Activin A, p15INK4b Signaling, and Cell Competition Promote Stem/Progenitor Cell Repopulation of Livers in Aging Rats

BACKGROUND & AIMS: Highly proliferative fetal liver stem/progenitor cells (FLSPCs) repopulate livers of normal recipients by cell competition. We investigated the mechanisms by which FLSPCs repopulate livers of older compared with younger rats. METHODS: Fetal liver cells were transplanted from DPPIV+ F344 rats into DPPIV- rats of different ages (2, 6, 14, or 18 months); liver tissues were analyzed 6 months later. Cultured cells and liver tissues were analyzed by reverse transcription polymerase chain reaction, immunoblot, histochemistry, laser-capture microscopy, and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling analyses. RESULTS: We observed 4- to 5-fold increases in liver repopulation when FLSPCs were transplanted into older compared with younger rats. Messenger RNA levels of cyclin-dependent kinase inhibitors increased progressively in livers of older rats; hepatocytes from 20-month-old rats had 6.1-fold higher expression of p15INK4b and were less proliferative in vitro than hepatocytes from 2-month-old rats. Expression of p15INK4b in cultured hepatocytes was up-regulated by activin A, which increased in liver during aging. Activin A inhibited proliferation of adult hepatocytes, whereas FLSPCs were unresponsive because they had reduced expression of activin receptors (eg, ALK-4). In vivo, expanding cell clusters derived from transplanted FLSPCs had lower levels of ALK-4 and p15INK4b and increased levels of Ki-67 compared with the host parenchyma. Liver tissue of older rats had 3-fold more apoptotic cells than that of younger rats. CONCLUSIONS: FLSPCs, resistant to activin A signaling, repopulate livers of older rats; hepatocytes in older rats have less proliferation because of increased activin A and p15INK4b levels and increased apoptosis than younger rats. These factors and cell types might be manipulated to improve liver cell transplantation strategies in patients with liver diseases in which activin A levels are increased.