The Ins and Outs of Steroid Hormone Transport Across the Plasma Membrane: Insight From an Insect

An intriguing study of the Drosophila fruitfly may change how we view steroid biology and possibly have a wider impact on the field of endocrinology and disease. Okamoto et al. (1) showed that cellular uptake of the molting hormone ecdysone, the fly’s primary steroid hormone, requires a plasma membrane importer, the protein Oatp74D. Oatp74D belongs to the organic anion–transporting polypeptide branch of a solute carrier organic anion (SLCO) superfamily of membrane proteins that is widely conserved across animal species. Oatp74D was identified in two independent screens and verified by genetic and biochemical experiments. The authors showed that the phenotype of Oatp74D-mutant larvae resembles that caused by ecdysone receptor deficiency. This demonstration of a membrane carrier for ecdysone and its biological relevance supports a model of facilitated entry into target cells and challenges prevailing views that steroid hormones because of their lipophilic nature cross the plasma membrane by simple diffusion.

The authors are notably cautious about the implications of their finding for steroid hormone action in mammals, perhaps reflecting widely held assumptions about the passive diffusion of steroid hormones into cells.

Several decades ago, initial suggestions that steroid and thyroid hormones acted in the nucleus prompted interest in how these hormones entered the cell to reach their site of action. The need to traverse the plasma membrane represented an additional step in the pathway of hormone action, in contrast to that of polypeptide hormones acting on receptors at the plasma membrane. Biochemical observations on the entry of steroid hormones into cells in culture led to opposing views for passive diffusion or membrane carrier-mediated mechanisms (2). In the absence of further evidence the debate lay unresolved, although assumptions about passive diffusion became common over the years. The harder task was to obtain proof for carrier-mediated transport because this would require identification of a specific membrane protein. The new report on the Oatp74D carrier protein as an insect steroid transporter (1) therefore represents a potentially key advance.

An interesting perspective is offered by the thyroid field, which has gone through a similar change of paradigm regarding how thyroid hormone is thought to cross the plasma membrane (3). Thyroid hormones (the active ligand triiodothyronine, T3, and its precursor thyroxine, T4) are hydrophobic and are taken up by many cell types in culture such that import by passive diffusion was often assumed. Nonetheless, biochemical arguments supported carrier-mediated transport, including the fact that T3 and T4 are derivatives of amino acids, a zwitterionic class of molecules that are known to require transporters for transmembrane passage. A breakthrough came from human genetics in 2004 and provided crucial physiological support for the concept of carrier-mediated uptake of T3 and T4. Two reports identified mutations in monocarboxylate transporter 8 (MCT8, SLC16A2), a membrane transporter of T3 and T4, in a syndrome of abnormal thyroid status and psychomotor retardation (Allan-Herndon-Dudley syndrome) (4, 5). Today the concept of carrier-mediated, transmembrane transport of T3 and T4 is supported by substantial genetic and biochemical evidence and is generally accepted as biologically important in a range of tissues.

Broadly speaking, steroid hormones resemble thyroid hormones in size, hydrophobic properties and mechanism of action through binding to members of the nuclear receptor superfamily. Would carrier-mediated transport be expected for steroid hormone entry into mammalian cells? Although much work is needed to obtain direct evidence, some observations are consistent with the proposal.

Certain biochemical studies previously suggested carrier-mediated uptake of steroid hormones by cells in culture. For example, as early as 1976, a carrier-mediated process for the uptake of cortisol into liver cells was proposed by Marie Luise Rao and Govind Rao at the University of Bonn (2). More recently, molecular analyses have suggested that SLCO1B3, another membrane transporter of the OATP family, mediates uptake of testosterone in prostate cancer cells in culture and that polymorphisms in SLCO1B3 alter testosterone uptake in assays in vitro (6). Wider considerations lend support to the need for regulated mechanisms for uptake of steroid hormones. For example, a distinct, specialized mechanism of endocytic uptake has been reported for sex steroids. In this case, the megalin/LRP2 receptor mediates uptake of sex steroids when bound in a complex with sex hormone binding globulin. Megalin-deficient mice display sex steroid–related phenotypes in testicular descent and vaginal opening (7).

Despite the plausibility of the arguments, we still await direct evidence for an ortholog of Drosophila Oatp74D with biological function in humans or mammalian models. It is conceivable that membrane carriers for steroids will be identified by exome sequencing of patients who display unexplained steroid-related abnormalities or symptoms of steroid insensitivity. This approach recently identified mutation of another membrane protein, OATP1C1, a transporter for T4 and conjugated steroids, in a human patient with juvenile neurodegeneration (8). A lesson from the thyroid field is that different membrane proteins with varying affinities and distinct tissue expression patterns may contribute to hormonal uptake across the plasma membrane.

The finding of mammalian transporters for steroid hormones would generate interest in pharmaceuticals that might block transmembrane transport. For example, in any syndrome associated with steroid excess, such blockers may provide alternatives to nuclear receptor antagonists, in particular, if the transporter is cell type–specific. The finding of a membrane transporter for a steroid hormone in Drosophila (1) adds impetus to the search for mammalian counterparts.

Glossary

Abbreviation:

SLCO

solute carrier organic anion