Angiogenesis is required for
tumor formation. Several studies have demonstrated that
tumor angiogenesis is regulated by a balance between proangiogenesis and antiangiogenesis factors and that this balance varies in different organ environments. To investigate whether expression of an
angiogenesis inhibitor by
cancer cells could alter this balance and prevent
tumor formation in different organ environments, we engineered stable transfectants from RenCa mouse
renal carcinoma cells and SW620 human colon
carcinoma cells to constitutively secrete a mouse
endostatin protein with c-myc and
polyhistidine (His) tags. Production and secretion of the
endostatin-c-myc-His fusion
protein by
endostatin-transfected cells were confirmed by immunofluorescence staining and Western blot analysis. The
endostatin transfectants and control transfectants, stably transfected with a control plasmid, had similar in vitro growth rates compared with their parental cell lines.
Conditioned medium from
endostatin-transfected cells inhibited human umbilical vein endothelial cell proliferation by 36-51% compared with
conditioned medium from control cells. After inoculation into mice, flank
tumors from
endostatin-transfected cells were 73-91% smaller than flank
tumors from control cells after 3 weeks. Inoculation of a cell mixture containing 25%
endostatin-transfected cells and 75% control cells resulted in inhibition of flank
tumor formation as effective as after inoculation of 100%
endostatin-transfected cells. Formation of lung
metastases by RenCa
endostatin-transfected cells and formation of liver
metastases by SW620
endostatin-transfected cells were dramatically inhibited compared with formation of
metastases by control cells. These findings demonstrate that
endostatin can inhibit
tumor formation in different organ environments and that gene delivery of
endostatin into even a minority of
tumor cells may be an effective strategy to prevent progression of
micrometastases to macroscopic disease.