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Protein kinase C (PKC) is a family of ten isozymes that play distinct and in some cases opposing roles in cell growth and survival. We recently reported that diamide, a diazene carbonyl derivative which oxidizes thiols to disulfides through addition/displacement reactions at the diazene bond, induces potent GSH-dependent inactivation of several PKC isozymes, including the oncogenic isozyme PKC epsilon, via S-glutathiolation. PKC delta, a pro-apoptotic isozyme, was distinguished by its resistance to inactivation. In this report, we show that PKC-regulatory S-thiolation modifications produced by physiological disulfides elicit opposing effects on PKC delta and PKC epsilon activity. We report that PKC delta is stimulated 2.0-2.5 fold by GSSG, (Cys-Gly)(2) and cystine, under conditions where PKC gamma and PKC epsilon are fully inactivated by cystine, and PKC alpha activity is affected marginally or not at all by the disulfides. Focusing on cystine, we show that DTT quenches cystine-induced PKC delta stimulation and PKC gamma and PKC epsilon inactivation, indicative of oxidative regulation. By analyzing DTT-reversible isozyme radiolabeling by [(35)S]cystine, we demonstrate that PKC gamma, PKC delta and PKC epsilon are each [(35)S] S-cysteinylated in association with the concentration-dependent regulation of isozyme activity by cystine. The restricted reactivity of cystine, together with the effects of DTT and thioredoxin on cystine-induced PKC isozyme regulation reported here, indicate that the cystine-induced PKC-regulatory effects entail isozyme S-cysteinylation. We recently hypothesized that antagonism of tumor promotion/progression by small cellular thiols may involve PKC regulation via oxidant-induced S-thiolation reactions with PKC isozymes. The findings of cystine-induced PKC isozyme regulation by S-cysteinylation reported here offer correlative support to the hypothetical model. Thus, PKC delta, a potent antagonist of DMBA-TPA-induced tumor promotion/progression in mouse skin, is stimulated by S-cysteinylation, PKC epsilon, an important mediator of the tumor promotion/progression response, is inactivated by S-cysteinylation, and PKC alpha, which is not influential in DMBA-TPA-induced tumor promotion/progression, is not regulated by cystine. Furthermore, PKC gamma has oncogenic activity, and S-cysteinylation inactivated PKC gamma and PKC epsilon similarly. These findings provide evidence that S-cysteinyl acceptor-sites in PKC isozymes may offer attractive targets for development of novel cancer preventive agents.

Cell signaling entails a host of post-translational modifications of effector-proteins. These modifications control signal transmission by regulating the activity, localization or half-life of the effector-protein. Prominent oxidative modifications induced by cell-signaling reactive oxygen species (ROS) are cysteinyl modifications such as S-nitrosylation, sulfenic acid and disulfide formation. Disulfides protect protein sulfhydryls against oxidative destruction and simultaneously influence cell signaling by engaging redox-regulatory sulfhydryls in effector-proteins. The types of disulfides implicated in signaling span (1) protein S-glutathionylation, e.g. as a novel mode of Ras activation through S-glutathionylation at Cys-118 in response to a hydrogen-peroxide burst, (2) intra-protein disulfides, e.g. in the regulation of the stability of the protein phosphatase Cdc25C by hydrogen-peroxide, (3) inter-protein disulfides, e.g. in the hydrogen peroxide-mediated inactivation of receptor protein-tyrosine phosphatase alpha (RPTPalpha) by dimerization and (4) protein S-cysteaminylation by cystamine. Cystamine is a byproduct of pantetheinase-catalyzed pantothenic acid recycling from pantetheine for biosynthesis of Coenzyme A (CoA), a ubiquitous and metabolically indispensable cofactor. Cystamine inactivates protein kinase C-epsilon (PKCepsilon), gamma-glutamylcysteine synthetase and tissue transglutaminase by S-cysteaminylation-triggered mechanisms. The importance of protein S-cysteaminylation in signal transmission in vivo is evident from the ability of cystamine administration to rescue the intestinal inflammatory-response deficit of pantetheinase knockout mice. These mice lack the predominant epithelial pantetheinase isoform and have sharply reduced levels of cystamine/cysteamine in epithelial tissues. In addition, intraperitoneal administration of cystamine significantly delays neurodegenerative pathogenesis in a Huntington's disease mouse model. Thus, cystamine may serve as a prototype for the development of novel therapeutics that target effector-proteins regulated by S-cysteaminylation.

Previous observations have shown that, during Martian dust storms, the peak of the ionosphere rises in altitude. Observational studies of this type, however, have been extremely limited. Using 13 years of ionospheric peak altitude data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on Mars Express, we study how the peak altitude responded to dust storms during six different Mars years (MY). The peak altitude increased ∼10-15 km during all six events, which include a local dust storm (MY 33), three regional dust storms (MYs 27, 29, and 32), and two global dust storms (MYs 28 and 34). The peak altitude's orbit-to-orbit variability was exceptionally large at the apexes of the MY 29 and MY 32 dust seasons and dramatically increased during the MY 28 and MY 34 global dust storms. We conclude that dust storms significantly increase upper atmospheric variability, which suggests that they enhance dynamical processes that couple the lower and upper atmospheres, such as upward propagating gravity waves or atmospheric tides. Plain Language SummaryLimited observations have shown that dust storms at Mars significantly affect the upper atmosphere and ionosphere. In particular, the expansion of the atmosphere in response to solar heating of dust causes fixed pressure levels in the upper atmosphere to rise in altitude. The peak of the ionosphere-where the maximum electron density occurs-can be used as a diagnostic of this expansion because it forms at a fixed pressure level in the upper atmosphere (120-150 km). In this work, we use 13 years of observations from the radar sounder on the Mars Express spacecraft to evaluate how the peak altitude of the ionosphere varied during six different dust storms. We found that the peak altitude increased by 10-15 km during each dust storm. Additionally, the orbit-to-orbit variability of the peak altitude increased significantly during the dust storms, which suggests that dynamical processes that couple the lower and upper atmospheres were enhanced.