Effect of dexamethasone on the carbohydrate chains of the insulin receptor

Quantitative approach to lectin-based glycoprofiling of thymic tissues in the control- and the dexamethasone-treated mice

Dexamethasone (DEX) is the most commonly used synthetic glucocorticoid in treatment of various inflammatory conditions. Here we focused on evaluating the effect of DEX on apoptosis and glycan profile in the mouse thymic tissues. Histological examinations revealed that the DEX treatment cause severe alterations in thymus, such as disruption of thymic capsule, impaired epithelial cell-thymocyte contacts, cellular loss and increased apoptosis. The identification of thymic glycans in the control- and the DEX-treated mice was carried out by using a panel of five plant lectins, Maackia amurensis agglutinin (MAA), peanut agglutinin (PNA), Sambucus nigra agglutinin (SNA), Concanavalin A (ConA) and wheat germ agglutinin (WGA). Lectin histochemistry results showed that glycosylation pattern of thymus changes upon DEX treatment. For further detailed quantitative analyses of the binding intensities for each lectin, histochemical data were scored as high positive (HP), mild positive (MP) and low positive (LP) and differences among signaling densities were investigated. The staining patterns of thymic regions observed with lectin histochemistry suggest that DEX can affect the thymic glycan profile as well as thymocyte apoptosis. These results are consistent with the opinion that not only sialic acid, but also other sugar motifs may be responsible for thymocyte development.

Altered sialylation on the cell-surface proteins of dexamethasone-treated human macrophages contributes to augmented uptake of apoptotic neutrophils

Macrophages eliminate apoptotic granulocytes before their secondary necrosis during resolution of inflammation. A well-known glucocorticoid, the anti-inflammatory dexamethasone augments phagocytosis capacity of macrophages with a so far not fully clarified mechanism. We have found that sialylation of cell-surface proteins on human macrophages is markedly altered by dexamethasone. Compared to non-treated cells, dexamethasone-treated macrophages can bind significantly less Sambucus nigra lectin specific for sialic acids on their surfaces as a result of undersialylation of annexin-II and an HLA-II protein. Non-treated macrophages covered by S. nigra lectin had increased uptake of apoptotic cells; however, the significantly higher phagocytosis capacity of dexamethasone-treated macrophages could not be stimulated further this way. Our results suggest that dexamethasone treatment leads to decreased number of sialic acids on the surfaces of human macrophages promoting recognition and uptake of apoptotic cells.

Long-term effects of corticosterone on behavior, oxidative and energy metabolism of parietotemporal cerebral cortex and hippocampus of rats: comparison to intracerebroventricular streptozotocin

We studied the effect of long-term application of corticosterone (CORT) s.c. the equivalent of cortisol in rats, on behavior, oxidative and energy metabolism in brain parietotemporal cortex and hippocampus of 1-year-old male Wistar rats. The data were compared with results derived from long-term and low dose intracerebroventricular application of the diabetogenic drug streptozotocin (STZ) known to inhibit the function of the neuronal insulin receptor and generating an insulin resistant brain state. CORT reduced both working and reference memory increasingly with time and running parallel to the STZ-induced deficit. The effect of CORT on the activities of the glycolytic enzymes hexokinase, phosphofructokinase, pyruvate kinase, glyceraldehyde-3-phosphodehydrogenase, lactate dehydrogenase and, in tricarboxylic acid cycle, alpha-ketoglutarate dehydrogenase equaled in both experimental conditions and in both regions studied: significant decreases of all enzyme activities except lactate dehydrogenase which increased between three and fourfold of normal. The CORT- and STZ-induced marked fall in ATP was in the same range in the regions studied. Differences became obvious in the concentration of creatine phosphate in parietotemporal cerebral cortex showing no decrease after CORT obviously due to a different susceptibility of the CORT-receptor. It is discussed that both CORT and STZ may act on the neuronal insulin receptor in a similar way. However, further studies are needed on the gene expression of insulin and the insulin receptor and its protein levels to clarify the exact action of CORT on the neuronal insulin receptor function.

The B isoform of the insulin receptor signals more efficiently than the A isoform in HepG2 cells

We have demonstrated previously that dexamethasone treatment of HepG2 cells caused an enhancement of insulin's metabolic effects (Kosaki, A., and Webster, N. J. (1993) J. Biol. Chem. 268, 21990-21996). This correlated with increased expression of the mRNA encoding the B isoform of the insulin receptor (IR). In the present study, we have demonstrated that dexamethasone treatment caused in addition an enhancement in insulin-stimulated immediate-early gene expression (c-fos and egr-1). Dexamethasone treatment caused an increase in in vivo IR autophosphorylation and insulin receptor substrate-1 (IRS-1) phosphorylation both early events in the insulin signaling pathway. Furthermore, the IRS-1 phosphorylation was distinctly left shifted, although the level of IRS-1 protein was unchanged. Total cellular tyrosine phosphatase activity was unaltered when assayed with 32P-labeled IR and IRS-1. Studies in vitro on wheat-germ agglutinin-purified receptors showed that the B isoform of the IR had increased kinase activity both toward itself and the exogenous substrates poly.glu4:tyr1 and recombinant IRS-1 protein. In addition, two-dimensional tryptic phosphopeptide maps indicated that the B isoform has an additional phosphopeptide that is not seen for the A isoform. In conclusion, it appears that the B isoform of the IR signals more efficiently than the A isoform in HepG2 cells.