Infertility is common among obese women, but the reasons remain poorly understood and few treatments exist. Now a team of Johns Hopkins Children's Center scientists, conducting experiments in mice, has uncovered what they consider surprising evidence that insulin resistance, long considered a prime suspect, has little to do with infertility in women with type-2 diabetes, polycystic ovary syndrome (PCOS) and metabolic syndrome, all obesity-related conditions in which the body becomes desensitized to insulin and loses the ability to regulate blood sugar.
In a report, published online Nov.10 in the journal Diabetes, the Johns Hopkins scientists say the real culprit appears to be insulin sensitivity in the ovaries and the pituitary.
The Johns Hopkins team said its findings show that these organs escape insulin resistance and, awash with high levels of circulating insulin common in obesity, develop abnormal cell signaling that disrupts ovulation and eventually leads to infertility.
"Our findings suggest that the focus should shift from treating insulin resistance in peripheral tissue to taming insulin sensitivity in the pituitary and ovaries," says lead investigator Sheng Wu, Ph.D., of the Johns Hopkins Children's Center. Scientists traditionally have treated obesity-induced infertility by lowering blood insulin to counter the effects of insulin resistance.
A 2010 study by the same team discovered that the pituitary gland, insensitive to insulin in lean mice, became sensitive to elevated levels of insulin seen in human and rodent obesity. By knocking out the insulin receptors in the pituitary glands of obese mice, the researchers were able to partially restore fertility, thus proving that abnormal insulin signaling in the pituitary was only part of the story.
"In the original study, disrupting insulin signaling in the pituitary restored 50 percent of fertility in obese mice, but the search was on for the accomplice," says senior investigator Andrew Wolfe, Ph.D., an endocrinologist at the Johns Hopkins Children's Center. "Our new findings point to the ovaries."
In the pituitary, faulty insulin signaling stimulates increased secretion of luteinizing hormone, the researchers say. In the ovary, it puts testosterone production into overdrive. Both disrupt ovulation, the researchers explain.