**1. Introduction**

#### **1.1. Global burden of chronic liver disease**

Chronic liver disease has globally risen due to mainly a prevalent hepatitis C virus (HCV) infection rate and an epidemic of obesity [1–7]. During the last 2 decades, global viral hepatitis

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

has increased by 163% (from 0.89 million to 1.45 million) and in 2013, viral hepatitis infections became the 7th leading cause of death [7]. While HBV infection is decreasing in most endemic areas due to successful vaccination policies, HCV infection lacks the benefits of a vaccine [7]. Even though HVC antiviral therapies recently introduced in clinical practice are highly successful, its implementation is limited due to access and/or financial constraints. Morphological studies in HCV showed hepatocyte lipid accumulation similar to the one that occurs in obesity.

degree of liver dysfunction or additional medical conditions at the time of diagnosis and the scarcity of available liver grafts. The role of cellular lipid management and metabolism in human health and disease is taking a center stage [13]. Higher fat intake, lower physical activity and a progressively aging population are among the behavioral and social factors of this phenomenon that add to the genetic load [1]. An overview of the role of cell aging and senescence in liver meta-

Cellular Senescence and Their Role in Liver Metabolism in Health and Disease...

http://dx.doi.org/10.5772/intechopen.71659

71

The liver is a unique organ with an innate ability to regenerate through mass compensation to satisfy portal flow and metabolic demands [14]. After injury and cell necrosis, immune recall of resting cells occurs and activation of oval-precursor cells in conjunction with platelets migration switch to a cell division renewal cycle [15, 16]. Mitosis is more prominent at the peri-portal stem cell niche site (zone 1) assuring clonal expansion until reaching zone 3 (peri-central vein) [15, 16]. In health, liver mass homeostasis is closely regulated through a delicate balance among regeneration, apoptosis (programmed cell termination), and senescence. During states of acute liver injury, the pendulum moves towards a regenerative and repair phase, however, during chronic states of liver injury collagen synthesis and deposition persists leading to organ fibrosis. In addition, natural processes of organ aging play a main role in organ response to both acute and chronic injuries. Primary cell life span is determined by a limited number of cell duplications, the so called Hayflick limit [17]. After such limited divisions, cells enter a state of cell replicative senescence which is believed to be triggered by shortening of telomere ends. Replicative cellular senescence is a stable form of cell arrest characterized by a lack of cell proliferation activity and apoptosis resistance mediated through a lack of mitogen response even though the cells remain metabolically active. On the other hand, cells can be induced to a senescence status by a variety of cellular stressors such as DNA

[17, 18]. Senescent cells undergo morphological changes as they acquire an enlarged and flattened morphology, in addition to an increase expression of the senescence associated markers ß-galactosidase (SA-ß-GAL), an accumulation of the senescence associated heterochromatic foci (SAHF) and DNA damage foci, and the expression of the senescence associated secretary phenotype (SASP) [18]. Senescent status is achieved and maintained by active signaling of p53, a tumor suppressor gene that exercises its effects through activation of p21, a potent cell cycle inhibitor, and the p16-retinoblastoma protein [18]. Cells induced into an irreversible cell cycle arrest at the G1 phase will undergo metabolic disturbances with an increase reactive oxygen species (ROS) production, decrease adenosine triphosphate (ATP) synthesis and

Changes in the content of daily oral intake can influence life span and thus cell aging. The cell death-inducing DNA fragmentation factor α-subunit-like effector A (Cidea), is a transcriptional coactivator implicated in lipid accumulation, cell stress and cell aging. Authors showed in rodents, that a high lipid diet up-regulated Cidea with hepatic lipid accumulation, cell stress, mitochondrial dysfunction and genetic upregulation of aging [19]. Other studies, in support of this findings have shown a life span reduction up to 30% in genetically obese mice (ob/ob) and this reduction was reversed by a caloric restricted diet [19]. Lipid enriched diets are associated in humans with DM type 2, HTN and cardiovascular events all

O2

production and heat stress

bolic responses to high caloric intake will be performed in the pages to follow.

damage, UV light, radiation, oncogene activation, increased H<sup>2</sup>

accumulation of lipofuscin [17].

**1.2. Regeneration, necrosis, apoptosis and senescent: a constant changing balance**

Our group has estimated that 2.2 billion people will be overweight and 1.1 billion people will be obese globally by 2030 [8, 9]. In addition, 36.1% of adult men and 32.4% of adult women were diagnosed with the metabolic syndrome in 2010 [10]. Obesity represents the core component of the metabolic syndrome, a cluster of metabolic disarrangements including dyslipidemias, insulin resistance status, hypertension, diabetes and organ metabolic disturbances such as non-alcoholic fatty liver disease (NAFLD) and its inflammatory component non-alcoholic fatty steatohepatitis (NASH), nephropathy, cardiomyopathy and muscle dysfunction [10]. Hepatocellular carcinoma (HCC) has been reported more often in non-cirrhotic livers in the background of NASH and its risks factors include male gender, older age, cigarette smoking, obesity and insulin-resistant states [3, 4, 11]. Overweight and obesity are associated with an overall increase in liver primary cancers of 17% and 89%, respectively [2, 11, 12] and males with a BMI > 35 had a 3.5–4 increase risk of liver malignancy [12] (**Figure 1**). Optimal management of HCC with curative intent includes resection or liver transplantation. Nevertheless, these therapies are limited because the

**Figure 1.** *Main causes of end stage liver disease (ESLD) and hepatocellular carcinoma (HCC).* Hepatitis C virus infection (HCV) and obesity are the main cause of the global and Western increase in ESLD and HCC. In USA 1 out of 3 adults is overweight and approximately 5 M people is HCV seropositive. While prevention relays in stopping virus transmission and implementing programs of healthy caloric intake and exercise programs, treatments of established ESLD and its malignant consequence are similar. Nevertheless, the most effective surgical treatment, liver transplantation is limited due to scarcity of donors and loco-regional therapies have limited survival effect due to malignant recurrence or progression of liver dysfunction.

degree of liver dysfunction or additional medical conditions at the time of diagnosis and the scarcity of available liver grafts. The role of cellular lipid management and metabolism in human health and disease is taking a center stage [13]. Higher fat intake, lower physical activity and a progressively aging population are among the behavioral and social factors of this phenomenon that add to the genetic load [1]. An overview of the role of cell aging and senescence in liver metabolic responses to high caloric intake will be performed in the pages to follow.

#### **1.2. Regeneration, necrosis, apoptosis and senescent: a constant changing balance**

has increased by 163% (from 0.89 million to 1.45 million) and in 2013, viral hepatitis infections became the 7th leading cause of death [7]. While HBV infection is decreasing in most endemic areas due to successful vaccination policies, HCV infection lacks the benefits of a vaccine [7]. Even though HVC antiviral therapies recently introduced in clinical practice are highly successful, its implementation is limited due to access and/or financial constraints. Morphological studies in HCV showed hepatocyte lipid accumulation similar to the one that occurs in obesity. Our group has estimated that 2.2 billion people will be overweight and 1.1 billion people will be obese globally by 2030 [8, 9]. In addition, 36.1% of adult men and 32.4% of adult women were diagnosed with the metabolic syndrome in 2010 [10]. Obesity represents the core component of the metabolic syndrome, a cluster of metabolic disarrangements including dyslipidemias, insulin resistance status, hypertension, diabetes and organ metabolic disturbances such as non-alcoholic fatty liver disease (NAFLD) and its inflammatory component non-alcoholic fatty steatohepatitis (NASH), nephropathy, cardiomyopathy and muscle dysfunction [10]. Hepatocellular carcinoma (HCC) has been reported more often in non-cirrhotic livers in the background of NASH and its risks factors include male gender, older age, cigarette smoking, obesity and insulin-resistant states [3, 4, 11]. Overweight and obesity are associated with an overall increase in liver primary cancers of 17% and 89%, respectively [2, 11, 12] and males with a BMI > 35 had a 3.5–4 increase risk of liver malignancy [12] (**Figure 1**). Optimal management of HCC with curative intent includes resection or liver transplantation. Nevertheless, these therapies are limited because the

70 Hepatocellular Carcinoma - Advances in Diagnosis and Treatment

**Figure 1.** *Main causes of end stage liver disease (ESLD) and hepatocellular carcinoma (HCC).* Hepatitis C virus infection (HCV) and obesity are the main cause of the global and Western increase in ESLD and HCC. In USA 1 out of 3 adults is overweight and approximately 5 M people is HCV seropositive. While prevention relays in stopping virus transmission and implementing programs of healthy caloric intake and exercise programs, treatments of established ESLD and its malignant consequence are similar. Nevertheless, the most effective surgical treatment, liver transplantation is limited due to scarcity of donors and loco-regional therapies have limited survival effect due to malignant recurrence or progression of liver dysfunction.

The liver is a unique organ with an innate ability to regenerate through mass compensation to satisfy portal flow and metabolic demands [14]. After injury and cell necrosis, immune recall of resting cells occurs and activation of oval-precursor cells in conjunction with platelets migration switch to a cell division renewal cycle [15, 16]. Mitosis is more prominent at the peri-portal stem cell niche site (zone 1) assuring clonal expansion until reaching zone 3 (peri-central vein) [15, 16]. In health, liver mass homeostasis is closely regulated through a delicate balance among regeneration, apoptosis (programmed cell termination), and senescence. During states of acute liver injury, the pendulum moves towards a regenerative and repair phase, however, during chronic states of liver injury collagen synthesis and deposition persists leading to organ fibrosis. In addition, natural processes of organ aging play a main role in organ response to both acute and chronic injuries. Primary cell life span is determined by a limited number of cell duplications, the so called Hayflick limit [17]. After such limited divisions, cells enter a state of cell replicative senescence which is believed to be triggered by shortening of telomere ends. Replicative cellular senescence is a stable form of cell arrest characterized by a lack of cell proliferation activity and apoptosis resistance mediated through a lack of mitogen response even though the cells remain metabolically active. On the other hand, cells can be induced to a senescence status by a variety of cellular stressors such as DNA damage, UV light, radiation, oncogene activation, increased H<sup>2</sup> O2 production and heat stress [17, 18]. Senescent cells undergo morphological changes as they acquire an enlarged and flattened morphology, in addition to an increase expression of the senescence associated markers ß-galactosidase (SA-ß-GAL), an accumulation of the senescence associated heterochromatic foci (SAHF) and DNA damage foci, and the expression of the senescence associated secretary phenotype (SASP) [18]. Senescent status is achieved and maintained by active signaling of p53, a tumor suppressor gene that exercises its effects through activation of p21, a potent cell cycle inhibitor, and the p16-retinoblastoma protein [18]. Cells induced into an irreversible cell cycle arrest at the G1 phase will undergo metabolic disturbances with an increase reactive oxygen species (ROS) production, decrease adenosine triphosphate (ATP) synthesis and accumulation of lipofuscin [17].

Changes in the content of daily oral intake can influence life span and thus cell aging. The cell death-inducing DNA fragmentation factor α-subunit-like effector A (Cidea), is a transcriptional coactivator implicated in lipid accumulation, cell stress and cell aging. Authors showed in rodents, that a high lipid diet up-regulated Cidea with hepatic lipid accumulation, cell stress, mitochondrial dysfunction and genetic upregulation of aging [19]. Other studies, in support of this findings have shown a life span reduction up to 30% in genetically obese mice (ob/ob) and this reduction was reversed by a caloric restricted diet [19]. Lipid enriched diets are associated in humans with DM type 2, HTN and cardiovascular events all

cellular components, support redox-state and regulate the function of highly metabolic active cells as in hepatocyte and immune cells [20, 21]. ROS in excess from over-oxidation of lipids, proteins, nucleic acids and other macromolecules is associated with a misalignment of their functional activity, reactions that if they last through the cell cycle can lead to permanent cell dysfunction and/or accelerated aging cell process. Thus, an excess of food intake in form of continuous lipid charge will test the oxi-redox systems that keeps the fragile mitochondrial equilibrium in balance. Continuous metabolic stress changes the equilibrium towards lower levels of antioxidants (glutathione sp.) with further increase of ROS that in turn accelerates processes of apoptosis and senescence. Former processes further lead to arrest of regeneration and activation of hepatic stellate cells (HSC) and therefore fibrogenesis (**Figure 2A**).

Cellular Senescence and Their Role in Liver Metabolism in Health and Disease...

http://dx.doi.org/10.5772/intechopen.71659

73

The hormonal mielau modulates cellular response to caloric intake. Insulin and somatotropin signaling are critical not only in the control of cell aging and longevity under conditions of abundant food supply but also in mediating the effects of caloric restriction on life span. In a rodent model of thyroxine induced aging, thermogenesis was directly correlated with increased mitochondrial function, increased ROS production, decreased concentration of glutathione reduced, downregulation in the activity of antioxidants enzymes and increased senescent marker expression in the liver as well as in other organs [21]. Estrogen influences lipid metabolism through nuclear receptors which enhances apoptosis of mutated cells, improves mitochondrial function, and decreases the metabolic syndrome phenotype [10, 22]. Actions that may explain, at least in part the constant disparity of overall life expectancy by gender.

Lifestyle changes such as exercise and caffeine supplementation have shown to increase the ratio of reduced/oxidized glutathione in liver and muscle tissue in the rodent model [23]. Although liver enzymes were identical in experimental and controls groups, plasma levels of cytokines associated with inflammation (IL-1ß, IL-6, TNF-α and INF-α) and cell aging were found to be significantly decreased in the experimental group when compared to controls [23]. It was noted that although exercise increased the production of ROS, exercise also evoked a beneficial increase in levels of cell antioxidants, and lowered levels of oxidative damage when cells were exposed to a second injury, i.e. lipid charge. Thus, the concept of exercise inducing gene expression of antioxidant enzymes that may protect the cell from other insults was called 'hormesis' [23]. Although caffeine, a member of the methyl-xanthine family increased the ratio of reduced-oxidized glutathione, no other markers of cell stress were modified. Perhaps,

The reduced tri-peptide glutathione (GSH) is the major antioxidant in the body responsible for maintaining the intracellular redox balance. 90% of the GSH in plasma derives from the liver [24] and aging is associated with a progressive decline in the levels of GSH in humans and rodents [25]. Senescent liver cells in culture showed elevated ROS leading to a state of chronic oxidative stress. In addition, age associated decline in GSH has been linked to an activation of neural sphingolipid hydrolase enzyme (NSMase) and the accumulation of bioactive ceramide, a precursor of inflammation [25]. The availability of L-cysteine is the rate-limiting

caffeine potentiates further the beneficial effects of exercise.

**2. Liver metabolism in health and disease**

**Figure 2.** (A) *Morphological changes observed in the mice model of high fat diet (HFD) plus fructose (Western Diet) in the microbiota, fat content tissue and liver.* Liver cells accumulate FA in form of TG from the spill-over of lipid excess in the fat compartment and after saturated the normal processes of liver lipid metabolism. (B) *Local liver inflammatory response from lipid excess.* Lipotoxicity an addition to increase LPS activates SEC, HSC and Kupffer cells inducing more parenchymal cells into senescence and apoptosis which changes the local mielue into an inflammatory microenvironment. Continuous HFD decreased further mitochondrial function with lower ATP production and increase collagen deposition leading to progressive liver fibrosis, liver dysfunction. The state of progressive liver fibrosis due to a local and systemic inflammatory state results in an increasing insulin resistance status with the full metabolic syndrome phenotype. Its progression results in decompensated ESLD and the development of malignancy.

of which limit life span. Caloric restriction without malnutrition can extend life span while caloric excess has the opposite effect [20]. Thus, the choice of oral intake has a profound impact on life span.

The free radical intermediates hypothesis of cell aging still remains the most reasonable in the induction and maintenance of the senescent status [17, 21]. ROS, reactive nitrogen sp., and lipid peroxide are important regulators of cell signaling that provides reliable maintenance of cellular components, support redox-state and regulate the function of highly metabolic active cells as in hepatocyte and immune cells [20, 21]. ROS in excess from over-oxidation of lipids, proteins, nucleic acids and other macromolecules is associated with a misalignment of their functional activity, reactions that if they last through the cell cycle can lead to permanent cell dysfunction and/or accelerated aging cell process. Thus, an excess of food intake in form of continuous lipid charge will test the oxi-redox systems that keeps the fragile mitochondrial equilibrium in balance. Continuous metabolic stress changes the equilibrium towards lower levels of antioxidants (glutathione sp.) with further increase of ROS that in turn accelerates processes of apoptosis and senescence. Former processes further lead to arrest of regeneration and activation of hepatic stellate cells (HSC) and therefore fibrogenesis (**Figure 2A**).

The hormonal mielau modulates cellular response to caloric intake. Insulin and somatotropin signaling are critical not only in the control of cell aging and longevity under conditions of abundant food supply but also in mediating the effects of caloric restriction on life span. In a rodent model of thyroxine induced aging, thermogenesis was directly correlated with increased mitochondrial function, increased ROS production, decreased concentration of glutathione reduced, downregulation in the activity of antioxidants enzymes and increased senescent marker expression in the liver as well as in other organs [21]. Estrogen influences lipid metabolism through nuclear receptors which enhances apoptosis of mutated cells, improves mitochondrial function, and decreases the metabolic syndrome phenotype [10, 22]. Actions that may explain, at least in part the constant disparity of overall life expectancy by gender.

Lifestyle changes such as exercise and caffeine supplementation have shown to increase the ratio of reduced/oxidized glutathione in liver and muscle tissue in the rodent model [23]. Although liver enzymes were identical in experimental and controls groups, plasma levels of cytokines associated with inflammation (IL-1ß, IL-6, TNF-α and INF-α) and cell aging were found to be significantly decreased in the experimental group when compared to controls [23]. It was noted that although exercise increased the production of ROS, exercise also evoked a beneficial increase in levels of cell antioxidants, and lowered levels of oxidative damage when cells were exposed to a second injury, i.e. lipid charge. Thus, the concept of exercise inducing gene expression of antioxidant enzymes that may protect the cell from other insults was called 'hormesis' [23]. Although caffeine, a member of the methyl-xanthine family increased the ratio of reduced-oxidized glutathione, no other markers of cell stress were modified. Perhaps, caffeine potentiates further the beneficial effects of exercise.
