Small Rho GTPases Mediate Tumor-Induced Inhibition of Endocytic Activity of Dendritic Cells

Abstract

The generation, maturation, and function of dendritic cells (DC) have been shown to be markedly compromised in the tumor microenvironment in animals and humans. However, the molecular mechanisms and intracellular pathways involved in the regulation of the DC system in cancer are not yet fully understood. Recently, we have reported on the role of the small Rho GTPase family members Cdc42, Rac1, and RhoA in regulating DC adherence, motility, and Ag presentation. To investigate involvement of small Rho GTPases in dysregulation of DC function by tumors, we next evaluated how Cdc42, Rac1, and RhoA regulated endocytic activity of DC in the tumor microenvironment. We revealed a decreased uptake of dextran 40 and polystyrene beads by DC generated in the presence of different tumor cell lines, including RM1 prostate, MC38 colon, 3LL lung, and B7E3 oral squamous cell carcinomas in vitro and by DC prepared from tumor-bearing mice ex vivo. Impaired endocytic activity of DC cocultured with tumor cells was associated with decreased levels of active Cdc42 and Rac1. Transduction of DC with the dominant negative Cdc42 and Rac1 genes also led to reduced phagocytosis and receptor-mediated endocytosis. Furthermore, transduction of DC with the constitutively active Cdc42 and Rac1 genes restored endocytic activity of DC that was inhibited by the tumors. Thus, our results suggest that tumor-induced dysregulation of endocytic activity of DC is mediated by reduced activity of several members of the small Rho GTPase family, which might serve as new targets for improving the efficacy of DC vaccines.

Ammonia Drives Dendritic Cells into Dysfunction

“…In terms of the tumor-induced suppression of the endocytic activity of DCs, Cdc42, and Rac1 (Rho GTPase)-mediated pathway, which could regulate the cytoskeleton architecture, is thought to be involved (42). These data support our research, which provided ammonia as an inhibitor of DC phagocytosis, with cell volume regulating potential.…”

Low-dose Chemotherapeutic Agents Regulate Small Rho GTPase Activity in Dendritic Cells

“…20,21,38,39 Recently, the involvement of Rho GTPases in DC dysfunction in the tumor microenvironment has also been proven. 22 This suggests that small Rho GTPases in DCs might serve as novel targets for improving function and longevity of DC vaccines in cancer. However, there are no data on pharmacologic regulation of Rho proteins in DCs for therapeutic purposes.…”

“…To evaluate small Rho GTPases in DCs, we assessed membrane-bound and cytoplasmic levels of RhoG proteins, reflecting active and inactive pools, respectively, or used the pool-down assay earlier. 21,22 Here, to test whether chemotherapeutic agents might affect small Rho GTPases in DCs, we used novel Activation Assay kits, which allow screening of multiple effector molecules. The low nontoxic concentrations of cytotoxic agents were estimated in Table 1 and were used in some pilot studies.…”

“…The low molecular weight Rho GTPases control DC adherence, antigen processing and presentation, migration, chemotaxis, and endocytosis. [19][20][21][22] Differential role of the small Rho GTPase family members, including RhoA and RhoB, Rac1/2/3, and Cdc42, in the capacity of DCs to phagocytose apoptotic cells (efferocytosis) and prime T cells via cross-presentation has been also reported. [23][24][25][26] Furthermore, cell stimulation with PS induces translocation and activation of Rac1 and promotes Cdc42 activation and phosphorylation of mitogen-activated protein kinase through Rac1/PS signaling, 27 confirming the earlier conclusion that PS is capable of signaling changes in the actin cytoskeleton and these changes require both Cdc42 and Rac1.…”

“…28 Despite emerging evidence to suggest that efferocytosis of dying tumor cells by DCs requires membrane targeting of specific small Rho GTPases, nothing is known with regard to the direct effect of chemotherapeutic agents on low molecular weight Rho GTPases in DCs. We previously reported regulation of small G proteins in DCs in the tumor environment, 22 and modulation of DC maturation and function by low-dose chemotherapeutic drug paclitaxel. 29 Herein, we studied putative regulatory roles for various chemotherapeutic agents in modulating small Rho GTPases in DCs.…”