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Estrogen Is Evolution's Most Ancient Steroid Hormone
May 3, 2001 7:50 CDT

A research biologist at Columbia University has reconstructed a DNA sequence that existed over 450 million years ago to reveal how new hormones emerged during evolution. The finding concludes that the female hormone estrogen is the most ancient of all steroid hormones, but that the role it has in differentiating the sexes from each other developed much later on.

The results of the study were reported Tuesday in the Proceedings of the National Academy of Sciences by Joe Thornton, a research scientist and evolutionary biologist at Columbia's Earth Institute. The findings complete a piece of a deep evolutionary puzzle by explaining how new hormones and other molecules emerged as parts of complex biological systems.

Steroid hormones are important to many biological processes, but their evolutionary origins have been a mystery. Steroid hormones ******* the sex hormones estrogen, testosterone, and progesterone and the adrenal hormones cortisol and aldosterone, which regulate behavior, immunity, and the body's response to stress and changing environments.

These hormones circulate within the blood and diffuse into target cells, where each binds to its own specific receptor -- a protein that turns genes on and off when its hormone partner is present. All vertebrates, ranging from humans to fish, have versions of the same six steroid hormone receptors. Invertebrates such as insects and nematodes have none.

The goal of Thornton's research was to answer the question of how hormones and their receptors evolved - it poses sort of an evolutionary chicken-or-the-egg puzzle. "Darwinian theory explains how a feature that improves an organism's fitness -- a mutation in some essential protein, for instance -- spreads through a population, but it doesn't explain how the body's proteins evolved in the first place," Thornton said. "The function of many proteins is to interact specifically with other molecules in tightly integrated regulatory systems. When the function of each part depends on the prior existence of all the other parts, the Darwinian model breaks down."

The puzzle was elegantly illustrated through the evolution of hormones and their receptors. Thornton explained: "What's the selective pressure that drives the evolution of a new hormone if there's not already a receptor to give it a function? Conversely, how does a new receptor evolve unless there's a hormone present? "I wanted to reconstruct the series of events by which new hormones and receptors emerged during vertebrate evolution to find a solution to this problem."

Thornton studied steroid receptors present in lamprey -- jawless fish that diverged from the rest of the vertebrate lineage about 450 million years ago. He used a gene identification technique called the polymerase chain reaction (PCR) to establish that lampreys have just three of the six hormone receptors that are found in humans and other jawed vertebrates -- an estrogen receptor, a progesterone receptor and a single corticoid receptor, but no androgen receptor.

Thornton's results showed that lampreys represent an intermediate stage in the evolution of the endocrine system, before steroids regulated the development of secondary sexual differences, as they do in other vertebrates. "Long before steroids were involved in making the sexes look strikingly different, they served the same reproductive and developmental functions in both males and females," he said, which was supported by the fact that estrogen regulates the transition to reproductive maturity in both male and female lamprey.

He then used molecular phylogenetic methods to reconstruct the evolutionary relationships among the lamprey receptors and those of dozens of other vertebrate species based on their gene sequences. The results, together with mapping data on the receptors' positions in the human genome, indicate that the full complement of hormone receptors evolved in two complete duplications of the vertebrate genome.

One genome expansion occurred before the divergence of lampreys from other vertebrates 450 million years ago, and the other happened after that event but before the divergence of fish from the lineage that led to mammals, reptiles, birds and amphibians, about 50 million years later.

Genome duplication proved to be a way for new receptors to evolve, but the order in which the hormones and their receptors appeared remained unknown. Thornton used a sophisticated computational technique and a statistical model of protein evolution to back-calculate the gene sequence of the ancestral hormone receptor at the root of the steroid receptor evolutionary tree. The analysis of that sequence made its identity clear. "It's quite convincing that the ancestral steroid receptor was an estrogen receptor of some sort," Thornton said, "because its gene sequence is extremely similar to that of the estrogen receptors found today, but it's not at all like those of the other hormone receptors."

The similarity was striking at sites in the sequence known to confer the ability to recognize specific hormones and target genes. The ancient nature of the estrogen receptor suggested a solution to the chicken-and-egg problem. Steroid hormones are produced in a common biochemical pathway in which progesterone is converted to testosterone, which is then transformed into estrogen.

Thornton found that the last hormone in the pathway was the first to have a receptor, which was a surprising result in a field whose accepted wisdom is that complex systems and structures are gradually elaborated and optimized in a step-by-step fashion. However, this same result suggested a solution to the puzzle, because it implied that less ancient steroid hormones -- progesterone and testosterone in particular -- had been present as biochemical intermediates before the receptors that recognize them evolved. "Once ancient organisms had estrogen and an estrogen receptor, they had to produce the other steroids in the process of making estrogen. When new receptors were created by gene duplication and then evolved affinity for these steroids, they turned what had been mere biochemical steppingstones into bona fide hormones," Thornton said.

"The solution to the which-came-first problem is that the hormones came first, but they weren't hormones and had no function per se until their receptor partners emerged in the vertebrate genome," Thornton explained.

This model has practical implications for scientists studying human receptors that control important aspects of development and disease. "If this is a general dynamic, then hormones for the many orphan receptors whose hormones haven't been identified will be found among intermediates formed in the synthesis of hormones for evolutionarily related receptors," Thornton said.

And if estrogen is the oldest hormone, it must also be the most widely distributed among organisms. "Sensitivity to estrogenic pesticides and industrial chemicals in the environment may therefore be quite broad, and it's possible that the endocrine systems of a very large variety of animal taxa could be disrupted by these pollutants," Thornton said.

The two-year study was supported by the National Science Foundation and was completed seven months ago. Thornton is now working to chemically synthesize the gene sequence he reconstructed, the functions of which will then be tested with techniques that molecular endocrinologists use to study present-day hormone receptors. He is also trying to figure out exactly when during evolution the estrogen receptor first appeared.

The research spans two related areas. Besides studying gene family evolution, he focuses on the public health and policy implications of global toxic pollution. Thornton's book, "Pandora's Poison: Chlorine, Health and a New Environmental Strategy" (MIT Press: 2000) analyzed global chemical contamination and its impact on human and wildlife health. The scientific journal Nature described it as a "landmark book" and the Washington Post said it was "one of the most impressive environmental books in recent years."

Source: UniSci; Image courtesy of University of Illinois

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~~Samantha~~
AIM: MercuryAdore

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