HDLs activate ADAM17-dependent shedding

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.