Suppression of apoptosis by glucocorticoids in glomerular endothelial cells: effects on proapoptotic pathways

1 Tumour necrosis factor-alpha (TNF-alpha)- and lipopolysaccharide (LPS)-in duced apoptosis of bovine glomerular endothelial cells is now recognized as an important part in the pathogenesis of glomerulonephritis characterized by early mitochondrial cytochrome c release, mitochondrial permeability tra nsition, Bak protein upregulation, Bcl-X-L protein downregulation and caspa se-3 activation. 2 Co-treatment of cells with 10 nM dexamethasone and TNF-alpha or LPS block ed roughly 90% of apoptotic cell death in glomerular endothelial cells. The action of glucocorticoids could be documented in that they prevented all a poptotic markers such as DNA laddering, DNA fragmentation measured by the d iphenylamine assay as well as morphological alterations. 3 To mechanistically elucidate the action of glucocorticoids we evaluated w hether glucocorticoids elicit a time-dependent effect. For dexamethasone, t o maximally inhibit DNA fragmentation a preincubation period was not requir ed. Even if dexamethasone was supplemented 6 h following TNF-alpha or LPS w e observed a maximal inhibitory effect. 4 Concerning its influence on TNF-alpha and LPS signal transduction, we fou nd that dexamethasone only partially prevented cytochrome-c-release as a fi rst sign of apoptotic cell death but efficiently blocked mitochondrial perm eability transition. Moreover, TNF-alpha- and LPS-induced Bak upregulation, Bcl-X-L-downregulation, and the activation of caspase-3-like proteases, me asured fluorometrically using DEVD-AMC and PARP cleavage, were efficiently blocked by dexamethasone. 5 We postulate that glucocorticoids exert their inhibitory action upstream of the terminal death pathways but downstream of primary receptor mediated signals by blocking pro-apoptotic signals pre- and/or post cytochrome c rel ease and mitochondrial signalling.