Impact of apolipoprotein(a) on in vitro angiogenesis

O-glycan structures in apo(a) subunit of human lipoprotein(a) suppresses the pro-angiogenic activity of galectin-1 on human umbilical vein endothelial cells

Apolipoprotein(a) [apo(a)] is a highly polymorphic O-glycoprotein circulating in human plasma as lipoprotein(a) [Lp(a)]. The O-glycan structures of apo(a) subunit of Lp(a) serve as strong ligands of galectin-1, an O-glycan binding pro-angiogenic lectin abundantly expressed in placental vascular tissues. But the pathophysiological significance of apo(a)-galectin-1 binding is not yet been revealed. Carbohydrate-dependent binding of galectin-1 to another O-glycoprotein, neuropilin-1 (NRP-1) on endothelial cells activates vascular endothelial growth factor receptor 2 (VEGFR2) and mitogen-activated protein kinase (MAPK) signaling. Using apo(a), isolated from human plasma, we demonstrated the potential of the O-glycan structures of apo(a) in Lp(a) to inhibit angiogenic properties such as proliferation, migration, and tube-formation in human umbilical vein endothelial cells (HUVECs) as well as neovascularization in chick chorioallantoic membrane. Further, in vitro protein-protein interaction studies have confirmed apo(a) as a superior ligand to NRP-1 for galectin-1 binding. We also demonstrated that the protein levels of galectin-1, NRP-1, VEGFR2, and downstream proteins in MAPK signaling were reduced in HUVECs in the presence of apo(a) with intact O-glycan structures compared to that of de-O-glycosylated apo(a). In conclusion, our study shows that apo(a)-linked O-glycans prevent the binding of galectin-1 to NRP-1 leading to the inhibition of galectin-1/neuropilin-1/VEGFR2/MAPK-mediated angiogenic signaling pathway in endothelial cells. As higher plasma Lp(a) level in women is an independent risk factor for pre-eclamsia, a pregnancy-associated vascular complication, we propose that apo(a) O-glycans-mediated inhibition of the pro-angiogenic activity of galectin-1 may be one of the underlying molecular mechanism of pathogenesis of Lp(a) in pre-eclampsia.

Lipoprotein(a) in Atherosclerotic Diseases: From Pathophysiology to Diagnosis and Treatment

Lipoprotein(a) (Lp(a)) is a low-density lipoprotein (LDL) cholesterol-like particle bound to apolipoprotein(a). Increased Lp(a) levels are an independent, heritable causal risk factor for atherosclerotic cardiovascular disease (ASCVD) as they are largely determined by variations in the Lp(a) gene (LPA) locus encoding apo(a). Lp(a) is the preferential lipoprotein carrier for oxidized phospholipids (OxPL), and its role adversely affects vascular inflammation, atherosclerotic lesions, endothelial function and thrombogenicity, which pathophysiologically leads to cardiovascular (CV) events. Despite this crucial role of Lp(a), its measurement lacks a globally unified method, and, between different laboratories, results need standardization. Standard antilipidemic therapies, such as statins, fibrates and ezetimibe, have a mediocre effect on Lp(a) levels, although it is not yet clear whether such treatments can affect CV events and prognosis. This narrative review aims to summarize knowledge regarding the mechanisms mediating the effect of Lp(a) on inflammation, atherosclerosis and thrombosis and discuss current diagnostic and therapeutic potentials.

Relationship between lipoprotein(a) and colorectal cancer among inpatients: a retrospective study

The present study was to explore the association between lipoprotein(a) [Lp(a)] and colorectal cancer (CRC) among inpatients. This study included 2822 participants (393 cases vs. 2429 controls) between April 2015 and June 2022. Logistic regression models, smooth curve fitting, and sensitivity analyses were performed to investigate the relationship between Lp(a) and CRC. Compared with the lower Lp(a) quantile 1 (<79.6 mg/L), the adjusted odds ratios (ORs) in quantile 2 (79.6-145.0 mg/L), quantile 3 (146.0-299.0 mg/L), and quantile 4 (≥300.0 mg/L) were 1.41 (95% confidence interval [CI]: 0.95-2.09), 1.54 (95% CI: 1.04-2.27), 1.84 (95% CI: 1.25-2.7), respectively. A linear relationship between lipoprotein(a) and CRC was observed. The finding that Lp(a) has a positive association with CRC supports the "common soil" hypothesis of cardiovascular disease (CVD) and CRC.

The association of lipoprotein(a) and intraplaque neovascularization in patients with carotid stenosis: a retrospective study

Background

Lipoprotein(a) is genetically determined and increasingly recognized as a major risk factor for arteriosclerotic cardiovascular disease. We examined whether plasma lipoprotein(a) concentrations were associated with intraplaque neovascularization (IPN) grade in patients with carotid stenosis and in terms of increasing plaque susceptibility to haemorrhage and rupture.

Methods

We included 85 patients diagnosed with carotid stenosis as confirmed using carotid ultrasound who were treated at Guangdong General Hospital. Baseline data, including demographics, comorbid conditions and carotid ultrasonography, were recorded. The IPN grade was determined using contrast-enhanced ultrasound through the movement of the microbubbles. Univariate and multivariate binary logistic regression analyses were used to evaluate the association between lipoprotein(a) and IPN grade, with stepwise adjustment for covariates including age, sex, comorbid conditions and statin therapy (model 1), total cholesterol, triglyceride, low-density lipoprotein cholesterol calculated by Friedwald's formula, high-density lipoprotein cholesterol, apolipoprotein A and apolipoprotein B (model 2), maximum plaque thickness and total carotid maximum plaque thickness, degree of carotid stenosis and internal carotid artery (ICA) occlusion (model 3).

Results

Lipoprotein(a) was a significant predictor of higher IPN grade in binary logistic regression before adjusting for other risk factors (odds ratio [OR] 1.238, 95% confidence interval [CI] (1.020, 1.503), P = 0.031). After adjusting for other risk factors, lipoprotein(a) still remained statistically significant in predicting IPN grade in all model. (Model 1: OR 1.333, 95% CI 1.074, 1.655, P = 0.009; Model 2: OR 1.321, 95% CI 1.059, 1.648, P = 0.014; Model 3: OR 1.305, 95% CI 1.045, 1.628, P = 0.019). Lp(a) ≥ 300 mg/L is also significantly related to IPN compare to < 300 mg/L (OR 2.828, 95% CI 1.055, 7.580, P = 0.039) as well as in model 1, while in model 2 and model 3 there are not significant difference.

Conclusions

Plasma lipoprotein(a) concentrations were found to be independently associated with higher IPN grade in patients with carotid stenosis. Lowering plasma lipoprotein(a) levels may result in plaque stabilization by avoiding IPN formation.

Apolipoprotein(a), an enigmatic anti-angiogenic glycoprotein in human plasma: A curse or cure?

Angiogenesis is a finely co-ordinated, multi-step developmental process of the new vascular structure. Even though angiogenesis is regularly occurring in physiological events such as embryogenesis, in adults, it is restricted to specific tissue sites where rapid cell-turnover and membrane synthesis occurs. Both excessive and insufficient angiogenesis lead to vascular disorders such as cancer, ocular diseases, diabetic retinopathy, atherosclerosis, intra-uterine growth restriction, ischemic heart disease, stroke etc. Occurrence of altered lipid profile and vascular lipid deposition along with vascular disorders is a hallmark of impaired angiogenesis. Among lipoproteins, lipoprotein(a) needs special attention due to the presence of a multi-kringle protein subunit, apolipoprotein(a) [apo(a)], which is structurally homologous to many naturally occurring anti-angiogenic proteins such as plasminogen and angiostatin. Researchers have constructed different recombinant forms of apo(a) (rhLK68, rhLK8, RHACK2, KV-11, and AU-6) and successfully exploited its potential to inhibit unwanted angiogenesis during tumor metastasis and retinal neovascularization. Similar to naturally occurring anti-angiogenic proteins, apo(a) can directly interfere with angiogenic signaling pathways. Besides this, apo(a) can also exert its anti-angiogenic effect indirectly by inducing endothelial cell apoptosis, by inhibiting endothelial progenitor cell functions or by upregulating nuclear factors in endothelial cells via apo(a)-bound oxPLs. However, the impact of the anti-angiogenic potential of native apo(a) during physiological angiogenesis in embryos and wounded tissues is not yet explored. In this context, we review the studies so far done to demonstrate the anti-angiogenic activity of apo(a) and the recent developments in using apo(a) as a therapeutic agent to treat impaired angiogenesis during vascular disorders, with emphasis on the gaps in the literature.

Lipoprotein(a) the Insurgent: A New Insight into the Structure, Function, Metabolism, Pathogenicity, and Medications Affecting Lipoprotein(a) Molecule

Lipoprotein(a) [Lp(a)], aka "Lp little a", was discovered in the 1960s in the lab of the Norwegian physician Kåre Berg. Since then, we have greatly improved our knowledge of lipids and cardiovascular disease (CVD). Lp(a) is an enigmatic class of lipoprotein that is exclusively formed in the liver and comprises two main components, a single copy of apolipoprotein (apo) B-100 (apo-B100) tethered to a single copy of a protein denoted as apolipoprotein(a) apo(a). Plasma levels of Lp(a) increase soon after birth to a steady concentration within a few months of life. In adults, Lp(a) levels range widely from <2 to 2500 mg/L. Evidence that elevated Lp(a) levels >300 mg/L contribute to CVD is significant. The improvement of isoform-independent assays, together with the insight from epidemiologic studies, meta-analyses, genome-wide association studies, and Mendelian randomization studies, has established Lp(a) as the single most common independent genetically inherited causal risk factor for CVD. This breakthrough elevated Lp(a) from a biomarker of atherosclerotic risk to a target of therapy. With the emergence of promising second-generation antisense therapy, we hope that we can answer the question of whether Lp(a) is ready for prime-time clinic use. In this review, we present an update on the metabolism, pathophysiology, and current/future medical interventions for high levels of Lp(a).

Suppression of colorectal cancer liver metastasis by apolipoprotein(a) kringle V in a nude mouse model through the induction of apoptosis in tumor-associated endothelial cells

The formation of liver metastases in colorectal cancer patients is the primary cause of patient death. Current therapies directed at liver metastasis from colorectal cancer have had minimal impact on patient outcomes. Therefore, the development of alternative treatment strategies for liver metastasis is needed. In the present study, we demonstrated that recombinant human apolipoprotein(a) kringle V, also known as rhLK8, induced the apoptotic turnover of endothelial cells in vitro through the mitochondrial apoptosis pathway. The interaction of rhLK8 with glucose-regulated protein 78 (GRP78) may be involved in the induction of apoptosis because the inhibition of GRP78 by GRP78-specific antibodies or siRNA knockdown inhibited the rhLK8-mediated apoptosis of human umbilical vein endothelial cells in vitro. Next, to evaluate the effects of rhLK8 on angiogenesis and metastasis, an experimental model of liver metastasis was established by injecting a human colorectal cancer cell line, LS174T, into the spleens of BALB/c nude mice. The systemic administration of rhLK8 significantly suppressed liver metastasis from human colorectal cancer cells and improved host survival compared with controls. The combination of rhLK8 and 5-fluorouracil substantially increased these survival benefits compared with either therapy alone. Histological observation showed significant induction of apoptosis among tumor-associated endothelial cells in liver metastases from rhLK8-treated mice compared with control mice. Collectively, these results suggest that the combination of rhLK8 with conventional chemotherapy may be a promising approach for the treatment of patients with life-threatening colorectal cancer liver metastases.

Apolipoprotein (a) impairs endothelial progenitor cell-mediated angiogenesis

Improvement of blood flow and promotion of angiogenesis are important therapeutic measures for the treatment of ischemic peripheral vascular diseases. Since apolipoprotein (a) (apo (a)) is a glycoprotein with repetitive kringle domains exhibiting 75% to 98% structural homology with plasminogen (Plg), apo (a) may also have a negative effect on endothelial progenitor cell (EPC)-induced angiogenesis through Plg-like inhibitory effects on EPC proliferation, adhesion, migration, and angiogenesis. To evaluate the effect of apo (a) on EPCs-induced angiogenesis, EPCs were isolated from the bone marrow of apo (a) transgenic mice, wild-type litter mates, and normal mice. These cells were cultured without or with apo (a) before transplantation. Hindlimb ischemia models were surgically induced in mice, which then received an intravenous injection of 3×10(5) EPCs. At 3, 7, and 14 days post EPC transplantation, the adhesion, migration abilities, and capillary density in calf muscles were assessed. Results indicate that apo (a) significantly reduced the adhesion and migration abilities of EPCs. Furthermore, the tubule-like formation of EPCs on Matrigel gels was damaged. In vivo experiments showed the homing of EPCs to ischemic peripheral vascular, and the number of capillary vessels decreased significantly in apo(a) transgenic mice. This study demonstrated that apo (a) could attenuate the adhesion, migration, and homing abilities of EPCs and could impair the angiogenesis ability of EPCs.

Apolipoprotein(a) inhibits in vitro tube formation in endothelial cells: identification of roles for Kringle V and the plasminogen activation system

Elevated plasma concentrations of lipoprotein(a) are associated with increased risk for atherothrombotic diseases. Apolipoprotein(a), the unique glycoprotein component of lipoprotein(a), is characterized by the presence of multiple kringle domains, and shares a high degree of sequence homology with the serine protease zymogen plasminogen. It has been shown that angiostatin, a proteolytic fragment of plasminogen containing kringles 1–4, can effectively inhibit angiogenesis. Moreover, proteolytic fragments of plasminogen containing kringle 5 are even more potent inhibitors of angiogenesis than angiostatin. Despite its strong similarity with plasminogen, the role of apolipoprotein(a) in angiogenesis remains controversial, with both pro- and anti-angiogenic effects reported. In the current study, we evaluated the ability of apolipoprotein(a) to inhibit VEGF- and angiopoietin-induced tube formation in human umbilical cord endothelial cells. A 17 kringle-containing form of recombinant apo(a) (17K), corresponding to a well-characterized, physiologically-relevant form of the molecule, effectively inhibited tube formation induced by either VEGF or angiopoietin-1. Using additional recombinant apolipoprotein(a) (r-apo(a)) variants, we demonstrated that this effect was dependent on the presence of an intact lysine-binding site in kringle V domain of apo(a), but not on the presence of the functional lysine-binding site in apo(a) kringle IV type 10; sequences within in the amino-terminal half of the molecule were also not required for the inhibitory effects of apo(a). We also showed that the apo(a)-mediated inhibition tube formation could be reversed, in part by the addition of plasmin or urokinase plasminogen activator, or by removal of plasminogen from the system. Further, we demonstrated that apo(a) treated with glycosidases to remove sialic acid was significantly less effective in inhibiting tube formation. This is the first report of a functional role for the glycosylation of apo(a) although the mechanisms underlying this observation remain to be determined in the context of angiogenesis.

A novel peptide derived from human apolipoprotein E is an inhibitor of tumor growth and ocular angiogenesis

Angiogenesis is a hallmark of tumor development and metastasis and now a validated target for cancer treatment. We previously reported that a novel dimer peptide (apoEdp) derived from the receptor binding region of human apolipoprotein E (apoE) inhibits virus-induced angiogenesis. However, its role in tumor anti-angiogenesis is unknown. This study demonstrates that apoEdp has anti-angiogenic property in vivo through reduction of tumor growth in a mouse model and ocular angiogenesis in a rabbit eye model. Our in vitro studies show that apoEdp inhibits human umbilical vein endothelial cell proliferation, migration, invasion and capillary tube formation. We document that apoEdp inhibits vascular endothelial growth factor-induced Flk-1 activation as well as downstream signaling pathways that involve c-Src, Akt, eNOS, FAK, and ERK1/2. These in vitro data suggest potential sites of the apoE dipeptide inhibition that could occur in vivo.

This is the first evidence that a synthetic dimer peptide mimicking human apoE has anti-angiogenesis functions and could be an anti-tumor drug candidate.

Thrombospondin-1 as a Paradigm for the Development of Antiangiogenic Agents Endowed with Multiple Mechanisms of Action

Uncontrolled neovascularization occurs in several angiogenesis-dependent diseases, including cancer. Neovascularization is tightly controlled by the balance between angiogenic growth factors and antiangiogenic agents. The various natural angiogenesis inhibitors identified so far affect neovascularization by different mechanisms of action. Thrombospondin-1 (TSP-1) is a matricellular modular glycoprotein that acts as a powerful endogenous inhibitor of angiogenesis. It acts both indirectly, by sequestering angiogenic growth factors and effectors in the extracellular environment, and directly, by inducing an antiangiogenic program in endothelial cells following engagement of specific receptors including CD36, CD47, integrins and proteoglycans (all involved in angiogenesis ). In view of its central, multifaceted role in angiogenesis, TSP-1 has served as a source of antiangiogenic tools, including TSP-1 fragments, synthetic peptides and peptidomimetics, gene therapy strategies, and agents that up-regulate TSP-1 expression. This review discusses TSP-1-based inhibitors of angiogenesis, their mechanisms of action and therapeutic potential, drawing our experience with angiogenic growth factor-interacting TSP-1 peptides, and the possibility of exploiting them to design novel antiangiogenic agents.

Lipoprotein(a) level and its association with tumor stage in male patients with primary lung cancer

BACKGROUND

Recently, attention has been focused on the effect of lipoprotein(a) [Lp(a)] on tumors because of its possible role in development of tumor angiogenesis. The aim of this study was to investigate Lp(a) serum levels in patients with lung cancer and its association with the stages of disease.

METHODS

Fasting venous blood samples were collected from 418 untreated male patients with stages I-IV lung carcinoma and were analyzed for Lp(a). The results were compared with the data from 65 healthy male controls.

RESULTS

Lp(a) levels were elevated (median 157 mg/L, range 16-1497 mg/L) in patients with lung carcinoma compared to control subjects (median 110 mg/L, range 35-706 mg/L) (p=0.004). Subgroup analysis showed that patients with stages II-IV disease had significantly higher Lp(a) concentrations than did healthy controls (p-0.05). There was an independently positive correlation between tumor stage and Lp(a) levels among patients with stages I-III (r=0.162, p=0.006). However, there was a decrease in Lp(a) in stage IV compared to stage III patients (p=0.03).

CONCLUSIONS

There is a significant association between Lp(a) and the presence and stage of lung cancer. Additional investigations with a larger number of lung cancer patients are needed to confirm these findings.

Apolipoprotein(a) stimulates vascular endothelial cell growth and migration and signals through integrin alphaVbeta3

Elevated plasma concentrations of Lp(a) [lipoprotein(a)] are an emerging risk factor for atherothrombotic disease. Apo(a) [apolipoprotein(a)], the unique glycoprotein component of Lp(a), contains tandem repeats of a plasminogen kringle (K) IV-like domain. In the light of recent studies suggesting that apo(a)/Lp(a) affects endothelial function, we evaluated the effects of apo(a)/Lp(a) on growth and migration of cultured HUVECs (human umbilical-vein endothelial cells). Two full-length r-apo(a) [recombinant apo(a)] variants (12K and 17K), as well as Lp(a), were able to stimulate HUVEC growth and migration to a comparable extent; 17K r-apo(a) also decreased the levels of total and active transforming growth factor-beta secreted by these cells. Using additional r-apo(a) variants corresponding to deletions and/or site-directed mutants of various kringle domains in the molecule, we were able to determine that the observed effects of full-length r-apo(a) on HUVECs were dependent on the presence of a functional lysine-binding site(s) in the apo(a) molecule. With respect to signalling events elicited by apo(a) in HUVECs, we found that 17K treatment of the cells increased the phosphorylation level of FAK (focal adhesion kinase) and MAPKs (mitogen-activated protein kinases), including ERK (extracellular-signal-regulated kinase), p38 and JNK (c-Jun N-terminal kinase). In addition, we showed that LM609, the function-blocking antibody to integrin alphaVbeta3, abrogated the effects of 17K r-apo(a) and Lp(a) on HUVECs. Taken together, the results of the present study suggest that the apo(a) component of Lp(a) signals through integrin alphaVbeta3 to activate endothelial cells.

Lipoprotein[a] and cancer: Anti-neoplastic effect besides its cardiovascular potency

While the death rate from cancer has substantially decreased over the past decade, the search for effective and tolerable therapies is a great challenge as yet. The evidence that malignant cells cannot grow to a clinically detectable tumor mass and spread in the absence of an adequate vascular support, has opened a new area of research towards the selective inhibition or even destruction of tumor vessels. Angiostatin and angiostatin-related proteins are a family of specific angiogenesis inhibitors produced by tumors from a family of naturally occurring proteins, which also includes plasminogen and lipoprotein[a]. The anti-angiogenic activity of these proteins resides in cryptic and highly-repetitive molecular domains hidden within the protein moiety, called kringles. Lipoprotein[a] is an intriguing molecule consisting of a low-density lipoprotein core in addition to the covalently bound apolipoprotein[a]. Apolipoprotein[a] is characterized by an inactive protease domain, a single copy of the plasminogen kringle V and multiple repeats of domains homologous to the plasminogen kringle IV. Reliable studies on animal models indicate that the proteolytic break-down products of apolipoprotein[a] would posses anti-angiogenic and anti-tumoral properties both in vitro and in vivo, a premise to develop novel therapeutic modalities which may efficiently suppress tumor growth and metastasis. This review is focused on the biochemical structure, metabolism and the anti-angiogenic activity of this unique and elusive kringle-containing lipoprotein.

Adeno-associated virus-mediated expression of apolipoprotein (a) kringles suppresses hepatocellular carcinoma growth in mice

Hepatocellular carcinoma (HCC) constitutes more than 90% of all primary liver cancers. HCC is a hypervascular tumor that develops from dedifferentiation of small avascular HCC and is therefore a good target for anti-angiogenic gene therapy. Recent studies have identified apolipoprotein(a) [apo(a)] kringles LK68 and LK8 (LKs) as having a potential antiangiogenic and anti-tumor activity, and the current study evaluates the therapeutic potential of gene therapy with recombinant adeno-associated virus carrying genes encoding LKs (rAAV-LK) in the treatment of hypervascular HCC. We generated rAAV-LK to obtain persistent transgene expression in vivo, which is essential for anti-angiogenic therapy. The rAAV-produced LKs substantially inhibited proliferation and migration of human umbilical vein endothelial cells (HUVECs) in vitro, validating their anti-angiogenic potential. Intramuscular administration of rAAV-LK gave 60% to 84% suppression (P < .05) of tumor growth in mice bearing subcutaneously transplanted HCC derived from Huh-7 and Hep3B cells, respectively. Histological and immunohistochemical analyses of HCC tumor sections showed that a single administration of rAAV-LK gave rise to persistent expression of LKs that inhibited tumor angiogenesis and triggered tumor apoptosis, and, thus, significantly suppressed tumor growth. The administration of rAAV-LK provided a significant survival benefit (P < .05), and 3 of 10 rAAV-LK-treated mice were still alive without visible tumors and without clinical symptoms 188 days after treatment. In conclusion, rAAV-LK is a potential candidate for anti-angiogenic gene therapy in the treatment of HCC.

Therapy of hyper-Lp(a)

Lipoprotein (a) [Lp(a)] appears to be one of the most atherogenic lipoproteins. It consists of a low-density lipoprotein (LDL) core in addition to a covalently bound glycoprotein, apolipoprotein (a) [apo(a)]. Apo(a) exists in numerous polymorphic forms. The size polymorphism is mediated by the variable number of kringle-4 Type-II repeats found in apo(a). Plasma Lp(a) levels are determined to more than 90% by genetic factors. Plasma Lp(a) levels in healthy individuals correlate significantly high with apo(a) biosynthesis and not with its catabolism. There are several hormones known to have a strong impact on Lp(a) metabolism. In certain diseases, such as kidney disease, Lp(a) catabolism is impaired leading to up to fivefold elevations. Lp(a) levels rise with age but are otherwise influenced only little by diet and lifestyle. There is no safe and efficient way of treating individuals with elevated plasma Lp(a) concentrations. Most of the lipid-lowering drugs have either no significant influence on Lp(a) or exhibit a variable effect in patients with different forms of primary and secondary hyperlipoproteinemia. There is without doubt a strong need to concentrate on the development of specific medications to selectively target Lp(a) biosynthesis, Lp(a) assembly and Lp(a) catabolism. So far only anabolic steroids were found to drastically reduce Lp(a) plasma levels. This class of substance cannot, of course, be used for treatment of patients with hyper-Lp(a). We recommend that the mechanism of action of these drugs be studied in more detail and that the possibility of synthesizing derivatives which may have a more specific effect on Lp(a) without having any side effects be pursued. Other strategies that may be of use in the development of drugs for treatment of patients with hyper-Lp(a) are discussed in this review.

Expression of human apolipoprotein(a) kringles in colon cancer cells suppresses angiogenesis-dependent tumor growth and peritoneal dissemination

BACKGROUND

Anti-angiogenesis therapy has been regarded as a promising treatment of cancer based on the fact that most tumors and their metastasis are angiogenesis-dependent. Gene therapy can potentially expand the horizons of tumor angiogenesis therapy by virtue of its ability to produce high concentrations of therapeutic agents in a local area for a sustained period. The present study was performed to evaluate the therapeutic potential of gene therapy for the treatment of cancer and metastasis.

METHODS

The murine colon carcinoma cell line CT26 was manipulated ex vivo to express an anti-angiogenic molecule, LK68, consisting of human apolipoprotein(a) kringle domains, KIV(9)-KIV(10)-KV, using retrovirus-mediated gene transfer. Its effects on colon tumor growth and metastasis were evaluated in experimental animal models established by injecting LK68-expressing and control CT26 cells subcutaneously or into the peritoneal cavity of BALB/c mice, respectively.

RESULTS

Expression of LK68 significantly suppressed colon tumor growth in mice, but did not influence the growth of tumor cells in vitro. Immunohistochemical analysis of tumor tissues revealed a significant reduction in microvessel density in LK68-expressing tumors. Thus, the suppression of tumor growth appears to result mainly from inhibition of tumor angiogenesis. This decrease in vessel density is correlated with a notable increase in tumor cell apoptosis in vivo, but has no influence on proliferation. Moreover, expression of LK68 prevents peritoneal dissemination, and consequently improves overall host survival.

CONCLUSIONS

These results collectively indicate that a gene therapy strategy using LK68 cDNA is useful for the treatment for both colon tumor growth and peritoneal dissemination.

Suppression of Colorectal Cancer Liver Metastasis and Extension of Survival by Expression of Apolipoprotein(a) Kringles

The formation of hepatic metastases in colorectal cancer is the main cause of patient death. Current therapies directed at hepatic metastasis of colorectal cancer have had minimal impact on outcome. Therefore, alternative treatment strategies for liver metastasis require development. The present study was performed to evaluate the application of cDNA of LK68 encoding apolipoprotein(a) kringles IV-9, IV-10, and V as possible candidates for gene therapy treatment of this life-threatening disease. The murine colorectal cancer cell line CT26 was transduced ex vivo with LK68 cDNA via retroviral gene transfer, and an experimental model of hepatic metastasis was established by injecting LK68-expressing and control cells into the spleens of BALB/c mice. Expression of LK68 did not affect the growth characteristics and viability of transduced CT26 cells in vitro. LK68 produced from CT26 cells substantially inhibited the migration of endothelial cells in vitro. In vivo, substantial suppression of liver metastasis and prolonged survival were observed in mice bearing LK68-expressing CT26 cells, compared with controls. LK68-expressing liver metastases were restricted to smaller sizes and displayed decreased microvessel density and increased tumor cell apoptosis. Our data collectively indicate that LK68 suppresses angiogenesis-dependent progression of prevascular micrometastases to macroscopic tumors and their growth, which are clinically accessible and biologically relevant therapeutic targets. We propose that antiangiogenic gene therapy with LK68 is a promising strategy for the treatment of colorectal cancer liver metastasis.

A Truncated Kringle Domain of Human Apolipoprotein(a) Inhibits the Activation of Extracellular Signal-regulated Kinase 1 and 2 through a Tyrosine Phosphatase-dependent Pathway

Most proangiogenic factors exert their biological effects primarily by activating extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3-K)/Akt signaling pathways. These pathways appear to play a critical role in endothelial cell migration, because selective inhibition of either ERK or PI3-K/Akt signaling almost completely prevented endothelial cell migration. Recently, we demonstrated that a truncated kringle domain of human apolipoprotein(a), termed rhLK68, inhibits endothelial cell migration in vitro. However, its mechanism of action was not well defined. In this study, we determined the effects of rhLK68 on ERK1/2 and PI3-K/Akt signaling pathways to explore the molecular mechanism of rhLK68-mediated inhibition of endothelial cell migration. Treatment with rhLK68 inhibited ERK1/2 phosphorylation but did not influence Akt activation. Interestingly, an inhibitor of protein-tyrosine phosphatase, sodium orthovanadate, dose-dependently reversed both rhLK68-induced dephosphorylation of ERK1/2 and decreased migration of endothelial cells, whereas rhLK68 showed no significant effects on MEKs phosphorylation. In conclusion, these results indicate that inhibition of endothelial cell migration by rhLK68 may be achieved by interfering with ERK1/2 activation via a protein-tyrosine phosphatase-dependent pathway.

Human apolipoprotein(a) kringle V inhibits angiogenesis in vitro and in vivo by interfering with the activation of focal adhesion kinases

Apolipoprotein(a) [apo(a)] contains the largest numbers of kringle domains identified to date. Of these, apo(a) kringle V shows significant sequence homology with plasminogen kringle 5, which is reported to be a potent angiogenesis inhibitor. To determine the effects of apo(a) kringle V on angiogenesis, it was expressed as a soluble protein (termed rhLK8) in Pichia pastoris and its in vitro and in vivo anti-angiogenic properties were examined. rhLK8 inhibited the migration of human umbilical vein endothelial cells in vitro in a dose-dependent manner. This function was associated with the down-regulation of the activation of focal adhesion kinase and the inhibition of the consequent formation of actin stress fibers/focal adhesions. rhLK8 also inhibited new capillary formation in vivo, as assessed by the chick chorioallantoic membrane assay and the Matrigel plug assay. These results indicate that rhLK8 may be an effective angiogenesis inhibitor both in vitro and in vivo.

Inhibition of angiogenesis and angiogenesis-dependent tumor growth by the cryptic kringle fragments of human apolipoprotein(a)

Apolipoprotein(a) (apo(a)) contains tandemly repeated kringle domains that are closely related to plasminogen kringle 4, followed by a single kringle 5-like domain and an inactive protease-like domain. Recently, the anti-angiogenic activities of apo(a) have been demonstrated both in vitro and in vivo. However, its effects on tumor angiogenesis and the underlying mechanisms involved have not been fully elucidated. To evaluate the anti-angiogenic and anti-tumor activities of the apo(a) kringle domains and to elucidate their mechanism of action, we expressed the last three kringle domains of apo(a), KIV-9, KIV-10, and KV, in Escherichia coli. The resultant recombinant protein, termed rhLK68, exhibited a dose-dependent inhibition of basic fibroblast growth factor-stimulated human umbilical vein endothelial cell proliferation and migration in vitro and inhibited the neovascularization in chick chorioallantoic membranes in vivo. The ability of rhLK68 to abrogate the activation of extracellular signal-regulated kinases appears to be responsible for rhLK68-mediated anti-angiogenesis. Furthermore, systemic administration of rhLK68 suppressed human lung (A549) and colon (HCT-15) tumor growth in nude mice. Immunohistochemical examination and in situ hybridization analysis of the tumors showed a significant decrease in the number of blood vessels and the reduced expression of vascular endothelial growth factor, basic fibroblast growth factor, and angiogenin, indicating that suppression of angiogenesis may have played a significant role in the inhibition of tumor growth. Collectively, these results suggest that a truncated apo(a), rhLK68, is a potent anti-angiogenic and anti-tumor molecule.

Atorvastatin lowers lipoprotein(a) but not apolipoprotein(a) fragment levels in hypercholesterolemic subjects at high cardiovascular risk

The effect of statins on Lp(a) levels is controversial; furthermore, the potential action of statins on apo(a) fragmentation is indeterminate. We therefore determined the circulating levels of Lp(a) and of apo(a) fragments in hypercholesterolemic patients before and after treatment (6 weeks) with Atorvastatin 10 mg/day (A10) or Simvastatin 20 mg/day (S20). In a double blind study, hypercholesterolemic patients (n=391) at high cardiovascular risk (LDL-C>=4.13 mmol/l; TG<2.24 mmol/l; 34% with documented CHD; 45% hypertensive; and 29% current smokers) were assigned to treatment with A10 (n=199) or S20 (n=192). Plasma Lp(a) and apo(a) fragment levels (n=206) were measured prior to and after treatment. At baseline, A10 and S20 groups did not differ in plasma levels of lipids, Lp(a) (A10: 0.45+/-0.48 mg/ml, S20: 0.46+/-0.5), and apo(a) fragments (A10: 3.88+/-5.22 microg/ml; S20: 3.25+/-3), and equally in apo(a) isoform size (A10: 26+/-5 kr, S20: 25.5+/-5.3). After treatment, both statins significantly reduced Lp(a) levels (A10: 0.42+/-0.47 mg/ml, 6% variation, P<0.001; S20: 0.45+/-0.53 mg/ml, 0.02% variation, P=0.046). A10 and S20 did not significantly differ in their efficacy to lower Lp(a) levels. In a multivariate logistic regression analysis, the reduction of Lp(a) levels was independently associated with Lp(a) baseline concentration, but not to other variables, including LDL-C reduction. Plasma levels of apo(a) fragments were not modified by either statin. In conclusion, both A10 and S20 significantly lowered Lp(a), although this effect was of greater magnitude in atorvastatin-treated patients.

Lipoprotein(a): still an enigma?

PURPOSE OF REVIEW

Lipoprotein(a) belongs to the class of the most atherogenic lipoproteins. Despite intensive research - in the last year more than 80 papers have been published on this topic - information is still lacking on the physiological function of lipoprotein(a) and the site of its catabolism. Important advances have been made in the knowledge of these points, which may have some therapeutic implications.

RECENT FINDINGS

The association of high lipoprotein(a) values with an increase in risk for coronary events has been documented in further prospective studies. This increased risk may relate to recent findings that apolipoprotein(a) is produced in situ within the vessel wall. In addition, lipoprotein(a) binds and inactivates the tissue factor pathway inhibitor and induces plasminogen activator inhibitor type 2 expression in monocytes. A new antisense oligonucleotide strategy has been proposed which efficiently inhibits apolipoprotein(a) expression in vitro and in vivo. Apolipoprotein(a), however, suppresses angiogenesis and thus may interfere with the infiltration of tumor cells. Finally, the enzymatic activity leading to the formation of apolipoprotein(a) fragments in plasma and their catabolism have been further elucidated.

SUMMARY

We are still far away from understanding the pathways involved in lipoprotein(a) catabolism, and the physiological function of this lipoprotein. Recent findings, however, provide new insight into pathomechanisms in patients with increased lipoprotein(a) related to hemostasis, which may serve as a basis for designing new treatment strategies.

Plasma apolipoprotein(a) co-deposits with fibrin in inflammatory arthritic joints

Extravascular coagulation and diminished fibrinolysis are processes that contribute to the pathology of both inflammatory arthritis and atherosclerosis. We hypothesized that, given its homology with plasminogen, apolipoprotein (apo) (a), the distinctive glycoprotein of the atherogenic lipoprotein (Lp) (a), may be equally implicated in inflammatory arthritis. We detected the presence of apo(a) as part of Lp(a) in human arthritic synovial fluid. The abundance of apo(a) in synovial fluid rose in proportion to plasma apo(a) levels and was higher in inflammatory arthritides than in osteoarthritis. In addition, apo(a) immunoreactive material, but not apo(a) transcripts, was detected in inflammatory arthritic synovial tissues. These data indicated that synovial fluid apo(a) originates from circulating Lp(a) and that diffusion of Lp(a) through synovial tissue is facilitated in inflammatory types of arthritis. In synovial tissues, apo(a) co-localized with fibrin. These observations could be reproduced in a model of antigen-induced arthritis, using transgenic mice expressing human Lp(a). Although in this mouse model the presence of apo(a) did not change the severity of arthritis, the co-localization of apo(a) with fibrin in synovial tissue suggests that, in humans, apo(a) may modulate locally the fibrinolytic activity and may thus contribute to the persistence of intra-articular fibrin in inflammatory arthritis.