A PROTEIN PRODUCED BY A MONOCYTIC HUMAN CELL-LINE CAN INDUCE APOPTOSIS ON TUMOR-CELLS

A serum-free medium conditioned by U937, a human cell line of monocyte /macrophage origin, was found capable of inducing apoptosis on exponen tially growing U937 cells themselves (autocrine suicide). The apoptosi s-inducing agent is a macromolecule and possibly a protein (SKT factor ), with a relative molecular mass in the range of 18-25 kDa. All human tumor cell lines examined have been induced to apoptosis with high ef ficiency, whereas non transformed human lymphocytes and monocytes are insensitive to the apoptosis-inducing activity; moreover, partially di fferentiated U937 are not killed but induced to full maturation. These observations suggest that the SKT factor could possibly be a cytokine with a specific cytotoxic tropism, that resembles in many respects th e cytokine tumor necrosis factor (TNF), even though no TNF is detectab le in the conditioned medium.