**10. Conclusions**

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. 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

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

increase in the expression of anti-apoptotic Bcl-2 [98].

472 Mitochondrial Diseases

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 and cellular loci dictates their specific function.

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

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 understanding of how estrogen induces carcinogenesis.

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 the varied effects of estrogen treatment described in this review. We also understand that despite the protective role of estrogen in inflammation, cardiovascular disease, and neurodegeneration, it promotes breast cancer through both nuclear and mitochondrial regulation. Further research is needed to understand the specific mechanisms of estrogen-induced mitochondrial changes in health and disease.

[7] Hirata S, Shoda T, Kato J, Hoshi K. Isoform/variant mRNAs for sex steroid hormone receptors in humans. Trends in Endocrinology and Metabolism. 2003;**14**(3):124-129

Estrogen and Mitochondrial Function in Disease http://dx.doi.org/10.5772/intechopen.73015 475

[8] Hirata S, Shoda T, Kato J, Hoshi K. Novel isoforms of the mRNA for human female sex steroid hormone receptors. The Journal of Steroid Biochemistry and Molecular Biology.

[9] Lewandowski S, Kalita K, Kaczmarek L. Estrogen receptor beta. Potential functional sig-

[10] Post WS, Goldschmidt-Clermont PJ, Wilhide CC, Heldman AW, Sussman MS, Ouyang P, et al. Methylation of the estrogen receptor gene is associated with aging and atheroscle-

[11] Ying AK, Hassanain HH, Roos CM, Smiraglia DJ, Issa JJ, Michler RE, et al. Methylation of the estrogen receptor-alpha gene promoter is selectively increased in proliferating

[13] Dupont S, Krust A, Gansmuller A, Dierich A, Chambon P, Mark M. Effect of single and compound knockouts of estrogen receptors alpha (ERalpha) and beta (ERbeta) on mouse

[14] Krege JH, Hodgin JB, Couse JF, Enmark E, Warner M, Mahler JF, et al. Generation and reproductive phenotypes of mice lacking estrogen receptor beta. Proceedings of the National Academy of Sciences of the United States of America. 1998;**95**(26):15677-15682

[15] Luconi M, Forti G, Baldi E. Genomic and nongenomic effects of estrogens: Molecular mechanisms of action and clinical implications for male reproduction. The Journal of

[16] Noteboom WD, Gorski J. Stereospecific binding of estrogens in the rat uterus. Archives

[17] Yang SH, Liu R, Perez EJ, Wen Y, Stevens SM, Valencia T, et al. Mitochondrial localization of estrogen receptor beta. Proceedings of the National Academy of Sciences of the

[18] Psarra AM, Sekeris CE. Steroid and thyroid hormone receptors in mitochondria. IUBMB

[19] Chen JQ, Eshete M, Alworth WL, Yager JD. Binding of MCF-7 cell mitochondrial proteins and recombinant human estrogen receptors alpha and beta to human mitochondrial DNA estrogen response elements. Journal of Cellular Biochemistry. 2004;**93**(2):358-373

[20] Sharma G, Mauvais-Jarvis F, Prossnitz ER. Roles of G protein-coupled estrogen receptor GPER in metabolic regulation. The Journal of Steroid Biochemistry and Molecular

[21] Revankar CM, Cimino DF, Sklar LA, Arterburn JB, Prossnitz ER. A transmembrane intracellular estrogen receptor mediates rapid cell signaling. Science. 2005;**307**(5715):1625-1630

rosis in the cardiovascular system. Cardiovascular Research. 1999;**43**(4):985-991

human aortic smooth muscle cells. Cardiovascular Research. 2000;**46**(1):172-179 [12] Couse JF, Korach KS. Estrogen receptor null mice: What have we learned and where will

they lead us? Endocrine Reviews. 1999;**20**(3):358-417

of Biochemistry and Biophysics. 1965;**111**(3):559-568

United States of America. 2004;**101**(12):4130-4135

Life. 2008;**60**(4):210-223

Biology. 2018;**176**(1879-1220):31-37

reproductive phenotypes. Development. 2000;**127**(19):4277-4291

Steroid Biochemistry and Molecular Biology. 2002;**80**(4-5):369-381

nificance of a variety of mRNA isoforms. FEBS Letters. 2002;**524**(1-3):1-5

2002;**83**(1-5):25-30
