostring analysis, we observed increased expression of pro-inflammatory genes such as IL-23 in the plaque and increased IL-23 in serum suggesting lipin-1 transcriptional co-regulatory activity in macrophages promotes anti-inflammatory responses to reduce atherosclerosis. Our work highlights the dynamic nature of lipid metabolism on macrophage function and contribution to disease progression.

Lipin-1 coordinates macrophage lipid metabolism to allow macrophage wound healing function

Excess lipid burden within macrophages disrupts cellular metabolism and alters their cellular function allowing these macrophages to contribute to both inappropriate and/or unresolving inflammation. The molecular mechanisms that align lipid metabolism to macrophage function are not well understood. Lipin-1 is a phosphatidic acid phosphatase that regulates the penultimate step of glycerolipid metabolism. Lipin-1 is also a transcriptional coregulator that regulates the expression of lipid catabolism genes including beta-oxidation. We and others have demonstrated that lipin-1 promotes macrophage pro-inflammatory responses. The objective of this study was to determine the contribution of lipin-1 transcriptional coregulator function within macrophages activity. We used mice that lack either lipin-1 enzymatic activity or both lipin-1 functions in myeloid cells to define how lipin-1 transcriptional coregulator function contributes to macrophage function. We demonstrated that lipin-1 transcriptional coregulator function is required for IL-4 mediated macrophage polarization and effective efferocytosis through the promotion of beta-oxidation/oxidative phosphorylation via activation of PPARγ. Loss of both lipin-1 activities from myeloid cells leads to increased atherosclerotic burden and reduction in wound closure. Furthermore, we observed enhanced pro-inflammatory responses both in the plaques and the serum of atherosclerotic mice lacking myeloid associated lipin-1. Our work provides evidence that lipin-1 aligns cellular lipid metabolism to promote effective macrophage responses.

Effect of maternal obesity on neonatal immunity

Introduction

Maternal obesity has been shown to enhance infection, autoimmunity, and allergic sensitization in murine offspring. These data suggest that maternal obesity may be altering the neonatal immune system, but this process has not yet been elucidated. In this study, we are investigating the differences between the immune systems of neonatal mice born to obese versus lean mothers. We are also investigating how maternal obesity influences neonatal immune responses to vaccines since most pediatric vaccines are introduced during the first year of life.

Methods

Flow cytometry is used to compare splenocyte populations from neonatal offspring of high-fat and control diet parents. Immune responses of neonates to tetanus toxoid conjugated pneumococcal type 14 polysaccharide vaccines (PPS14-TT) are assessed. RNAseq is performed to investigate differences in peritoneal macrophage inflammatory states in neonatal offspring of high-fat and control-diet parents.

Results

7-day old neonatal mice from high-fat diet parents have a higher percentage of T cells (CD3+) in the spleen compared to neonates from control-diet parents. The high-fat diet offspring also have an increased B cell (B220+) population. Interestingly, although IgG antibody responses to PPS14-TT vaccine was comparable in the neonates from the two groups, those born to obese mothers developed significantly higher levels of IgM antibodies.

Conclusions

Data obtained so far suggest that maternal obesity influences the development of the adaptive immune cell populations and alter the antibody responses to vaccines in neonatal offspring. More detailed analysis of neonatal immune responses to vaccines will be presented at the meeting.

Macrophage-associated lipin-1 enzymatic activity promotes inflammation in vitro and in vivo

During atherosclerotic progression, macrophage lipid handling dictates macrophage responses. As macrophages process modified low-density lipoproteins (modLDLs), glycerolipid synthesis is turned on to process lipids into storage sites for excess cholesterol. The key regulatory enzyme in this pathway is lipin-1, a phosphatidic acid phosphohydrolase. In vitro studies provide evidence that lipin-1 enzymatic activity initiates a PKCα/βII-ERK1/2-cJun signaling cascade that primes the macrophage to be hyper-responsive to subsequent pro-inflammatory stimulus. Our in vivo data demonstrated that mice lacking lipin-1 enzymatic activity in myeloid-derived cells (lipin-1mEnzyKO) had decreased atherosclerosis. However, it is currently unknown if the loss of lipin-1 and as a result the decreased inflammatory response is responsible for the decreased severity of atherosclerosis observed in our lipin-1mEnzyKO mouse model. We defined the transcriptome regulated by lipin-1 within modLDL/lipopolysaccharide stimulated wild-type and lipin-1mEnzyKO macrophages. The loss of lipin-1 led to a reduction of pro-inflammatory transcripts. To link the decrease in inflammation in the lipin-1mEnzyKO BMDMs with the decrease in atherosclerosis severity seen in vivo, we collected RNA from the aortic arch of lipin-1mEnzyKO and wild-type mice. Analysis of the RNA using an Atherosclerosis RT2 Profiler Array indicated that the lipin-1mEnzyKO mice showed increases in anti-apoptotic gene transcripts and decreases in transcripts related to extracellular matrix degradation suggesting lipin-1mEnzyKO mice may have more stable plaques.

AAV8-mediated overexpression of mPCSK9 in liver differs between male and female mice

BACKGROUND AND AIMS

The recombinant adeno-associated viral vector serotype 8 expressing the gain-of-function mutation of mouse proprotein convertase subtilisin/kexin type 9 (AAV8- PCSK9) is a new model for the induction of hypercholesterolemia. AAV8 preferentially infects hepatocytes and the incorporated liver-specific promoter should ensure expression of PCSK9 in the liver. Since tissue distribution of AAVs can differ between male and female mice, we investigated the differences in PCSK9 expression and hypercholesterolemia development between male and female mice using the AAV8-PCSK9 model.

METHODS

Male and female C57BL/6 mice were injected with either a low-dose or high-dose of AAV8-PCSK9 and fed a high-fat diet. Plasma lipid levels were evaluated as a measure of the induction of hypercholesterolemia.

RESULTS

Injection of mice with low dose AAV8-PCSK9 dramatically elevated both serum PCSK9 and cholesterol levels in male but not female mice. Increasing the dose of AAV8-PCSK9 threefold in female mice rescued the hypercholesterolemia phenotype but did not result in full restoration of AAV8-PCSK9 transduction of livers in female mice compared to the low-dose male mice. Our data demonstrate female mice respond differently to AAV8-PCSK9 injection compared to male mice.

CONCLUSIONS

These differences do not hinder the use of female mice when AAV8-PCSK9 doses are taken into consideration. However, localization to and production of AAV8-PCSK9 in organs besides the liver in mice may introduce confounding factors into studies and should be considered during experimental design.

Abstract 188: Macrophage-associated Lipin-1 Enzymatic Activity Primes Macrophages to Be Hyper-responsive to Additional Proinflammatory Stimulus

Macrophage proinflammatory responses induced by oxidized low-density lipoproteins (oxLDL) contribute to atherosclerotic progression. Our previous data using mice lacking lipin-1 enzymatic activity in myeloid-derived cells demonstrated that stimulation of bone marrow-derived macrophages with oxLDL activates a lipin-1 dependent persistent PKCα/βII-ERK1/2-cJun signaling cascade that primes the macrophage to be hyper-responsive to subsequent proinflammatory stimulus. To further investigate the impact of lipin-1 enzymatic activity on oxLDL-induced macrophage proinflammatory responses, bone marrow-derived macrophages (BMDMs) were collected from mice lacking lipin-1 enzymatic activity in myeloid-derived cells and littermate control mice. Both the control BMDMs and BMDMs lacking lipin-1 enzymatic activity were then stimulated with oxLDL, lipopolysaccharide (LPS), or oxLDL and LPS. RNA was collected from the BMDMs and RNA sequencing was performed. The goal was to identify transcripts that are altered between the control BMDMs and BMDMs lacking lipin-1 enzymatic activity to define transcription factors and signaling pathways regulated by lipin-1 enzymatic activity on a global level. Our data demonstrates that the loss of lipin-1 enzymatic activity in BMDMs results in a reduction of proinflammatory transcripts, via loss of cJun activity, in response to the combination treatment of oxLDL and LPS when compared to the control BMDMs.

Abstract 433: Difference in pAAV/D377Y-mPCSK9-induced Expression of mPCSK9 Between Male and Female Mice

Objective: The recombinant adeno-associated viral vector serotype 8 (AAV8) expressing the gain-of-function mutation of mouse proprotein convertase subtilisin/kexin type 9 (PCSK9) is a new model for the induction of hypercholesterolemia and atherosclerosis development. AAV8 tends to preferentially infect hepatocytes and the liver-specific promoter should ensure expression of PCSK9 results in reduced hepatic low-density lipoprotein receptor levels. However, the tissue distribution of adeno-associated viral vectors can differ between male and female mice. Therefore, we set out to investigate differences in PCSK9 expression and hypercholesterolemia development in male and female mice using the AAV8-PCSK9 model.

Approach and Results: Male and female C57BL/6 mice were retro-orbitally injected with a low-dose (3x10 10 vector genomes) of AAV8-PCSK9 and fed a high-fat diet for 8 weeks. Inoculation of male mice with low dose AAV8-PCSK9 dramatically elevated both serum PCSK9 and cholesterol levels, which was not observed in female mice. Increasing the inoculation dose of AAV8-PCSK9 threefold (9x10 10 vector genomes) in female mice induced serum cholesterol and PCSK9 concentration to levels equivalent with low-dose inoculated male mice. Although increasing the dose of AAV8-PCSK9 induces a hypercholesterolemia phenotype in female mice, it did not result in an increased amount of AAV8 virus nor increased mRNA expression of mPCSK9 in female livers but did increase mRNA expression of mPCSK9 in the brains of these mice.

Conclusions: Our data demonstrate that AAV8-PCSK9 inoculation results in differences between male and female mice in the localization and production of mPCSK9. These differences do not hinder the use of female mice in hypercholesterolemia and atherosclerosis studies when AAV8-PCSK9 doses are taken into consideration. However, localization to and production of AAV8-PCSK9 in organs besides the liver in female mice may introduce confounding factors into studies and should be considered during experimental design.

Macrophage-associated lipin-1 enzymatic activity contributes to modified low-density lipoprotein-induced pro-inflammatory signaling and atherosclerosis

Macrophage proinflammatory responses induced by modified low-density lipoproteins (modLDL) contribute to atherosclerotic progression. How modLDL causes macrophages to become proinflammatory is still enigmatic. Macrophage foam cell formation induced by modLDL requires glycerolipid synthesis. Lipin-1, a key enzyme in the glycerolipid synthesis pathway, contributes to modLDL-elicited macrophage proinflammatory responses in vitro. The objective of this study was to determine if macrophage-associated lipin-1 contributes to atherogenesis and to assess its role in modLDL-mediated signaling in macrophages. We developed mice lacking lipin-1 in myeloid-derived cells and used adeno-associated viral vector 8 expressing the gain-of-function mutation of mouse proprotein convertase subtilisin/kexin type 9 (AAV8-PCSK9) to induce hypercholesterolemia and plaque formation. Mice lacking myeloid-associated lipin-1 had reduced atherosclerotic burden compared to control mice despite similar plasma lipid levels. Stimulation of bone marrow-derived macrophages with modLDL activated a persistent PKCα/βII-ERK1/2-cJun signaling cascade that contributed to macrophage proinflammatory responses that was dependent on lipin-1 enzymatic activity. Our data demonstrate that macrophage-associated lipin-1 is atherogenic, likely through persistent activation of a PKCα/βII-ERK1/2-cJun signaling cascade that contributes to foam cell proinflammatory responses. Taken together these results suggest modLDL-induced foam cell formation and modLDL-induced macrophage proinflammatory responses are not independent consequences of modLDL stimulation, but rather are both directly influenced by enhanced lipid synthesis.

Absence of Nicotinamide Nucleotide Transhydrogenase in C57BL/6J Mice Exacerbates Experimental Atherosclerosis

BACKGROUND

Mitochondrial reactive oxygen species (ROS) contribute to inflammation and vascular remodeling during atherosclerotic plaque formation. C57BL/6N (6N) and C57BL/6J (6J) mice display distinct mitochondrial redox balance due to the absence of nicotinamide nucleotide transhydrogenase (NNT) in 6J mice. We hypothesize that differential NNT expression between these animals alters plaque development.

METHODS

6N and 6J mice were treated with AAV8-PCSK9 (adeno-associated virus serotype 8/proprotein convertase subtilisin/kexin type 9) virus leading to hypercholesterolemia, increased low-density lipoprotein, and atherosclerosis in mice fed a high-fat diet (HFD). Mice were co-treated with the mitochondria-targeted superoxide dismutase mimetic MitoTEMPO to assess the contribution of mitochondrial ROS to atherosclerosis.

RESULTS

Baseline and HFD-induced vascular superoxide is increased in 6J compared to 6N mice. MitoTEMPO diminished superoxide in both groups demonstrating differential production of mitochondrial ROS among these strains. PCSK9 treatment and HFD led to similar increases in plasma lipids in both 6N and 6J mice. However, 6J animals displayed significantly higher levels of plaque formation. MitoTEMPO reduced plasma lipids but did not affect plaque formation in 6N mice. In contrast, MitoTEMPO surprisingly increased plaque formation in 6J mice.

CONCLUSION

These data indicate that loss of NNT increases vascular ROS production and exacerbates atherosclerotic plaque development.

Macrophage-Associated Lipin-1 Enzymatic Activity Contributes to Modified Low-Density Lipoprotein-Induced Proinflammatory Signaling and Atherosclerosis

OBJECTIVE

Macrophage proinflammatory responses induced by modified low-density lipoproteins (modLDL) contribute to atherosclerotic progression. How modLDL causes macrophages to become proinflammatory is still enigmatic. Macrophage foam cell formation induced by modLDL requires glycerolipid synthesis. Lipin-1, a key enzyme in the glycerolipid synthesis pathway, contributes to modLDL-elicited macrophage proinflammatory responses in vitro. The objective of this study was to determine whether macrophage-associated lipin-1 contributes to atherogenesis and to assess its role in modLDL-mediated signaling in macrophages.

APPROACH AND RESULTS

We developed mice lacking lipin-1 in myeloid-derived cells and used adeno-associated viral vector 8 expressing the gain-of-function mutation of mouse proprotein convertase subtilisin/kexin type 9 (adeno-associated viral vector 8-proprotein convertase subtilisin/kexin type 9) to induce hypercholesterolemia and plaque formation. Mice lacking myeloid-associated lipin-1 had reduced atherosclerotic burden compared with control mice despite similar plasma lipid levels. Stimulation of bone marrow-derived macrophages with modLDL activated a persistent protein kinase Cα/βII-extracellular receptor kinase1/2-jun proto-oncogene signaling cascade that contributed to macrophage proinflammatory responses that was dependent on lipin-1 enzymatic activity.

CONCLUSIONS

Our data demonstrate that macrophage-associated lipin-1 is atherogenic, likely through persistent activation of a protein kinase Cα/βII-extracellular receptor kinase1/2-jun proto-oncogene signaling cascade that contributes to foam cell proinflammatory responses. Taken together, these results suggest that modLDL-induced foam cell formation and modLDL-induced macrophage proinflammatory responses are not independent consequences of modLDL stimulation but rather are both directly influenced by enhanced lipid synthesis.

Abstract 608: Macrophage-Associated Lipin-1 Contributes to Atherosclerosis

Objective: Macrophage pro-inflammatory responses induced by oxidized low-density lipoproteins (oxLDL) mediate atherosclerosis progression. However, how oxLDL causes macrophages to become pro-inflammatory is still enigmatic. Macrophage foam cell formation induced by oxLDL requires glycerolipid synthesis. Lipin-1, a key enzyme in the glycerolipid synthesis pathway, contributes to oxLDL-elicited pro-inflammatory responses in a macrophage cell line. The objective of this study was to determine if myeloid-associated lipin-1 contributes to atherogenesis and asses its role in oxLDL-mediated signaling in macrophages

Approach and Results: We developed mice lacking lipin-1 in myeloid cells and used adeno-associated viral vector 8 expressing the gain-of-function mutation of mouse proprotein convertase subtilisin/kexin type 9 (AAV8-PCSK9) to induce hypercholesterolemia and plaque formation. Mice lacking myeloid-associated lipin-1 had reduced atherosclerotic burden compared to control mice despite similar plasma lipid levels. Stimulation of bone marrow-derived macrophages with oxLDL activated a persistent PKCα/βII-ERK1/2-cJun signaling cascade that contributed to macrophage pro-inflammatory responses. Bone marrow-derived macrophages lacking lipin-1 failed to activate this PKCα/βII-ERK1/2-cJun signaling cascade.

Conclusions: Our data demonstrates that myeloid-associated lipin-1 is atherogenic, likely through persistent activation of a PKCα/βII-ERK1/2-cJun signaling cascade that contributes to foam cell pro-inflammatory responses. Taken together these results suggest that oxLDL-induced foam cell formation and oxLDL-induced macrophage pro-inflammatory responses are not separate outcomes of oxLDL-stimulation of macrophages, but rather lipid synthesis that contributes to foam cell formation also influences macrophage inflammatory responses.

Abstract 591: Absence of Nicotinamide Nucleotide Transhydrogenase Exacerbates Atherosclerosis in Mice

Oxidant stress contributes to endothelial cell injury and inflammation that are hallmarks of early stage atherosclerosis. Emerging evidence implicates mitochondrial reactive oxygen species (ROS) as important contributors to this oxidant stress and differences in mitochondrial function may augment this process. We have shown that variation in mitochondrial function and ROS production associated with ethnicity contributes to endothelial and vascular dysfunction. To model these distinct mitochondrial redox phenotypes we used C57Bl/6N (6N) and C57Bl/6J (6J) mice that also display unique mitochondrial functional properties due to the differential expression nicotinamide nucleotide transhydrogenase (NNT). Mice were treated with adeno-associated virus encoding a gain-of-function form of proprotein convertase subtilisin/kexin type 9 (PCSK9) that leads to hypercholesterolemia, increased LDL levels, and atherosclerosis in mice. PCSK9 treatment and 8 weeks of high fat diet led to increases in plasma lipids in both 6N and 6J mice. However, 6J animals displayed significantly higher levels of fat deposition in the vasculature and increased plaque size in the carotid sinus. 6N mice co-treated with the mitochondria targeted superoxide dismutase mimetic MitoTEMPO for the final 4 weeks of the experiment displayed reduced plasma lipids, but no impact on fat deposition or plaque size was observed. In contrast, MitoTEMPO increased vascular fat deposition and plaque size in 6J mice consistent with a more severe atherosclerotic phenotype. Increased mitochondrial ROS was confirmed by demonstrating elevated vascular superoxide in 6J versus 6N animals and that this difference is exacerbated on high fat diet. MitoTEMPO diminished vascular superoxide production to near baseline levels in both groups, yet increased plaque size and fat deposition in 6J mice suggests a role for hydrogen peroxide in this process. These data indicate that loss of NNT and changes in mitochondrial function increase vascular ROS production and exacerbate lipid deposition and plaque development in the early stages of atherosclerosis.

Abstract 663: Modified Low Density Lipoproteins Elicited Macrophage InflammatoryResponses is Regulated by the Glycerolipid Synthesis Enzyme Lipin-1

Modified low density lipoproteins (modLDL) elicit macrophage generation into foam cells that release pro-inflammatory mediators driving atherosclerotic lesion progression causing cardiovascular disease. The molecular mechanisms that elicit foam cell inflammatory responses have not yet been fully elucidated. The lipid-laden phenotype that is characteristic of macrophage foam cells is due to lipid droplet biogenesis in response to excess cholesterol. Lipid droplet biogenesis is a process that is thought to be symptomatic of, but not drive atherosclerosis. Lipid droplet biogenesis requires glycerolipid synthesis, during which, lipin-1 converts phosphatidate into diglyceride as the penultimate step of lipid droplet generation. We had previously demonstrated lipin-1 is also required for modLDL-elicited pro-inflammatory response from macrophages. We hypothesized that modLDL elicits chronic diglyceride generation, via lipin-1 enzymatic activity, that activates signaling cascades responsible for foam cell pro-inflammatory responses. To test our hypotheses we stimulated wild type and lipin-1 depleted bone marrow-derived macrophages (BMDMs) with oxidized LDLs (oxLDLs). Stimulation of wild type BMDMs resulted in chronic activation of the signaling kinases PKCα/βII, ERK1/2 and the AP-1 transcription factor subunit cJun (up to 48 hours after stimulation). This pathway was not observed to be active in BMDMs depleted of lipin-1 either genetically or with siRNA. The pharmacological inhibition of lipin-1, PKCα/βII, ERK1/2 strongly suggest lipin-1- PKCα/βII-ERK1/2-cJun represents a signaling axis. Finally, each of these proteins were required for oxLDL-elicited pro-inflammatory responses by macrophages. These results suggest that augmented glycerolipid synthesis in macrophages due to modLDL stimulation is not just symptomatic of atherosclerosis but promote inflammatory responses that drive lesion progression

Lipin-1 contributes to modified low-density lipoprotein-elicited macrophage pro-inflammatory responses

Atherosclerosis is a chronic inflammatory disease of large and medium-sized arteries and the underlying cause of cardiovascular disease, a major cause of mortality worldwide. The over-accumulation of modified cholesterol-containing low-density lipoproteins (e.g. oxLDL) in the artery wall and the subsequent recruitment and activation of macrophages contributes to the development of atherosclerosis. The excessive uptake of modified-LDL by macrophages leads to a lipid-laden "foamy" phenotype and pro-inflammatory cytokine production. Modified-LDLs promote foam cell formation in part by stimulating de novo lipid biosynthesis. However, it is unknown if lipid biosynthesis directly regulates foam cell pro-inflammatory mediator production. Lipin-1, a phosphatidate phosphohydrolase required for the generation of diacylglycerol during glycerolipid synthesis has recently been demonstrated to contribute to bacterial-induced pro-inflammatory responses by macrophages. In this study we present evidence demonstrating the presence of lipin-1 within macrophages in human atherosclerotic plaques. Additionally, reducing lipin-1 levels in macrophages significantly inhibits both modified-LDL-induced foam cell formation in vitro, as observed by smaller/fewer intracellular lipid inclusions, and ablates modified-LDL-elicited production of the pro-atherogenic mediators tumor necrosis factor-α, interleukin-6, and prostaglandin E2. These findings demonstrate a critical role for lipin-1 in the regulation of macrophage inflammatory responses to modified-LDL. These data begin to link the processes of foam cell formation and pro-inflammatory cytokine production within macrophages.

Abstract 382: Lipin-1 Links Pro-inflammatory Responses and Foam Cell Formation by Oxidized-Low Density Lipoprotein-Elicited Macrophages

Atherosclerosis is a chronic inflammatory disease of large and medium-sized arteries and one of the underlying causes of cardiovascular disease (CVD). Macrophages participate decisively in the development and promotion of atherosclerosis. Macrophages infiltrate the arterial intima to ingest modified low density lipoproteins (e.g. oxLDLs) via scavenger receptors. The scavenging of oxLDLs results in foam cell formation due to enhanced lipid droplet biogenesis. These foam cells eventually release pro-inflammatory cytokines that promote atherosclerosis. However, it is currently unknown whether there is a link between lipid droplet biogenesis and pro-inflammatory cytokine production in macrophages that scavenge oxLDL. Lipin-1, a phosphatidate phosphohydrolase enzyme, partially contributes to macrophage pro-inflammatory cytokine production following stimulation with bacteria. Lipin-1 is also required for lipid droplet biogenesis in macrophages. Finally, we observed lipin-1 protein within macrophages from human atherosclerotic plaques. Thus, we hypothesized that lipid droplet biogenesis, via lipin-1 activity, directly contributes to foam cell pro-inflammatory cytokine production. To test this hypothesis we compared lipid droplet biogenesis and pro-inflammatory cytokine responses of oxLDL-stimulated wild type and lipin-1-depleted macrophages. Depletion of lipin-1 inhibited oxLDL-induced foam cell generation by reducing lipid droplet number, area, and staining intensity. There were no differences in scavenger receptor expression or uptake of oxLDL between wild type and lipin-1-depleted cells. In addition, depletion of lipin-1 also ablated oxLDL-elicited production of the pro-atherogenic cytokines tumor necrosis factor-α and interleukin-6. These findings demonstrate a critical role for lipin-1 in the regulation of macrophage inflammatory responses to oxLDL. Furthermore, these data begin to link foam cell formation, via lipid droplet biogenesis, and pro-inflammatory cytokine production within oxLDL stimulated macrophages. Thus, our studies suggest that lipid droplet biogenesis may be an ideal therapeutic target to inhibit inflammation associated with atherosclerosis to treat CVD.

Natural killer T (NKT) cells accelerate Shiga toxin type 2 (Stx2) pathology in mice

Shiga toxin-producing Escherichia coli (STEC) is a leading cause of childhood renal disease Hemolytic Uremic Syndrome (HUS). The involvement of renal cytokines and chemokines is suspected to play a critical role in disease progression. In current article, we tested the hypothesis that NKT cells are involved in Stx2-induced pathology in vivo. To address this hypothesis we compared Stx2 toxicity in WT and CD1 knockout (KO) mice. In CD1KO mice, which lack natural killer T (NKT) cells, Stx2-induced pathologies such as weight loss, renal failure, and death were delayed. In WT mice, Stx2-specific selective increase in urinary albumin occurs in later time points, and this was also delayed in NKT cell deficient mice. NKT cell-associated cytokines such as IL-2, IL-4, IFN-γ, and IL-17 were detected in kidney lysates of Stx2-injected WT mice with the peak around 36 h after Stx2 injection. In CD1KO, there was a delay in the kinetics, and increases in these cytokines were observed 60 h post Stx2 injection. These data suggest that NKT cells accelerate Stx2-induced pathology in mouse kidneys. To determine the mechanism by which NKT cells promote Stx2-associated disease, in vitro studies were performed using murine renal cells. We found that murine glomerular endothelial cells and podocytes express functional CD1d molecules and can present exogenous antigen to NKT cells. Moreover, we observed the direct interaction between Stx2 and the receptor Gb₃ on the surface of mouse renal cells by 3D STORM-TIRF which provides single molecule imaging. Collectively, these data suggest that Stx2 binds to Gb₃ on renal cells and leads to aberrant CD1d-mediated NKT cell activation. Therefore, strategies targeting NKT cells could have a significant impact on Stx2-associated renal pathology in STEC disease.