Cell signalling and the control of gene transcription

Ethanol inhibition of inducible nitric oxide synthase activity in C6 glioma cells

Nitric oxide (NO.), a free radical gas, has been implicated in the CNS actions of ethanol. The brain contains several cell types that can produce NO., including neurons and glia. This study examined the effect of acute and chronic ethanol exposure on the activity of the inducible isoform of nitric oxide synthase (iNOS) found in neuroglia. Experiments were performed using intact rat C6 glioma cells, and NO. production was assessed by nitrite accumulation after iNOS induction by coadministration of phorbol 12-myristate 13-acetate (PMA) and lipopolysaccharide (LPS). Ethanol was inhibitory at high concentrations (IC50 approximately 150 mM) when acutely present during the 24-hr period subsequent to initiation of enzyme induction. In contrast, cells exposed to ethanol were inhibited chronically at clinically relevant lower concentrations (IC50 approximately 30 mM with 10 days exposure). Chronic inhibition was both time- and concentration-dependent. Inhibition by ethanol seems to be a consequence of interference with LPS signal transduction. Acutely, ethanol did not affect the ability of PMA to synergize with LPS to induce activity, but it attenuated the ability of LPS to synergize with the PMA. Ten days exposure to 50 mM ethanol decreased the LPS potency by 4-fold in the presence of a maximally activating concentration of PMA, although not significantly changing PMA potency. Inhibition by chronic ethanol exposure was long-lasting, being retained over 24 hr in cells returned to control conditions. Thus, chronic ethanol may downregulate key components needed for iNOS expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium, calmodulin and cell cycle progression

Proliferation of mammalian cells both in vivo and in vitro is dependent upon physiological concentrations of extracellular Ca2+. Growth factor stimulation of quiescent cells at the G0/G1 border usually results in a rapid mobilization of Ca2+ from both intra- and extracellular pools. However, Ca2+ influx is also required for later phases of cell cycle transition, especially in the late G1 phase for initiation of DNA synthesis. Available evidence indicates that calmodulin plays the major and essential roles in the Ca(2+)-dependent regulation of cell proliferation. Ca2+ and calmodulin act at multiple points in the cell cycle, including the initiation of the S phase and both initiation and completion of the M phase. Ca2+ and calmodulin stimulate the expression of genes involved in the cell cycle progression, leading to activation of cyclin-dependent kinases p33cdk2 and p34cdc2. Ca2+ and calmodulin are also involved in activation of enzymes participating in nucleotide metabolism and DNA replication, as well as nuclear envelope breakdown and cytokinesis. Ca2+/calmodulin-dependent protein kinase II and protein phosphatase calcineurin are both involved in the Ca2+ and calmodulin-mediated signalling of growth regulation. As compared to normal cells, growth of transformed cells is independent of extracellular Ca2+ and much less sensitive to calmodulin antagonists, suggesting the existence of derangements in the Ca2+ and calmodulin-mediated growth regulation mechanisms.