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Rabbit liver apolipoprotein A-I synthesis is under nonparenchymal cell paracrine control
T J Rea1, C L Bisgaier, R B DeMattos
1Division of Therapeutics, Parke-Davis Pharmaceutical Research Division, Warner-Lambert, Ann Arbor, MI 48105.
Abstract:
Apolipoprotein A-I (apoA-I), the primary protein of high density lipoprotein, originates from intestine and liver of almost all mammalian species. In contrast to most species, intact rabbit liver is only capable of producing minute amounts of apoA-I mRNA and protein. In this report we demonstrate that purified rabbit hepatic parenchymal cells have apoA-I mRNA levels approximately 50-fold higher than intact liver after 48 h in monolayer culture. Investigations of the differential between in vivo and in vitro expression showed that conditioned media from nonparenchymal cells, a cell population essentially absent in parenchymal cell cultures, inhibited the elevation of apoA-I mRNA in a specific, concentration-dependent, and reversible fashion. Furthermore, at a concentration of nonparenchymal cell-conditioned media that inhibited apoA-I mRNA levels by > 80% compared to control, there were only slight changes in apoB, apoE, LDL receptor, LCAT, 7 alpha-hydroxylase, hepatic lipase, HMG-CoA reductase, and albumin mRNA levels. Metabolic labeling of parenchymal cell secreted proteins with [35S]methionine followed by apoA-I immunoprecipitation revealed that apoA-I synthesis and secretion corresponded to the changes observed for apoA-I mRNA. Initial biochemical characterization of the nonparenchymal cell media revealed that the inhibitory factor was > 30 kDa, heat-stable to 70 degrees C, and still active after urea denaturation and renaturation. These data suggest that, in rabbits, hepatic parenchymal-nonparenchymal communication in the form of a secreted factor may attenuate liver apoA-I expression in vivo.
Insights
Rabbit liver produces very little Apolipoprotein A-I (apoA-I). However, isolated liver cells show high apoA-I production, suggesting communication with other liver cells regulates this crucial protein. This finding reveals a novel regulatory mechanism for apoA-I expression.
Area of Science:
- Biochemistry
- Cell Biology
- Molecular Biology
Background:
- Apolipoprotein A-I (apoA-I) is the main protein in high-density lipoprotein (HDL).
- Rabbit liver exhibits unusually low apoA-I production compared to other mammals.
- Understanding apoA-I regulation is crucial for cardiovascular health.
Purpose of the Study:
- To investigate the low in vivo expression of apoA-I in rabbit liver.
- To identify factors regulating apoA-I mRNA and protein levels in rabbit hepatocytes.
- To elucidate the role of cell-cell communication in hepatic apoA-I synthesis.
Main Methods:
- Primary rabbit hepatocytes were cultured to assess apoA-I mRNA levels.
- Conditioned media from nonparenchymal cells were used to study inhibitory effects.
- Metabolic labeling and immunoprecipitation were employed to measure apoA-I synthesis and secretion.
- mRNA levels of various liver proteins were analyzed using quantitative methods.
Main Results:
- Isolated rabbit hepatic parenchymal cells showed a 50-fold increase in apoA-I mRNA compared to intact liver.
- Conditioned media from nonparenchymal cells specifically inhibited apoA-I mRNA elevation in a dose-dependent manner.
- The inhibitory factor from nonparenchymal cells was heat-stable and larger than 30 kDa.
- ApoA-I synthesis and secretion in cultured cells mirrored apoA-I mRNA levels.
Conclusions:
- Rabbit hepatic nonparenchymal cells secrete a factor that suppresses apoA-I expression in parenchymal cells.
- This paracrine signaling mechanism likely explains the low in vivo apoA-I production in rabbit liver.
- The findings reveal a novel regulatory pathway for apoA-I, impacting HDL metabolism.