chondrial population. The likely reason cell viability is decreased during PHB knockdown coupled with autophagy inhibition or TNFa treatment is that the threshold of damaged mitochondria exceeds the capacity of the cell. Our finding that decreased PHB levels coupled with inhibited autophagy increased cell death has important clinical implications. Patients with inflammatory bowel disease exhibit decreased expression of PHB in intestinal epithelial cells. Dysfuntion in autophagy genes such as ATG16L1, IRGM, and LRRK2, are emerging as potential susceptibility traits in patients with inflammatory bowel disease. We speculate that decreased expression of PHB during active inflammatory bowel disease is a signal to the epithelial cell that there is inflammatory stress and that autophagy is subsequently induced to maintain cell viability and return homeostasis. Thus, these findings support an important role PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22179956 of autophagy in intestinal health and lend further insight into the mechanisms of dysfunctional 
autophagy via PHB in inflammatory bowel diseases. In the setting of PHB overexpression coupled with autophagy inhibition, ROS levels were not further increased and in fact are decreased during loss of ATG16L1. These data support the therapeutic concept of repletion of PHB levels in the setting of dysfunctional autophagy. Basal autophagy was reduced by PHB overexpression in Caco2BBE cells. We have shown previously that exogenous PHB expression in Caco2-BBE cells induced the expression of glutathione-S-transferase p, an antioxidant enzyme that catalyzes the conjugation of electrophiles to GSH, and modulates the activity of nuclear factor erythroid 2-related factor 2, a transcriptional regulator of antioxidant response. PHB overexpression may reduce basal autophagy through an increased antioxidant response on ROS production during normal physiological respiration. This issue warrants further investigation. Although PHB protein levels inversely correlated with IFNcinduced autophagy, overexpression of PHB in Caco2-BBE cells did not affect IFNc-induced autophagy. This is in contrast to the response to TNFa. NF-kB, a major downstream signaling Prohibitin Modulation of Autophagy pathway of TNFa, has been implicated in modulating autophagy. Our previous work demonstrated that exogenous PHB expression reduced basal and TNFa-stimulated NF-kB activation. It is possible that this effect on NF-kB reduces TNFainduced autophagy as compared to IFNc since IFNc is not a major activator of NF-kB. Furthermore, TNFa promotes autophagy via mitochondrial ROS generation. TNFa treatment during inhibition of autophagy exacerbated intracellular ROS levels in cells with knockdown of PHB. Since PHB overexpression did not decrease IFNc induced autophagy, this would suggest that the IFNc autophagy pathway is distinct from that of TNFa. Exogenous PHB expression can likely prevent TNFa-induced autophagy since PHB overexpression reduces ROS generation. The mechanism whereby PHB overexpression dampens basal and TNFa-induced, but not IFNc-induced, autophagy requires further investigation. It is widely accepted that the tumor suppressor p53 regulates autophagy depending upon is subcellular localization. Normal levels of p53 maintain a tonic inhibition of autophagy; autophagy is induced via a reduction in cytoplasmic p53 levels. p53 can activate genes that induce autophagy including damage-regulated autophagy modulator and sestrins 1 and 2. Since Caco2-BBE cells have buy 1260907-17-2 mutated p53, we in
