Xenohormone
Xenohormones or environmental hormones are compounds produced outside of the human body that exhibit endocrine hormone-like properties. They may be either of natural origin, such as phytoestrogens, which are derived from plants, or of synthetic origin. These compounds can cause endocrine disruption by multiple mechanisms including acting directly on hormone receptors, affecting the levels of natural hormones in the body, and by altering the expression of hormone receptors. The most commonly occurring xenohormones are xenoestrogens, which mimic the effects of estrogen. Other xenohormones include xenoandrogens and xenoprogesterones. Xenohormones are used for a variety of purposes including contraceptive & hormonal therapies, and agriculture. However, exposure to certain xenohormones early in childhood development can lead to a host of developmental issues including infertility, thyroid complications, and early onset of puberty. Exposure to others later in life has been linked to increased risks of testicular, prostate, ovarian, and uterine cancers.
Etymology
The term is derived from the Greek words ξένος, meaning "stranger". The prefix "xeno-" is added because xenohormones are foreign to the body, even though they mimic natural hormones.Endocrinology
When present in excessive amounts in the human body, xenohormones can cause a host of health issues due to their disruption of the endocrine system. The name given to these exogenous hormones is endocrine disruptors, due to their tendency to mimic the behaviors of naturally produced bodily hormones. Endocrine disruptors have also been found to affect the levels and behaviors of a number of other bodily hormones. Because of this, it is difficult to establish a definitive relationship between xenohormones and health problems, making effects hard to predict.Xenohormones pose a problem because they are retained by the body in fat tissue for a very long time. As we are exposed more and more to these chemicals, they build up within the body in ever-increasing amounts, a process known as bioaccumulation. With the amount of xenohormones we are exposed to every day, the health effects of xenohormones are becoming more relevant.
The human body's endocrine system functions through hormones that act as messengers within the body. Hormones, upon release, travel through the body to their receptors and trigger a physiological response. Hormones naturally work at very low concentrations in the body. This means that even low concentrations of xenohormones in the body can act as an excess and have a profound effect on the body's endocrine system.
The levels of hormones present in the body at any given time are tightly controlled through feedback mechanisms. When xenohormones are present in the body, they alter the levels of hormones in the body and therefore alter the feedback mechanisms that the endocrine system relies on.
Xenohormones can interact with the human endocrine system because they are structurally similar to natural hormones. This similarity allows for xenohormones to act on hormone receptors, usually either as an agonist or antagonist. Agonists activate a receptor by binding to the receptor, enhancing the effect of the natural hormone. Antagonists inhibit the activation of a receptor by preventing the binding of the natural hormone to its receptor. In this way, xenohormones act as endocrine disruptors by increasing or decreasing the activation of hormone receptors in the body.
Xenohormones can often act on multiple hormone receptor types and enact multiple different effects. For example, BPA acts as an agonist of estrogen receptors and as an antagonist of androgen receptors. Methoxychlor is an organochlorine pesticide that can act on both estrogen receptors and androgen receptors.
Other than having effects by directly acting on the endocrine receptors, xenohormones can also act to decrease the availability of natural hormones. Phthalates inhibit testosterone synthesis and decrease the production of natural androgens in the body. Dioxins and some organochlorine pesticides can cause increased metabolism of estrogen, decreasing the amount of estrogen in the body. Xenohormones can also alter the expression of hormone receptors to either increase or decrease the amount of receptors available in tissues.
When xenohormone exposure occurs during the early developmental stages of life, the effects tend to be permanent. The consequences of excessive xenohormone exposure in adulthood are different, and typically more temporary in nature. This is to say that the health risks can be minimized if the individual is removed from their state of excessive exposure. Xenohormone-related issues in adults frequently take the form of increased cancer risk in reproductive/secondary sexual areas.
Xenoestrogens
Xenoestrogens are xenohormones that mimic the effects of natural estrogen. When present in the body, xenoestrogens can bind with estrogen receptors in the brain, leading to a disruption in the gonadal endocrine system.Xenoestrogen exposure during different developmental periods can have differing effects on the reproductive system. Prenatal and perinatal exposure results in greater reproductive defects than exposure in adult life.
The negative effects of excessive xenoestrogen involve a long list of developmental abnormalities, especially when the exposure occurs during a critical postnatal period. When high levels of xenoestrogen are experienced shortly after birth, urogenital tract and nervous system development are hindered, as they are known to be especially sensitive to hormonal disruption.
Known xenoestrogens include bisphenol A, the organochlorine pesticide methoxychlor, and the insecticide endosulfan.
Xenoandrogens
Xenoandrogens are xenohormones that mimic the effects of natural androgen hormones. Androgen hormones are often associated with males and include the major hormone testosterone. Androgens work on the metabolic system playing roles in muscle growth, bone formation, and endocrine function.There are not many compounds found in nature that are capable of interacting with human androgen receptors, so humans are most likely to come into contact with man-made xenoandrogens by taking anabolic steroids or through pollutants that contain xenoandrogens. Organochlorine pesticides, polychlorinated biphenyls, and polychlorinated dibenzo-p-dioxins/dibenzofurans are several pesticides known to contain xenoandrogens.