Fatty acids alter time dependent loss of apolipoprotein E expression by primary cultures of rat hepatocytes

Although the phenomenon of intracellular apolipoprotein E (apoE) degradatio n has been reported in other cell types, the fate of newly synthesized apoE in the fiver is not well understood. In the present study, we examined the expression (the balance of synthesis, secretion, and degradation) of apoE in primary cultures of rat hepatocytes and compared it with albumin, a typi cal secretory protein. Synthesis and secretion of [S-35]apoE was diminished in primary hepatocytes cultured for more than 2 days, in agreement with an observed decrease in apoE mRMA. Cells cultured for 1 day and labeled for u p to 4 hours secreted total protein, apoE, and albumin, linearly. The appar ent rates of synthesis for apoE and albumin were similar (1,158 vs. 1,334 d pm/mg/min) but rates of their secretion differed significantly (225 vs. 1,1 59 dpm/mg/min). Pulse-chase experiments indicated that cell-associated [S-3 5]albumin was secreted without degradation, whereas significant quantities of newly synthesized apoE were degraded. The overall synthesis and secretio n of total proteins, including secretion of apoE, was enhanced by oleic aci d (1 mmol/L). However, this effect may not be limited to oleic acid because other fatty acids showed a similar effect on apoE mRNA abundance. In contr ol cells, apoE was found to associate with high density lipoproteins predom inantly, although the fraction associated with very low density lipoprotein was increased in hepatocytes incubated with oleic acid Overall, the findin gs from this study suggest that the level of apoE expression by primary hep atocytes is dependent on the age of the culture. The study also indicates t hat the phenomenon of apoE degradation occurs in primary hepatocytes. (J. N utr. Biochem. 10:306-313, 1999) (C) Elsevier Science Inc. 1999. All rights reserved.