High-yield expression in E. coli and refolding of the bZIP domain of activating transcription factor 5

Targeting Transcription Factors ATF5, CEBPB and CEBPD with Cell-Penetrating Peptides to Treat Brain and Other Cancers

Developing novel therapeutics often follows three steps: target identification, design of strategies to suppress target activity and drug development to implement the strategies. In this review, we recount the evidence identifying the basic leucine zipper transcription factors ATF5, CEBPB, and CEBPD as targets for brain and other malignancies. We describe strategies that exploit the structures of the three factors to create inhibitory dominant-negative (DN) mutant forms that selectively suppress growth and survival of cancer cells. We then discuss and compare four peptides (CP-DN-ATF5, Dpep, Bpep and ST101) in which DN sequences are joined with cell-penetrating domains to create drugs that pass through tissue barriers and into cells. The peptide drugs show both efficacy and safety in suppressing growth and in the survival of brain and other cancers in vivo, and ST101 is currently in clinical trials for solid tumors, including GBM. We further consider known mechanisms by which the peptides act and how these have been exploited in rationally designed combination therapies. We additionally discuss lacunae in our knowledge about the peptides that merit further research. Finally, we suggest both short- and long-term directions for creating new generations of drugs targeting ATF5, CEBPB, CEBPD, and other transcription factors for treating brain and other malignancies.

Self-assembly of the bZIP transcription factor ΔFosB

ΔFosB is a highly stable transcription factor that accumulates in specific brain regions upon chronic exposure to drugs of abuse, stress, or seizures, and mediates lasting behavioral responses. ΔFosB reportedly heterodimerizes with JunD forming a canonical bZIP leucine zipper coiled coil that clamps onto DNA. However, the striking accumulation of ΔFosB protein in brain upon chronic insult has brought its molecular status into question. Here, we demonstrate through a series of crystal structures that the ΔFosB bZIP domain self-assembles into stable oligomeric assemblies that defy the canonical arrangement. The ΔFosB bZIP domain also self-assembles in solution, and in neuron-like Neuro 2a cells it is trapped into molecular arrangements that are consistent with our structures. Our data suggest that, as ΔFosB accumulates in brain in response to chronic insult, it forms non-canonical assemblies. These species may be at the root of ΔFosB's striking protein stability, and its unique transcriptional and behavioral consequences.

Regression/eradication of gliomas in mice by a systemically-deliverable ATF5 dominant-negative peptide

Malignant gliomas have poor prognosis and urgently require new therapies. Activating Transcription Factor 5 (ATF5) is highly expressed in gliomas, and interference with its expression/function precipitates targeted glioma cell apoptosis in vitro and in vivo. We designed a novel deliverable truncated-dominant-negative (d/n) form of ATF5 fused to a cell-penetrating domain (Pen-d/n-ATF5-RP) that can be intraperitoneally/subcutaneously administered to mice harboring malignant gliomas generated; (1) by PDGF-B/sh-p53 retroviral transformation of endogenous neural progenitor cells; and (2) by human U87-MG xenografts. In vitro Pen-d/n-ATF5-RP entered into glioma cells and triggered massive apoptosis. In vivo, subcutaneously-administered Pen-d/n-ATF5-RP passed the blood brain barrier, entered normal brain and tumor cells, and then caused rapid selective tumor cell death. MRI verified elimination of retrovirus-induced gliomas within 8-21 days. Histopathology revealed growth-suppression of intracerebral human U87-MG cells xenografts. For endogenous PDGF-B gliomas, there was no recurrence or mortality at 6-12 months versus 66% mortality in controls at 6 months. Necropsy and liver-kidney blood enzyme analysis revealed no adverse effects on brain or other tissues. Our findings thus identify Pen-d/n-ATF5-RP as a potential therapy for malignant gliomas.

claMP Tag: a versatile inline metal-binding platform based on the metal abstraction peptide

Molecularly targeted research and diagnostic tools are essential to advancing understanding and detection of many diseases. Metals often impart the desired functionality to these tools, and conjugation of high-affinity chelators to proteins is carried out to enable targeted delivery of the metal. This approach has been much more effective with large lanthanide series metals than smaller transition metals. Because chemical conjugation requires additional processing and purification steps and yields a heterogeneous mixture of products, inline incorporation of a peptide tag capable of metal binding is a highly preferable alternative. Development of a transition metal binding tag would provide opportunity to greatly expand metal-based analyses. The metal abstraction peptide (MAP) sequence was genetically engineered into recombinant protein to generate the claMP Tag. The effects of this tag on recombinant epidermal growth factor (EGF) protein expression, disulfide bond formation, tertiary structural integrity, and transition metal incorporation using nickel were examined to confirm the viability of utilizing the MAP sequence to generate linker-less metal conjugates.

Human cytochrome P450 17A1 conformational selection: modulation by ligand and cytochrome b5

Crystallographic studies of different membrane cytochrome P450 enzymes have provided examples of distinct structural conformations, suggesting protein flexibility. It has been speculated that conformational selection is an integral component of substrate recognition and access, but direct evidence of such substate interconversion has thus far remained elusive. In the current study, solution NMR revealed multiple and exchanging backbone conformations for certain structural features of the human steroidogenic cytochrome P450 17A1 (CYP17A1). This bifunctional enzyme is responsible for pregnenolone C17 hydroxylation, followed by a 17,20-lyase reaction to produce dehydroepiandrosterone, the key intermediate in human synthesis of androgen and estrogen sex steroids. The distribution of CYP17A1 conformational states was influenced by temperature, binding of these two substrates, and binding of the soluble domain of cytochrome b5 (b5). Notably, titration of b5 to CYP17A1·pregnenolone induced a set of conformational states closely resembling those of CYP17A1·17α-hydroxypregnenolone without b5, providing structural evidence consistent with the reported ability of b5 to selectively enhance 17,20-lyase activity. Solution NMR thus revealed a set of conformations likely to modulate human steroidogenesis by CYP17A1, demonstrating that this approach has the potential to make similar contributions to understanding the functions of other membrane P450 enzymes involved in drug metabolism and disease states.

Influence of the valine zipper region on the structure and aggregation of the basic leucine zipper (bZIP) domain of activating transcription factor 5 (ATF5)

Protein aggregation is a major problem for biopharmaceuticals. While the control of aggregation is critically important for the future of protein pharmaceuticals, mechanisms of aggregate assembly, particularly the role that structure plays, are still poorly understood. Increasing evidence indicates that partially folded intermediates critically influence the aggregation pathway. We have previously reported the use of the basic leucine zipper (bZIP) domain of activating transcription factor 5 (ATF5) as a partially folded model system to investigate protein aggregation. This domain contains three regions with differing structural propensity: a N-terminal polybasic region, a central helical leucine zipper region, and a C-terminal extended valine zipper region. Additionally, a centrally positioned cysteine residue readily forms an intermolecular disulfide bond that reduces aggregation. Computational analysis of ATF5 predicts that the valine zipper region facilitates self-association. Here we test this hypothesis using a truncated mutant lacking the C-terminal valine zipper region. We compare the structure and aggregation of this mutant to the wild-type (WT) form under both reducing and nonreducing conditions. Our data indicate that removal of this region results in a loss of α-helical structure in the leucine zipper and a change in the mechanism of self-association. The mutant form displays increased association at low temperature but improved resistance to thermally induced aggregation.

Regulated ATF5 loss-of-function in adult mice blocks formation and causes regression/eradication of gliomas

Glioblastomas are among the most incurable cancers. Our past findings indicated that glioblastoma cells, but not neurons or glia, require the transcription factor ATF5 (activating transcription factor 5) for survival. However, it was unknown whether interference with ATF5 function can prevent or promote regression/eradication of malignant gliomas in vivo. To address this issue, we created a mouse model by crossing a human glial fibrillary acidic protein (GFAP) promoter-tetracycline transactivator mouse line with tetracycline operon-dominant negative-ATF5 (d/n-ATF5) mice to establish bi-transgenic mice. In this model, d/n-ATF5 expression is controlled by doxycycline and the promoter for GFAP, a marker for stem/progenitor cells as well as gliomas. Endogenous gliomas were produced with high efficiency by retroviral delivery of platelet-derived growth factor (PDGF)-B and p53-short hairpin RNA (shRNA) in adult bi-transgenic mice in which expression of d/n-ATF5 was spatially and temporally regulated. Induction of d/n-ATF5 before delivery of PDGF-B/p53-shRNA virus greatly reduced the proportion of mice that formed tumors. Moreover, d/n-ATF5 induction after tumor formation led to regression/eradication of detectable gliomas without evident damage to normal brain cells in all 24 mice assessed.

Effects of disulfide bond formation and protein helicity on the aggregation of activating transcription factor 5

Amorphous aggregation is a major problem for protein biopharmaceuticals, and aggregate formation in a drug formulation can have serious health implications for the patient. In many cases, an immunogenic response is generated from the administration of a drug product containing aggregated protein. This becomes especially significant when the patient requires long-term or repeated administration of the drug, because the likelihood of a severe immune response increases. While the prevention of protein aggregation is critically important for the future of protein pharmaceuticals, the mechanism of amorphous aggregation is still poorly understood. The lack of understanding regarding nonfibrillar aggregation is largely due to the fact that assembly is difficult to study. In particular the role that various structural features (i.e., alpha-helix, beta-structure, disulfide bonds) play in the aggregation process varies with the amino acid sequence and is dependent upon tertiary structure and solution conditions. Well-structured proteins do not readily aggregate in solution, whereas partially unfolded proteins tend to aggregate rapidly and often become insoluble. Here, we present a unique and simple system for studying amorphous protein aggregation. We have previously reported the isolation of the basic leucine zipper (bZIP) domain of activating transcription factor 5 (ATF5), a protein notable for its potential as a pharmaceutical target for treatment of glioblastoma multiforme. This domain consists of a single alpha-helix and possesses a single cysteine residue. It is only partially structured and displays marginal stability in solution under physiological conditions. We have modulated solution conditions that affect backbone solubility and the oxidation state of the thiol to successfully investigate the role that alpha-helical structure and disulfide bond formation play in protein stability. Our data indicate that covalent cross-linking helps to retain ATF5's helicity, which inhibits the formation of large aggregates. These studies have led to the identification of stabilizing conditions for ATF5, which will enable further study of the protein as a pharmaceutical target. Moreover, this work has general implications for analyzing stability of helical proteins in vitro as well as the specific atomic-level interactions in ATF5 that contribute to instability and self-association.