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) 3ß inhibitor, and siRNA, suggesting that K5 increased VDAC1 phosphorylation via the AKT-GSK3ß 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-GSK3ß 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-GSK3ß-VDAC1," which may provide new perspectives on the mechanisms of K5-induced apoptosis.

Pigment epithelial-derived factor (PEDF)-triggered lung cancer cell apoptosis relies on p53 protein-driven Fas ligand (Fas-L) up-regulation and Fas protein cell surface translocation.

Pigment epithelium-derived factor (PEDF), a potent antiangiogenesis agent, has recently attracted attention for targeting tumor cells in several types of tumors. However, less is known about the apoptosis-inducing effect of PEDF on human lung cancer cells and the underlying molecular events. Here we report that PEDF has a growth-suppressive and proapoptotic effect on lung cancer xenografts. Accordingly, in vitro, PEDF apparently induced apoptosis in A549 and Calu-3 cells, predominantly via the Fas-L/Fas death signaling pathway. Interestingly, A549 and Calu-3 cells are insensitive to the Fas-L/Fas apoptosis pathway because of the low level of cell surface Fas. Our results revealed that, in addition to the enhancement of Fas-L expression, PEDF increased the sensitivity of A549 and Calu-3 cells to Fas-L-mediated apoptosis by triggering the translocation of Fas protein to the plasma membrane in a p53- and FAP-1-dependent manner. Similarly, the up-regulation of Fas-L by PEDF was also mediated by p53. Furthermore, peroxisome proliferator-activated receptor γ was determined to be the upstream regulator of p53. Together, these findings uncover a novel mechanism of tumor cell apoptosis induced by PEDF and provide a potential therapeutic strategy for tumors that are insensitive to Fas-L/Fas-dependent apoptosis because of a low level of cell surface Fas.

Activation of AMPK/MnSOD signaling mediates anti-apoptotic effect of hepatitis B virus in hepatoma cells.

AIM: To investigate the anti-apoptotic capability of the hepatitis B virus (HBV) in the HepG2 hepatoma cell line and the underlying mechanisms. METHODS: Cell viability and apoptosis were measured by MTT assay and flow cytometry, respectively. Targeted knockdown of manganese superoxide dismutase (MnSOD), AMP-activated protein kinase (AMPK) and hepatitis B virus X protein (HBx) genes as well as AMPK agonist AICAR and antagonist compound C were employed to determine the correlations of expression of these genes. RESULTS: HBV markedly protected the hepatoma cells from growth suppression and cell death in the condition of serum deprivation. A decrease of superoxide anion production accompanied with an increase of MnSOD expression and activity was found in HepG2.215 cells. Moreover, AMPK activation contributed to the up-regulation of MnSOD. HBx protein was identified to induce the expression of AMPK and MnSOD. CONCLUSION: Our results suggest that HBV suppresses mitochondrial superoxide level and exerts an anti-apoptotic effect by activating AMPK/MnSOD signaling pathway, which may provide a novel pharmacological strategy to prevent HCC.

Pigment epithelium-derived factor gene loaded in cRGD-PEG-PEI suppresses colorectal cancer growth by targeting endothelial cells.

Pigment epithelium-derived factor (PEDF) recombinant protein has been investigated in many kinds of solid tumors due to its potent antiangiogenic activity. However, the complexity of protein purification, instability of recombinant protein and requirement of repeated injections are obstacles for the recombinant PEDF therapy for solid tumors. We successfully synthesized polyethyleneglycol-polyetherimide (PEG-PEI) and cRGD-PEG-PEI which was coupled with a cyclic RGD peptide, a special ligand for integrin αvß3 receptor, as the vehicle for PEDF gene therapy in this study. In vitro, the competitive binding assay showed that cRGD contributed to the enhanced gene transfection efficiency of PEG-PEI in human umbilical vein endothelial cells (HUVECs). PEDF gene delivered by cRGD-PEG-PEI apparently suppressed growth of tumor with a 67.4% reduction and decreased microvessel density in nude mice bearing SW620 human colorectal xenografts. Accordingly, SW620 tumors from cRGD-PEG-PEI/PEDF-pcDNA3.1 (+)-treated mice expressed more PEDF than that of the control groups. Our study demonstrated that cRGD-PEG-PEI transported the PEDF gene into endothelia cells more efficiently than PEG-PEI, resulting in more effective inhibitory effects on tumor growth by anti-angiogenesis. Therefore, for the first time, we have explored an effective non-viral vehicle for PEDF gene therapy by targeting endothelial cells.