The role of notch signaling in regulating nuclear hormone receptor activity in mammalian T lymphocytes and <i>Drosophila</i>
Regulation of programmed cell death is crucial for proper development and maintaining homeostasis of organisms. This thesis work attempts to elucidate the relationships between two evolutionarily well-conserved pathways, i.e. nuclear hormone receptor and Notch signaling pathway, using programmed cell death in T lymphocytes and Drosophila melanogaster as model systems.
During metamorphosis of insects, all larval tissues undergo apoptotic cell death, a process triggered by steroid hormone ecdysone. In larval tissues, ecdysone pulses directly regulate expression of cell death genes. In this thesis work, the effect of Notch signaling on ecdysone receptor (EcR) activity was examined in larval salivary gland undergoing apoptosis. Aberrant expression of activated Notch or mutations in Suppressor of Hairless (Su(H)), a downstream effector protein of Notch, resulted in inhibition of cell death in the larval salivary glands. This inhibitory effect was due to the inability of EcR to induce ecdysone responsive genes during prepupal ecdysone pulse. Genetic interaction experiments revealed an allele-specific genetic interaction between Su(H) and EcR. These results suggest the cross talk between Notch signaling and EcR in Drosophila.
There are two signalings involved in apoptosis of thymocytes during thymic development, steroid hormone, glucocorticoid, and orphan nuclear receptor Nur77. Previously, aberrant expression of Notch was shown to inhibit apoptosis induced by both signalings in T cell hybridomas. In this thesis work, the role for nuclear hormone receptor in regulating Notch activity was examined. Glucocorticoid treatment of thymocytes resulted in rapid down-modulation of Notch I proteins. This event was inhibited by caspase and proteasome inhibitors, suggesting that glucocorticoid-induced down-modulation of Notch protein is mediated through caspase and the proteasome pathways. Furthermore, thymocytes from transgenic mice expressing a Notch antisense construct, showed an increased in susceptibility to glucocorticoid-induced apoptosis. Lastly, upregulation of Notch proteins, and activation of Notch signaling were observed in T lymphocytes upon stimulation. This observation suggests a novel role of Notch signaling in regulating functions of peripheral T cells. The work described in this thesis provides an insight into the link between Notch signaling and nuclear hormone receptors.