Nucleophosmin1 is a negative regulator of the small GTPase Rac1.

The Rac1 GTPase is a critical regulator of cytoskeletal dynamics and controls many biological processes, such as cell migration, cell-cell contacts, cellular growth and cell division. These complex processes are controlled by Rac1 signaling through effector proteins. We have previously identified several effector proteins of Rac1 that also act as Rac1 regulatory proteins, including caveolin-1 and PACSIN2. Here, we report that Rac1 interacts through its C-terminus with nucleophosmin1 (NPM1), a multifunctional nucleo-cytoplasmic shuttling protein with oncogenic properties. We show that Rac1 controls NPM1 subcellular localization. In cells expressing active Rac1, NPM1 translocates from the nucleus to the cytoplasm. In addition, Rac1 regulates the localization of the phosphorylated pool of NPM1 as this pool translocated from the nucleus to the cytosol in cells expressing activated Rac1. Conversely, we found that expression of NPM1 limits Rac1 GTP loading and cell spreading. In conclusion, this study identifies NPM1 as a novel, negative regulator of Rac1.

Regulation of CXCR4 conformation by the small GTPase Rac1: implications for HIV infection.

The chemokine receptor CXCR4 is a critical regulator of cell migration and serves as a coreceptor for HIV-1. The chemokine stromal cell derived factor-1, also known as CXCL12, binds to CXCR4 and exerts its biologic functions partly through the small guanosine triphosphate hydrolase (GTPase) Rac1 (ras-related C3 botulinum toxin substrate 1). We show in different cell types, including CD34(+) hematopoietic stem and progenitor cells, that inhibition of Rac1 causes a reversible conformational change in CXCR4, but not in the related receptors CXCR7 or CCR5. Biochemical experiments showed that Rac1 associates with CXCR4. The conformational change of CXCR4 on Rac1 inhibition blocked receptor internalization and impaired CXCL12-induced Gα(i) protein activation. Importantly, we found that the conformation adopted by CXCR4 after Rac1 inhibition prevents HIV-1 infection of both the U87-CD4-CXCR4 cell line and of primary peripheral blood mononuclear cells. In conclusion, our data show that Rac1 activity is required to maintain CXCR4 in the responsive conformation that allows receptor signaling and facilitates HIV-1 infection; this implies that Rac1 positively regulates CXCR4 function and identifies the Rac1-CXCR4 axis as a new target for preventing HIV-1 infection.

Binding of human papilloma virus L1 virus-like particles to dendritic cells is mediated through heparan sulfates and induces immune activation.

Immunization using human papilloma virus (HPV)-L1 virus-like particles (VLPs) induces a robust and effective immune response, which has recently resulted in the implementation of the HPV-L1 VLP vaccination in health programs. However, during infection, HPV can escape immune surveillance leading to latency and disease. Dendritic cells (DCs) induce effective immune responses after vaccination, but might also induce immune modulation during infection. The interaction of HPV-L1 VLPs with mucosal DCs determines the immune response. However, little is known about the receptors on mucosal DC subsets involved in HPV-L1 VLP binding. Therefore, we set out to investigate the interaction of HPV-L1 VLPs with the different mucosal DC subsets; the subepithelial DCs and Langerhans cells (LCs). We observed strong binding of HPV-L1 VLPs to both DCs and LCs. We did not observe an involvement for C-type lectins such as dendritic cell-specific ICAM-3 grabbing non-integrin (DC-SIGN) and langerin. The HPV-L1 VLP binding to DCs was mediated through heparan sulfates, since it was abrogated by heparinase-II treatment. The heparan sulfate proteoglycan syndecan-3 binds VLPs and is expressed on both DCs and LCs. Binding of VLPs to DCs, but not to LCs, strongly correlated with the levels of heparan sulfates and syndecan-3, suggesting that syndecan-3 is the main receptor for HPV-L1 VLPs on DCs. VLP interaction with DCs resulted in the up-regulation of co-stimulatory molecules and the production of the cytokines IL-6, IL-8, IL-10 and IL-12p40. Our results support an important role for syndecan-3 as a HPV receptor on DCs, which could be important for both vaccine development and understanding HPV pathogenesis.