DEFECTIVE SECRETION OF ALBUMIN ENCODED BY EXON-SKIPPED MESSENGER-RNA FOUND IN AGED ANALBUMINEMIC RAT-LIVER

Remarkable increases in the amounts of exon H-skipped (DELTAH), exons G and H-skipped (DELTAGH), and exons H and I-skipped (DELTAHI) mRNAs o f rat albumin have been found in the livers of aged or mutagen-treated analbuminemic rats (Kaneko, T., H. Shima, H. Esumi, M. Ochiai, S. Nag ase, T. Sugimura, M. Nagao: Proc. Natl. Acad. Sci. USA 98, 2707-2711 ( 1991)), in association with increases in numbers of immunohistochemica lly albumin-positive hepatocytes and the accumulation of albumin(s) in the cells. To determine which type of mRNA is responsible for the acc umulation of albumin(s), expression vector systems for intact mRNA and three kinds of exon-skipped albumin mRNAs were constructed and transf ected into COS-1 and HepG2 cells. Transient expression and secretion o f albumins were examined in these cells. DELTAHI albumin and intact al bumin were efficiently expressed by both types of transfected cells. T he DELTAHI albumin expressed had a similar molecular weight to the maj or albumin isolated from the microsome fraction of the liver of aged a nalbuminemic rats, as judged by SDS-PAGE. The rate of secretion of DEL TAHI albumin was very low in both types of transfected cells, whereas those of DELTAH and DELTAGH albumins were normal. Immunocytochemical a nalyses of the transfected COS-1 cells with antiserum against rat albu min showed that the distribution of intact albumin had a compact Golgi -like pattern, whereas that of DELTAHI albumin had a diffuse endoplasm ic reticulum-like pattern. These distribution patterns were similar to those in aged analbuminemic rat hepatocytes. These findings suggest t hat the albumin accumulated in the hepatocytes of aged and carcinogen- treated analbuminemic rats is DELTAHI albumin.