Steroid hormones like estrogen are signalling molecules that are involved in a vast array of physiological processes in many species. Steroid hormone effects can be divided into slow genomic and rapid non-genomic responses. The slow response to steroid hormones is reasonably well understood; it is mediated by soluble, cytoplasmic receptors that enter the nucleus after binding with their ligand, where they affect transcription of steroid hormone-responsive genes. In contrast, the second response type, which occurs too quickly to be explained by changes in gene expression, is still poorly understood. But evidence accumulated over the past ten years suggests that membrane-associated steroid hormone receptors play a role in these rapid non-genomic signalling events. Now, an exciting finding by Eric Prossnitz and his co-workers supports this idea. They have discovered, in parallel with a Texas research group, that a G-Protein coupled receptor called GPR30 is a new class of membrane-associated estrogen receptor.
While the suggestion that rapid estrogen responses somehow involve GPR30 has been around for several years, nobody had really looked into this. Prossnitz's team set out to investigate the proposed function of GPR30 in estrogen-mediated cell activation. First, they located where GPR30 is expressed in the cell, using confocal fluorescence microscopy to examine monkey kidney cells expressing fusions of GPR30 and a fluorescent marker protein. It turned out that GPR30 predominantly resides in the endoplasmic reticulum (ER) membranes.
Can a transmembrane ER-protein like GPR30 really act as a functional estrogen receptor? The team came up with a nifty trick to answer this question. They simply synthesized fluorescent derivatives of estrogen to demonstrate that the hormone binds to GPR30 in living cells. They looked for co-localization of the fluorescent-labeled GPR30 and the fluorescent estrogen derivative; if GPR30 really is an estrogen receptor, they should see a match between the fluorescence of the receptor and the hormone. Indeed, the team found that the fluorescence signals matched almost perfectly within the cells. Moreover, when they performed competition-binding assays they found a high binding affinity between GPR30 and estrogen. The team concluded that GPR30 is an estrogen binding protein.
But does GPR30 also mediate rapid estrogen responses? To address this question, the team analysed intracellular Ca2+ and phosphatidylinositol 3,4,5-trisphosphate (PIP3), both known to be involved in estrogen-dependent signalling. Activation of GPR30 by estrogen resulted in Ca2+ mobilization and PIP3 synthesis, so GPR30 does indeed mediate rapid estrogen responses. However, the signalling pathways of GPR30-mediated estrogen responses appear to be different to those of the classical intracellular estrogen receptors.
The above findings are generally in line with those of the Texas team,except that the latter scientists detected GPR30 in the cell's outer plasma membrane rather than in the endoplasmic reticulum. But regardless of the actual cellular location, the discovery of a novel class of steroid hormone receptors calls for an update of physiological textbooks, which is already overdue because they still only discuss the existence of nuclear steroid receptors and not membrane-associated receptors. Importantly, the novel receptors may also broaden the palette of choices to interfere with steroid hormone responsiveness of certain tumour cells, since Prossnitz's team found that GPR30 is the sole functional estrogen receptor in SKBr3 cells, a breast cancer cell line.