Elevation of serum albumin concentration in analbuminemic rats by administration of 3'-methyl-4-dimethylaminoazobenzene

Marked increases of two kinds of two-exon-skipped albumin mRNAs with aging and their further increase by treatment with 3'-methyl-4-dimethylaminoazobenzene in Nagase analbuminemic rats

Nagase analbuminemic rats (NARs) have a 7-base-pair deletion at the 5' splice site of the HI intron of the albumin gene. The level of immunohistochemically albumin-positive hepatocytes is about 1 per 10(5) cells in neonatal NARs, increases with age, and further increases with chronic oral treatment with 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). The mechanisms involved in the increase in albumin-positive hepatocytes during aging of NARs and their treatment with 3'-MeDAB were analyzed. NARs were found to have four species of albumin mRNA: intact mRNA and those lacking the regions corresponding to exon H, exon G-H, and exon H-I. In 4-week-old NARs, the level of intact albumin mRNA was about 1/4000 of that in normal rats and mRNA lacking the exon H sequence was the major species. In aged and 3'-MeDAB-treated aged NARs, all four species of mRNA increased and the relative proportion of mRNAs lacking two exon sequences to mRNAs lacking one exon sequence was greatly increased, suggesting that aging is associated with changes of the splicing pattern and that 3'-MeDAB treatment enhanced these changes. In aged NARs and 3'-MeDAB-treated aged NARs, there was an increase in the amount of aberrant 60-kDa albumin. The 60-kDa protein could be a translation product of mRNAs lacking two exons, the amount of which increases in aged NARs and 3'-MeDAB-treated NARs.

Appearance of albumin-producing cells in the liver of analbuminemic rats on aging and administration of mutagens

The lack in serum albumin in analbuminemic rats, a strain derived from Sprague-Dawley rats, was found to be due to deficient synthesis of albumin in the liver caused by a disturbance in the processing of albumin mRNA. The serum albumin gene was cloned from analbuminemic rats and from parental normal Sprague-Dawley rats. Analyses of the nucleotide sequences of these albumin genes revealed that there is a seven base pair deletion in the HI intron of the albumin gene of analbuminemic rats. This deletion extends from the 5th to the 11th base of the 5'-end of the intron causing change in the nucleotide sequence of the 5'-end of the HI intron from GTAGGTT to GTAGCGA. The HI intron sequence was found to be accumulated in the nuclear RNA of analbuminemic rat liver indicating blocking of mRNA splicing. Although analbuminemic rats are almost completely deficient in serum albumin, a small but appreciable amount of "albumin" was detected in their serum. This protein was purified by immunoprecipitation and SDS-gel electrophoresis and shown to have the same immunological crossreactivity and digestion patterns with V8 protease and papain as those of normal rat serum albumin. The concentration of "albumin" increased slightly upon aging of analbuminemic rats. The existence of serum albumin in hepatocytes of analbuminemic rats was studied immunohistochemically by the peroxidase anti-peroxidase method. There were about 1/10(5) albumin-positive cells, presumably albumin-producing hepatocytes at birth, and their number increased gradually to 100 approximately 200/10(4) about 24 months after birth. When the hepatocarcionogenic mutagen 3'-methyl-4-dimethylaminoazobenzene was administered to analbuminemic rats, the number of albumin-positive cells in the liver increased 8-fold in 5 weeks and 10-fold in 15 weeks. A similar increase was observed after administration of acetylaminofluorene, but not after partial hepatectomy or administration of diethylnitrosamine.