The Inhibitor of VEGF-Receptors Tyrosine Kinase PTK-787 Blocks the Invasive Potential of Adult T-Cell Leukemia Cells through the Endothelium.

Extravasation of tumor cells through the endothelial barrier is a critical step in cancer metastasis. HTLV-I associated adult T-cell leukemia/lymphoma (ATL) is an aggressive disease characterized by multiple organ invasion. We have shown that ATL-derived cells induce angiogenesis and communicate with endothelial cells through gap junctions. This results in induction of endothelial-derived metalloproteinases, sub-endothelial basement membrane degradation followed by endothelial cell retraction allowing neoplastic lymphocytes extravasation. In this study we have used the specific tyrosine kinase inhibitor of VEGF receptors PTK787, functional blocking antibodies against VEGF and the gap junction communication inhibitor18-a-G to dissect the respective role of VEGF signaling pathway and cell-to-cell communication in endothelial cell retraction and tumor cell extravasation. PTK787 has mild growth inhibitory and cytotoxic effects on both endothelial and tumor cells at a concentration of 50μM and significantly inhibited cell proliferation at 100μM. Hence, 20μM PTK787 which did not exhibit any cytotoxic or antiproliferative effects on both cell types was used in experiments that last 24 hours or longer (in vitro tube formation assay and invasion assay). Endothelial cells monolayers incubated with either ATL derived cells or their cell-free conditioned medium for 48 hours induced in vitro tube formation in Matrigel angiogenesis assay. This was inhibited by both anti-VEGF antibody and 18-a-G and to a greater extent and in a dose dependent manner by PTK787. Importantly, PTK787, and to a lesser extent anti-VEGF antibody and 18-alpha-G significantly attenuated the ability of ATL-derived cells to traverse endothelial cell monolayers in double compartment invasion assays. Signaling cascade that is activated upon VEGF stimulation of endothelial cells was investigated in confluent endothelial cells cocultured with ATL derived cells or cell-free conditioned medium in the absence or presence of PTK787 or anti-VEGF antibody, using phospho-specific antibodies against FAK Tyr 861, FAK Tyr 397, Src, ERK, PLCgamma, and Akt/PKB, known to be signal mediators in the Ras-Raf-MEKK-ERK signaling cascade. The addition of ATL derived cells or cell-free conditioned medium resulted in a transient activation of the above signal mediators with a peak increase at 15 minutes and gradual return to baseline after 60 minutes. This activation was down regulated in the presence of anti-VEGF antibody and to a greater extent in the presence of PTK787. This data establishes a novel role of VEGF signaling in tumor cell invasion and supports the potential use of PTK787 in the treatment of ATL and other hematological malignancies with angiogenic and invasive properties.