Recent studies have shown that angiogenesis, which is induced by VEGF, may be involved in the pathogenesis of hematopoietic malignancies. A human leukemia model consisting of T-lymphoblastic CEM/0, 7 monoclonal refractory clones resistant to both cytosine arabinoside (ara-C) and L-asparaginase (ASNase), Jurkat/E6-1 and U937, representing the leukemic blasts from relapsed patients with leukemias was investigated for secretion of VEGF before and after treatment with various agents. The T-lymphoblastic cell line, Jurkat/E6-1, was used as the negative control, which has been characterized as not expressing mRNA nor the VEGF protein, and did not secrete VEGF. With no treatment, U937, the positive control, secreted the highest VEGF concentration of 1612.7 pg/ml. The CEM/O wild type cell line and 5 other drug-resistant clones secreted VEGF at levels ranging from 180.9 to 414.2 pg/ml. Two CEM drug-resistant clones, CEM/ara-C/G/ASNase-0.5-1 and CEM/ara-C/G/ASNase-1-1, lacked VEGF production. Docetaxel (Taxotere, TXR), Vincristine (VCR), ASNase, and the Fit-1/Fc chimera, a specific inhibitor of VEGF-dependent human umbilical vein endothelial cell (HUVEC) proliferation, were tested for inhibition of VEGF secretion. Treatment of the leukemic cell lines with 2 microg/ml Flt-1/Fc chimera for 24 hours completely inhibited VEGF secretion to the detection limit of the assay (<10pg/ml). After 24 hours incubation with Flt-1/Fc chimera, the leukemic cells appeared to be undergoing apoptosis, based on microphotography examination, suggesting that VEGF could be used in an autocrine loop to promote cell survival by the leukemic cells. Treatment with 0.5, 1, and 2 microg/ml Flt-1/FC chimera for 48 hours demonstrated a 15-25% growth inhibition by MTT assay. Strong inhibition of VEGF secretion in the culture media was observed after 10 microM TXR or 0.1 microM VCR for 24 hours in the wild-type and drug-resistant clones, except CEM/ara-C/I, in comparison with controls. In contrast, treatment with 1 IU/ml ASNase, a specific T-cell protein inhibitor, in 5 cell lines for 24 hours demonstrated no inhibition of VEGF in CEM/0 3 drug-resistant clones and the myeloid U937 line. We conclude that the leukemia cell lines actively secrete VEGF, in vitro. TXR and VCR, but not ASNase, strongly inhibit the VEGF production, suggesting that inhibition of this growth factor may be a mechanism of antileukemic activity. Moreover, the leukemic cell lines examined here may constitute a useful model to study antiangiogenic drugs, alone or in combination with established drug regimens used against refractory leukemias.
