Molecular mechanisms of impaired stimulation of DNA synthesis in cultured hepatocytes of aged rats

Proteomic Analysis of eIF5B Silencing-Modulated Proteostasis

Protein translational machinery is an important component of the proteostasis network that maintains cellular proteostasis and regulates aging and other cellular processes. Ample evidence indicates that inhibition of translation initiation factor activities enhances stress resistance in model organisms. Eukaryotic translation initiation factor 5B (eIF5B) acts by joining the pre-40S subunit with the 60S ribosomal unit to form an 80S-like complex during protein translational initiation. Reduced eIF5B expression may disrupt proteostasis and trigger cellular processes associated with stress responses. In this study, the physiological effects of altered eIF5B expression were examined in 293T and HepG2 cells. Cells with eIF5B-knockdown (eIF5B-KN) grew more slowly than control cells, and had a lower level of intracellular reactive oxygen species (ROS), increased resistance to oxidative stress and enhanced autophagy. Proteomic analysis showed that eIF5B knockdown resulted in upregulation of 88 proteins and downregulation of 130 proteins compared with control cells. The differentially expressed proteins were associated with diverse cellular processes including amino acid metabolism, RNA processing and protein metabolism, and DNA synthesis. Autonomous downregulation of the mitogen-activated protein kinase (MAPK) signaling pathway was identified as confirmed by western blotting and qPCR. We proposed that deactivation of MAPK pathway modulated proteostasis and induced prolonged S-phase of the cell-cycle, contributing to the slow growth of eIF5B-KN cells. eIF5B silencing also inactivated the mTOR pathway, downregulated glutamine transporters, enhanced autophagy, and decreased 28S rRNA and 5.8S rRNA expression levels which were reversed by restoration of eIF5B expression. Taken together, these results suggest that eIF5B silencing provides a negative feedback to deactivate MAPK signaling, leading to reduced cell growth. These findings provide a useful resource to further biological exploration of the functions of protein synthesis in regulation of proteostasis and stress responses.

A rat primary hepatocyte culture model for aging studies

The purpose of this protocol is to establish a primary hepatocyte culture system as a suitable model to examine age-related changes in Phase II detoxication gene expression. Hepatocytes are isolated using a two-step collagenase perfusion technique from young (3 to 6 months) and old (24 to 28 months) rats and placed in primary culture using collagen (Type I)-coated plates as the extracellular matrix. A supplemented William's E Medium is used as the medium. This culture system maintains hepatocyte viability from both young and old rats for ∼60 hr, as measured by lactate dehydrogenase activity, while also maintaining their respective phenotypes relative to Phase II detoxification. We thus conclude that a collagen-based cell culture system is suitable to study age-associated deficits in Nrf2/ARE-mediated Phase II gene regulation provided that experiments can be conducted within 60 hr after cell isolation.

Impact of cell cycle disruption on impaired hepatic regeneration in aged livers with ischemic insult

BACKGROUND

The number of elderly patients with hepatobiliary malignancies has increased with the steady growth of elderly population. However, the safety of major hepatectomy for elderly patients remains controversial. This study investigated the effect of aging on the hepatic regenerative response after partial resection of livers subjected to ischemic insult.

METHODS

Two-thirds hepatectomy following 1-h hepatic ischemia was performed in young (12-wk-old) and old (18-mo-old) rats under portosystemic shunt establishment by subcutaneous transposition of the spleen.

RESULTS

The survival rate 48 h after hepatectomy of the old rats was significantly lower (20%) than that of the young rats (53%), whereas all animals without hepatic ischemia were alive at 48 h. Hepatic necrosis and hepatocyte apoptosis during the early post-hepatectomy phase were more severe in the aged livers, which also showed delayed Akt activation. Liver mass restoration was significantly retarded in the old rats, despite higher plasma IL-6 levels, rapid and prolonged activation of hepatic STAT3, and increased hepatocyte nuclear cyclin D1 levels. In the young livers, cyclin E, which is essential for G1/S transition, and cyclin A, a marker of S phase, were observed in the nucleus from 24 h, reaching peaks 48 h after hepatectomy. In contrast, the old livers showed greatly delayed and decreased nuclear cyclin E and cyclin A levels.

CONCLUSION

Age-related reductions in the regenerative ability of ischemically damaged livers may be caused by cell cycle disruption at either the late G1 phase or the G1/S transition, despite increased cyclin D1 levels and compensatory IL-6/STAT3 activation.

Activation of sphingolipid turnover and chronic generation of ceramide and sphingosine in liver during aging

Aging leads to a decreased ability of liver to metabolize drugs and increased expression and secretion of acute phase proteins, such as serum amyloid A (SAA), C-reactive protein (CRP), and alpha-1-acid glycoprotein (AGP). This phenomenon resembles some aspects of the acute phase response of host to inflammation; however, the molecular basis for the similarity is unclear. Ceramide and sphingosine are second messenger mediators of cellular responses to stress and inflammation. In liver, they play important role in mediating acute phase responses to IL1-beta. In this study, we use HPLC and thin layer chromatography to evaluate the effects of aging on steady-state levels of ceramide and sphingosine. We report that both lipids are elevated in liver of old (24 months) as compared to young (5 months) male Fisher 344 rats. To elucidate the mechanism(s) for ceramide elevation, we test the acidic (ASMase) and neutral sphingomyelinase (NSMase) in vitro using NBD-sphingomyelin as an exogenous substrate. SM synthase is also analyzed in vitro using NBD-ceramide and [3H]-dipalmitoylphosphatidylcholine (DPPC) as exogenous substrates. In accordance with the increases in the mass of ceramide, the activity of acid and neutral SMase is elevated in old animals. Michaelis-Menten analysis of NSMase implies that the apparent activation of this enzyme is caused by an increase in the Vmax of the enzyme. In contrast, SM synthase activity is lower in old animals as compared to young ones. These results show that aging is accompanied by an elevation in SM turnover and a decrease in its synthesis, resulting in accumulation of pro-inflammatory and growth inhibitory second messenger ceramide. Ceramidase, the only enzyme leading to sphingosine generation, is also measured in vitro using NBD-ceramide as a substrate and liver homogenate as an enzyme source. Its activity is higher in the old rats, as compared to young ones. The acid and neutral forms of the enzyme are affected the most, while the changes in the alkaline enzyme are not significant. The increases in the basal levels of ceramide and sphingosine in old animals may contribute to the onset of an inflammatory like state in liver during aging, exemplified by decreased P4502C11 mRNA expression and chronic induction of acute phase protein expression.

The role of protein kinase B and mitogen-activated protein kinase in epidermal growth factor and tumor necrosis factor alpha-mediated rat hepatocyte survival and apoptosis

Perturbation of hepatocyte growth regulation is associated with a number of liver diseases such as fibrosis and cancer. These diseases are mediated by a network of growth factors and cytokines that regulate the induction of hepatocyte proliferation and apoptosis. In this study, we have investigated the role of signaling pathways activated by tumor necrosis factor alpha (TNF-alpha) and epidermal growth factor (EGF) in the regulation of apoptosis induced by transforming growth factor beta(1) (TGF-beta(1)), because this physiological factor is believed to regulate spontaneous apoptosis in the liver. We show that pretreatment with (10 ng/mL) EGF or (25 ng/mL) TNF-alpha can suppress TGF-beta(1)-induced apoptosis by 73% and 50%, respectively, in isolated rat hepatocytes. However, suppression of TGF-beta(1)-induced apoptosis by EGF and TNF-alpha occurs via different protein kinase signaling pathways. Using specific inhibitors, we show that suppression of apoptosis by EGF is dependent on activation of phosphoinositide 3-kinase (PI 3-kinase) and the extracellular signal regulated kinase (ERK) mitogen-activated protein (MAP) kinase pathways, but not p38 MAP kinase. In contrast, suppression of TGF-beta(1)-induced apoptosis by TNF-alpha does not require PI 3-kinase and protein kinase B (PKB or Akt)-mediated pathways, but is dependent on ERK and p38 MAP kinase activity. These data contribute to our understanding of the intracellular survival signals that play a role in normal liver homeostasis and in diverse pathological conditions.

The mitogen-activated protein kinase kinase/extracellular signal-regulated kinase cascade activation is a key signalling pathway involved in the regulation of G(1) phase progression in proliferating hepatocytes

In this study, activation of the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) signalling pathway was analyzed in proliferating rat hepatocytes both in vivo after partial hepatectomy and in vitro following epidermal growth factor (EGF)-pyruvate stimulation. First, a biphasic MEK/ERK activation was evidenced in G(1) phase of hepatocytes from regenerating liver but not from sham-operated control animals. One occurred in early G(1) (30 min to 4 h), and the other occurred in mid-late G(1), peaking at around 10.5 h. Interestingly, the mid-late G(1) activation peak was located just before cyclin D1 induction in both in vivo and in vitro models. Second, the biological role of the MEK/ERK cascade activation in hepatocyte progression through the G(1)/S transition was assessed by adding a MEK inhibitor (PD 98059) to EGF-pyruvate-stimulated hepatocytes in primary culture. In the presence of MEK inhibitor, cyclin D1 mRNA accumulation was inhibited, DNA replication was totally abolished, and the MEK1 isoform was preferentially targeted by this inhibition. This effect was dose dependent and completely reversed by removing the MEK inhibitor. Furthermore, transient transfection of hepatocytes with activated MEK1 construct resulted in increased cyclin D1 mRNA accumulation. Third, a correlation between the mid-late G(1) MEK/ERK activation in hepatocytes in vivo after partial hepatectomy and the mitogen-independent proliferation capacity of these cells in vitro was established. Among hepatocytes isolated either 5, 7, 9, 12 or 15 h after partial hepatectomy, only those isolated from 12- and 15-h regenerating livers were able to replicate DNA without additional growth stimulation in vitro. In addition, PD 98059 intravenous administration in vivo, before MEK activation, was able to inhibit DNA replication in hepatocytes from regenerating livers. Taken together, these results show that (i) early induction of the MEK/ERK cascade is restricted to hepatocytes from hepatectomized animals, allowing an early distinction of primed hepatocytes from those returning to quiescence, and (ii) mid-late G(1) MEK/ERK activation is mainly associated with cyclin D1 accumulation which leads to mitogen-independent progression of hepatocytes to S phase. These results allow us to point to a growth factor dependency in mid-late G(1) phase of proliferating hepatocytes in vivo as observed in vitro in proliferating hepatocytes and argue for a crucial role of the MEK/ERK cascade signalling pathway.