The Role of a Membrane Androgen Receptron in the Brain
Gatson, Joshua Wayne, The Role of a Membrane Androgen Receptor in the Brain. Doctor of Philosophy (Biomedical Sciences), February 2007, pp187, 34 illustrations. In the brain, depending on the insult type, androgens have been shown to protect from or exacerbate the levels of cell death. This discrepancy is partly due to the array of receptors that androgens may activate during injury. For example, activation of intracellular androgen receptors (AR) leads to the activation of pro-survival pathways and protects from various toxins such beta-amyloid. In contrast, previous studies have demonstrated that testosterone causes an increase in lesion size following stroke. The damaging effects of androgens in the brain may be mediated by a membrane-associated AR (mAR), since activation of mAR in peripheral tissue results in a decrease in cell growth and an increase in apoptotic cell death during serum deprivation. Here, I hypothesize that activation of a mAR in cortical astrocytes, suppresses the ERK and Akt signaling pathways and increases cell death in the presence of a metabolic and oxidative stressor. In this study, we found that glia express both isoforms of the AR (AR-B and AR-A) and that dihydrotestosterone (DHT) elicits ERK and Akt phosphorylation in rat glioma (C6) cells. The effect of DHT on the activation of these signaling pathways is AR dependent, since flutamide blocked this effect. In contrast to the intracellular receptor, we concluded that DHT-BSA (membrane impermeant form of DHT) binds to DHT displaceable sites on the plasma membrane. Also, treatment with DHT-BSA in the C6 cells resulted in a significant decrease in phosphor-ERK and Akt levels, suggesting the existence of two different pathways through which DHT can influence the activity of these signaling pathways. With respect to cell survival, the C6 cells and primary cortical astrocytes were treated with the metabolic and oxidative insult, iodoacetic acid (IAA), in the presence or absence of DHT, DHT-BSA, or estradiol. Following treatment, DHT and estradiol protected the glia from IAA-induced toxicity, whereas DHT-BSA caused a significant increase in cell death in the presence of a sublethal concentration of IAA. These results indicate that activation of the intracellular pathway is protective and activation of a membrane pathway is damage-inducing during injury, further supporting our results from the ERK and Akt signaling studies. To further characterize this mAR in the brain, we decided to look for indices of apoptosis such as caspase activation and TUNEL staining. It was found that DHT-BSA treatment in the presence of IAA, resulted in an increase in caspase-3/7 activation and increased TUNEL staining. In addition, PKC-delta mediated DHT-BSA-induced cell death, since antagonism of PKC-delta with rottlerin afforded protection. In conclusion, we have partially characterized a novel mAR in astrocytes during injury. Here, the damaging effects of androgens, at least in astrocytes, may in fact be mediated by a mAR, which may be a therapeutic target stroke or reperfusion injury.
Biology, Cell and Developmental Biology, Cell Biology, Cells, Cellular and Molecular Physiology, Life Sciences, Medical Cell Biology, Medical Neurobiology, Medical Sciences, Medicine and Health Sciences, Molecular and Cellular Neuroscience, Nervous System, Neuroscience and Neurobiology, Neurosciences, Other Cell and Developmental Biology, Membrane androgen receptor, brain, apoptotic cell death, rats, dihydrotestosterone, DHT-BSA, glioma, c6 cells, iodoacetic acid, IA, astrocytes, injury
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