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It is well established that phosphorylation and dephosphorylation are key cellular events which regulate important metabolic activities such as gene expression, cell cycle progression, and apoptosis. The polyether fatty acid, okadaic acid has been shown previously to activate apoptosis in a variety of cell lines. Although this marine sponge toxin is known to inhibit protein phosphatase (PP)-2A and PP-1, it is not certain in most cases whether inhibition of PP-1 or PP-2A is necessary to activate apoptosis. Furthermore, it is not clear how inhibition of these phosphatases leads to apoptosis. Here we present evidence that inhibition of PP-2A by okadaic acid does not activate apoptosis in the lens system. However, when PP-1 is inhibited by okadaic acid, rabbit lens epithelial cells undergo rapid apoptosis. Associated with this process is the several-fold up-regulation of the tumor suppressor gene p53 and the pro-apoptotic gene bax at both mRNA and protein levels. Analyses of the temporal pattern of expression of the two genes reveal that the up-regulation is maximized in a few hours after treatment with okadaic acid, when the majority of the treated cells become committed to apoptosis. A brief treatment of the cells with a protein synthesis inhibitor can abolish okadaic acid-induced up-regulation of both P53 and Bax proteins. Concomitant with this inhibition, okadaic acid-induced apoptosis is also temporarily blocked. These results suggest that okadaic acid-induced expression of p53, bax, and other genes are necessary for the activation of the apoptotic programs in lens systems.Keywords : apoptosis; p53 ; bax ; phosphatase-1; lens.Apoptosis is a distinct form of cell death regulated by internal genetic programs [1,2]. The morphological changes associated with apoptosis include blebbing of cytoplasmic membranes, condensation of nucleoplasm and cytoplasm, and fragmentation of the dying cell into apoptotic bodies which are phagocytosed by neighboring cells [3,4]. The biochemical hallmarks of apoptosis are marked by DNA fragmentation into nucleosomal fragments [4,5], activation of the interleukin-1β-converting enzyme family of proteases [6], and cleavage of various substrates of that family of proteases such as poly(ADPribose) polymerase [7,8] and nuclear lamin [9].Apoptosis normally occurs as a physiological process during certain stages of animal development or during tissue homeostasis in an adult organism [10,11]. It can also be triggered by various external signals such as hormone [4], γ-irradiation [12] and withdrawal of growth factors [13] and other factors (reviewed in [2]). Normal physiological apoptosis helps to remove

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Abstract:It is unclear what mechanisms lead to the degeneration of basal forebrain cholinergic neurons in Alzheimer's or other human brain diseases. Some brain cholinergic neurons express neuronal nitric oxide (NO) synthase (nNOS), which produces a free radical that has been implicated in some forms of neurodegeneration. We investigated nNOS expression and NO toxicity in SN56 cells, a clonal cholinergic model derived from the medial septum of the mouse basal forebrain. We show here that, in addition to expressing choline acetyltransferase (ChAT), SN56 cells express nNOS. Treatment of SN56 cells with retinoic acid (RA; 1 M) for 48 h increased ChAT mRNA (ϩ126%), protein (ϩ88%), and activity (ϩ215%) and increased nNOS mRNA (ϩ98%), protein (ϩ400%), and activity (ϩ15%). After RA treatment, SN56 cells became vulnerable to NO excess generated with S-nitro-N-acetyl-DL-penicillamine (SNAP) and exhibited increased nuclear DNA fragmentation that was blocked with a caspase-3 inhibitor. Treatment with dexamethasone, which largely blocked the RA-mediated increase in nNOS expression, or inhibition of nNOS activity with methylthiocitrulline strongly potentiated the apoptotic response to SNAP in RA-treated SN56 cells. Caspase-3 activity was reduced when SNAP was incubated with cells or cell lysates, suggesting that NO can directly inhibit the protease. Thus, whereas RA treatment converts SN56 cells to a proapoptotic state sensitive to NO excess, endogenously produced NO appears to be antiapoptotic, possibly by tonically inhibiting caspase-3.