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

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.

Neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD) are characterized by the progressive deterioration and death of specific neuron types in the brain [69]. It is well known that mitochondrial dysfunction plays a pathogenic role in neurodegeneration [69]. AD is characterized by atrophy of the cortex and hippocampus. Additionally, increases in glucose metabolism and a reduction in the activities of mitochondrial Complexes I, III, and IV in the brain have been reported [69, 70]. Plaques containing amyloid beta and tau protein can become incorporated in the remaining brain cells [69]. These inclusions are

In PD, the loss of dopaminergic neurons in the substantia nigra contributes to the development of motor disturbances. The protein alpha synuclein becomes entangled with these neurons, resulting in ROS production, mitochondrial dysfunction, and neuronal cell death [71]. A decrease in mitochondrial Complex I activity has been observed in postmortem brains of PD patients that is linked to a reduction in oxidative phosphorylation [72]. *In vivo* and *in vitro* models of PD utilize Complex I inhibitors, such as rotenone, to induce PD-like systems to study disease mechanisms [73, 74]. Most PD cases are of unknown etiology; however, data suggest that defects in the expression of the mitochondrial clearance genes parkin, pink1, and

Gender differences have been identified in the pathology of both AD and PD [75, 76]. These include alterations in brain weight, regional atrophy, distribution of white and gray matter, cerebral blood flow, expression of neurotransmitter transporters and receptors, and age of onset [77]. In the case of PD, there is also an increased incidence of the disease in men compared to women [78, 79]. The gender difference in neurodegenerative phenotypes has led to the hypothesis that estrogen exerts neuroprotective effects and that these effects are mediated at the level of the mitochondrion [80]. In mouse spinal cord neurons, estrogen treatment increased mRNA levels of nuclear-encoded mitochondrial electron transport chain genes ND1, CytB, Cox2, and ATP6 [80]. ERα has been identified in the mitochondria of endothelial cells in the brain and forebrain. In these cells, estrogen increased expression of cytochrome C and reduced ROS formation [81]. ERβ has been localized to hippocampal mitochondria [17, 81, 82]. Hippocampal cells isolated from ERβ KO mice had lower mitochondrial membrane potential and were more resistant to oxidative stress compared to control mice [83]. Taken together, these data suggest opposing effects of ERα and ERβ in the brain. More cell-type specific studies are needed to better understand the role of these ERs in the brain and to help clarify these opposing views. Estrogen also interacts with mitochondrial proteins in a non-genomic manner. Under *in vitro* conditions, estrogen had no effect on sodium dependent calcium influx from mitochondria isolated from synaptosomes and increased mitochondrial calcium efflux [84]. Thus, estrogen

**8. Estrogen, mitochondria, and neurodegeneration**

470 Mitochondrial Diseases

known to increase oxidative stress and mitochondrial dysfunction.

DJ-1 may be an underlying mechanism [69].

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 in the cytoskeleton that indirectly influences mitochondrial structure [92]. Alteration in mitochondrial structure not only affects the capacity of the energy production but also influences other crucial functions such as calcium signaling, ROS production, or biosynthetic processes.

peroxide formation. These data suggest that tamoxifen induces the mitochondrial cell death pathway. While current breast cancer therapeutics inhibit both estrogen signaling and mitochondrial function, the development of next generation drugs that can more efficiently inhibit

Estrogen and Mitochondrial Function in Disease http://dx.doi.org/10.5772/intechopen.73015 473

Estrogen is a multi-functional hormone that exerts its effects by both transcriptional and nongenomic mechanisms. Nuclear transcriptional responses are classically induced by binding of estrogen to ERα and ERβ. Estrogen-dependent gene transcription plays an important role in the development of female reproductive structures and regulation of the estrous cycle. The local production of estrogen is now also thought to regulate physiological responses in males. Non-genomic responses to estrogen are more rapid and include induction of signaling pathways that promote cell proliferation. The differential expression of ERs in different cell types

Studies in recent years have defined a role for estrogen in the regulation of mitochondrial structure and function. Estrogen increases expression of respiratory complexes, antioxidant molecules, and anti-apoptotic factors that directly impact mitochondrial structure and function. Aging in women is associated with a reduction in estrogen formation and the development of mitochondrial dysfunction. An increase in free radical damage in cells also occurs with aging. Damaged mitochondria are more likely to produce additional ROS, thus initiating a vicious cycle that progressively degrades cellular function. This includes estrogen biosynthesis. Transgenic mice with a mutation in the inner mitochondrial membrane peptidase-2 (IMMP-2) had hyperpolarized mitochondria, which produced increased levels of superoxide and ATP and resulted in impaired ovulation and reduced fertility [101]. Other data show that defective mitochondrial DNA polymerase activity induces mitochondrial dysfunction and infertility in mice [102]. It follows that cytoprotective responses to estrogen at the level of the mitochondrion are ablated. Thus, a reciprocal relationship exists between estrogen and mitochondrial function. Under normal physiological conditions, mitochondria are critical media-

Strong evidence suggests that estrogen plays a major role in promoting the proliferation of both normal and the neoplastic breast cancer cells. However, cancer represents a unique scenario in which estrogen exerts tumorigenic responses in susceptible cells. Prolonged exposure to high levels of estrogen is associated with an increased incidence of breast cancer, which supports models of estrogen-induced carcinogenesis. In breast cancer cells, estrogen is shown to not only stimulate the cell proliferation but also inhibit apoptotic pathways, which can therefore lead to uncontrolled tumor growth. Estrogen increases mitochondrial ROS production that can also promote cancer progression. Further research is needed to expand our

Estrogen and ER signaling in the mitochondria play an important role in health and disease. We have shown that estrogen effects are cell type and receptor type specific, which explains

these processes is required.

and cellular loci dictates their specific function.

tors of estrogen biosynthesis and are also targets for estrogen action.

understanding of how estrogen induces carcinogenesis.

**10. Conclusions**

Second, estrogen induces cellular ROS production in cancer cells by three main pathways: (1) direct inhibition of respiratory chain complexes; (2) accumulation of calcium within mitochondria; and (3) inhibition of the antioxidant response element (ARE) [92]. The increase in ROS generation by the mitochondria and the decrease in antioxidant capacity cause a cellular shift to high ROS production that plays an important role in cancer cell proliferation and cell damage. Estrogen also promotes ROS formation in breast cancer cells by inducing cyclin D1 gene expression [93]. NRF-1 regulates the expression of several nuclear-encoded mitochondrial genes [94] that encode respiratory protein subunits, mtDNA transcription/replication machinery, components of heme biosynthesis, and mitochondrial protein import [95]. Cyclin D1 phosphorylates NRF-1, resulting in repression of its activity. This inactivation of NRF-1 reduces mitochondrial activity and shift glucose metabolism toward glycolysis [96]. The reduction in mitochondrial ATP production, in part, increases mitochondrial ROS production.

Another characteristic of cancer cells is that they highjack apoptotic pathways in order to evade cell death. When normal cells are exposed to UV radiation, the generation of mitochondrial ROS activates c-jun N-terminal kinase (JNK) and protein kinase C (PKC)-δ. These signaling molecules trigger the translocation of *Bax* to the mitochondria and induce apoptosis. In breast cancer cells exposed to UV radiation, estrogen attenuates cytochrome C release, preserves mitochondrial membrane potential, and inhibits apoptotic cell death [97]. Other data show that addition of estrogen to MCF-7 breast cancer cells induced apoptotic signaling through the extrinsic cell death *Fas* ligand pathway. This response was accompanied by an increase in the expression of anti-apoptotic Bcl-2 [98].

The role of estrogen receptors in breast cancer development has been known for almost over 30 years. Subsequent studies strongly suggested that ER status is the single most important predictive and prognostic biomarker in breast cancer. Clinicians use several strategies to battle estrogen-sensitive breast cancer, which affect not only estrogen levels but also mitochondrial function. One approach is to block ovarian function. Ovarian ablation can either be performed surgically to remove the ovaries (oophorectomy) or by radiation. An alternative approach is to temporarily suppress the ovarian function pharmacologically using gonadotropin-releasing hormone (GnRH) agonists. GnRH interferes with signals that are produced by the pituitary gland that stimulate the ovaries to produce estrogen. GnRH agonists also act by inducing mitochondrial depolarization, thereby decreasing mitochondrial oxidative capacity. Aromatase inhibitors represent another pharmacological approach to inhibit estrogen synthesis. Addition of aromatase inhibitors to MCF-7 cells was shown to induce caspase 9 expression [99]. Thus, activation of the intrinsic cell death pathway may be an alternative mechanism by which aromatase inhibitors decrease cancer progression. Selective estrogen receptor modulators (SERMs) are another class of drugs that are used for treatment of breast cancers. The SERM tamoxifen blocks the ability of estrogen to stimulate the growth of breast cancer cells. Tamoxifen has also been shown to induce mitochondrial ROS and apoptosis by increasing mitochondrial nitric oxide synthase (mtNOS) [100]. Tamoxifen decreases cellular respiration, increases mitochondrial cytochrome C release, and increases mitochondrial lipid peroxide formation. These data suggest that tamoxifen induces the mitochondrial cell death pathway. While current breast cancer therapeutics inhibit both estrogen signaling and mitochondrial function, the development of next generation drugs that can more efficiently inhibit these processes is required.
