**7. Estrogen, mitochondria, and inflammation**

isolated from the heart than other tissues. Estrogen induced a further increase in cardiac ATPase activity implying a direct link between estrogen stimulation and ATP production [45, 46]. Estrogen can therefore exert different effects on mitochondria from different cell types.

Cardiovascular disease (CVD) is the leading cause of death in men and women, and American Heart Association statistics reveal a significant increase in CVD mortality in women compared to men [47]. Mitochondrial dysfunction has been implicated as a causative factor in CVD, with mitochondrial DNA damage being significantly increased in the heart and aorta of patients with CVD compared to healthy controls [25, 48–50]. Women do not generally present with CVD until the seventh decade, while the incidence of death due to CVD is high in men throughout life. This age-dependent increase in CVD in women has been linked to the onset of menopause and a reduction in circulating estrogen levels. The "Free Radical Theory of Aging" proposes that, with increased age, an increase in free radical formation initiates a vicious cycle of ROS formation that causes progressive cell injury [51]. Data suggest that loss of the antioxidant and anti-inflammatory effects of estrogen after menopause contributes to the development of mitochondrial injury [52, 53]. Thus, maintaining high levels of estrogen

Studies using experimental animal models of CVD show that OVX increases vascular inflammation/injury in a manner that is prevented by estrogen treatment [54, 55]. Analysis of mitochondria isolated from hearts of OVX rats reveals increased levels of apoptotic markers compared to mitochondria of intact animals. Administration of estrogen to these animals significantly attenuated apoptosis [56]. Since mitochondrial damage and apoptosis can be mediated by ROS, it has been hypothesized that the estrogen can decrease ROS by activating the antioxidant pathway. Treatment of human aortic endothelial cells (HAECs) with estradiol upregulates the mitochondrial antioxidant MnSOD by an ERα-dependent mechanism. The ability of estrogen to increase MnSOD levels is ablated in ERα KO mice but not in ERβ KO mice. Interestingly, while ERβ does not regulate MnSOD expression, it was shown to be essential for preventing atherosclerotic progression *in vivo* [57, 58]. These data show that ERs modulate mitochondrial antioxidant production and have distinctive vasoprotective mechanisms.

Gender differences have been identified in mitochondrial genes isolated from rat hearts [59]. Whole genome microarray analysis showed that expression of genes associated with mitochondrial apoptosis pathways is significantly elevated in male mice compared to females. In contrast, genes associated with fatty acid and glucose metabolism were upregulated in females. Female rats also displayed higher transcription levels for mitochondrial Complexes I and IV. These data suggest that genes related to cellular metabolism, including mitochondrial respiration, are upregulated in cardiac mitochondria from female rats while genes associated with mitochondrial apoptosis are increased in males. Whether this difference is directly

These differential responses may impact disease pathogenesis.

468 Mitochondrial Diseases

**6. Estrogen, mitochondria, and cardiovascular disease**

may increase lifespan and/or health in postmenopausal women.

related to the circulating levels of estrogen *in vivo* is unclear.

Macrophages contribute to the chronic inflammation associated with many diseases including CVD and neurodegeneration. Macrophages display plasticity in that they may adopt various phenotypes. The differentiation of these cells is highly dependent on the local microenvironment in which they are situated. M1 macrophages are pro-inflammatory and are induced by cytokines and lipopolysaccharide (LPS). M2 macrophages are anti-inflammatory, play a role in wound healing, and are induced by IL-4 and IL-13 [63, 64]. The metabolic characteristics of M1 and M2 macrophages are different. M1 macrophages rely on glycolysis for ATP formation while M2 macrophages are dependent on mitochondrial oxidative phosphorylation for energy [63, 64]. Damage to mitochondria induced by inflammatory stimuli can exacerbate cellular injury [65]. It was shown that both ERα knockout and mitochondrial dysfunction inhibit the IL-4 mediated conversion to macrophages from an M1 to an M2 phenotype [64, 66].

Treatment of macrophages with LPS/interferon-γ (IFN-γ) favors an increase in the M1 phenotype. In macrophages isolated from premenopausal women, estrogen treatment was shown to reduce the M1/M2 ratio in cells exposed to LPS/IFN-γ to a greater extent than macrophages isolated from postmenopausal women [67]. Recent studies from our group have shown that there is a significant decrease in ERα expression in macrophages from postmenopausal women compared to premenopausal women while estrogen therapy was able to preserve ERα expression [68]. These data imply that estrogen and ER levels play a crucial role in macrophage polarization, but the role of estrogen on the mitochondrial function in these groups is still unknown. Determining the role of estrogen on the mitochondria and how it affects macrophage phenotype can help us to better understand the anti-inflammatory roles of estrogen.

prevented mitochondrial calcium overload. Higher levels of cytoplasmic calcium increase mitochondrial ATP production and cause neuron-specific changes in cellular signaling. In aged, post-reproductive rodents, loss of estrogen is associated with a decrease in brain weight and a concomitant increase in the utilization of ketone bodies and fatty acids [85]. This was associated with a decrease in metabolic substrates for mitochondrial ATP production that was further decreased in an AD mouse model [85]. Taken together, we and others propose that reductions in estrogen levels that cause decreased mitochondrial function during the postmenopausal period may explain the increased incidence of AD in women at this stage.

Estrogen and Mitochondrial Function in Disease http://dx.doi.org/10.5772/intechopen.73015 471

Estrogen preserves mitochondrial structure/function by upregulating the mitochondrial antioxidant enzyme MnSOD in the brain of female rodents [37, 86, 87]. In SK-N-SH neuroblastoma cells, estrogen inhibits the effects of the mitochondrial Complex II inhibitor, 3-nitroprionic acid (3-NPA), by preserving mitochondrial ATP production and inhibiting the 3-NPA induced hydrogen peroxide and peroxynitrite formation [88]. These data suggest that estrogen also plays an anti-oxidant role in the brain. This has led many to hypothesize that the anti-oxidant

Estrogen also regulates mitochondrial dynamics in astrocytes in a gender-dependent manner. It reduces expression of the fusion protein Mfn1 in astrocytes isolated from male rodents but has no effect on astrocytes obtained from females [80]. Treatment of cortical primary astrocytes with estrogen increases the expression of fission (Dyn 1 and Fis 1) and fusion (Mfn2) proteins to a greater extent in female mice than males. The upregulation of both fission and fusion proteins suggests that mitochondrial network is more dynamic in females than males. Although the exact mechanisms and reasons for the differences in fission and fusion regulation between male and female rodents are unknown, these responses may explain the sexual

Cancer is the second leading cause of death in the United States, and, among all cancers, breast cancer is the second most commonly diagnosed cancer in women. Breast tumor cells that express estrogen receptors are classified as ER-positive and account for 80% of all breast cancers [89]. Further, the Women's Health Initiative Study showed that menopausal hormone therapy (MHT) increases the incidence of breast cancer in women compared to controls [90]. Modern day cancer treatment principally focuses on identifying estrogen signatures in breast cancers, and suppression of estrogen receptor function is a routine therapeutic strategy.

Estrogen is known to regulate mitochondrial function in the context of breast cancer by several mechanisms. First, it has been shown to alter mitochondrial morphology. Administration of physiologically relevant doses of estrogen to MCF-7 breast cancer cell lines results in enlargement of mitochondria [91]. Mitochondrial cristae adopt an abnormal structure that is reminiscent of mitochondria that are oxygen-deprived and rely on glycolysis for ATP formation. This change in structure was associated with a 2.5-fold increase in the mitochondrial content of ERα and ERβ and an increase in the mitochondrial expression of cytochrome C oxidase subunits I and II. Alternatively, activation of cell membrane estrogen receptors is reported to induce changes

effects of estrogen may play a role in slowing disease pathogenesis.

dimorphism seen in neurodegenerative diseases and other pathologies.

**9. Estrogen, mitochondria, and cancer**
