**4. Mitochondrial senescence**

The mitochondria, a double membrane cell organelle varies in number and its presence is linearly associated with the metabolic activity of the organ and its required energy requirements in form of ATP. Within the mitochondrial matrix a series of biochemical reactions occur. Acetyl-choline primer is reduced through the tricarboxylic acid cycle converting glycolysis-derived pyruvate into NADH and succinate. The former compounds couple another set of reactions at the inner membrane border where the electron transport chain (ETC) is present to boil an oxidative phosphorylation process. The ETC is composed of five enzymatic complexes (I to V; NADH-CoQ, succinate-CoQ, CoQ-cytochrome reductases, cytochrome c oxidase and ATP synthase, respectively) where NADH is the substrate of ETC-C1 and succinate the substrate of ETC-CII [10]. After oxidation, electrons are transferred from Complex I to CII to CIII and finally to Complex IV where oxygen is reduced to form H<sup>2</sup> O. The electron transport process is coupled to a proton pumping process creating a proton gradient between the mitochondrial membranes, gradient that is dissipated by Complex V (ATP synthase) through ATP synthesis. A control mechanism is created by the "proton leak", mechanism that generates heat instead of ATP [10]. Much of the leak is a catalytic reaction generated by the uncoupling proteins (UCP's) which play an important role in reducing proton gradient, heat and ROS [10]. Mitochondrial aging and senescence are linked to reduced ATP production and increase ROS production, i.e. superoxide (O2−), hydrogen peroxide (H<sup>2</sup> O2 ) and hydroxyl radical (OH− ) which are mostly produced because electron leakage at the level of CI and CIII [17]. Mitochondria function benefit from the role of estrogen in plasma through its binding to the nuclear estrogen receptor that enhances a signaling to prevent oxidant stress and also inhibits the renin-angiotensin-aldosterone system [10]. Thus, sex differences in mitochondrial function may explain the disparity in overall survival between men and women, differences that may be taken into account during animal models studies.

scaffold where organ cells get structural support with a dynamic and continuous traffic of water, ions, metabolites, proteins and cells on passant to maintain organ physiology. As such, ECM interactions with organ cell components regulate cell differentiation, adhesion, proliferation, migration and survival [64]. The collagen family is the major fibrillar proteins of the ECM and the body (approx. 30% of the total protein contain) [64]. There are three main classes of collagen, fibril-forming which include types I, II, III, XI, XXIV and XXVII the most common varieties and their role is mainly mechanical by conferring tensile strength to both tissue and organs. Fibrilassociated collagens with interrupted triple helix (FACIT's) includes type IX, XII, XIV, XIX, XX, XXI and XXII; this subclass of proteins do not form fibrils themselves but bind to the surface of pre-existing collagen favoring fibril enlargement. Finally, type III collagen serves as anchoring collagen between the epithelial cells and the lamina reticularis constituting the basement membrane where type IV collagen is most abundant. Non-collagenous proteins include fibronectin, tenascin, laminins, fibrillins and matrix-cellular proteins. While the former peptides play a major role in cell differentiation, cell growth, adhesion and migration, matrix-cellular proteins, i.e. thrombospondin-1 and 2, osteonectin, osteopontin and cyr-61/connective tissue growth factor (CTGF) serve mainly as a vehicle for cell signaling. Proteoglycans are carbohydrate enriched proteins which retain large quantities of water regulating the smooth trafficking of molecules to

The ECM continuous remodeling is a complex process that integrates proteins and cellular components from local and distal environments [64]. The degradation of ECM proteins are closely controlled by matrix metalloproteinases (MMP's), a superfamily of zinc-dependent endopeptidases highly regulated by specific inhibitors such as the tissue inhibitor of metalloproteinases (TIMP's). In the liver, cellular component involved in collagen synthesis and deposition included HSC, MF and vascular smooth muscle cells [64]. In chronic liver injury, an override mechanism of collagen deposition regulation promotes massive ECM expansion. The characteristic features of abnormal liver fibrogenesis as a consequence of continuous liver injury and activation of collagen secreting cells include damage to the epithelial/endothelial barrier, recruitment of inflammatory cells, secretion of cytokines and other inflammatory mediators, further generation of ROS, progressive deposition of collagen with expansion of ECM and worsening organ fibrosis and subsequent metabolic changes of portal hypertension.

The mitochondria, a double membrane cell organelle varies in number and its presence is linearly associated with the metabolic activity of the organ and its required energy requirements in form of ATP. Within the mitochondrial matrix a series of biochemical reactions occur. Acetyl-choline primer is reduced through the tricarboxylic acid cycle converting glycolysis-derived pyruvate into NADH and succinate. The former compounds couple another set of reactions at the inner membrane border where the electron transport chain (ETC) is present to boil an oxidative phosphorylation process. The ETC is composed of five enzymatic complexes (I to V; NADH-CoQ, succinate-CoQ, CoQ-cytochrome reductases, cytochrome c oxidase and ATP synthase, respectively) where NADH is the substrate of ETC-C1 and succinate the substrate of ETC-CII [10]. After oxidation, electrons are transferred from Complex I to CII to CIII and finally to Complex

and from the cell with numerous signaling active sites for growth factors.

88 Hepatocellular Carcinoma - Advances in Diagnosis and Treatment

**4. Mitochondrial senescence**

The reasons why the mitochondrion conserves a cell independent genome are not clear, but it is intuitive to imply self-energy regulatory processes are united through a fine tune mechanism between energy expenditure (ATP use) and energy production (ATP synthesis) at every organelle level. It may provide an overall advantage for survival of the cell, the organ and entire biological living system. The gradual ROS response theory of aging argues a protective role of ROS in early life, when cell oxidative damage and ROS production are low; however, later in life ROS reaches a level where its beneficial effects (as the one observed in dietary restriction and/or exercise) are overcome by its detrimental effects elicited by a higher cell oxidative stress (as the one observed in high fat diet and sedentary habits) [17]. The effects that are amplified include loss of genomic controls (p53), microRNA dysregulation, loss of function of telomerase reverse transcriptase (TERT) and a lower immune-surveillance status. Although the role of p53 in the mitochondrion is not completely clear, p53 binds to the Peroxisome proliferator-activated receptor Gamma-Coactivator 1 alpha and Beta (PGC-1α and ß) fomenting their inhibition of expression and therefore downregulated oxidative function. In addition, p53 target p16 and p21, factors that triggers G1-phase cycle arrest by inhibiting cell cycle regulatory kinases Cdk4 and Cdk2 [17]. The third known effect of p53 at the mitochondrion level is to promote cell apoptosis by increasing mitochondrial membrane permeability with leakage of cytochrome proteins, a direct activator of the caspase cascade. The function of TERT is highly affected by levels of ROS production and its protective patterns are only observed with low ROS levels. The role of microRNA in the mitochondrial environment remains to be elucidated.

The Mitochondrial Free Radicals Theory of Aging (MFRTA) has been the most popular theory to explain the cell aging process where increasing production of mitochondrial ROS with lower ATP production are the main factors responsible for cell aging and corresponding mitochondrial ultrastructure changes [17, 91]. As mentioned, leakage of electrons at the level of CI and CIII transfer are larger with age and the higher potential for DNA damage. 8-oxo-7.8-dihydro-2′deoxyguanosine (8-oxodG) is one of the most abundant DNA mutations caused by oxidative conversion to guanosine. Furthermore, its accumulation follows an inverse and exponential curve against life expectancy in several mammals [17]. Recently, it was described that humans with longer longevity have a higher content of mitochondrial DNA (mtDNA) per cell in different organs, and support the notion of ethnic background on mtDNA influence and life span. The frequency of mtDNA mutations occurs at different rate depending on the organ. Skeletal and cardiac muscles, liver and kidney are more affected by somatic mtDNA mutations compared to other organs such as the skin and lung [17]. Furthermore, the clonal expansion of mtDNA mutations occurs via a phenomenon called genetic drift, a random propagation and expansion of DNA mutations occurring at each DNA replication. The drift of mutations may be more important in metabolically more active organs that require more energy expenditure and therefore more ATP synthesis. The expansion of mtDNA mutations may be enhanced not only by its duplication and drift but also by a lower state of DNA damage repair mechanisms [17]. The Base Excision Repair (BER) process is impaired in senescence and aging due to a loss of function to BER associated proteins CSA and CSB. Thus, the increase production of ROS creates a vicious loop of mtDNA mutations than in turn favor an increase production of ROS perpetuating and enhanced organelle dysfunction by defective reparative mechanisms. A naturally occurring thymidine to cytidine mutation in the mitochondrial stressors tRNAILE gene is associated with phenotypes of hypertension, hypercholesterolemia and hypomagnesemia [10]. Furthermore, the DNA A3243G mutation causes impaired insulin secretion and polymorphisms in the promoter of the UCP2 protein, alterations associated with increased incidence of obesity, reduced insulin secretion and DMII [10].

plasma based monitoring of such changes on disease progression and treatment response as well as for tumor screening may be possible by metabolomic liver prints in the near future [30].

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

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

91

There have been a myriad or reports on compounds that not only prevent but reverse cell aging and some even malignant development in the animal model [25, 93–102]. Curcumin, the major bioactive compound of turmeric spice, through its antioxidant and anti-inflammatory properties has been claimed to retard tumorigenesis and diabetes and to modulate lipid metabolism [103]. Furthermore, curcumin prevents the development of atherosclerosis and NASH, perhaps by the upregulation of a fatty acid binding protein present in adipocytes (aP2) but also found in macrophages (FABP-4). This protein is a cytosolic protein present in adipocytes and macrophages which modulates the trafficking of lipids/cholesterol processes and activation of inflammatory mechanisms through CD36 upregulation and reduced expression of NF-kß thus, decreasing cytokine secretion [103]. Prior studies showed that high fat diet and obesity promoted liver tumorigenesis by inducing chronic inflammation through the IL6/STAT3 pathway and, STAT3 activated tumors has been showed to be more aggressive in humans. Lycopene attenuated HCC occurrences in the animal model through downregulation of the STAT3 signaling [95]. The aqueous extract of *Ligustrum lucidum* fruit induced apoptosis through the activation of the caspase cascade and cellular senescence by upregulation of p21 and downregulation of RB phosphorylation [102].

Other molecules with promising cell aging and tumor repression properties included the COX-2 and a Na/K/ATP signaling mechanisms. Inhibition of 15-hydroxyprostaglandin dehydrogenase (15-PGDH), a prostaglandin-degrading enzyme, potentiates tissue regeneration in multiple organs in mice [43]. During a chemical screen, a low molecular compound was identified capable of selectively inhibiting 15-PGDH with the subsequent increase of PGE2 levels in bone marrow and other organs, accelerating hematopoietic recovery in mice receiving bone marrow transplant and tissue regeneration in the colon and liver. It also promoted tissue regeneration in mouse models of colon and liver injury. Selective COX-2 products may have rescued telomere dysfunction, cell senescence and tissue regenerative potential [74]. However, its mechanism and signal transduction remains to be determined. pNaKtide is a synthetic peptide that conserves the active sequence for the ligand-binding capacity to the ß-subunit of the transmembrane Na/K-ATPase. Although the Na/K-ATPase mainly exercise its function as an ion exchanger pump vital for cell survival, recently it was shown to elicit nuclear signaling that regulates mitochondrial function and cell energy production through a Src/ERK pathway [104–112]. Furthermore, pNaKtide prevents the development of atherosclerosis and fatty liver disease in the HFD mice model with significant amelioration of ROS. In addition, it downregulates collagen synthesis and inhibit growth of human cancer cells in vitro. Translation of promising compounds to the treatment of patients with NAFLD/NASH is expected in the near future to further prevent the consequences of advanced liver fibrosis and HCC development.

**Abbreviations**

4R-BP Factor 4E binding protein ADP Adenosine diphosphate

AGPAT sn-1-acyl-glycerol-3-phosphatase acyltransferase

Mitochondrial function may be impaired in chronic high fat diet challenge as a result of a decrease in ß-lipid oxidation. Indirect evidence showed an accumulation of diacylglycerol and fatty-Acyl-CoA which in turn activates stress-related serine/threonine kinase activity and inhibit glucose transport [10]. Oxidative stress contributes further to impaired insulin signaling increasing UCP2 activity which in turn enhances "proton leak" with uncoupling of the glucose metabolism pathway and decreased ATP production. A progressive higher lipid peroxidation may favor further oxidative stress with DNA damage and low DNA repair by affecting members of the Bcl-2 family, triggering an influx of Ca2+ with subsequent opening of the mitochondrial permeability transition pore, cytochrome-c leakage to the cytosol and activation of the caspace-3 complex. Cell self-digestion and nuclear DNA fragmentation overcomes with the typical cell fragments morphology [10]. Alternatively, DNA damage and telomerase shortening results in mutations that may affect mitochondrial function to a level of organelle survival but inefficient ATP production assuring the "apoptosis switch" and diverting biochemical reactions to a cytosolic site for ATP production. The later assumption may find some support in the observations that tumor development and early growth is favored in low O<sup>2</sup> delivery zones and that tumor development is associate with increase lactate production, the Warburg effect [30, 69].
