Fatty acids alter time dependent loss of apolipoprotein E expression by primary cultures of rat hepatocytes.
Although the phenomenon of intracellular apolipoprotein E (apoE) degradation has been reported in other cell types, the fate of newly synthesized apoE in the liver 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 typical secretory protein. Synthesis and secretion of [(35)S]apoE was diminished in primary hepatocytes cultured for more than 2 days, in agreement with an observed decrease in apoE mRNA. Cells cultured for 1 day and labeled for up to 4 hours secreted total protein, apoE, and albumin, linearly. The apparent rates of synthesis for apoE and albumin were similar (1,158 vs. 1,334 dpm/mg/min) but rates of their secretion differed significantly (225 vs. 1,159 dpm/mg/min). Pulse-chase experiments indicated that cell-associated [(35)S]albumin was secreted without degradation, whereas significant quantities of newly synthesized apoE were degraded. The overall synthesis and secretion of total proteins, including secretion of apoE, was enhanced by oleic acid (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 control cells, apoE was found to associate with high density lipoproteins predominantly, although the fraction associated with very low density lipoprotein was increased in hepatocytes incubated with oleic acid. Overall, the findings from this study suggest that the level of apoE expression by primary hepatocytes is dependent on the age of the culture. The study also indicates that the phenomenon of apoE degradation occurs in primary hepatocytes.