Hepatocyte senescence in vivo following preconditioning for liver repopulation

Abstract

In the retrorsine (RS)-based model of massive liver repopulation, preexposure to this naturally occurring alkaloid is sufficient to prime normal host parenchymal cells to be slowly replaced by transplanted normal hepatocytes. The basis for this striking effect is yet to be fully elucidated. In the present studies the possible involvement of cell senescence was investigated. Fischer 344 rats were treated according to the RS-based protocol for hepatocyte transplantation, i.e., two doses of RS, 2 weeks apart, and were killed at 4 or 8 weeks after treatment. Control groups were given saline. Expression of senescence-associated beta-galactosidase was greatly induced in hepatocytes exposed to RS. In addition, several other changes that have been related to cell senescence were observed: these included markers of persistent activation of a DNA damage response, an increased expression of mammalian target of rapamycin, and positive regulators of the cell cycle, together with the induction of p21 and p27 cyclin-dependent kinase inhibitors. Furthermore, RS treatment increased levels of interleukin-6 in the liver, consistent with the activation of a senescenceassociated secretory phenotype. Conclusion: These findings indicate that RS induces hepatocyte senescence in vivo. We propose that cell senescence and the associated secretory phenotype can contribute to the selective growth of transplanted hepatocytes in this system. (HEPATOLOGY 2012;56:760-768)

Clearance of senescent hepatocytes in a neoplastic-prone microenvironment delays the emergence of hepatocellular carcinoma

“…Furthermore, a main component of SASP/SMS, namely the pro-inflammatory cytokine IL-6, was also over-expressed in DENA+RS-treated animals. Both findings were in agreement with those reported following exposure to RS alone [10]. …”

“…As mentioned in the Introduction, recent findings have indicated that exposure to RS induces extensive hepatocyte senescence in rat liver [10]. Although cell senescence can represent a fail safe mechanism to alt neoplastic progression of altered cells [17], it is now well established that it can also contribute to the emergence of the neoplastic phenotype, possibly through secretion of a host of factors, variably referred to as senescence-associated secretory phenotype (SASP) [18] or senescence-messaging secretome (SMS) [19], and comprising cytokines, growth factors and proteases.…”

“…Recent studies, aimed at defining the biological and molecular determinants of the RS-induced effect, revealed the presence of extensive hepatocyte senescence in rat liver exposed to the alkaloid. Based on those findings, it was suggested that cell senescence and the associated SASP/SMS are possibly involved in the induction of the RS-associated neoplastic-prone tissue microenvironment [10]. In fact, it is now widely recognized that the senescence phenotype, while representing a fail-safe mechanism to avoid the risk of malignant transformation in cells harbouring damaged DNA or activated oncogenes [16,17,22,23], can also foster the emergence of premalignant and malignant cells [18,19,24,25], including their acquisition of metastatic potential [26] and resistance to chemotherapy [27,28].…”

“…Along this line, studies from our laboratory have indicated that a growth-constrained/senescent tissue environment is able to generate a powerful driving force for the selective expansion of pre-neoplastic hepatocytes in the liver, leading to their progression to HCC [8]. Exposure to retrorsine (RS), a naturally-occurring pyrrolizidine alkaloid, impairs liver regeneration and induces extensive hepatocyte senescence in rat liver [9-10]. When pre-neoplastic cells isolated from hepatic nodules were transplanted in RS-treated livers, they grew rapidly and evolved into HCC; however, the same cell preparation was unable to expand and progress following injection into untreated, syngeneic normal hosts [8].…”

Chemokine-Mediated Robust Augmentation of Liver Engraftment: A Novel Approach

“…Despite clinical improvements in patients, significant problems have arisen because of inefficient engraftment, death or ectopic distribution of cells that did not engraft in the target tissue, emboli formation, immunological rejection, and transient effects of transplanted cells [34][35][36]. A number of preconditioning regimens including irradiation or chemicals have been tried but discontinued for various reasons such as senescence, radiation hazards, or an increased risk of cancer [37,38].…”