212 - Thresholds and Limitations of Adaptive Homeostasis in Stressed and Unstressed Cells

Abstract

In response to stress-related signaling cells transiently and reversibly expand a wide suite of defense mechanisms, as part of the process of adaptive homeostasis. Included in the array of adaptive responses are alterations to existing proteasomes and increased de novo synthesis of 20S proteasomes, immuno-proteasomes, and PA28 (11S) proteasome regulators that enable more efficient and rapid removal of oxidatively damaged proteins from cells. Our laboratory has shown that this proteasomal adaptive response confers greater survival capacity against oxidative stress in cultured mammalian cells, Caenorhabditis elegans nematode worms, and Drosophila melanogaster fruit flies. There are, however, there are limits and thresholds to adaptive homeostasis stress responses. If cells (or organisms) are already experiencing a degree of increased stress, they may have only a limited ability to further adapt to additional stresses. When we started to explore the adaptive homeostasis phenomenon in mammalian cells, for example, we found that cells grown in room air with 20% ambient O2 were much less able to adapt to H2O2 than were cells grown at 5% ambient O2; importantly, however, the cells grown 20% O2 were initially more H2O2 resistant than were the cells grown at 5% O2. In practice, of course, many cell types used in laboratories throughout the world were derived from tissues where physiological normoxia is 3 - 5% O2, yet they are cultured and studied at ambient 20% O2. This study examined the adaptive response to H2O2 and cellular growth rate of Mouse Embryonic Fibroblasts (MEFs), cultured continuously at either 5% O2, 20% O2, or 40% O2. Our goal was to define thresholds and limitations to adaptive homeostasis imposed by pre-existing oxidative stress levels. Our results show that adaptive homeostasis responses in general, and proteasome induction as a specific marker, decrease with increasing O2 culturing concentrations: in other words, with increasing baseline pre-existing oxidative stress.