Abstract
Our own and other previous studies demonstrate marked anti-proliferative activity of the tyrophostin adaphostin (NSC680410) in a variety of hematologic malignancies including chronic myelocytic leukemia (CML), chronic lymphcytic leukemia (CLL), acute myelocytic leukemia (AML), and Multiple Myeloma. Here we show that adaphostin (NSC680410), similar to bortezomib, additionally inhibits tumor angiogenesis within the MM bone marrow (BM) microenvironment. This effect is elicited both indirectly by inhibition of VEGF production and secretion in MM cells, as well as directly by abrogation of endothelial cell growth. Specifically, adaphostin triggers marked downregulation of nuclear c-Myc expression in MM cells. Both adaphostin, as well as specific downregulation of c-Myc using siRNA, lead to a decrease in cobalt chloride- induced Hif-1alpha- expression and Hif-1alpha activity, as evidenced by western blot analysis and expression of Hif-1alpha- driven luciferase, respectively. Indeed secretion of the Hif-1alpha target gene VEGF is markedly inhibited in a dose- and time- dependent manner. Importantly, neither knockdown of c-Abl expression nor exogenous overexpression of caspase- cleavage- induced c-Abl fragment abrogates drug- induced Hif-1alpha downregulation or inhibition of its activity. Taken together, these results indicate the existence of a c-Myc/ Hif-1alpha- dependent, but c-Abl- independent, pathway modulating MM cell production and secretion of VEGF. In contrast, we demonstrate a direct antiangiogenic effect of adaphostin on endothelial cells, similar to H2O2, is mediated via c-Jun upregulation, inhibition of cell proliferation, and the induction of cell apoptosis. Moreover, our data further demonstrate activity of adaphostin within the BM microenvironment. Adaphostin, similar to bortezomib, significantly inhibits VEGF secretion triggered by adhesion of MM cells to BMSCs and endothelial cells. Consequently, conditioned medium derived from adaphostin- treated co-cultures markedly inhibits endothelial cell growth and tubule formation in a dose- dependent manner. Finally, we confirmed these in vitro results using an in vivo xenograft mouse model of human MM. Specifically, western blot analysis, as well as immunohistochemistry, demonstrate marked downregulation of both Hif-1alpha and CD31 in tumors isolated from adaphostin- treated animals versus control animals, confirming the in vivo antiangiogenic effect of adaphostin. Similar effects were obtained using a SCIDhu mouse model as well as a significant decrease of MM- related bone disease, due to anti- VEGF activity of adaphostin. Taken together, these data provide the rationale for the clinical evaluation of adaphostin to target both MM cells and the BM milieu to improve patient outcome in Multiple Myeloma.
A mechanism of Rap1-induced stabilization of endothelial cell–cell junctions
