A fresh look at augmenter of liver regeneration in rats

Augmenter of liver regeneration (ALR) is a hepatotrophic protein originally identified by bioassay in regenerating rat and canine livers following par tial hepatectomy and in the hyperplastic livers of weanling rats, but not i n resting adult livers. The ALR gene and gene product were subsequently des cribed, but little is known about the cellular/subcellular sites of ALR syn thesis in the liver, or about the release and dissemination of the peptide, To obtain this information in rats, we raised antibodies in rabbits agains t rat ALR for development of an enzyme-linked immunosorbent assay (ELISA), ALR concentrations were then determined in intact livers of unaltered weanl ing and adult rats; in regenerating residual liver after partial hepatectom y; in cultured hepatocytes and nonparenchymal cells (NPCs); and in culture medium and serum. ALR in the various liver cells was localized with immunoh istochemistry. In addition, hepatic ALR and ALR mRNA were assayed with West ern blotting and reverse-transcriptase polymerase chain reaction (RT-PCR), respectively, The hepatocyte was the predominant liver cell in which ALR wa s synthesized and stored; the cultured hepatocytes secreted ALR into the me dium in a time-dependent fashion, Contrary to previous belief, the ALR pept ide and ALR mRNA were present in comparable concentrations in the hepatocyt es of both weanling and resting adult livers, as well as in cultured hepato cytes, A further unexpected finding was that hepatic ALR levels decreased f or 12 hours after 70% hepatectomy in adult rats and then rose with no corre sponding change in mRNA transcripts, In the meantime, circulating (serum) A LR levels increased up to 12 hours and declined thereafter, Thus, ALR appea rs to be constitutively expressed in hepatocytes in an inactive form, and r eleased from the cells in an active form by unknown means in response to pa rtial hepatectomy and under other circumstances of liver maturation (as in weanling rats) or regeneration.