Casein Kinase 1 Phosphomimetic Mutations Negatively Impact Connexin-43 Gap Junctions in Human Pluripotent Stem Cell-Derived Cardiomyocytes

The transplantation of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) has shown promise in preclinical models of myocardial infarction, but graft myocardium exhibits incomplete host-graft electromechanical integration and a propensity for pro-arrhythmic behavior. Perhaps contributing to this situation, hPSC-CM grafts show low expression of connexin 43 (Cx43), the major gap junction (GJ) protein, in ventricular myocardia. We hypothesized that Cx43 expression and function could be rescued by engineering Cx43 in hPSC-CMs with a series of phosphatase-resistant mutations at three casein kinase 1 phosphorylation sites (Cx43-S3E) that have been previously reported to stabilize Cx43 GJs and reduce arrhythmias in transgenic mice. However, contrary to our predictions, transgenic Cx43-S3E hPSC-CMs exhibited reduced Cx43 expression relative to wild-type cells, both at baseline and following ischemic challenge. Cx43-S3E hPSC-CMs showed correspondingly slower conduction velocities, increased automaticity, and differential expression of other connexin isoforms and various genes involved in cardiac excitation-contraction coupling. Cx43-S3E hPSC-CMs also had phosphorylation marks associated with Cx43 GJ internalization, a finding that may account for their impaired GJ localization. Taken collectively, our data indicate that the Cx43-S3E mutation behaves differently in hPSC-CMs than in adult mouse ventricular myocytes and that multiple biological factors likely need to be addressed synchronously to ensure proper Cx43 expression, localization, and function.

Bright ferritin for long-term MR imaging of human embryonic stem cells

BACKGROUND

A non-invasive imaging technology that can monitor cell viability, retention, distribution, and interaction with host tissue after transplantation is needed for optimizing and translating stem cell-based therapies. Current cell imaging approaches are limited in sensitivity or specificity, or both, for in vivo cell tracking. The objective of this study was to apply a novel ferritin-based magnetic resonance imaging (MRI) platform to longitudinal tracking of human embryonic stem cells (hESCs) in vivo.

METHODS

Human embryonic stem cells (hESCs) were genetically modified to stably overexpress ferritin using the CRISPR-Cas9 system. Cellular toxicity associated with ferritin overexpression and manganese (Mn) supplementation were assessed based on cell viability, proliferation, and metabolic activity. Ferritin-overexpressing hESCs were characterized based on stem cell pluripotency and cardiac-lineage differentiation capability. Cells were supplemented with Mn and imaged in vitro as cell pellets on a preclinical 3 T MR scanner. T1-weighted images and T1 relaxation times were analyzed to assess contrast. For in vivo study, three million cells were injected into the leg muscle of non-obese diabetic severe combined immunodeficiency (NOD SCID) mice. Mn was administrated subcutaneously. T1-weighted sequences and T1 mapping were used to image the animals for longitudinal in vivo cell tracking. Cell survival, proliferation, and teratoma formation were non-invasively monitored by MRI. Histological analysis was used to validate MRI results.

RESULTS

Ferritin-overexpressing hESCs labeled with 0.1 mM MnCl2 provided significant T1-induced bright contrast on in vitro MRI, with no adverse effect on cell viability, proliferation, pluripotency, and differentiation into cardiomyocytes. Transplanted hESCs displayed significant bright contrast on MRI 24 h after Mn administration, with contrast persisting for 5 days. Bright contrast was recalled at 4-6 weeks with early teratoma outgrowth.

CONCLUSIONS

The bright-ferritin platform provides the first demonstration of longitudinal cell tracking with signal recall, opening a window on the massive cell death that hESCs undergo in the weeks following transplantation before the surviving cell fraction proliferates to form teratomas.

Methods for Transepicardial Cell Transplantation in a Swine Myocardial Infarction Model

The transplantation of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) has garnered significant attention as a potential means of restoring lost muscle mass and contractile function in injured hearts. Early preclinical work with hPSC-CMs employed rodent models, but the field has recently advanced to transplantation studies in more translationally relevant large animal models including non-human primates and swine. The pig is a particularly attractive model for such studies because the size, structure, and physiology of the porcine heart is very similar to that of humans. The pig model has reasonably high throughput, is readily amenable to clinically available cell delivery methods and imaging modalities and has been used frequently to test the safety and efficacy of new cardiac therapies. Here, we describe methods that were established in our laboratory for the specific purpose of testing hPSC-CM transplantation in a pig model of subacute myocardial infarction, but these same techniques should be broadly applicable to the transepicardial delivery of other biologicals including other candidate cell populations, biomaterials, and/or viral vectors.

Transplanted microvessels improve pluripotent stem cell-derived cardiomyocyte engraftment and cardiac function after infarction in rats

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) offer an unprecedented opportunity to remuscularize infarcted human hearts. However, studies have shown that most hiPSC-CMs do not survive after transplantation into the ischemic myocardial environment, limiting their regenerative potential and clinical application. We established a method to improve hiPSC-CM survival by cotransplanting ready-made microvessels obtained from adipose tissue. Ready-made microvessels promoted a sixfold increase in hiPSC-CM survival and superior functional recovery when compared to hiPSC-CMs transplanted alone or cotransplanted with a suspension of dissociated endothelial cells in infarcted rat hearts. Microvessels showed unprecedented persistence and integration at both early (~80%, week 1) and late (~60%, week 4) time points, resulting in increased vessel density and graft perfusion, and improved hiPSC-CM maturation. These findings provide an approach to cell-based therapies for myocardial infarction, whereby incorporation of ready-made microvessels can improve functional outcomes in cell replacement therapies.

Correction to: Optical mapping of human embryonic stem cell-derived cardiomyocyte graft electrical activity in injured hearts

An amendment to this paper has been published and can be accessed via the original article.

Omnipolarity applied to equi-spaced electrode array for ventricular tachycardia substrate mapping

Aims 

Bipolar electrogram (Bi_EGM)-based substrate maps are heavily influenced by direction of a wavefront to the mapping bipole. In this study, we evaluate high-resolution, orientation-independent peak-to-peak voltage (Vpp) maps obtained with an equi-spaced electrode array and omnipolar EGMs (OT_EGMs), measure its beat-to-beat consistency, and assess its ability to delineate diseased areas within the myocardium compared against traditional Bi_EGMs on two orientations: along (AL) and across (AC) array splines.

Methods and results

The endocardium of the left ventricle of 10 pigs (three healthy and seven infarcted) were each mapped using an Advisor™ HD grid with a research EnSite Precision™ system. Cardiac magnetic resonance images with late gadolinium enhancement were registered with electroanatomical maps and were used for gross scar delineation. Over healthy areas, OT_EGM Vpp values are larger than AL bipoles by 27% and AC bipoles by 26%, and over infarcted areas OT_EGM Vpp values are 23% larger than AL bipoles and 27% larger than AC bipoles (P < 0.05). Omnipolar EGM voltage maps were 37% denser than Bi_EGM maps. In addition, OT_EGM Vpp values are more consistent than bipolar Vpps showing less beat-by-beat variation than Bi_EGM by 39% and 47% over both infarcted and healthy areas, respectively (P < 0.01). Omnipolar EGM better delineate infarcted areas than traditional Bi_EGMs from both orientations.

Conclusion

An equi-spaced electrode grid when combined with omnipolar methodology yielded the largest detectable bipolar-like voltage and is void of directional influences, providing reliable voltage assessment within infarcted and non-infarcted regions of the heart.

Optical mapping of human embryonic stem cell-derived cardiomyocyte graft electrical activity in injured hearts

BACKGROUND

Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) show tremendous promise for cardiac regeneration, but the successful development of hESC-CM-based therapies requires improved tools to investigate their electrical behavior in recipient hearts. While optical voltage mapping is a powerful technique for studying myocardial electrical activity ex vivo, we have previously shown that intra-cardiac hESC-CM grafts are not labeled by conventional voltage-sensitive fluorescent dyes. We hypothesized that the water-soluble voltage-sensitive dye di-2-ANEPEQ would label engrafted hESC-CMs and thereby facilitate characterization of graft electrical function and integration.

METHODS

We developed and validated a novel optical voltage mapping strategy based on the simultaneous imaging of the calcium-sensitive fluorescent protein GCaMP3, a graft-autonomous reporter of graft activation, and optical action potentials (oAPs) derived from di-2-ANEPEQ, which labels both graft and host myocardium. Cardiomyocytes from three different GCaMP3+ hESC lines (H7, RUES2, or ESI-17) were transplanted into guinea pig models of subacute and chronic infarction, followed by optical mapping at 2 weeks post-transplantation.

RESULTS

Use of a water-soluble voltage-sensitive dye revealed pro-arrhythmic properties of GCaMP3+ hESC-CM grafts from all three lines including slow conduction velocity, incomplete host-graft coupling, and spatially heterogeneous patterns of activation that varied beat-to-beat. GCaMP3+ hESC-CMs from the RUES2 and ESI-17 lines both showed prolonged oAP durations both in vitro and in vivo. Although hESC-CMs partially remuscularize the injured hearts, histological evaluation revealed immature graft structure and impaired gap junction expression at this early timepoint.

CONCLUSION

Simultaneous imaging of GCaMP3 and di-2-ANEPEQ allowed us to acquire the first unambiguously graft-derived oAPs from hESC-CM-engrafted hearts and yielded critical insights into their arrhythmogenic potential and line-to-line variation.

Human Embryonic Stem Cell-Derived Cardiomyocytes Regenerate the Infarcted Pig Heart but Induce Ventricular Tachyarrhythmias

Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) show considerable promise for regenerating injured hearts, and we therefore tested their capacity to stably engraft in a translationally relevant preclinical model, the infarcted pig heart. Transplantation of immature hESC-CMs resulted in substantial myocardial implants within the infarct scar that matured over time, formed vascular networks with the host, and evoked minimal cellular rejection. While arrhythmias were rare in infarcted pigs receiving vehicle alone, hESC-CM recipients experienced frequent monomorphic ventricular tachycardia before reverting back to normal sinus rhythm by 4 weeks post transplantation. Electroanatomical mapping and pacing studies implicated focal mechanisms, rather than macro-reentry, for these graft-related tachyarrhythmias as evidenced by an abnormal centrifugal pattern with earliest electrical activation in histologically confirmed graft tissue. These findings demonstrate the suitability of the pig model for the preclinical development of a hESC-based cardiac therapy and provide new insights into the mechanistic basis of electrical instability following hESC-CM transplantation.

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hESC-CM transplantation partially remuscularizes the infarcted pig heart

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hESC-CM recipients show frequent tachyarrhythmias at early time points

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Graft-related arrhythmias arise from focal mechanisms rather than macro-reentry

Inhibition of pericellular plasminogen activation by apolipoprotein(a): Roles of urokinase plasminogen activator receptor and integrins αMβ2 and αVβ3

BACKGROUND AND AIMS

Lipoprotein(a) (Lp(a)) is a causal risk factor for cardiovascular disorders including coronary heart disease and calcific aortic valve stenosis. Apolipoprotein(a) (apo(a)), the unique glycoprotein component of Lp(a), contains sequences homologous to plasminogen. Plasminogen activation is markedly accelerated in the presence of cell surface receptors and can be inhibited in this context by apo(a).

METHODS

We evaluated the role of potential receptors in regulating plasminogen activation and the ability of apo(a) to mediate inhibition of plasminogen activation on vascular and monocytic/macrophage cells through knockdown (siRNA or blocking antibodies) or overexpression of various candidate receptors. Binding assays were conducted to determine apo(a) and plasminogen receptor interactions.

RESULTS

The urokinase-type plasminogen activator receptor (uPAR) modulates plasminogen activation as well as plasminogen and apo(a) binding on human umbilical vein endothelial cells (HUVECs), human acute monocytic leukemia (THP-1) cells, and THP-1 macrophages as determined through uPAR knockdown and overexpression. Apo(a) variants lacking either the kringle V or the strong lysine binding site in kringle IV type 10 are not able to bind to uPAR to the same extent as wild-type apo(a). Plasminogen activation is also modulated, albeit to a lower extent, through the Mac-1 (α_Mβ₂) integrin on HUVECs and THP-1 monocytes. Integrin α_Vβ₃ can regulate plasminogen activation on THP-1 monocytes and to a lesser extent on HUVECs.

CONCLUSIONS

These results indicate cell type-specific roles for uPAR, α_Mβ₂, and α_Vβ₃ in promoting plasminogen activation and mediate the inhibitory effects of apo(a) in this process.

Characterization of the I4399M variant of apolipoprotein(a): implications for altered prothrombotic properties of lipoprotein(a)

Essentials Elevated lipoproteinp(a) is an independent and causal risk factor for atherothrombotic diseases. rs3798220 (Ile/Met substitution in apo(a) protease-like domain) is associated with disease risk. Recombinant I4399M apo(a) altered clot structure to accelerate coagulation/delay fibrinolysis. Evidence was found for increased solvent exposure and oxidation of Met residue.

SUMMARY

Background Lipoprotein(a) (Lp[a]) is a causal risk factor for a variety of cardiovascular diseases. Apolipoprotein(a) (apo[a]), the distinguishing component of Lp(a), is homologous with plasminogen, suggesting that Lp(a) can interfere with the normal fibrinolytic functions of plasminogen. This has implications for the persistence of fibrin clots in the vasculature and hence for atherothrombotic diseases. A single-nucleotide polymorphism (SNP) (rs3798220) in the gene encoding apo(a) has been reported that results in an Ile→Met substitution in the protease-like domain (I4399M variant). In population studies, the I4399M variant has been correlated with elevated plasma Lp(a) levels and higher coronary heart disease risk, and carriers of the SNP had increased cardiovascular benefit from aspirin therapy. In vitro studies suggested an antifibrinolytic role for Lp(a) containing this variant. Objectives We performed a series of experiments to assess the effect of the Ile→Met substitution on fibrin clot formation and lysis, and on the architecture of the clots. Results We found that the Met variant decreased coagulation time and increased fibrin clot lysis time as compared with wild-type apo(a). Furthermore, we observed that the presence of the Met variant significantly increased fibrin fiber width in plasma clots formed ex vivo, while having no effect on fiber density. Mass spectrometry analysis of a recombinant apo(a) species containing the Met variant revealed sulfoxide modification of the Met residue. Conclusions Our data suggest that the I4399M variant differs structurally from wild-type apo(a), which may underlie key differences related to its effects on fibrin clot architecture and fibrinolysis.

Programming cells for cardiac repair

Because the heart is a poorly regenerative organ, there has been considerable interest in developing novel cell-based approaches to restore lost contractile function after myocardial infarction (MI). While a wide variety of candidate cell types have been tested in animal MI models, the vast majority of clinical trials have used adult stem cell types, usually derived from bone marrow. These studies have generally yielded disappointing results, an outcome that may reflect in part the limited cardiogenic potential of the adult stem cell sources employed. Post-MI heart failure is ultimately a disease of cardiomyocyte deficiency, so better outcomes may be possible with more cardiogenic approaches that may 'remuscularize' the infarct scar with new, electrically-integrated myocardium. In this review, we summarize work in the field to 'program' exogenous or endogenous cells into such a cardiogenic state, as well as efforts to test their capacity to mediate true heart regeneration.

Apolipoprotein(a) inhibits hepatitis C virus entry through interaction with infectious particles

The development of different cell culture models has greatly contributed to increased understanding of the hepatitis C virus (HCV) life cycle. However, it is still challenging to grow HCV clinical isolates in cell culture. If overcome, this would open new perspectives to study HCV biology, including drug-resistant variants emerging with new antiviral therapies. In this study we hypothesized that this hurdle could be due to the presence of inhibitory factors in patient serum. Combining polyethylene glycol precipitation, iodixanol gradient, and size-exclusion chromatography, we obtained from HCV-seronegative sera a purified fraction enriched in inhibitory factors. Mass spectrometric analysis identified apolipoprotein(a) (apo[a]) as a potential inhibitor of HCV entry. Apo(a) consists of 10 kringle IV domains (KIVs), one kringle V domain, and an inactive protease domain. The 10 KIVs are present in a single copy with the exception of KIV type 2 (KIV₂ ), which is encoded in a variable number of tandemly repeated copies, giving rise to numerous apo(a) size isoforms. In addition, apo(a) covalently links to the apolipoprotein B component of a low-density lipoprotein through a disulfide bridge to form lipoprotein(a). Using a recombinant virus derived from the JFH1 strain, we confirmed that plasma-derived and recombinant lipoprotein(a) as well as purified recombinant apo(a) variants were able to specifically inhibit HCV by interacting with infectious particles. Our results also suggest that small isoforms are less inhibitory than the large ones. Finally, we observed that the lipoprotein moiety of HCV lipoviroparticles was essential for inhibition, whereas functional lysine-binding sites in KIV₇ , KIV₈ , and KIV₁₀ were not required.

CONCLUSIONS

Our results identify apo(a) as an additional component of the lipid metabolism modulating HCV infection. (Hepatology 2017;65:1851-1864).

Activated thrombin-activatable fibrinolysis inhibitor (TAFIa) attenuates breast cancer cell metastatic behaviors through inhibition of plasminogen activation and extracellular proteolysis

Background

Thrombin activatable fibrinolysis inhibitor (TAFI) is a plasma zymogen, which can be converted to activated TAFI (TAFIa) through proteolytic cleavage by thrombin, plasmin, and most effectively thrombin in complex with the endothelial cofactor thrombomodulin (TM). TAFIa is a carboxypeptidase that cleaves carboxyl terminal lysine and arginine residues from protein and peptide substrates, including plasminogen-binding sites on cell surface receptors. Carboxyl terminal lysine residues play a pivotal role in enhancing cell surface plasminogen activation to plasmin. Plasmin has many critical functions including cleaving components of the extracellular matrix (ECM), which enhances invasion and migration of cancer cells. We therefore hypothesized that TAFIa could act to attenuate metastasis.

Methods

To assess the role of TAFIa in breast cancer metastasis, in vitro migration and invasion assays, live cell proteolysis and cell proliferation using MDA-MB-231 and SUM149 cells were carried out in the presence of a TAFIa inhibitor, recombinant TAFI variants, or soluble TM.

Results

Inhibition of TAFIa with potato tuber carboxypeptidase inhibitor increased cell invasion, migration and proteolysis of both cell lines, whereas addition of TM resulted in a decrease in all these parameters. A stable variant of TAFIa, TAFIa-CIIYQ, showed enhanced inhibitory effects on cell invasion, migration and proteolysis. Furthermore, pericellular plasminogen activation was significantly decreased on the surface of MDA-MB-231 and SUM149 cells following treatment with various concentrations of TAFIa.

Conclusions

Taken together, these results indicate a vital role for TAFIa in regulating pericellular plasminogen activation and ultimately ECM proteolysis in the breast cancer microenvironment. Enhancement of TAFI activation in this microenvironment may be a therapeutic strategy to inhibit invasion and prevent metastasis of breast cancer cells.

Abstract 262: Lipoprotein(a) Stimulates Pro-Inflammatory Phenotypes in Vascular Cells: Demonstration of a Role for the Lysine-Binding Properties of Apolipoprotein(a)

Elevated plasma Lp(a) concentrations are a causal risk factor for a variety of atherosclerotic disorders. Although the role of Lp(a) in vascular disease remains poorly understood, the atherogenicity of Lp(a) may stem from the direct atherosclerotic effects of its LDL-like component, or from the presence of the unique protein component apolipoprotein(a) (apo(a)). It has been hypothesized that Lp(a) contributes to early atherogenesis by initiating a program of endothelial cell (EC) dysfunction which results in increased EC permeability and the adoption of a pro-coagulant and pro-inflammatory phenotype. In support of the latter, studies have shown that Lp(a) stimulates expression of cell adhesion molecules (CAMs) on ECs, and induces monocyte recruitment to these cells. In the present study we demonstrate that a recombinant 17 kringle form of apo(a) (17K) stimulates expression of CAMs on primary human umbilical vein endothelial cells (HUVECs) in both a time- and dose-dependent manner, as determined by qRT-PCR analysis. We found that 17K was able to induce the expression of ICAM1, E-Selectin, VCAM1, but not PE-CAM at pathophysiologically-relevant concentrations of apo(a). However, 17KD57A, lacking the strong lysine-binding site (sLBS) in KIV10, lacked these effects. 17K was also able to induce a dose dependant increase in ICAM1 expression in the THP-1 monocyte cell line, an effect that was completely abolished by the NF-B inhibitor BAY-11-7082. In a fluorescent monocyte adhesion assay, both Lp(a) and 17K apo(a) were able to stimulate increased monocyte adhesion to ECs. Using 17KD57A, we were able to show that this effect is dependent on the sLBS in KIV10. The presence of the sLBS in KIV10 has also been found to be required for the covalent oxidized phospholipid modification on apo(a). We report that multiple KIV10 sLBS mutants lack the oxPL modification, further supporting a pro-inflammatory role for an intact KIV10 domain. Taken together, these studies contribute to a more complete understanding of the pathophysiological role that the apo(a) component of Lp(a) plays in the initiation and progression of atherosclerosis, and has the potential to aid in the development new therapeutics aimed at reducing the harmful effects of Lp(a) in the vasculature.

Mechanistic insights into Lp(a)-induced IL-8 expression: a role for oxidized phospholipid modification of apo(a)

Elevated lipoprotein (a) [Lp(a)] levels are a causal risk factor for coronary heart disease. Accumulating evidence suggests that Lp(a) can stimulate cellular inflammatory responses through the kringle-containing apolipoprotein (a) [apo(a)] component. Here, we report that recombinant apo(a) containing 17 kringle (17K) IV domains elicits a dose-dependent increase in interleukin (IL)-8 mRNA and protein expression in THP-1 and U937 macrophages. This effect was blunted by mutation of the lysine binding site in apo(a) kringle IV type 10, which resulted in the loss of oxidized phospholipid (oxPL) on apo(a). Trypsin-digested 17K had the same stimulatory effect on IL-8 expression as intact apo(a), while enzymatic removal of oxPL from apo(a) significantly blunted this effect. Using siRNA to assess candidate receptors, we found that CD36 and TLR2 may play roles in apo(a)-mediated IL-8 stimulation. Downstream of these receptors, inhibitors of MAPKs, Jun N-terminal kinase and ERK1/2, abolished the effect of apo(a) on IL-8 gene expression. To assess the roles of downstream transcription factors, luciferase reporter gene experiments were conducted using an IL-8 promoter fragment. The apo(a)-induced expression of this reporter construct was eliminated by mutation of IL-8 promoter binding sites for either NF-κB or AP-1. Our results provide a mechanistic link between oxPL modification of apo(a) and stimulation of proinflammatory intracellular signaling pathways.

Prevention of fluid effusion in kidney transplantation with the use of hemostatic biomaterials

BACKGROUND

Fluid effusion (blood, lymph, or urine) in kidney transplantation may give rise to several complications, directly, such as hematoma, seroma, lymphocele, and/or urinoma, or consequently, such as increased infection risk, longer hospital stay, graft compression--with or without functional impairment--and necessity of further hospitalizations. The aim of this study was to evaluate effectiveness of hemostatic biomaterials in prevention of fluid effusions, especially lymphocele in kidney transplant patients.

METHODS

We selected 40 patients who underwent kidney transplantation from 2009 to 2012 in which we used hemostatic biomaterials, and compared their results with those of other transplant patients from our center in which we did not used these biomaterials. Evaluated parameters were: fluid effusion, graft function, quality and quantity of drainage, blood count, and operative time.

RESULTS

There was no difference in operative time. The incidence of complications on which biomaterials can have a role decreased; particularly, we observed a reduction from 24.21% to 7.5% of fluid effusions (lymphocele). There was no evidence of complications due to biomaterials.

CONCLUSIONS

Hemostasis is important in surgery, and in kidney transplantations lymphostasis also has a significant role. In addition to the traditional hemostatic methods, recently some biomaterials, with the purpose of providing atraumatic hemostasis, were added. In our experience they are easy to use, and their use has proved to be effective for both hemostasis and lymphostasis with consequent reduction of fluid effusions.

Lipoprotein(a) catabolism is regulated by proprotein convertase subtilisin/kexin type 9 through the low density lipoprotein receptor

Elevated levels of lipoprotein(a) (Lp(a)) have been identified as an independent risk factor for coronary heart disease. Plasma Lp(a) levels are reduced by monoclonal antibodies targeting proprotein convertase subtilisin/kexin type 9 (PCSK9). However, the mechanism of Lp(a) catabolism in vivo and the role of PCSK9 in this process are unknown. We report that Lp(a) internalization by hepatic HepG2 cells and primary human fibroblasts was effectively reduced by PCSK9. Overexpression of the low density lipoprotein (LDL) receptor (LDLR) in HepG2 cells dramatically increased the internalization of Lp(a). Internalization of Lp(a) was markedly reduced following treatment of HepG2 cells with a function-blocking monoclonal antibody against the LDLR or the use of primary human fibroblasts from an individual with familial hypercholesterolemia; in both cases, Lp(a) internalization was not affected by PCSK9. Optimal Lp(a) internalization in both hepatic and primary human fibroblasts was dependent on the LDL rather than the apolipoprotein(a) component of Lp(a). Lp(a) internalization was also dependent on clathrin-coated pits, and Lp(a) was targeted for lysosomal and not proteasomal degradation. Our data provide strong evidence that the LDLR plays a role in Lp(a) catabolism and that this process can be modulated by PCSK9. These results provide a direct mechanism underlying the therapeutic potential of PCSK9 in effectively lowering Lp(a) levels.

Abstract 437: Lipoprotein(a) Internalization in HepG2 Cells is Regulated by Proprotein Convertase Subtilisin Kexin Type 9 Through the Low-Density Lipoprotein Receptor

Elevated levels of lipoprotein(a) (Lp(a)) have been identified as an independent and causal risk factor for coronary heart disease. Lp(a) consists of an LDL-like moiety covalently linked to the unique glycoprotein apolipoprotein(a) (apo(a)). The mechanism by which Lp(a) is catabolized is currently unknown, but may form the basis for the development of drug therapy to reduce high levels of plasma Lp(a). Although the role of the LDL receptor (LDLR) in Lp(a) catabolism is controversial, recent evidence has shown that Lp(a) levels are significantly reduced with an antibody against proprotein convertase subtilisin kexin type 9 (PCSK9) in patients with hypercholesterolemia receiving statin therapy. Therefore, we explored the role of PCSK9 in Lp(a)/apo(a) internalization by hepatic cells. Lp(a) or apo(a) internalization is significantly reduced in HepG2 (human hepatoma) cells either by overexpressing PCSK9 or by treatment with purified PCSK9. The ability of Lp(a) and apo(a) to be internalized was significantly reduced in the presence of the lysine analogue, ε-ACA, indicating lysine-dependent interactions with cellular receptors. Mutation of the strong lysine binding site in a recombinant apo(a) variant resulted in a reduced ability to be internalized. While LDL can bind to PCSK9 and inhibit its ability to degrade the LDLR, we found that Lp(a) lacked these properties. Interestingly, overexpressing the LDLR on HepG2 cells significantly increased the ability of Lp(a) to be internalized, an effect that was partially reduced by the addition of PCSK9. This indicates a potential key role for the LDLR in regulating Lp(a) catabolism. Furthermore, knockdown of clathrin heavy chain resulted in a significant decrease in apo(a) internalization and apo(a) internalization was not further reduced by pre-treatment of PCSK9 in the context of clathrin heavy chain knockdown. Treatment of HepG2 cells with a lysosomal inhibitor, but not a proteosomal inhibitor, resulted in accumulation of Lp(a) in HepG2 cells indicating that Lp(a) is potentially targeted for degradation through lysosomes. Taken together, these results indicate that Lp(a)/apo(a) uptake can be regulated in HepG2 cells by PCSK9 and the LDLR through clathrin-mediated endocytosis and lysosomal degradation.

Inhibition of plasminogen activation by apo(a): role of carboxyl-terminal lysines and identification of inhibitory domains in apo(a)

Apo(a), the distinguishing protein component of lipoprotein(a) [Lp(a)], exhibits sequence similarity to plasminogen and can inhibit binding of plasminogen to cell surfaces. Plasmin generated on the surface of vascular cells plays a role in cell migration and proliferation, two of the fibroproliferative inflammatory events that underlie atherosclerosis. The ability of apo(a) to inhibit pericellular plasminogen activation on vascular cells was therefore evaluated. Two isoforms of apo(a), 12K and 17K, were found to significantly decrease tissue-type plasminogen activator-mediated plasminogen activation on human umbilical vein endothelial cells (HUVECs) and THP-1 monocytes and macrophages. Lp(a) purified from human plasma decreased plasminogen activation on THP-1 monocytes and HUVECs but not on THP-1 macrophages. Removal of kringle V or the strong lysine binding site in kringle IV10 completely abolished the inhibitory effect of apo(a). Treatment with carboxypeptidase B to assess the roles of carboxyl-terminal lysines in cellular receptors leads in most cases to decreases in plasminogen activation as well as plasminogen and apo(a) binding; however, inhibition of plasminogen activation by apo(a) was unaffected. Our findings directly demonstrate that apo(a) inhibits pericellular plasminogen activation in all three cell types, although binding of apo(a) to cell-surface receptors containing carboxyl-terminal lysines does not appear to play a major role in the inhibition mechanism.

Overexpression of CD99 Increases the Migration and Invasiveness of Human Malignant Glioma Cells

The malignant glioma is the most common primary human brain tumor, and its migration and invasiveness away from the primary tumor mass are considered a leading cause of tumor recurrence and treatment failure. Recently, gene expression profiling revealed that the transmembrane glycoprotein CD99 is more highly expressed in malignant glioma than in normal brain. Although its function is not completely understood, CD99 is implicated in cell adhesion and migration in a variety of different cell types. CD99 has wild-type and splice variant isoforms. Previous studies have shown that wild-type CD99 may be an oncosuppressor in some tumors, distinct from the role of the splice variant isoform. In this study, our data reveal that only wild-type CD99 is expressed in human glioma cells and tissues. Using a tissue microarray, we validated that gliomas demonstrate higher expression of CD99 compared with nonneoplastic brain. To assess the role of CD99 in glioma migration and invasion, we inhibited CD99 expression by siRNA and demonstrated decreased glioma migration and invasion. In contrast, when CD99 was overexpressed in glioma cells, we observed enhancement of cell migration and invasiveness. An orthotopic brain tumor model demonstrates that CD99 overexpression significantly increases invasiveness and decreases survival rate. Interestingly, Rac activity was decreased and Rho activity was increased in CD99 overexpressing glioma cells, and the proportion of amoeboid cells to mesenchymal cells was significantly increased. Taken together, our findings suggest that CD99 may play an important role in the migration and invasion of human gliomas independent of Akt, ERK, or JNK signaling pathways. Moreover, CD99 might be involved in amoeboid-mesenchymal transition in glioma migration. CD99 may be an important future target to inhibit migration and invasion, especially in CD99-expressing gliomas.

Apolipoprotein(a) stimulates nuclear translocation of β-catenin: a novel pathogenic mechanism for lipoprotein(a)

Apolipoprotein(a) (apo(a)), the distinguishing component of lipoprotein(a), accelerates atherosclerosis by provoking endothelial cell dysfunction. This study unravels proatherosclerotic and proinflammatory apo(a)-mediated signaling pathways involving PTEN/PI3K/Akt/GSK3β that result in β-catenin nuclear translocation and up-regulation of COX-2.

Lipoprotein(a) (Lp(a)) is associated with cardiovascular disease risk. This may be attributable to the ability of Lp(a) to elicit endothelial dysfunction. We previously reported that apolipoprotein(a) (apo(a); the distinguishing kringle-containing component of Lp(a)) elicits cytoskeletal rearrangements in vascular endothelial cells, resulting in increased cellular permeability. These effects require a strong lysine-binding site (LBS) in apo(a). We now report that apo(a) induces both nuclear β-catenin–mediated cyclooxygenase-2 (COX-2) expression and prostaglandin E2 secretion, indicating a proinflammatory role for Lp(a). Apo(a) caused the disruption of VE-cadherin/β-catenin complexes in a Src-dependent manner, decreased β-catenin phosphorylation, and increased phosphorylation of Akt and glycogen synthase kinase-3β, ultimately resulting in increased nuclear translocation of β-catenin; all of these effects are downstream of apo(a) attenuation of phosphatase and tensin homologue deleted on chromosome 10 activity. The β-catenin–mediated effects of apo(a) on COX-2 expression were absent using a mutant apo(a) lacking the strong LBS. Of interest, the normal and LBS mutant forms of apo(a) bound to human umbilical vein endothelial cells in a similar manner, and the binding of neither was affected by lysine analogues. Taken together, our findings suggest a novel mechanism by which apo(a) can induce proinflammatory and proatherosclerotic effects through modulation of vascular endothelial cell function.

Role of the cofilin activity cycle in astrocytoma migration and invasion

The cofilin pathway plays a central role in the regulation of actin polymerization and the formation of cell membrane protrusions that are essential for cell migration. Overexpression of cofilin has been linked to the aggressiveness of a variety of different cancers. In these cancers, the phosphorylation of cofilin at Ser3 is a key regulatory mechanism modulating cofilin activity. The activation status of cofilin has been directly linked to tumor invasion. Accordingly, in this study, we examined the expression of cofilin and its activation status in astrocytoma cell lines and astrocytic tumors. We show that cofilin expression was increased and correlated with increasing grade malignant astrocytoma. In addition, both cofilin and LIMK had elevated expression in astrocytoma cell lines. Knockdown of cofilin by siRNA altered astrocytoma cell morphology and inhibited astrocytoma migration and invasion. Conversely, overexpression of a cofilin phosphorylation mutant in an in vivo intracranial xenograft model resulted in a more highly invasive phenotype than those xenographs expressing wild-type cofilin. Animals harboring astrocytomas stably expressing the cofilin phosphorylation mutant (cofilin-S3A) demonstrated marked local invasiveness and spread across the corpus callosum to the contralateral hemisphere in all animals. Taken together, these data indicate that the cofilin activity pathway may represent a novel therapeutic target to diminish the invasion of these highly malignant tumors.

Intravascular lipoma from iliofemoral tract

Lipoma is a benign tumour composed of well-differentiated adipocytes and is the most common soft tissue mass. We present a case of an intraluminal lipoma of the iliofemoral axis presenting as lower limb venous obstruction. In our case, definite diagnosis is impossible as there is aspecific symptomatology and radiologic imaging methods are not able to provide diriment data, considering rarity of the disease. Therefore, we could make correct diagnosis only intraoperatively. We preferred a direct reconstruction of the wall of vein. At 12-month follow-up, we have had complete disappearance of symptoms and functional diseases.