Differential expression of heat shock protein 70 (hsp70) in human monocytes rendered apoptotic by IL-4 or serum deprivation

Apoptosis of monocytes is regulated by the balance between pro- and antiapo ptotic triggers and pathways and may strongly influence inflammatory disord ers, The major heat shock protein, hsp70, is all effective inhibitor of apo ptosis in lymphocytic and monocytic tumor cell Lines, but the implications in the regulation of apoptosis of freshly isolated human monocytes have not been elucidated. In this study, we examined whether two different triggers of monocyte apoptosis, serum deprivation and IL-4, respectively, altered h sp70 expression and whether expression levels correlated with monocyte surv ival. Monocyte apoptosis was determined quantitatively by flow cytometry de tecting annexin V binding or nuclear stainability with propidium iodide (PI ). Hsp70 expression was analyzed by semiquantitative RT-PCR and immunoblott ing, Exposing monocytes to heat shock (47 degreesC, 20 min) induced a rapid and marked upregulation of hsp70 without evoking injury or apoptosis, sugg esting that hsp70 conferred protection and survival. In accordance, when mo nocytes were rendered apoptotic by serum deprivation, a drastic downregulat ion of hsp70 occurred, which was accompanied by a reduced synthesis of the constitutive family member hsc70, However, induction of monocyte apoptosis by IL-4 increased hsP70 expression in a concentration and time-dependent fa shion. A neutralizing antibody against IL-4 abolished hsp70 expression and apoptosis induction after IL-4 treatment and so excluded indirect effects. LPS rescued monocytes from apoptosis but did. not alter hsp70 formation sig nificantly, These findings suggest that, in monocytes, distinct apoptotic t riggers induce different responses of hsp70 so that this molecule does not exert protection against cell death directly or hi general.