Amino acid and hormonal control of macromolecular turnover in perfused rat liver. Evidence for selective autophagy.

The control of RNA degradation by amino acids, insulin, and glucagon was investigated in perfused livers of fed rats previously labeled in vivo with [6-14C] orotic acid; rates were determined from the release of [14C]cytidine in the presence of 0.5 mM cytidine to suppress reutilization. Studies with cyclically perfused livers showed that plasma amino acids at 10 times (10X) normal concentrations inhibited RNA breakdown by 85%. Similar inhibition was obtained with a known regulatory amino acid mixture (Leu, Met, Pro, Trp, and His), whereas leucine alone (0.8 mM) decreased degradation by 47%. Perfusions carried out in the single-pass mode with graded levels of plasma amino acids revealed that the acceleration of RNA degradation over the full range of amino acid deprivation (0 to 10X normal levels) was the same as that for protein breakdown (3.19 and 3.15% h-1, respectively), and both were equally suppressed by insulin (2.4 micrograms h-1). Glucagon (10 micrograms h-1), though, was far less effective in stimulating RNA than protein turnover. A direct comparison of the two dose responses revealed a strong dissociation at 1 and 2 times normal amino acid levels. These findings support the notion that RNA and protein are degraded within a single macroautophagic compartment during amino acid and insulin deprivation. Glucagon, however, appeared to induce a second pathway in which the proportion of sequestered RNA to protein was selectively reduced. Electron micrographs showed that the ratio of vacuoles containing rough as compared with smooth endoplasmic reticulum was decreased by nearly 80% under these conditions.