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The tumor necrosis factor-alpha (TNF) converting enzyme (ADAM17) is a metalloprotease that cleaves several transmembrane proteins, including TNF and its receptors (TNFR1 and TNFR2). We recently showed that the shedding activity of ADAM17 is sequestered in lipid rafts and that cholesterol depletion increased the shedding of ADAM17 substrates. These data suggested that ADAM17 activity could be regulated by cholesterol movements in the cell membrane. We investigated if the membrane cholesterol efflux induced by high-density lipoproteins (HDLs) was able to modify the shedding of ADAM17 substrates. HDLs added to different cell types, increased the ectodomain shedding of TNFR2, TNFR1, and TNF, an effect reduced by inhibitors active on ADAM17. The HDLs-stimulated TNF release occurred also on cell-free isolated plasma membranes. Purified apoA1 increased the shedding of TNF in an ABCA1-dependent manner, suggesting a role for the cholesterol efflux in this phenomenon. HDLs reduced the cholesterol and proteins (including ADAM17) content of lipid rafts and triggered the ADAM17-dependent cleavage of TNF in the non-raft region of the membrane. In conclusion, these data demonstrate that HDLs alter the lipid raft structure, which in turn activates the ADAM17-dependent processing of transmembrane substrates. J. Cell. Physiol. 214: 687–693, 2008. © 2007 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

The level of adhesion molecules expressed at the endothelial cell surface is critical in the control of inflammation. Adenylate cyclase (AC) activity allowing cyclic adenosine monophosphate (cAMP) production can modulate the inflammatory process. We investigated the AC-dependent modulation of ICAM-1 surface expression in human umbilical venous endothelial cells (HUVEC). Pretreatment of HUVEC with forskolin significantly upregulated tumor necrosis factor alpha (TNF-alpha)- and interleukin-1 alpha (IL1-alpha)-induced ICAM-1 surface expression exclusively after a prolonged time of incubation with forskolin (at least 7-8 h). A poorly metabolizable analog of cAMP, dibutyryl-cAMP, mimicked forskolin effect on ICAM-1 surface expression. Protein kinase A (PKA) inhibitor H89 prevented forskolin effect on ICAM-1 surface expression. Upregulation of ICAM-1 surface level occurred through the increase in its mRNA levels and also to a subsequent activation of ICAM-1 intracellular trafficking towards cell surface. Stimulation by agonists of beta-adrenergic receptors did not alter the TNF-alpha-induced ICAM-1 surface expression. Pretreatment of HUVEC with pertussis toxin, which is known to activate AC through Gialpha inhibition, upregulated mRNA levels and surface expression of ICAM-1 induced by TNF-alpha. This effect was serum-dependent, since ICAM-1 expression was no more upregulated by pertussis toxin in cells cultured in 1% instead of 20% serum-enriched medium. However, forskolin treatment of HUVEC did not modify their overall adhesive properties. In conclusion, a persistent cAMP level elevation activating PKA is able to enhance ICAM-1 expression at the cell surface of endothelial cells placed under pro-inflammatory conditions. Combination of activation of gene transcription and membrane targeting may account for this augmentation., (2004 Wiley-Liss, Inc.)