Characterization of a streptococcal antitumor glycoprotein (SAGP)

Abstract

An acidic antitumor glycoprotein (SAGP) was purified from a crude extract of Streptococcus pyogenes, Su strain. Intraperitoneal injection with SAGP (20 mg protein/kg/day for 4 consecutive days) prolonged the life span of mice inoculated i.p. with Ehrlich ascite carcinoma cells and methylcholanthrene-induced fibrosarcoma cells (Meth A) up to 244% and 169% of that of the control mice, respectively. These in vivo antitumor effects were reduced in immunosuppressed mice. The effector spleen cells from the Meth A-inoculated and SAGP-injected mice showed a considerable cytostatic activity on Meth A cells in vitro, and immunosuppression 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 including transformed hamster embryonic lung cells, murine leukemia L1210, Meth A and human promyelocytic leukemia HL60 cells. The IC 50s for the cell growth of these cells were all below 0.1 μg protein/ml. SAGP inhibited the incorporation of nucleic acid precursors into Meth A cells. It seems that sulfhydryl groups of the SAGP molecule are essential for the expression of the antitumor action of SAGP. The cell growth-inhibitory activity of SAGP was diminished in Meth A cells preincubated with pertussis toxin (IAP), whereas it was augmented in the cells preincubated 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 neither in Meth A cells incubated with heat-inactivated SAGP nor in SAGP-insensitive L929 cells. SAGP induced apoptosis in Meth A and HL60 cells as assessed by DNA fragmentation. A single dose injection of SAGP (100mg protein/kg, i.v., s.c., or i.p.) into mice produced no tixic signs except occasional pain responses observed for one week after the injection. Thus, SAGP is a low toxic substance that shoes in vivo antitumor activity by modulating immune responses of the host, and also exhibits in vitro cell-growth inhibition through IAP-sensitive G-protein.