9-CIS RETINOIC ACID IS A DIRECT HEPATOCYTE MITOGEN IN RATS

We recently suggested that peroxisome proliferators (PP)-induced hepat ocyte DNA synthesis may be mediated by a specific peroxisome prolifera tor activated receptor (PPAR). Heterodimers of the PPAR with the retin oid nuclear receptor, RXR, activate transcription after binding to DR1 response elements of the target genes. DR1 elements are also activate d by RXR homodimers formed in the presence of 9-cis retinoic acid (9 c is RA) suggesting that PP and 9 cis RA might regulate an overlapping s et of target genes. The present study was therefore designed to test w hether 9-cis RA stimulates hepatocyte DNA synthesis. Male Wistar rats were given a single intragastric dose of 9-cis RA (10-100 mg/Kg) or al l trans retinoic acid -(RA) (200 mg/Kg and 100 mg/Kg), and levels of h epatocyte DNA synthesis after 24 hours were determined by BrdU immunoh istochemistry. Effects of 9-cis RA and RA (10(-9) - 10(-5)M) on hepato cyte DNA synthesis in primary culture were also examined. Over 10 fold increases in the levels of BrdU incorporation were noted 24 hours aft er a single dose of 9 cis RA at a dose of 60 and 100 mg/Kg. RA at a do se of 200 mg/Kg induced a 5-6 fold increases in BrdU labeling, while a dose of 100 mg/Kg had no significant effects. Since the RA effect onl y occurs at higher doses, it may be only after conversion to 9-cis RA. In primary culture of hepatocytes, neither 9-cis RA nor RA with or wi thout EGF had stimulatory effects on hepatocyte DNA synthesis. This is the first report to demonstrate a potent stimulatory effect of 9-cis RA on DNA synthesis of rat hepatocytes in vivo. It is suggested that 9 -cis RA exerts this effect through receptor mediated mechanisms simila r to PP, both activating genes that regulate hepatocyte proliferation.