En muchas especies de mamíferos, incluida la humana, las hembras viven más tiempo que los machos. Como ya se ha comentado anteriormente, según la teoría del envejecimiento por radicales libres, el principal factor influyente en la variación de la duración de la vida es la producción de ROS, especialmente en la mitocondria. En consecuencia, la diferencia entre sexos en la esperanza de vida se ha atribuido a que esta producción en las mitocondrias de las hembras es menor que en las de los machos (Viña et al., 2005). Esta divergencia en la producción de ROS podría obedecer a diferencias mitocondriales más profundas. En este sentido, se ha atribuido a los estrógenos un efecto importante en la morfología y biogénesis mitocondriales, observándose mitocondrias mayores y con crestas mejor definidas en hembras (Rodriguez-Cuenca et al., 2002; Chen et al., 2005). Además, el 17β-estradiol, incrementa la transcripción de genes que codifican para proteínas de la CRM y su ensamblaje en los complejos, lo que aumenta la función de la CRM y la fosforilación oxidativa (Chen et al., 2005). Paralelamente, el 17β-estradiol muestra un efecto compensatorio estimulando la expresión de genes que codifican para enzimas antioxidantes como la Mn-SOD y la GPx y estimulando también sus actividades (Chen et al., 2005; Viña et al., 2005), mostrando así, las hembras, el doble de expresión y de función de dichas enzimas que los machos (Strehlow et al., 2003; Viña et al., 2005).
Estudios realizados en el tejido adiposo marrón comparando ambos sexos han confirmado que las ratas hembras poseen mitocondrias de mayor tamaño y más funcionales, además de mayor capacidad termogénica que los machos (Rodriguez et
Introducción / Introduction
39 al., 2001; Rodriguez-Cuenca et al., 2002). También se han observado mitocondrias con más proteína, es decir, más diferenciadas, en el tejido hepático de ratas hembra (Valle et al., 2007). Igualmente, se ha publicado mayor capacidad oxidativa y fosforilativa y mayor actividad de la enzima GPx en el tejido adiposo marrón de las hembras (Justo et al., 2005; Valle et al., 2007) y también de la SOD (Pinto et al., 1969;
Borras et al., 2003), así como menores niveles de ADN oxidado (Borras et al., 2003).
El músculo esquelético también presenta mayor contenido de mitocondrias, capacidad oxidativa y fosforilativa y actividad GPx (Colom et al., 2007a). En el músculo cardiaco, también se ha puesto de manifiesto menor contenido de mitocondrias pero con mayor eficiencia fosforilativa, indicando un mayor nivel de diferenciación mitocondrial en las ratas hembra, así como una menor producción de ROS y en consecuencia menor daño oxidativo (Colom et al., 2007b).
Por otro lado, los estrógenos tienen efectos neuroprotectores (McEwen, 2001;
Behl, 2002b; Nilsen et al., 2004). Así, se ha descrito que una terapia de reemplazo de estrógenos retrasa la aparición de enfermedades neurodegenerativas (Behl, 2002a;
Czlonkowska et al., 2003). Por tanto, la bien conocida actividad neuroprotectora de los estrógenos podría estar relacionada con la diferente prevalencia entre géneros de determinadas enfermedades neurodegenerativas.
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Introducción / Introduction
41 1. Aging
Aging is a biological process characterized by a generalized physiological decline and an increase in susceptibility to diseases. According to the ‘disposable soma’ theory, organisms would use needed metabolic resources for effective somatic maintenance in order to keep the organism in sound physiological condition for as long as they have a reasonable chance of survival and reproduction (Kirkwood, 1977).
Given that, in the wild, animal death generally happens due to different causes other than aging, natural selection would have favoured pleiotropic genes with good early effects even if these genes had bad effects at later ages (Williams, 1957) or would simply have allowed mutations with late-acting deleterious effects to accumulate (Martin et al., 1996). So, it is clear that there are many genes influencing the aging process, which is the final step of ontogenesis.
A wide range of cellular alterations in somatic tissues has been observed in aging, especially in mitochondria and their DNA. Mitochondria are the main organelles providing the host cell with ATP as energy needed for its operation. Mitochondria also play a central role in intracellular Ca2+ homeostasis, intermediary metabolism, intracellular signalling and regulation pathways, steroid synthesis, free radical oxygen species (ROS) production and apoptotic cell death. As a consequence, mitochondrial dysfunction has devastating effects on cell integrity and is therefore involved in aging and degenerative diseases (Kann et al., 2007).
In aging organisms, cellularity decreases due to the prevalence of apoptosis over proliferation. Thus, cellularity would be the main difference between young and old organisms, that is, the number of functional cells in organs or tissues, rather than their quality (Severin et al., 2009).
2. Mitochondria
The mitochondrial inner membrane contains the mitochondrial respiratory chain (MRC) complexes, or electron transport chain complexes (Figure 1). These complexes are integral membrane proteins with prosthetic groups that act as redox centres
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capable of catalyzing electron transfer from one complex to the next. The final component is ATP synthase, responsible for ATP formation.
MRC complexes, except for Complex II (succinate-ubiquinone oxidoreductase or succinate dehydrogenase) have, as well as their electron transport function, the capacity of proton translocation from the mitochondrial matrix into the intermembrane space. This function can be prevented by complex inhibitors such as rotenone, specific inhibitor of Complex I (NADH-ubiquinone oxidoreductase or NADH dehydrogenase), antimycin, specific inhibitor of Complex III (ubiquinol-cytochrome c oxidoreductase) or cyanide and carbon monoxide, specific inhibitors of Complex IV (cytochrome c-O2
oxidoreductase, cytochrome c oxidase or COX). The electron shuttle between complexes is carried out by smaller, more diffusible, molecules such as the lipidic ubiquinone coenzyme Q, from complex I and II to complex III, and cytochrome c, peripheral membrane protein, which shuttles electrons from complex III to complex IV - the final electron acceptor - which reduces O2 to H2O. ATP synthase, also known as complex V, is the channel through which protons go back into the matrix and can be specifically inhibited by oligomycin (Walker et al., 1995; Boyer, 1998).
Stoichiometry of MRC complexes shows a simple molar ratio, which means respiratory carriers and complexes are arranged in assemblies with specific protein-protein interactions, which are effective for rapid electron transfer by limiting intermolecular distances (Chance et al., 1956).
Mitochondria play a key role in energy metabolism since they are the organelles responsible for supplying most of the energy needed for cellular activity. Mitochondria consume approximately the 90% of the oxygen arriving at cell in oxidative phosphorylation, a mechanism that produces energy as high energy molecules: ATP, which constitutes the source of most ATP used by cell. Oxidative phosphorylation entails nutrient oxidation to CO2 and H2O. Together with substrate oxidation, reduction of nicotinamide adenine dinucleotide (NADH) in complex I and flavin adenine dinucleotide (FAD) in complex II takes place, thus becoming the initial donors of electrons, which then flow throw the MRC of the inner membrane following the oxidation potential of their components.
The differences in redox potential of the electron carriers define the reactions that are exergonic enough to provide the free energy required for the coupled H+ pumping from the matrix into the intermembrane space. This H+ translocation causes the matrix to become negatively charged and the intermembrane space positively
Introducción / Introduction
43 charged, producing an electrochemical gradient that generates a proton-motive force great enough to drive ATP synthesis when protons go back into the matrix through the ATP synthase complex.
Figura 1: Mitochondrial Respiratory Chain (MRC). Proton translocation through complex I, III and IV and ATP generation in ATP synthase or complex V. Q: ubiquinone. Cyt c: Cytochrome c.
Source: Lehninger et al.
The rate of mitochondrial electron transfer and ATP synthesis depend on cell energy requirements. When energy needs increase, ATP breakdown into ADP and Pi also rises. The increase in ADP availability speeds up the electron transfer generating ATP. Normally, phosphorylation capacity is tightly regulated, fluctuating only slightly in most tissues, although it is quite clear that oxidative phosphorylation is never completely coupled in mitochondria. MRC control and mitochondrial metabolic states were first defined in 1956 (Chance et al., 1956). State 4 or resting respiration is characterized by a great substrate but limited ADP availability, which causes a slow electron transfer rate leading to high ROS production, around 1 nmol H2O2/min·mg of
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protein. In State 3 or phosphorylating state, there are plenty of substrates and ADP in mitochondria, which causes a high electron transfer speed reaching the maximal physiologic rate of ATP production and O2 consumption and showing very low ROS production rate, around 0.1 nmol H2O2/min·mg of protein (Boveris et al., 2000). The electron transfer rate is between 3 and 8 times faster in State 3 than in State 4 in isolated mitochondria, thus the respiratory control rate is within this range (Chance et al., 1956). Mammalian mitochondria in physiological conditions have been estimated to be mainly in State 4, around 60-70%, and only the remaining 30-40% in State 3 (Boveris et al., 1999).
