Enhanced anti-angiogenic effect of a deletion mutant of plasminogen kringle 5 on neovascularization

Plasminogen kringle 5 suppresses gastric cancer via regulating HIF-1α and GRP78

Inhibition of tumour angiogenesis has an important role in antitumour therapy. However, a recent study indicates that antiangiogenesis therapy may lead to glucose-related protein 78 (GRP78) associated antiapoptotic resistance. The present study aims to elucidate the dual effects of plasminogen kringle 5 (K5) on tumour angiogenesis and apoptosis induction by targeting hypoxia-inducible factor 1α (HIF-1α) and GRP78. Co-immunoprecipitation and western blotting were used for examining the ubiquitination of HIF-1α and analysing angiogenesis and apoptosis-associated proteins. K5 promoted the sumo/ubiquitin-mediated proteasomal degradation of HIF-1α by upregulating von Hippel-Lindau protein under hypoxia, resulting in the reduction of vascular endothelial growth factor and thus suppressing tumour angiogenesis. Furthermore, K5 decreased GRP78 expression via downregulation of phosphorylated extracellular-regulated protein kinase, leading to caspase-7 cleavage and tumour cell apoptosis. Blocking voltage-dependent anion channel abrogated the effects of K5 on both HIF-1α and GRP78. K5 significantly inhibited the growth of gastric carcinoma xenografts by inhibiting both angiogenesis and apoptosis. The dual effects suggest that K5 might be a promising bio-therapeutic agent in the treatment of gastric cancer, particularly in patients who exhibit the induction of GRP78.

Nestin-mediated cytoskeletal remodeling in endothelial cells: novel mechanistic insight into VEGF-induced cell migration in angiogenesis

Nestin is highly expressed in poorly differentiated and newly formed proliferating endothelial cells (ECs); however, the role of this protein in angiogenesis remains unknown. Additionally, the cytoskeleton and associated cytoskeleton-binding proteins mediate the migration of vascular ECs. Therefore, the aim of the present study was to determine whether VEGF regulates the cytoskeleton, as well as other associated proteins, to promote the migration of vascular ECs. The coexpression of nestin and CD31 during angiogenesis in alkali-burned rat corneas was examined via immunohistochemical analysis. Western blot analyses revealed that the exposure of human umbilical vein endothelial cells (HUVECs) to hypoxia promoted nestin expression in vitro. Additionally, nestin silencing via siRNA significantly inhibited many of the process associated with VEGF-induced angiogenesis, including tube formation and the migration and proliferation of HUVECs. Moreover, FITC-phalloidin labeling revealed that F-actin filaments were successfully organized into microfilaments in VEGF-treated cells, suggesting a network rearrangement accomplished via F-actin that contrasted with the uniform and loose actin filament network observed in the siRNA-nestin cells. The results of the present study highlight the key role played by nestin in activated HUVECs during angiogenesis. The inhibition of the ERK pathway suppressed the nestin expression induced by VEGF in the HUVECs. Therefore, our study provides the first evidence that nestin-mediated cytoskeleton remodeling in ECs occurs via filopodia formation along the cell edge, facilitating both filopodia localization and cell polarization and ultimately promoting HUVEC migration via VEGF induction, which may be associated with ERK pathway activation.

Plasminogen kringle 5 induces endothelial cell apoptosis by triggering a voltage-dependent anion channel 1 (VDAC1) positive feedback loop

Human plasminogen kringle 5 (K5) is known to display its potent anti-angiogenesis effect through inducing endothelial cell (EC) apoptosis, and the voltage-dependent anion channel 1 (VDAC1) has been identified as a receptor of K5. However, the exact role and underlying mechanisms of VDAC1 in K5-induced EC apoptosis remain elusive. In the current study, we showed that K5 increased the protein level of VDAC1, which initiated the mitochondrial apoptosis pathway of ECs. Our findings also showed that K5 inhibited the ubiquitin-dependent degradation of VDAC1 by promoting the phosphorylation of VDAC1, possibly at Ser-12 and Thr-107. The phosphorylated VDAC1 was attenuated by the AKT agonist, glycogen synthase kinase (GSK) 3β inhibitor, and siRNA, suggesting that K5 increased VDAC1 phosphorylation via the AKT-GSK3β pathway. Furthermore, K5 promoted cell surface translocation of VDAC1, and binding between K5 and VDAC1 was observed on the plasma membrane. HKI protein blocked the impact of K5 on the AKT-GSK3β pathway by competitively inhibiting the interaction of K5 and cell surface VDAC1. Moreover, K5-induced EC apoptosis was suppressed by VDAC1 antibody. These data show for the first time that K5-induced EC apoptosis is mediated by the positive feedback loop of "VDAC1-AKT-GSK3β-VDAC1," which may provide new perspectives on the mechanisms of K5-induced apoptosis.

Acidic/neutral amino acid residues substitution in NH2 terminal of plasminogen kringle 5 exerts enhanced effects on corneal neovascularization

PURPOSE

Recent results showed that plasminogen kringle 5 (K5) has improved inhibitory effect on human umbilical vein endothelial cells (HUVECs) viability when 5 acidic amino acids in NH2 terminal outside kringle domain were replaced by 5 serine residues (mutant K5, mK5). This study was designed to identify the enhanced antiangiogenic activity of mK5 in corneal neovascularization (CNV).

METHODS

Alkali burn-induced CNV was induced and treated with K5 and mK5 for 11 days. CNV and inflammation were evaluated by the CNV area and the inflammatory index, respectively. At the end of treatment, the corneas were removed for terminal deoxynucleotidyl transferase dUTP nick end labeling detection and immunohistochemistry. The effects of mK5 and K5 on HUVECs apoptosis were tested by MTT, BrdU, and flow cytometry. The expression levels of pigment epithelium-derived factor (PEDF) and vascular endothelial growth factor (VEGF) were detected by Western blot.

RESULTS

In a rat model of CNV induced by alkali, topical treatment with mK5 significantly decreased the neovascular area and inflammation compared with the wild-type K5-treated group. Meanwhile, mK5 and K5 specifically inhibited the HUVECs proliferation and induced vascular endothelial cell apoptosis in vitro and in vivo, and mK5 exerted higher apoptosis induction. Toward the mechanism of action, both mK5 and K5 significantly upregulated the expression of PEDF and mildly downregulated the expression of VEGF. The elevation of PEDF/VEGF ratio induced by mK5 was higher than that by K5.

CONCLUSIONS

These findings suggest that mK5 has more effective therapeutic potential in CNV than wild-type K5.

Dual inhibition of plasminogen kringle 5 on angiogenesis and chemotaxis suppresses tumor metastasis by targeting HIF-1α pathway

We had demonstrated that plasminogen kringle 5 (K5), a potent angiogenic inhibitor, inhibited retinal neovascularization and hepatocellular carcinoma growth by anti-angiogenesis. The current study investigated the effects and the underlying mechanisms of K5 on both tumor growth and spontaneous pulmonary metastasis in Lewis lung carcinoma (LLC) implanted mouse model. Similarly, K5 could decrease expression of VEGF in LLC cells and grafted tissues and suppress tumor angiogenesis and growth. K5 had no direct effect on proliferation and apoptosis of LLC. However, K5 could significantly inhibit SDF-1α-induced chemotaxis movement of LLC cells and resulted in a great reduction of surface metastatic nodules and micrometastases in the lungs of LLC tumor-bearing mice. K5 also decreased expression of chemokine (C-X-C motif) receptor 4 (CXCR4) in LLC cells and grafted tissues. Furthermore, K5 down-regulated SDF-1α expression in metastatic lung tissues of LLC-bearing mice. Therefore, K5 may suppress tumor pulmonary metastasis through inhibiting SDF-1α-CXCR4 chemotaxis movement and down-regulation of VEGF. Moreover, the role of hypoxia inducible factor-1α (HIF-1α), a crucial transcriptional factor for both VEGF and CXCR4 expression, was evaluated. The siRNA of HIF-1α attenuated expression of VEGF and CXCR4 and inhibited LLC migration. K5 decreased HIF-1α protein level and impaired nuclear HIF-1α accumulation. These results showed for the first time that K5 inhibits LLC growth and metastasis via the dual effects of anti-angiogenesis and suppression of tumor cell motility by targeting the pivotal molecule, HIF-1α.

Plasminogen K5 activates mitochondrial apoptosis pathway in endothelial cells by regulating Bak and Bcl-x(L) subcellular distribution

Plasminogen Kringle 5(K5) is a proteolytic fragment of plasminogen, which displays potent anti-angiogenic activities. K5 has been shown to induce apoptosis in proliferating endothelial cells; however the exact mechanism has not been well explored. The present study was designed to elucidate the possible molecular mechanism of K5-induced endothelial cell apoptosis. Our results showed that K5 inhibited basic fibroblast growth factors activated in human umbilical vein endothelial cells (HUVECs), indicating proliferation in a dose-dependent manner and induced endothelial cell death via apoptosis. K5 exposure activated caspase 7, 8 and 9. These results suggested that both the intrinsic mitochondrial apoptosis pathway and extrinsic pathway might be involved in K5-induced apoptosis. K5 reduced mitochondrial membrane potential (MMP) of HUVECs, demonstrating mitochondrial depolarization in HUVECs. K5 increased the ratio of Bak to Bcl-x(L) on mitochondria, decreased the ratio in cytosol, and had no effect on the total amounts of these proteins. K5 also did not effect on Bax/Bcl-2 distribution. K5 increased the ratio of Bak to Bcl-x(L) on mitochondrial that resulted in mitochondrial depolarization, cytochrome c release and consequently the cleavage of caspase 9. These results suggested that K5 induces endothelial cell apoptosis at least in part via activating mitochondrial apoptosis pathway. The regulation of K5 on Bak and Bcl-x(L) distribution may play an important role in endothelial cell apoptosis. These results provide further insight into the anti-angiogenesis roles of K5 in angiogenesis-related ocular diseases and solid tumors.

High efficacy and minimal peptide required for the anti-angiogenic and anti-hepatocarcinoma activities of plasminogen K5

Kringle 5(K5) is the fifth kringle domain of human plasminogen and its anti-angiogenic activity is more potent than angiostatin that includes the first four kringle fragment of plasminogen. Our recent study demonstrated that K5 suppressed hepatocarcinoma growth by anti-angiogenesis. To find high efficacy and minimal peptide sequence required for the anti-angiogenic and anti-tumour activities of K5, two deletion mutants of K5 were generated. The amino acid residues outside kringle domain of intact K5 (Pro452-Ala542) were deleted to form K5mut1(Cys462-Cys541). The residue Cys462 was deleted again to form K5mut2(Met463-Cys541). K5mut1 specifically inhibited proliferation, migration and induced apoptosis of endothelial cells, with an apparent two-fold enhanced activity than K5. Intraperitoneal injection of K5mut1 resulted in more potent tumour growth inhibition and microvessel density reduction than K5 both in HepA-grafted and Bel7402-xenografted hepatocarcinoma mouse models. These results suggested that K5mut1 has more potent anti-angiogenic activity than intact K5. K5mut2, which lacks only the amino terminal cysteine of K5mut1, completely lost the activity, suggesting that the kringle domain is essential for the activity of K5. The activity was enhanced to K5mut1 level when five acidic amino acids of K5 in NH₂ terminal outside kringle domain were replaced by five serine residues (K5mut3). The shielding effect of acidic amino acids may explain why K5mut1 has higher activity. K5, K5mut1 and K5mut3 held characteristic β-sheet spectrum while K5mut2 adopted random coil structure. These results suggest that K5mut1 with high efficacy is the minimal active peptide sequence of K5 and may have therapeutic potential in liver cancer.