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Heat‐shock protein‐90 prolongs septic neutrophil survival by protecting c‐Src kinase and caspase‐8 from proteasomal degradation

The brief lifespan of the polymorphonuclear neutrophil (PMN) is regulated through its capacity to undergo apoptosis, a constitutive process that is actively inhibited during sepsis. We sought to define the cellular mechanisms through which Heat Shock Protein 90 (Hsp90) prolongs the survival of inflammatory PMN. We evaluated Hsp90 expression and interaction with client proteins in PMNs from patients with sepsis and in healthy control PMNs treated with LPS (1 μg/mL). Hsp90 activity was inhibited pharmacologically using radicicol (Rad; 1 μM), and Hsp90 transcription was silenced in septic PMN using siRNA. PMN apoptosis was evaluated by flow cytometry and expression of cleaved caspase‐8 and ‐3. Septic PMNs showed reduced rates of apoptosis compared with control PMNs 21 h after isolation, and Hsp90‐α mRNA was significantly more abundant in septic PMN. Caspase‐8 coimmunoprecipitated with Hsp90, c‐Src, and the p85 inhibitory subunit of PI3K in both septic and LPS‐treated PMN. Inhibition of Hsp90 activity with Rad or its translation using siRNA restored basal rates of apoptosis in both septic and LPS‐treated PMN. Radicicol further reduced c‐Src protein abundance, increased the ubiquitination of caspase‐8 and c‐Src, and enhanced the cleavage of caspase‐8 and ‐3. We conclude that Hsp90 prolongs the survival of activated neutrophils by stabilizing a molecular complex of c‐Src kinase and caspase‐8, preventing their ubiquitination, and resulting in inhibition of the catalytic activity of caspase‐8 and ‐3.