Modifications in lens protein biosynthesis signal the initiation of cataracts induced by buthionine sulfoximine in mice

Cataract induction in preweanling mice by L-buthionine sulfoximine (BSO), an inhibitor of glutathione biosynthesis, was correlated with perturbations in vitro protein biosynthesis. These were detected by incubation of late precataract and early cataract lenses with S-35-labeled amino acids, followed by dissection of lenses into capsule-epithelium and decapsulated fiber fractions, further processing of the fibers into water-soluble, urea-soluble and urea-insoluble fractions, and analysis by 2D electrophoresis and fluorography. Most of the protein labeling in control lenses was in the water-soluble fiber and capsule-epithelium fractions (80% and 14% of total cpm, respectively). Labeling in all three fiber fractions was decreased by cataract induction. The urea-insoluble fraction displayed a transient increase in labeled high molecular weight basic protein, as labeling of polypeptide monomers decreased. Densitometric analysis of fluorograms from the water-soluble and urea-soluble fiber fractions revealed a sharp decrease in fiber gamma-crystallin polypeptide labeling preceding and accompanying early cataract development, a delayed decrease in labeling of alpha A-crystallin and increased relative percentage of several labeled beta-crystallin polypeptides, especially in the urea-soluble fraction. By contrast with diminished labeling of the fiber fractions during cataract initiation, protein labeling of the corresponding capsule-epithelium fraction was stimulated dramatically and persisted at reduced levels during early opacification (stage 3), when nearly all of the protein labeling in the lens was found in capsule-epithelium. Capsule-epithelium polypeptides showing increased labeling during cataract initiation included alpha A-crystallin, several acidic polypeptides of M(r) = 40-50 kDa and a group of neutral to mildly acidic polypeptides of M(r) = 20-28 kDa. This transient activation, which was relatively non-specific, may relate to previously reported observations of polyribosome accumulation in lens epithelium during initial development of BSO cataracts. The labeled capsule-epithelium preparations are known to include newly differentiating fibers near the lens equator as well as epithelial cells. Both of these cell populations survive in mature BSO cataracts. It is suggested that modifications of the normal pattern of gene expression in the lens may be involved in initiation of the mouse BSO cataract and its subsequent pattern of development. (C) 1996 Academic Press Limited