Induction of ferritin synthesis in cells infected with Mengo virus.

We have recently identified ferritin as a cellular protein particle whose synthesis is stimulated in mouse or human cells infected by the picornavirus Mengo. Immunoprecipitation of the particle from infected murine L929 cells showed a 4- and 6-fold increase in the intracellular concentrations of H and L apoferritin subunits, respectively. This differential expression altered the H/L subunit ratio from 3.0 in uninfected cells to 2.2 in Mengo virus-infected cells. The induction is not due to an increase in transcription of the apoferritin L and H genes, nor is it due to an increase in stability of the apoferritin mRNAs. At the level of translation, the iron regulatory protein (IRP) remained intact, with similar amounts being detected in uninfected and infected cells. The Mengo virus RNA genome does not compete with the iron regulatory element (IRE) for the binding of IRP, and sequence analysis confirmed that there are no IREs in the virus RNA. The IRE binding activity of IRP in infected cells decreased approximately 30% compared with uninfected cells. The decrease in binding activity could be overcome by the addition of Desferal (deferoxamine mesylate; CIBA) an intracellular iron chelator, which suggests that virus infection causes an increase in intracellular free iron. Electron paramagnetic resonance (EPR) studies have confirmed the increase in free iron in Mengo virus infected cells. The permeability of cells for iron does not change in virus infected cells, suggesting that the induction of ferritin by Mengo virus is due to a change in the form of intracellular iron from a bound to a free state.

Impaired biosynthesis of phosphatidylcholine causes a decrease in the number of very low density lipoprotein particles in the Golgi but not in the endoplasmic reticulum of rat liver.

We have investigated the mechanism by which inhibition of phosphatidylcholine biosynthesis in rat hepatocytes by choline deprivation causes a reduction in the secretion of very low density lipoprotein (VLDL) (Yao, Z., and Vance, D. E. (1988) J. Biol. Chem. 263, 2998-3004). Rats ingested a choline-deficient or control diet for 3 days, and subcellular fractions of liver were prepared. No change in the amount of apolipoprotein B in the lumina of the endoplasmic reticulum was observed, but there was a 40-50% decrease of apolipoprotein B in the lumina of the Golgi from choline-deficient compared with control rats. Incubation of microsomes, derived from choline-deficient and -supplemented hepatocytes, with trypsin showed similar degradation of apolipoprotein B, indicating similar quantities of this protein are present on the surface and within the lumina. The VLDL particles in the Golgi of liver cells and in plasma, on average, were larger in samples derived from choline-deficient compared with choline-supplemented animals. Incubation of plasma VLDL with proteases demonstrated that the apolipoprotein B of plasma VLDL particles from choline-deficient animals had a different susceptibility to digestion than did VLDL from choline-supplemented animals. These data indicate that the number of VLDL particles assembled in the endoplasmic reticulum of liver is similar in choline-deficient and -supplemented rats, but the number of particles is decreased in the Golgi from choline-deficient animals.