A longstanding question in steroid physiology is how steroids mediate nongenomic, or transcription-independent, effects. Examples of rapid, steroid-triggered nongenomic signaling are myriad, including estrogen-mediated up-regulation of nitric oxide synthase (NOS) in endothelial cells, vitamin D-induced calcium mobilization in osteosarcoma cells, and progesterone-induced maturation of frog and fish oocytes. These signaling events may mediate important biological functions such as blood vessel relaxation, bone metabolism, and fertilization. Unfortunately, most of these processes are part of complex biological systems that are difficult to manipulate in vitro, which presents a serious problem when trying to understand the biology behind them. This proposal uses the phenomenon of steroid-induced maturation of frog oocytes as an experimental model for studying nongenomic steroid signaling. The maturation of an oocyte refers to the meiotic stage at which an oocyte rests. """"""""Immature"""""""" oocytes are arrested in prophase of meiosis I, while """"""""mature"""""""" oocytes rest in metaphase II. Steroids induce this re-entry into the cell cycle via a transcription-independent process that appears to involve membrane bound classical steroid receptors and possibly signaling via G proteins. Although controversial, the primary physiological mediator of oocyte maturation in Xenopus appears to be the androgen, testosterone. This system offers many advantages in studying nongenomic signaling by steroids. Steroid-induced oocyte maturation is reproducible, easy to detect, and biologically relevant. Furthermore, oocytes are easily manipulated in vitro for protein expression and signaling studies.
The aims of this proposal are: 1) to improve our knowledge of the early signaling pathways induced by steroids in oocytes, including the role of G proteins, in the maturation process; and 2) to elucidate the role of the classical androgen receptor in the maturation process. Understanding nongenomic steroid-induced signaling in oocytes should prove helpful in elucidating the mechanisms involved in other nongenomic signaling pathways and may lead to new insights toward controlling their associated biological processes.
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