Metabolic Regulation of Caspase-2

Abstract

Apoptosis is a form of programmed cellular "suicide" which is activated in response to a variety of pro-death stimuli. Apoptotic cell death is orderly and energy-dependent, and cellular constituents are packaged into membrane-bound vesicles for consumption by phagocytes. Toxic intracellular signals are never exposed to neighboring cells or to the extracellular environment, and a host inflammatory response does not occur. Apoptosis is executed by the coordinated activation of caspase family proteins. Caspase-2 is an apical protease in this family, and promotes cell death after receipt of cues from intracellular stressor signals. Caspase-2 helps to initiate apoptosis by responding to cellular death stimuli and signaling for downstream cytochrome c release and executioner caspase activation. Several years ago our lab determined that Xenopus laevis oocyte death is partly controlled by the activation of caspase-2. In the setting of oocyte or egg extract nutrient depletion, caspase-2 was observed to be activated upstream of mitochondrial cytochrome c. In fact, caspase-2 is suppressed in response to the nutrient status of the oocyte: nutrient-replete oocytes with healthy pentose phosphate pathway flux and abundant NADPH production are able to inhibit caspase-2 via S135 phosphorylation catalyzed by calcium/calmodulin-dependent protein kinase II. Phosphorylation of caspase-2 at S135 is critical in preventing oocyte cell death, and a caspase-2 mutant unable to be phosphorylated loses its ability to respond to suppressive NADPH signals. In this dissertation we examine the converse mechanism of metabolically-regulated caspase-2 activation in the Xenopus egg extract. We now show that caspase-2 phosphorylated at S135 binds the interactor 14-3-3 zeta, thus preventing caspase-2 dephosphorylation. Moreover, we determined that S135 dephosphorylation is catalyzed by protein phosphatase-1, which directly binds caspase-2. Although caspase-2 dephosphorylation is responsive to metabolism, neither PP1 activity nor binding is metabolically regulated. Rather, release of 14-3-3 zeta from caspase-2 is the point of metabolic control and allows for caspase-2 dephosphorylation. Accordingly, a caspase-2 mutant unable to bind 14-3-3 zeta is highly susceptible to activation. Although this mechanism was initially established in Xenopus, we now demonstrate similar control of murine caspase-2 by phosphorylation and 14-3-3 binding in mouse eggs. In the second part of this dissertation we examine the paradigm of caspase-2 metabolic regulation in a mammalian somatic cell context. We observed that mammalian caspase-2 is a metabolically-regulated phosphoprotein in somatic cells, and that the site of regulation is caspase-2 S164. Phosphorylation at S164 appears to inhibit mammalian caspase-2 by preventing its induced proximity oligomerization, thus also preventing procaspase-2 autocatalytic processing. We further identify some of the molecular machinery involved in S164 phosphorylation and demonstrate conservation with the validated Xenopus regulators. Interestingly, we extend the findings of caspase-2 phosphorylation to a study of ovarian cancer, and show that caspase-2 S164 phosphorylation might be involved in determining cancer cell chemosensitivity. We further provide evidence that chemosensitivity can be modulated by the cellular metabolic status in a caspase-2-dependent manner. Thus, we have identified a novel phosphorylation site on mammalian caspase-2 in somatic cells, and are working further to understand the implications of caspase-2 signaling in the context of cancer cell responsiveness to chemotherapeutic treatments.