Retinoids stimulate ApoA-I synthesis by induction of gene transcription in primary hepatocyte cultures from cynomolgus monkey (Macaca fascicularis)

Abstract

The influence of different retinoids on apolipoprotein A-I (apoA-I) synthesis and secretion was investigated in primary monolayer cultures of hepatocytes from cynomolgus monkeys. Addition of retinol (vitamin A) and retinoic acid to the culture medium resulted in a time- and dose-dependent increase in the secretion of apoA-I. No effect was observed during the first 24-hour incubation period; however, apoA-I secretion was enhanced 1.5-fold in the following 24-hour period in the presence of 10 mumol/L retinoic acid. Maximal stimulation (2.7-fold) was obtained at 10 mumol/L retinoic acid during a third 24-hour incubation. In these experiments apoB-100 secretion was unaffected. When [35S]methionine incorporation studies were performed de novo synthesis of apoA-I was increased, whereas total protein synthesis remained constant. These observations indicated that the induction of apoA-I synthesis is not part of a general effect of retinoic acid on hepatic protein synthesis. Among different natural and synthetic retinoids, retinoic acid and its 9-cis and 13-cis isomers were equally active and were the most potent inducers of apoA-I synthesis, whereas the maximal stimulation induced by retinol was lower (1.6-fold). ApoA-I mRNA abundance was increased threefold in hepatocytes exposed for 72 hours to 10 mumol/L retinoic acid, which was associated with a twofold increase in the transcriptional rate of the apoA-I gene. In contrast, no changes were found in the apoB-100 mRNA level and transcriptional activity of the apoB-100 gene. We conclude that retinoids enhance apoA-I synthesis in simian hepatocytes by transcriptional regulation.

Opposite in vitro and in vivo regulation of hepatic apolipoprotein A-I gene expression by retinoic acid. Absence of effects on apolipoprotein A-II gene expression.

“…A report that appeared during the preparation of this article demonstrated a similar induction on stimulation of isolated primary hepatocytes from cynomolgus monkeys. 41 These results extend previous reports showing that RA is able to transactivate the apoA-I gene promoter in transient transfection assays. 35 In all in vitro models tested, rather high RA doses are required to increase apoA-I mRNA levels, whereas lower, more physiological levels are not active.…”

Retinoids increase human apo C-III expression at the transcriptional level via the retinoid X receptor. Contribution to the hypertriglyceridemic action of retinoids.

“…Positive RAR/RXR DR-2 and RXR/RXR DR-1 sites have been identified in the apo A-I promoter A footprint (43,44). Furthermore, retinoids increase apo A-I mRNA levels and secretion in human, monkey, and rat hepatocytes (45)(46)(47). Similarly, a RXRE has been localized in the apo A-II promoter J-site, which mediates the induction of apo A-II expression in human hepatocytes (31).…”

Effect of retinoic acid on cell proliferation and differentiation as well as on lipid synthesis, lipoprotein secretion, and apolipoprotein biogenesis

“…The medium supplemented with antiproteases (as described above) was first mixed with a plasma lipid carrier (4:1, vol/vol) to efficiently isolate de novo lipoproteins synthesized. The lipoproteins were then isolated by sequential ultracentrifugation using a TL-100 ultracentrifuge (Beckman), as described previously (30). Briefly, chylomicrons (CM) were isolated after ultracentrifugation (20,000 rpm for 20 min).…”

“…These observations indicate that RA suppressed the expression of apo A-I in rats. In contrast, a stimulatory effect on apo A-I synthesis was noted in hepatoma cell lines (6) and in primary cultures from cynomolgus monkey (30). The reason for the discrepancy among studies is not fully clear.…”