As an approach to understanding how mammals regulate H 2 O 2 intracellular concentration to prevent its toxicity, we analyzed the genome-wide mRNA profile changes of human cells after treatment with a non-toxic H 2 O 2 concentration. We identified a large and essentially late H 2 O 2 response of induced and repressed genes that unexpectedly comprise few or no antioxidants but mostly apoptosis and cell cycle control activities. The requirement of the p53 regulator for regulating about a third of this H 2 O 2 stimulon and the lack of an associated enhancement of total cellular H 2 O 2 scavenging activity further suggest that H 2 O 2 elicits a stress antiproliferative/repair response that does not increase antioxidant defenses. We conclude that mammalian antioxidant defenses are constitutive, a finding that contrasts with the oxidant-inducibility of such defenses in microorganisms. This finding might be important in understanding the role of H 2 O 2 as a key signaling molecule in mammals.Oxygen is both essential for aerobic life, because it is the terminal electron acceptor in respiration, and dangerous, because of the potency of reactive oxygen species (ROS) 1 to indiscriminately oxidize biological molecules. ROS include the superoxide anion (O 2 . ) and hydrogen peroxide (H 2 O 2 ). Antioxidant systems that avert ROS toxicity comprise the scavenging enzymes superoxide dismutase, catalase, selenothiol-and thiolperoxidase, as well as associated electron donor systems that include the pentose phosphate, thioredoxin, and glutathione pathways (1-3). In microorganisms, antioxidant systems are part of so-called oxidative stress-inducible adaptive responses (1, 2, 4). Specialized regulators operate these responses by sensing minimal increases in ROS concentration and setting the transcriptional expression of oxidant scavenger genes in proportion. Such regulation, meant to prevent oxidative stressinduced cellular damage, is essential for the aerobic life of microorganisms and has the hallmarks of a homeostatic control. It is not clearly established, however, whether similar ROS-inducible genetic responses exist in mammals. This question is highly significant because, in addition to their toxic side, ROS have been attributed specific roles in mammalian signal transduction. H 2 O 2 , especially, acts as a key signaling molecule in cell growth and differentiation (5, 6), and many eukaryotic signaling pathways, by being redox-sensitive, could be influenced by ROS (7-9). The identification of mammalian ROS sensors should be an indication of the existence of ROS-inducible responses. However, as of today, no such function has been precisely identified in mammals. The Keap1-Nrf2 system senses electrophiles and controls the expression of phase II detoxifying enzymes through the antioxidant/electrophile response element (10 -12), but it has no demonstrated direct role in ROS sensing per se (13,14). Similarly, several stress-responsive regulators, including the c-Jun N-terminal kinase stressactivated mitogen activated protein kina...

Isoproterenol stimulates H-K-ATPase activity in rat cortical collecting duct β-intercalated cells through a PKA-dependent pathway. This study aimed at determining the signaling pathway underlying this effect. H-K-ATPase activity was determined in microdissected collecting ducts preincubated with or without specific inhibitors or antibodies against intracellular signaling proteins. Transient cell membrane permeabilization with streptolysin-O allowed intracellular access to antibodies. Isoproterenol increased phosphorylation of ERK in a PKA-dependent manner, and inhibition of the ERK phosphorylation prevented the stimulation of H-K-ATPase. Antibodies against the monomeric G protein Ras or the kinase Raf-1 curtailed the stimulation of H-K-ATPase by isoproterenol, whereas antibodies against the related proteins Rap-1 and B-Raf had no effect. Pertussis toxin and inhibition of tyrosine kinases with genistein also curtailed isoproterenol-induced stimulation of H-K-ATPase. It is proposed that activation of PKA by isoproterenol induces the phosphorylation of β-adrenergic receptors and the switch from Gs to Gi coupling. In turn, βγ-subunits released from Gi would activate a tyrosine kinase-Ras-Raf-1 pathway, leading to the activation of ERK1/2 and of H-K-ATPase.

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