Differential effects of invasion by and phagocytosis of Salmonella typhimurium on apoptosis in human macrophages: potential role of Rho-GTPases and Akt

In addition to direct activation of caspase-1 and induction of apoptosis by SipB, in- vasive Salmonella stimulates multiple signaling pathways that are key regulators of host cell survival. Nevertheless, little is known about the relative con- tributions of these pathways to Salmonella-medi- ated death of macrophages. We studied human monocytic U937 cells and found that apoptosis was induced by invading wild-type Salmonella typhi- murium but not by phagocytosed, serum-opso- nized, noninvasive Salmonella mutants. Pretreat- ing U937 cells with inhibitors of tyrosine kinases or phosphatidylinositol-3 kinase (PI-3K) completely blocked phagocytosis of opsonized Salmonella mu- tants but did not affect invasion by wild-type Salmo- nella or the apoptosis caused by invasion. However, pretreatment with GGTI-298, a geranylgeranyltrans- ferase-1 inhibitor that prevents prenylation of Cdc42 and Rac1, suppressed Salmonella-induced apopto- sis by 70%. Transduction of Tat fusion con- structs containing dominant-negative Cdc42 or Rac1 significantly inhibited Salmonella-induced cell death, indicating that the cytotoxicity of Sal- monella requires activation of Cdc42 and Rac. In contrast to phagocytosis of opsonized bacteria, in- vasion by S. typhimurium stimulated Cdc42 and Rac1, regardless of the activities of tyrosine- or PI-3K. Moreover, Salmonella infection activated Akt protein in a tyrosine-kinase or PI-3K-depen- dent manner, and a reduced expression of Akt by antisense transfection rendered the cells more sen- sitive to apoptosis induced by opsonized Salmo- nella. These results indicate that direct activation of Cdc42 and Rac1 by invasive Salmonella is a prerequisite of Salmonella-mediated death of U937 cells, whereas the simultaneous activation of Akt by tyrosine kinase and PI-3K during receptor- mediated phagocytosis protects cells from apoptosis. J. Leukoc. Biol. 74: 000-000; 2003.