CHARACTERIZATION OF A STREPTOCOCCAL ANTITUMOR GLYCOPROTEIN (SAGP)

An acidic antitumor glycoprotein (SAGP) was purified from a crude extr act of Streptococcus pyogenes, Su strain. Intraperitoneal injection wi th SAGP (20 mg protein/kg/day for 4 consecutive days) prolonged the li fe span of mice inoculated i.p. with Ehrlich ascite carcinoma cells an d methylcholanthrene-induced fibrosarcoma cells (Meth A) up to 244% an d 169% of that of the control mice, respectively. These in vivo antitu mor effects were reduced in immunosuppressed mice. The effector spleen cells from the Meth A-inoculated and SAGP-injected mice showed a cons iderable cytostatic activity on Meth A cells in vitro, and immunosuppr ession studies suggested that carrageenan-sensitive and/or asialo-GM1 positive spleen cells are responsible for the in vivo antitumor effect of SAGP. SAGP inhibited the cell growth of cultured cell lines includ ing transformed hamster embryonic lung cells, murine leukemia L1210, M eth A and human promyelocytic leukemia HL60 cells. The IC(50)s for the cell growth of these cells were all below 0.1 mu g protein/ml. SAGP i nhibited the incorporation of nucleic acid precursors into Meth A cell s. It seems that sulfhydryl groups of the SAGP molecule are essential for the expression of the antitumor action of SAGP. The cell growth-in hibitory activity of SAGP was diminished in Meth A cells preincubated with pertussis toxin (IAP), whereas it was augmented in the cells prei ncubated with cholera toxin (CTX), suggesting the involvement of toxin -sensitive GTP (G)-proteins in the SAGP-action. IAP and CTX-catalyzed ADP ribosylation assays confirmed that SAGP augmented the activity of IAP-sensitive G-protein. In addition, this augmentation was detected n either in Meth A cells incubated with heat-inactivated SAGP nor in SAG P-insensitive L929 cells. SAGP induced apoptosis in Meth A and HL60 ce lls as assessed by DNA fragmentation. A single dose injection of SAGP (100 mg protein/kg, i.v., s.c., or i.p.) into mice produced no toxic s igns except occasional pain responses observed for one week after the injection. Thus, SAGP is a low toxic substance that shows in vivo anti tumor activity by modulating immune responses of the host, and also ex hibits in vitro cell-growth inhibition through IAP-sensitive G-protein .