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

Abstract

Background & Aims Highly proliferative fetal liver stem/progenitor cells (FLSPC) repopulate livers of normal recipients by cell competition. We investigated the mechanisms by which FLSPC 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 PCR, immunoblot, histochemistry, laser-capture microscopy, and TUNEL analyses. Results We observed 4–5-fold increases in liver repopulation when FLSPC were transplanted into older, compared with younger, rats. mRNA 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 upregulated by activin A, which increased in liver during aging. Activin A inhibited proliferation of adult hepatocytes, whereas FLSPC were unresponsive because they had reduced expression of activin receptors (e.g. ALK-4). In vivo, expanding cell clusters derived from transplanted FLSPC had lower levels of ALK-4 and p15INK4b and increased levels of Ki-67, compared with the host parenchema. Liver tissue of older rats had 3-fold more apoptotic cells than of younger rats. Conclusions FLSPC, 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 of 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.

Engraftment and Repopulation Potential of Late Gestation Fetal Rat Hepatocytes

“…These cells have the ability to extensively repopulate injured and normal adult liver. 9,43,44 The present observation that late gestation fetal hepatocytes, despite their being far along in the differentiation process, possess a similar capacity for repopulation was unexpected. This is likely related to the presence of a significant proportion of hepatocyte marker-positive cells maintaining a less well differentiated phenotype.…”

“…7,8 These cells have been shown to repopulate adult liver through the process of cell competition. 9 Previous studies in our laboratory have shown that late gestation fetal hepatocytes isolated on ED19 (term, ED21) are highly proliferative and relatively well-differentiated along a hepatic lineage, expressing albumin, glycolytic enzymes and other indicators of mature hepatocyte function. 10–12 We have also shown that growth factor signaling pathways involving ERK1/2 and PI3K/Akt are uncoupled in late gestation fetal liver.…”

Stem cells in liver diseases and cancer: Recent advances on the path to new therapies

“…30 Similarly, as reported at the conference, adult LPC emerge and expand in a rat model of end-stage liver cirrhosis. However, the finding that hepatocyte function is impaired in these animals suggests that the cirrhotic liver environment may not only cause loss of hepatocyte differentiation, but may also prevent LPC maturation.…”

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

“…In many tissues, most notably skeletal muscle, liver, and kidneys, the age-related decline of regenerative potential is correlated with the loss of stem or progenitor cells via necrotic or apoptotic cell death (Collins et al, 2007; Giampietri et al, 2010; Menthena et al, 2011; Suzuki et al, 2012; Yang et al, 2010). In the case of the cochlea, such cell death may also be playing a role.…”