Non-genomic effects of thyroid hormone receptor and thyroid hormone

Intracellular Ca2+ signaling has been intimately linked to mitochondrial metabolism. A close proximity between the ER and mitochondria provides local Ca2+ signaling between the two organelles within cells. Several Ca2+-dependent enzymes in the citric acid cycle, which localize to the mitochondria matrix, have been identified. Control of metabolism by mitochondrial matrix Ca2+ appears to be a fundamental mechanism whereby cells meet their energy needs. Recently, there is increasing evidence suggesting that thyroid hormone (T₃) directly modulates mitochondrial metabolism. However, the underlying mechanism of action is unknown. Furthermore, a link between intracellular Ca2+ signaling and T₃-activated thyroid hormone receptor has not been investigated.; The Xenopus oocyte model was utilized throughout this study because there are no detectable levels of thyroid hormone receptor and retinoid X receptor protein. Receptor mRNA was prepared and injected directly into the oocytes. The oocytes were incubated for several days to insure optimal levels of protein expression. We present evidence demonstrating that T ₃-activated thyroid hormone receptor acutely regulates IP₃-mediated Ca2+ wave activity in the Xenopus oocyte model. The modulation of Ca2+ wave activity requires both T₃ and xTR_βA1. In contrast to transcriptional effects, we demonstrate that these non-genomic effects do not require DNA interaction and xRXR_α heterodimerization. We show that the xTR_β A1 mutant, with its DNA binding domain and nuclear localization signal deletion, is fully capable of Ca2+ wave activity modulation. Moreover, the co-expression of xTR_βA1 and xRXR_α do not affect the ability of xTR_βA1 to modulate Ca 2+ oscillations. Only expression of xTR_βA1 and the shortened form of rat TR_α1 (rTR_αΔF), which can target the mitochondria, are effective at modulating Ca2+ activity. No effects of T₃ on Ca2+ signaling are observed with the full-length rat TR_α1 isoform (rTR _α1), which does not localize to the mitochondria. We also demonstrate that inhibition of mitochondrial Ca2+ influx by Ruthenium red, a polycation that inhibits the Ca2+ uniporter, partially reverses the effects on Ca2+ signaling by TR/T₃ and energized mitochondria in the same manner. Together, these observations suggest a new, non-genomic pathway for the modulation of intracellular Ca2+ signaling via thyroid hormone and thyroid hormone receptor stimulated mitochondrial metabolism.