Functional principles and regulation of molecular chaperones

To be able to perform their biological function, a protein needs to be correctly folded into its three dimensional structure. The protein folding process is spontaneous and does not require the input of energy. However, in the crowded cellular environment where there is high risk of inter-molecular interactions that may lead to protein molecules sticking to each other, hence forming aggregates, protein folding is assisted. Cells have evolved robust machinery called molecular chaperones to deal with the protein folding problem and to maintain proteins in their functional state. Molecular chaperones promote efficient folding of newly synthesized proteins, prevent their aggregation and ensure protein homeostasis in cells. There are different classes of molecular chaperones functioning in a complex interplay. In this review, we discuss the principal characteristics of different classes of molecular chaperones, their structure-function relationships, their mode of regulation and their involvement in human disorders.

Regulated structural transitions unleash the chaperone activity of αB-crystallin

The small heat shock protein αB-crystallin is an oligomeric molecular chaperone that binds aggregation-prone proteins. As a component of the proteostasis system, it is associated with cataract, neurodegenerative diseases, and myopathies. The structural determinants for the regulation of its chaperone function are still largely elusive. Combining different experimental approaches, we show that phosphorylation-induced destabilization of intersubunit interactions mediated by the N-terminal domain (NTD) results in the remodeling of the oligomer ensemble with an increase in smaller, activated species, predominantly 12-mers and 6-mers. Their 3D structures determined by cryo-electron microscopy and biochemical analyses reveal that the NTD in these species gains flexibility and solvent accessibility. These modulated properties are accompanied by an increase in chaperone activity in vivo and in vitro and a more efficient cooperation with the heat shock protein 70 system in client folding. Thus, the modulation of the structural flexibility of the NTD, as described here for phosphorylation, appears to regulate the chaperone activity of αB-crystallin rendering the NTD a conformational sensor for nonnative proteins.

Inhibitory effect of the HPV-16mE6δ/mE7/TBhsp70δ vaccine on oral squamous cell carcinoma

AIM

To evaluate the inhibitory effect of a recombinant human papillomavirus (HPV) fusion protein vaccine on oral squamous cell carcinoma (OSCC).

MATERIALS AND METHODS

An animal model of OSCC was established using human peripheral blood lymphocyte reconstituted nonobese diabetic/severe combined immunodeficiency mice. HPV vaccine was subcutaneously injected into mice after tumor establishment. Tumors and spleens were measured, weighed and stained with hematoxylin and eosin. Lymphocyte subpopulations and cytotoxicity were analyzed with flow cytometric and cytotoxic T lymphocyte assay.

RESULTS

The average weight and volume of tumors were significantly lower in the vaccine group than in the control group from day 27. Mice in both groups had high percentages of human CD3+ and CD3+CD8+ T lymphocytes. An elevated percentage of human CD3+CD16+56+ natural killer cells was found in the vaccine group. Moreover, vaccine increased the infiltration of human CD3 and UCHL-1+ cells in tumor tissues and enhanced cytotoxicity.

CONCLUSIONS

The HPV fusion protein vaccine induces tumor cell death, lymphocyte infiltration and therefore suppresses tumor growth and protects against OSCC.

Conformational selection in substrate recognition by Hsp70 chaperones

Hsp70s are molecular chaperones involved in the folding and assembly of proteins. They recognize hydrophobic amino acid stretches in their substrate binding groove. However, a detailed understanding of substrate specificity is still missing. Here, we use the endoplasmic reticulum-resident Hsp70 BiP to identify binding sites in a natural client protein. Two sites are mutually recognized and form stable Hsp70-substrate complexes. In silico and in vitro analyses revealed an extended substrate conformation as a crucial factor for interaction and show an unexpected plasticity of the substrate binding groove. The basic binding mechanism is conserved among different Hsp70s.

Active-specific immunotherapy of human cancers with the heat shock protein Gp96-revisited

The passive administration of specific antibodies that selectively target tumors is a well-known strategy in cancer treatment. Active immunotherapy using peptide vaccines, in contrast, is expected to induce specific, cytolytic T cells in the patient, which react against tumor antigens and destroy malignant cells. Although several concepts exist, the identification and low immunogenicity of tumor-specific peptides remain a serious problem. Heat shock proteins (HSPs), notably glycoprotein (Gp) 96, are of special interest, because they are able to take molecular peptide-fingerprints of the protein array characteristic for a particular cell. Association of Gp96 with peptides has been shown to be essential for crosspresentation and activation of T cells. Consequently, Gp96-peptide complexes extracted from cancer cells harbor the tumor-specific peptides and are immunogenic, thus offering a tool for active immunization against the tumor. Already, several immunotherapy studies of human cancers have been carried out, showing no severe adverse effects but unfortunately only limited improvement in the clinical outcome. Vitespen, a commercial HSP-peptide complex vaccine based on tumor-derived Gp96, seems to induce an improved overall survival for subsets of early stage melanoma and kidney cancer patients. The limited access to vaccine material derived from the autologous tumor requires the development of alternative protocols. Moreover, counteracting immunosuppressive mechanisms induced by the malignancy might further improve the efficacy of vaccinations. This review critically analyzes the current state of clinical immunotherapy with Gp96, with special attention to Vitespen.

High affinity binding of hydrophobic and autoantigenic regions of proinsulin to the 70 kDa chaperone DnaK

BACKGROUND

Chaperones facilitate proper folding of peptides and bind to misfolded proteins as occurring during periods of cell stress. Complexes of peptides with chaperones induce peptide-directed immunity. Here we analyzed the interaction of (pre)proinsulin with the best characterized chaperone of the hsp70 family, bacterial DnaK.

RESULTS

Of a set of overlapping 13-mer peptides of human preproinsulin high affinity binding to DnaK was found for the signal peptide and one further region in each proinsulin domain (A- and B-chain, C-peptide). Among the latter, peptides covering most of the B-chain region B11-23 exhibited strongest binding, which was in the range of known high-affinity DnaK ligands, dissociation equilibrium constant (K'd) of 2.2 ± 0.4 μM. The B-chain region B11-23 is located at the interface between two insulin molecules and not accessible in insulin oligomers. Indeed, native insulin oligomers showed very low DnaK affinity (K'd 67.8 ± 20.8 μM) whereas a proinsulin molecule modified to prevent oligomerization showed good binding affinity (K'd 11.3 ± 7.8 μM).

CONCLUSIONS

Intact insulin only weakly interacts with the hsp70 chaperone DnaK whereas monomeric proinsulin and peptides from 3 distinct proinsulin regions show substantial chaperone binding. Strongest binding was seen for the B-chain peptide B 11-23. Interestingly, peptide B11-23 represents a dominant autoantigen in type 1 diabetes.

Therapeutic HPV DNA vaccines

It is now well established that most cervical cancers are causally associated with HPV infection. This realization has led to efforts to control HPV-associated malignancy through prevention or treatment of HPV infection. Currently, commercially available HPV vaccines are not designed to control established HPV infection and associated premalignant and malignant lesions. To treat and eradicate pre-existing HPV infections and associated lesions which remain prevalent in the U.S. and worldwide, effective therapeutic HPV vaccines are needed. DNA vaccination has emerged as a particularly promising form of therapeutic HPV vaccines due to its safety, stability and ability to induce antigen-specific immunity. This review focuses on improving the potency of therapeutic HPV vaccines through modification of dendritic cells (DCs) by [1] increasing the number of antigen-expressing/antigen-loaded DCs, [2] improving HPV antigen expression, processing and presentation in DCs, and [3] enhancing DC and T cell interaction. Continued improvement in therapeutic HPV DNA vaccines may ultimately lead to an effective DNA vaccine for the treatment of HPV-associated malignancies.