Amino-acid-dependent signal transduction.

Recent research carried out in several laboratories has indicated that, in addition to their role as intermediates in many metabolic pathways, amino acids can interact with insulin-dependent signal transduction. In this short review, the current state of this rapidly expanding field is discussed.

Sphingomyelinase treatment of rat hepatocytes inhibits cell-swelling-stimulated glycogen synthesis by causing cell shrinkage.

Breakdown of plasma-membrane sphingomyelin caused by TNF-alpha is known to inhibit glucose metabolism and insulin signalling in muscle and fat cells. In hepatocytes, conversion of glucose to glycogen is strongly activated by amino acid-induced cell swelling. In order to find out whether breakdown of plasma-membrane sphingomyelin also inhibits this insulin-independent process, the effect of addition of sphingomyelinase was studied in rat hepatocytes. Sphingomyelinase (but not ceramide) inhibited glycogen synthesis, caused cell shrinkage, decreased the activity of glycogen synthase a, but had no effect on phosphorylase a. Cell integrity was not affected by sphingomyelinase addition as gluconeogenesis and the intracellular concentration of ATP were unchanged. As a control, glycogen synthesis was studied in HepG2 cells. In these cells, the basal rate of glycogen production was high, could not be stimulated by amino acids, nor be inhibited by sphingomyelinase. Regarding the mechanism responsible for the inhibition of glycogen synthase a, sphingomyelinase did not affect amino acid-induced, PtdIns 3-kinase-dependent, phosphorylation of p70S6 kinase, but caused an increase in intracellular chloride, which is known to inhibit glycogen synthase phosphatase. It is concluded that the decrease in cell volume, following the breakdown of sphingomyelin in the plasma membrane of the hepatocyte, may contribute to the abnormal metabolism of glucose when TNF-alpha levels are high.

Interleukin-10, interleukin-12, and tumor necrosis factor-alpha differentially influence the proliferation of human CD8+ and CD4+ T-cell clones.

Activation and proliferation of human T lymphocytes in vitro can be obtained by various stimuli including specific antigens, mitogens, and cytokines. Here we describe the effect of interleukin-10, interleukin-12 and tumor necrosis factor-alpha on the interleukin-2 dependent proliferation and function of established human CD4+ and CD8+ alloreactive T-cell clones in the absence of antigen presenting cells. IL-12 and TNF-alpha both demonstrated an inhibitory effect on the proliferation of CD8+ cytotoxic T lymphocyte clones, whereas IL-10 enhanced the proliferation. IL-12-induced inhibition of CD8+ CTL clones was not mediated by the endogenous production of TNF-alpha by these clones. The strong inhibitory effect of IL-12 and TNF-alpha did not result in apoptosis. These cytokines did not alter the cytotoxicity of CD8+ CTL clones. When CD4+ T-cell clones were tested in the absence of APC, no significant change in IL-2-dependent proliferation due to IL-10, IL-12, and TNF-alpha could be measured. Since these effects on established CTL clones are in contrast to the effects of IL-10, IL-12, and TNF-alpha during the induction phase of immune responses, a dichotomy of immunomodulatory cytokines such as IL-10, IL-12, and TNF-alpha early and late in the immune response is suggested.