Inhibition of Transforming Growth Factor- 1 and UV Light-Induced Apoptosis by Prostanoids in Primary Cultures of Rat Hepatocytes Bernd Kroll, Susanne Kunz, Naxin Tu, and Leslie R. Schwarz 1 GSF-Forschungszentrum fu ¨r Umwelt und Gesundheit, Institut fu ¨r Toxikologie, D-85758 Neuherberg, Germany Received December 15, 1997; accepted June 15, 1998 Inhibition of Transforming Growth Factor-1 and UV Light- Induced Apoptosis by Prostanoids in Primary Cultures of Rat Hepatocytes. Kroll, B., Kunz, S., Tu, N., and Schwarz, L. R. (1998). Toxicol. Appl. Pharmacol. 152, 240 –250. Treatment of rat hepatocytes cultured in collagen gel with transforming growth factor-1 (TGF1) or with UV light strongly increased the frequency of apoptotic nuclei within 24 h; at doses of 0.5 ng/ml TGF1 or 90 J/m 2 UV light about 17 and 22% apoptotic nuclei were determined, respectively. DNA of the treated cells showed internucleosomal DNA fragmentation. Already the pres- ence of the cytokine for only 1 h significantly induced apoptosis. The prostanoids PGI 2 , PGD 2 , and PGE 1 decreased the frequency of apoptotic nuclei in a dose-dependent manner by up to 70 to 80% and suppressed internucleosomal DNA fragmentation. In con- trast, PGE 2 and PGF 2 elicited a smaller protective effect and arachidonic acid had none. In the case of PGE 1 it was shown that the prostaglandin was most effective when added together with TGF1 or within 2 h before or after treatment with this cytokine. An early increase of the tumor supressor gene product p53 is thought to play a decisive role in UV light-induced apoptosis. However, this increase in p53 was not affected by the strong cytoprotective prostacyclin PGI 2 . Our findings show a marked antiapoptotic activity of the pro- stanoids PGE 1 , PGI 2 , and PGD 2 and raise the question of whether these prostanoids may influence apoptosis in pathological pro- cesses in the liver. © 1998 Academic Press In principle, two basic types of cell death can be distin- guished, necrosis and active cell death (Buja et al., 1993). Among the various types of active cell death, apoptosis has received particular attention (Kerr et al., 1972; Schulte-Her- mann et al., 1995). Several lines of evidence suggest that transforming growth factor-1 (TGF1) plays an important role in the regulation of apoptosis in the liver: (1) apoptotic hepatocytes show an increased expression of TGF1 and im- munostaining for a latent form of TGF1 (Bursch et al., 1993; Oberhammer et al., 1996); (2) administration of TGF1 to rats induces a marked loss of hepatocytes within the pericentral region (Terrell et al., 1993) and increases the rate of apoptosis during liver regression due to withdrawal of the mitogen cypro- terone acetate (Oberhammer et al., 1992, 1993); (3) transgenic mice overexpressing TGF1 develop continuing apoptotic death of hepatocytes in the liver (Sanderson et al., 1995); and (4) TGF1 is a potent inducer of apoptosis in vitro in hepato- cyte cultures (Oberhammer et al., 1991, 1992; Bayly et al., 1994; Ohno et al., 1995; Benedetti et al., 1995; Oberhammer and Qin, 1995; Gressner et al., 1996; Yamamoto et al., 1996; Wo ¨rner and Schrenk 1996; Bour et al., 1996; Fasciati and Maier, 1997). The mechanisms by which TGF1 exerts its apoptotic action are still poorly unterstood. The fundamental event in the generation of TGF1 signals is the binding of the cytokine to specific membrane serine/threonine receptors and the phosphorylation of the receptor complex (Massague, 1996; Yingling et al. 1995). The downstream events are much less clear. Possibly the Mad (mothers against dpp) gene family is implicated in signal transduction of TGF1 (Massague, 1996), and recent findings suggest the involvement of reactive oxygen species in apoptosis elicited by TGF1 in fetal hepatocytes (Sanchez et al. 1996). In previous studies a cytoprotective action of the prostacy- clin PGI 2 and of the prostaglandins PGE 1 and PGE 2 (and their derivatives Iloprost, Misoprostol, Rioprostil, Enprostil, and 16,16'-dmPGE 2 ) has been demonstrated in vivo as well as in vitro (Bursch et al. 1989; Divald et al. 1990; Masaki et al. 1992; Mihas, 1992; Bergasa et al. 1992; Quiroga and Prieto, 1993; Matsumoto et al. 1992). In these studies, cell damage, which was induced by cytotoxic drugs such as CCl 4 , galac- tosamine, and aflatoxine B1, as well as ischemia, was attrib- uted to cell necrosis. It remains unclear, however, whether the prostanoids might also be capable of protecting hepatocytes against apoptosis induced by a physiological stimulus such as TGF1. This question may also be of particular relevance for a more detailed understanding of tumor promotion in the liver. The antiapoptotic action of hepatic tumor promoters such as phenopbarbital (PB) and lindane, which is thought to contrib- ute significantly to their tumor promoting activity, may at least be partially mediated by prostanoids via paracrine signaling (Bursch et al. 1984; Schulte-Hermann et al. 1990 and 1995). Thus, Denda et al. (1989) showed that inhibitors of cyclooxy- 1 To whom correspondence should be addressed. Fax: 89 –3187–3449; E- mail: Schwarz@gsf.de. TOXICOLOGY AND APPLIED PHARMACOLOGY 152, 240 –250 (1998) ARTICLE NO. TO988513 240 0041-008X/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.