In the past few years, headlines trumpeted a decline in sperm production in men around the world. Many observers single out certain synthetic chemicals, known as endocrine disruptors, as the culprits in this decline. But critics of this theory argue that there is no solid proof that sperm counts are actually dipping or, if they are, that synthetic chemicals play a role. Because of the uncertainties surrounding this issue, and its potential global importance, scientists have begun investigating endocrine disruptors and their effects on the reproductive system. Population Council reproductive biologist Matthew P. Hardy, along with Benson Akingbemi—a postdoctoral fellow in reproductive biomedicine at the Council’s Center for Biomedical Research—and other colleagues, studied the effect of a common pesticide on Leydig cells, which produce testosterone, a hormone essential to sperm production.
Mimics of estrogen
Chemicals used in everything from pesticides to plastics have been found to disturb the endocrine—or hormonal—system, hence the name endocrine disruptors. One class of endocrine disruptors, known as environmental estrogens, mimics the hormone estrogen. Hardy, Akingbemi, and their team investigated the chemical HPTE, a metabolite of the common pesticide methoxychlor. HPTE readily binds to estrogen receptors. Although estrogen is the primary female sex hormone, men’s bodies also use estrogen. Methoxychlor—a pesticide in the DDT family—gained popularity after DDT was banned in the United States in 1972. “Compared to DDT, methoxychlor is less persistent in the atmosphere, is excreted from the body more rapidly, and is less toxic,” says Akingbemi. It has also, however, been associated with decreased weight of the testis, seminal vesicles, and prostate in rats.
In test-tube experiments involving tissue from laboratory rats, the investigators examined the effect of HPTE on testosterone production in progenitor, immature, and adult Leydig cells. (As Leydig cells develop, they produce more testosterone.) The researchers treated each of these types of Leydig cells with graded concentrations of HPTE; they found that the more HPTE that Leydig cells were exposed to, the less testosterone the cells produced. HPTE inhibited testosterone production in progenitor and immature Leydig cells after ten hours of treatment, and in adult Leydig cells after 18 hours. These changes were not observed in cells unexposed to HPTE.
To determine the reversibility of HPTE-induced testosterone inhibition, the researchers incubated the three types of cells with the chemical for three, six, ten, 14, or 18 hours, then waited 18 hours in each case to see whether testosterone production in these cells recovered to the levels observed in control cells. When treated for three hours, testosterone production by immature and adult Leydig cells rebounded within the 18-hour recovery period, but remained significantly inhibited within progenitor Leydig cells. When treatment lasted six or more hours, however, immature and adult Leydig cells also failed to fully resume testosterone production within the 18-hour recovery period. According to the researchers, this suggests that, although all Leydig cells exhibit some sensitivity to HPTE, the less developed the Leydig cell, the more sensitive it is to this compound. This early sensitivity to HPTE is crucial given that other researchers have shown in rats that mothers can transfer methoxychlor and its metabolites to male offspring in utero and during lactation.
Estrogen receptors
The researchers determined that HPTE interferes with testosterone production by binding with estrogen receptors in Leydig cells. They discovered this by treating Leydig cells with a compound known as ICI 182,780, which binds to the two known types of estrogen receptor. In the presence of ICI 182,780, HPTE lost its ability to influence testosterone production.
Leydig cells convert cholesterol into testosterone through a series of reactions facilitated by various enzymes. When HPTE binds to the estrogen receptor, it sets off a reaction within the cell that decreases the activity of a crucial enzyme known as the cytochrome P450 cholesterol side-chain cleavage enzyme, which in turn reduces the amount of testosterone produced within the cell.
Because HPTE reduces testosterone production, the investigators believe that it could be a contributory factor in male infertility associated with methoxychlor and possibly other DDT-related compounds. Ironically, “while HPTE and similar agents are of concern for environmental reasons, studying them may also help us identify new molecular targets for male contraceptives,” says Hardy. In the future Hardy, Akingbemi, and their colleagues plan to study the effects of the endocrine disruptors bisphenol A and diethylhexylphthalate (DEHP) on Leydig cells. Bisphenol A is commonly used in dental sealants and in the lining of food cans, while DEHP is used as a plasticizer in the construction, food, and medical product industries.
Source
Akingbemi, Benson T., Ren-Shan Ge, Gary R. Klinefelter, Glen L. Gunsalus, and Matthew P. Hardy. 1999. “A metabolite of methoxychlor, 2,2-bis(p-hydroxyphenyl)-1,1,1- trichloroethane, reduces testosterone biosynthesis in rat Leydig cells through suppression of steady-state mRNA levels of the cholesterol side-chain cleavage enzyme,” subsequently published in Biology of Reproduction. 2002. 62: 571–578.
Outside funding
National Institute of Environmental Health Sciences
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