Imaging local estrogen production in single living cells with recombinant fluorescent indicators

Estrogens are steroid hormones with many systemic effects in addition to development and maintenance of the female reproductive system, and ligands of estrogen receptors are of clinical importance because of their use as oral contraceptive, hormone replacement and antitumoral therapy. In addition, tumoral tissues have been found to express aromatase and other steroidogenic enzymes synthesizing estradiol. To aid in the understanding of these processes, we have developed assays to image the local production of estrogens in isolated living mammalian cells. We constructed biosensors based on estrogen receptor α ligand binding domain and fluorescent proteins by following two approaches. First, the ligand binding domain and a short fragment of steroid receptor coactivator-1 were appended to a circularly permuted yellow fluorescent protein to construct an excitation ratio estrogen indicator. In the second strategy, we constructed emission ratio sensors based on fluorescence resonance energy transfer, containing the ligand binding domain flanked by donor and acceptor fluorescent proteins. Estrogens altered the fluorescence signal of cells transfected with the indicators in a dose-dependent manner. We imaged local estrogen production in adrenocortical H295 cells expressing aromatase and transfected with the fluorescent sensors. In addition, paracrine detection was observed in HeLa cells harboring the indicators and co-cultured with H295 cells. This imaging approach may allow detection of physiological levels of these hormones in suitable animal models.