We have recently shown that vascular endothelial growth factor-A (VEGFA), a major regulator of tumor vascularization, is essential for the organization of tumor cells into capillary-like structure (CLS), which is a hallmark of tumor vasculogenic mimicry (VM). Herein we further dissect the involvement of VEGFA and its downstream transducers, VEGF receptor 1 (VEGFR1), VEGFR2, and protein kinase C (PKC) in melanoma VM. The knockdown of VEGFR1 in three melanoma cell lines completely disrupts Matrigel-induced CLS formation, whereas inhibition of VEGFR2 kinase with a specific inhibitor, protein tyrosine kinase inhibitor II (PTKi-II), does not affect the process, indicating that VEGFR2 signaling is not involved in VEGFA-mediated melanoma VM. Furthermore, among tested PKC isoforms, only PKCα and δ are expressed in the melanoma cells during CLS formation. Pretreatment with selective PKCα and δ inhibitors blocked CLS formation. However, inhibition of PKCα, but not PKCδ, completely destroyed the previously formed CLS. Moreover, knockdown of PKCα, but not PKCδ, using small interfering RNAs abrogated CLS formation, suggesting that PKCα is the major contributory factor in melanoma VM. In-vivo experiments indicate that disruption of PKCα signaling significantly reduces the signs of VM in allografted B16/F10 melanoma. These findings may contribute to the development of new therapeutic agents that target melanoma VM.

Metalloproteinases are membrane-bound proteins that play a role in the cellular responses to antiglioma therapy. Previously, it has been shown that treatment of glioma cells with temozolomide (TMZ) and radiation (XRT) induces the expression of metalloproteinase 14 (MMP14). To investigate the role of MMP14 in gliomagenesis, we used several chemical inhibitors which affect MMP14 expression. Of all the inhibitors tested, we found that Marimastat not only inhibits the expression of MMP14 in U87 and U251 glioma cells, but also induces cell cycle arrest. To determine the relationship between MMP14 inhibition and alteration of the cell cycle, we used an RNAi technique. Genetic knockdown of MMP14 in U87 and U251 glioma cells induced G2/M arrest and decreased proliferation. Mechanistically, we show that TMZ and XRT regulated expression of MMP14 in clinical samples and in vitro models through downregulation of microRNA374. In vivo genetic knockdown of MMP14 significantly decreased tumor growth of glioma xenografts and improved survival of glioma-bearing mice. Moreover, the combination of MMP14 silencing with TMZ and XRT significantly improved the survival of glioma-bearing mice compared to a single modality treatment group. Therefore, we show that the inhibition of MMP14 sensitizes tumor cells to TMZ and XRT and could be used as a future strategy for antiglioma therapy.Glioblastoma remains an incurable form of brain cancer. In this manuscript, we show that inhibition of MMP14 can potentiate the efficacy of current standard of care which includes chemo- and radiotherapy.

Notch signaling plays an important role in tumor angiogenesis. Recent studies suggest that Notch signaling also regulates the progression of primary melanomas toward an aggressive phenotype. The aim of this study was to investigate the involvement of Notch signaling pathway in organization of tumor cells into capillary-like structures (CLS), the phenomenon also known as vasculogenic mimicry (VM). Here, we show that Notch signaling cascade was constitutively active in melanoma cell lines we used. Blocking Notch signaling with the γ-secretase inhibitors, DAPT, dibenzazepine or Jagged1 neutralizing antibody resulted in stabilization of CLS indicating that Notch signaling pathway attenuates melanoma VM. We further studied this phenomenon on melanomas grafted in nude mice. Compared to control, VM channels in DAPT-treated grafted melanoma became larger and more branched. DAPT-treated melanomas also exhibited an up-regulation of MMP-2 and VEGFR1, both known as VM mediators. Moreover, we did not observe necrosis in VM channels areas of DAPT-treated melanomas. These findings indicate that VM regulated by Notch signaling may present a novel target in melanoma therapy.