Hsp90-mediated cytosolic refolding of exogenous proteins internalized by dendritic cells

How ricin and Shiga toxin reach the cytosol of target cells: retrotranslocation from the endoplasmic reticulum

A number of protein toxins bind at the surface of mammalian cells and after endocytosis traffic to the endoplasmic reticulum, where the toxic A chains are liberated from the holotoxin. The free A chains are then dislocated, or retrotranslocated, across the ER membrane into the cytosol. Here, in contrast to ER substrates destined for proteasomal destruction, they undergo folding to a catalytic conformation and subsequently inactivate their cytosolic targets. These toxins therefore provide toxic probes for testing the molecular requirements for retrograde trafficking, the ER processes that prepare the toxic A chains for transmembrane transport, the dislocation step itself and for the post-dislocation folding that results in catalytic activity. We describe here the dislocation of ricin A chain and Shiga toxin A chain, but also consider cholera toxin which bears a superficial structural resemblance to Shiga toxin. Recent studies not only describe how these proteins breach the ER membrane, but also reveal aspects of a fundamental cell biological process, that of ER-cytosol dislocation.

Recombinant human TAT-OP1 to enhance NGF neurogenic potential: preliminary studies on PC12 cells

Osteogenic protein 1 (OP1), also known as bone morphogenic protein-7 (BMP7), is a multifunctional cytokine with demonstrated neurogenic potential. As the recombinant OP1 (rhOP1) was shown to provide axonal guidance cues and to prevent the reduction of dendritic growth in the injury-induced cortical cultures, it was suggested that an in vivo efficient rhOP1 delivery could enhance neurite growth and functional reconnectivity in the damaged brain. In the present work, we engineered a chimeric molecule in which rhBMP7 was fused to a protein transduction domain derived from HIV-1 TAT protein to deliver the denatured recombinant BMP7 into cells and obtain its chaperone-mediated folding, circumventing the expensive and not much efficient in vitro refolding procedures. When tested on rat PC12 cells, a widely used in vitro neurogenic differentiation model, the resulting fusion protein (rhTAT-OP1) demonstrated to enter fastly into the cells, lose HIV-TAT sequence and interact with membrane receptors activating BMP pathway by SMAD 1/5/8 phosphorylation. In comparison with nerve growth factor (NGF) and BMP7, it proved itself effective to induce the formation of more organized H and M neurofilaments. Moreover, if used in combination with NGF, it stimulated a significant (P < 0.05) and more precocious dendritic outgrowth with respect to NGF alone. These results indicate that rhTAT-OP1 fused with TAT transduction domain shows neurogenic activity and may be a promising enhancer factor in NGF-based therapies.

Essential role of endogenous heat shock protein 90 of dendritic cells in antigen cross-presentation

Extracellular HSP90 associated with Ag peptides have been demonstrated to efficiently cross-prime T cells, following internalization by dendritic cells (DCs). In addition, the nature of cell-associated Ags required for cross-priming is implicated as peptides and proteins chaperoned by heat shock protein (HSP). However, the role of endogenous HSP in DCs during cross-presentation remains elusive. In this paper, we show that endogenous HSP90 is essential for cross-presentation of both soluble and cell-associated Ags in DCs. Cross-presentation of soluble OVA and OVA-loaded transporter associated with Ag processing-1-deficient cells by bone marrow-derived DCs and DC-like cell line DC2.4 was profoundly blocked by HSP90 inhibitors, whereas presentation of endogenously expressed OVA was only partially suppressed. Assays using small interfering RNA and heat shock factor-1-deficient DCs (with defective expression of HSP90alpha) revealed the pivotal role of HSP90alpha in cross-presentation. The results suggest that in addition to HSP90 in Ag donor cells, endogenous HSP90 in DCs plays an essential role during Ag cross-presentation and, moreover, points to a link between heat shock factor-1-dependent induction of HSP90alpha within DC and cytotoxic T cell immunity.

Hsp90 is required for transfer of the cholera toxin A1 subunit from the endoplasmic reticulum to the cytosol

Cholera toxin (CT) is an AB(5) toxin that moves from the cell surface to the endoplasmic reticulum (ER) by retrograde vesicular transport. In the ER, the catalytic A1 subunit dissociates from the rest of the toxin and enters the cytosol by exploiting the quality control system of ER-associated degradation (ERAD). The driving force for CTA1 dislocation into the cytosol is unknown. Here, we demonstrate that the cytosolic chaperone Hsp90 is required for CTA1 passage into the cytosol. Hsp90 bound to CTA1 in an ATP-dependent manner that was blocked by geldanamycin (GA), an established Hsp90 inhibitor. CT activity against cultured cells and ileal loops was also blocked by GA, as was the ER-to-cytosol export of CTA1. Experiments using RNA interference or N-ethylcarboxamidoadenosine, a drug that inhibits ER-localized GRP94 but not cytosolic Hsp90, confirmed that the inhibitory effects of GA resulted specifically from the loss of Hsp90 activity. This work establishes a functional role for Hsp90 in the ERAD-mediated dislocation of CTA1.

Defective cross-presentation of viral antigens in GILT-free mice

Gamma-interferon-inducible lysosomal thiolreductase (GILT) promotes major histocompatibility complex (MHC) class II-restricted presentation of exogenous antigens containing disulfide bonds. Here, we show that GILT also facilitates MHC class I-restricted recognition of such antigens by CD8+ T cells, or cross-presentation. GILT is essential for cross-presentation of a CD8+ T cell epitope of glycoprotein B (gB) from herpes simplex virus 1 (HSV-1) but not for its presentation by infected cells. Initiation of the gB-specific CD8+ T cell response during HSV-1 infection, or cross-priming, is highly GILT-dependent, as is initiation of the response to the envelope glycoproteins of influenza A virus. Efficient cross-presentation of disulfide-rich antigens requires a complex pathway involving GILT-mediated reduction, unfolding, and partial proteolysis, followed by translocation into the cytosol for proteasomal processing.

Macrophage-derived AIM is endocytosed into adipocytes and decreases lipid droplets via inhibition of fatty acid synthase activity

Macrophages infiltrate adipose tissue in obesity and are involved in the induction of inflammation, thereby contributing to the development of obesity-associated metabolic disorders. Here, we show that the macrophage-derived soluble protein AIM is endocytosed into adipocytes via CD36. Within adipocytes, AIM associates with cytosolic fatty acid synthase (FAS), thereby decreasing FAS activity. This decreases lipid droplet size, stimulating the efflux of free fatty acids and glycerol from adipocytes. As an additional consequence of FAS inhibition, AIM prevents preadipocyte maturation. In vivo, the increase in adipocyte size and fat weight induced by high-fat diet (HFD) was accelerated in AIM-deficient (AIM(-)(/-)) mice compared to AIM(+/+) mice. Moreover, injection of recombinant AIM in AIM(-)(/-) mice suppresses the increase in fat mass induced by HFD. Interestingly, metabolic rates are comparable in AIM(-)(/-) and AIM(+/+) mice, suggesting that AIM specifically influences adipocyte status. Thus, this AIM function in adipocytes may be physiologically relevant to obesity progression.

Receptor-mediated phagocytosis elicits cross-presentation in nonprofessional antigen-presenting cells

In cross-presentation by dendritic cells (DCs), internalized proteins are retrotranslocated into the cytosol, degraded by the proteasome, and the generated antigenic peptides bind to MHC class I molecules for presentation on the cell surface. Endoplasmic reticulum (ER) contribution to phagosomal membranes is thought to provide antigen access to the ER-associated degradation (ERAD) machinery, allowing cytosolic dislocation. Because the ERAD pathway is present in all cell types and exogenous antigens encounter an ER-containing compartment during phagocytosis, we postulated that forcing phagocytosis in cell types other than DCs would render them competent for cross-presentation. Indeed, FcRgammaIIA expression endowed 293T cells with the capacity for both phagocytosis and ERAD-mediated cross-presentation of an antigen provided as an immune complex. The acquisition of this ability by nonprofessional antigen-presenting cells suggests that a function potentially available in all cell types has been adapted by DCs for presentation of exogenous antigens by MHC class I molecules.

Cancer immunotherapy by dendritic cells

Cancerous lesions promote tumor growth, motility, invasion, and angiogenesis via oncogene-driven immunosuppressive leukocyte infiltrates, mainly myeloid-derived suppressor cells, tumor-associated macrophages, and immature dendritic cells (DCs). In addition, many tumors express or induce immunosuppressive cytokines such as TGF-beta and IL-10. As a result, tumor-antigen crosspresentation by DCs induces T cell anergy or deletion and regulatory T cells instead of antitumor immunity. Tumoricidal effector cells can be generated after vigorous DC activation by Toll-like receptor ligands or CD40 agonists. However, no single immunotherapeutic modality is effective in established cancer. Rather, chemotherapies, causing DC activation, enhanced crosspresentation, lymphodepletion, and reduction of immunosuppressive leukocytes, act synergistically with vaccines or adoptive T cell transfer. Here, I discuss the considerations for generating promising therapeutic antitumor vaccines that use DCs.

Hepatitis B virus HBx protein localizes to mitochondria in primary rat hepatocytes and modulates mitochondrial membrane potential

Over 350 million people are chronically infected with hepatitis B virus (HBV), and a significant number of chronically infected individuals develop primary liver cancer. HBV encodes seven viral proteins, including the nonstructural X (HBx) protein. The results of studies with immortalized or transformed cells and with HBx-transgenic mice demonstrated that HBx can interact with mitochondria. However, no studies with normal hepatocytes have characterized the precise mitochondrial localization of HBx or the effect of HBx on mitochondrial physiology. We have used cultured primary rat hepatocytes as a model system to characterize the mitochondrial localization of HBx and the effect of HBx expression on mitochondrial physiology. We now show that a fraction of HBx colocalizes with density-gradient-purified mitochondria and associates with the outer mitochondrial membrane. We also demonstrate that HBx regulates mitochondrial membrane potential in hepatocytes and that this function of HBx varies depending on the status of NF-kappaB activity. In primary rat hepatocytes, HBx activation of NF-kappaB prevented mitochondrial membrane depolarization; however, when NF-kappaB activity was inhibited, HBx induced membrane depolarization through modulation of the mitochondrial permeability transition pore. Collectively, these results define potential pathways through which HBx may act in order to modulate mitochondrial physiology, thereby altering many cellular activities and ultimately contributing to the development of HBV-associated liver cancer.