Abstract
Cytosolic calcium-dependent phospholipase A2 (cPLA2) has multiple roles including production of arachidonic acid (a key player in cellular signaling pathways) and membrane remodeling. Additionally, since catabolism of arachidonic acid generates free radicals, the enzyme is also implicated in ischemic injury to mammalian organs. Regulation of cPLA2 could be important in the suppression and prioritization of cellular pathways in animals that undergo reversible transitions into hypometabolic states. The present study examines the responses and regulation of cPLA2 in skeletal muscle and liver of hibernating thirteen-lined ground squirrels, Spermophilus tridecemlineatus. cPLA2 activity decreased significantly by 43% in liver during hibernation, compared with euthermic controls, and Km values for arachidonoyl thio-PC substrate fell in both organs during hibernation to 61% in liver and 28% in muscle of the corresponding euthermic value. To determine whether these responses were due to a change in the phosphorylation state of the enzyme, Western blotting was employed using antibodies recognizing phospho-Ser505 on ฮฑ-cPLA2. The amount of phosphorylated ฮฑ-cPLA2 in hibernator liver was just 38% of the value in euthermic liver. Furthermore, incubation of liver extracts under conditions that enhanced protein phosphatase action caused a greater reduction in the detectable amount of phospho-Ser505 enzyme content in euthermic, versus hibernator, extracts. The data are consistent with a suppression of cPLA2 function during torpor via enzyme dephosphorylation, an action that may contribute to the well-developed ischemia tolerance and lack of oxidative damage found in hibernating species over cycles of torpor and arousal.