ARCHIVES OF BIWHEMISTRY AND BIOPHYSICS Vol. 270, No. 1, April, pp. 267-276,1989 Expression of the cr-Fetoprotein Gene in Adult Rat Liver YU-JUI YVONNE WAN AND JANICE YANG CHOU’ Human Genetics Branch, National Institute of Child Health and, Human Development, National Institutes of Health, Bethesda, Maryland 20892 Received August 24,1988, and in revised form December 5,1988 The adult rat liver contains three cy-fetoprotein (AFP) mRNAs of 2.2 (minor), 1.7, and 1.5 kb. These transcripts share a common 3’ sequence, but the 1.7- and 1.5-kb AFP mRNAs lack sequences present in the first seven 5’ exons of the 2.2-kb AFP mRNA. Sl nuclease analysis maps the 1.7-kb mRNA at the 5’ boundary of the eighth exon of the 2.2-kb AFP mRNA and the 1.5-kb mRNA in the middle of the eight exon. In a trans- formed fetal rat liver cell line, we have previously identified a 1.7-kb AFP mRNA which is encoded by an AFP cDNA (ARFP5) isolated from an adult rat liver cDNA library. The 90-bp 5’ sequence of ARFP5, which is located in the seventh intron of the rat AFP gene, is not present in the 2.2-kb fetal AFP mRNA, although ARFP5 does contain nucleo- tide sequence present in the 2.2-kb AFP mRNA extending from the beginning of its eighth exon (nucleotide 873) to the 3’ end. The 1.7-kb RNA in adult liver also hybridizes with the 90-bp 5’ sequence of ARFP5, suggesting that the two 1.7-kb AFP mRNAs are similar. The developmental profile of these AFP transcripts shows that fetal rat liver contains mainly the 2.2-kb mRNA which decreases to a very low level around the fifth week after birth. The 1.7- and 1.5-kb AFP mRNAs can be visualized about the third and fourth weeks after birth, respectively, and are the major AFP mRNAs in the liver of rats that are older than 4 weeks. The levels of the AFP mRNAs in adult liver are approxi- mately 0.4% of the AFP mRNA level in M-day-old fetal liver. Both the 1.7- and 1.5-kb AFP mRNAs are actively translated; they direct the cell-free synthesis of two polypep- tides of 50K and 44K. A 50K polypeptide is also the encoded product of the 1.7-kb mRNA in the transformed fetal rat liver cell line, suggesting that the 44K polypeptide may be encoded by the 1.5-kb AFP mRNA. o MS Academic press, I~C. AFP? the major serum protein of the fe- tus, is produced mainly by fetal liver and yolk sac (1, 2), although production has also been demonstrated in fetal intestine, lung, and gut (1,3). The AFP mRNA in fe- tal rat liver is about 2.2 kb in length (4, 5) and encodes two polypeptides of 69K and 73K (fetal AFP) which differ in carbohy- drate side chains (6). The levels of fetal AFP and AFP mRNA fall rapidly after 1 To whom correspondence should be addressed at: Building 10, Room 8C429, NIH, Bethesda, MD 20892. ’ Abbreviations used: AFP, a-fetoprotein; SDS, so- dium dodecyl sulfate; Pipes, 1,4-piperazineethanesul- fonic acid. birth and become barely detectable in 4- week-old rat liver (7-9). It has been shown that there is one AFP gene per haploid ge- nome in mouse (lo), rat (4, ll), and human (12). In normal adult rat, liver is the only organ that expresses low levels of this 2.2- kb fetal AFP mRNA (3). However, adult liver contains two additional AFP tran- scripts of 1.7 and 1.5 kb which are the ma- jor AFP mRNAs in adult life (l&13-15). The 1.7-kb AFP transcript was first de- tected in a fetal rat hepatocyte line (RLA209-15) established by viral transfor- mation (16, 17). Probe hybridization and Sl nuclease analysis indicate that the 1.7- kb AFP mRNA in RLA209-15 cells con- tains sequences similar to those of the 2.2- 267 0003-9861189 $3.00 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any formreserved.