Exploiting Overlapping Advantages of In Vitro and In Cellulo Selection Systems to Isolate a Novel High-Affinity cJun Antagonist

An Approach to Derive Functional Peptide Inhibitors of Transcription Factor Activity

We report the development of a high-throughput, intracellular "transcription block survival" (TBS) screening platform to derive functional transcription factor antagonists. TBS is demonstrated using the oncogenic transcriptional regulator cJun, with the development of antagonists that bind cJun and prevent both dimerization and, more importantly, DNA binding remaining a primary challenge. In TBS, cognate TRE sites are introduced into the coding region of the essential gene, dihydrofolate reductase (DHFR). Introduction of cJun leads to TRE binding, preventing DHFR expression by directly blocking RNA polymerase gene transcription to abrogate cell proliferation. Peptide library screening identified a sequence that both binds cJun and antagonizes function by preventing DNA binding, as demonstrated by restored cell viability and subsequent in vitro hit validation. TBS is an entirely tag-free genotype-to-phenotype approach, selecting desirable attributes such as high solubility, target specificity, and low toxicity within a complex cellular environment. TBS facilitates rapid library screening to accelerate the identification of therapeutically valuable sequences.

The Role of AP-1 Transcription Factors in Plasma Cell Biology and Multiple Myeloma Pathophysiology

Multiple myeloma (MM) is an incurable hematologic malignancy characterized by the clonal expansion of malignant plasma cells within the bone marrow. Activator Protein-1 (AP-1) transcription factors (TFs), comprised of the JUN, FOS, ATF and MAF multigene families, are implicated in a plethora of physiologic processes and tumorigenesis including plasma cell differentiation and MM pathogenesis. Depending on the genetic background, the tumor stage, and cues of the tumor microenvironment, specific dimeric AP-1 complexes are formed. For example, AP-1 complexes containing Fra-1, Fra-2 and B-ATF play central roles in the transcriptional control of B cell development and plasma cell differentiation, while dysregulation of AP-1 family members c-Maf, c-Jun, and JunB is associated with MM cell proliferation, survival, drug resistance, bone marrow angiogenesis, and bone disease. The present review article summarizes our up-to-date knowledge on the role of AP-1 family members in plasma cell differentiation and MM pathophysiology. Moreover, it discusses novel, rationally derived approaches to therapeutically target AP-1 TFs, including protein-protein and protein-DNA binding inhibitors, epigenetic modifiers and natural products.

Selective antagonism of cJun for cancer therapy

The activator protein-1 (AP-1) family of transcription factors modulate a diverse range of cellular signalling pathways into outputs which can be oncogenic or anti-oncogenic. The transcription of relevant genes is controlled by the cellular context, and in particular by the dimeric composition of AP-1. Here, we describe the evidence linking cJun in particular to a range of cancers. This includes correlative studies of protein levels in patient tumour samples and mechanistic understanding of the role of cJun in cancer cell models. This develops an understanding of cJun as a focal point of cancer-altered signalling which has the potential for therapeutic antagonism. Significant work has produced a range of small molecules and peptides which have been summarised here and categorised according to the binding surface they target within the cJun-DNA complex. We highlight the importance of selectively targeting a single AP-1 family member to antagonise known oncogenic function and avoid antagonism of anti-oncogenic function.

Coupling Computational and Intracellular Screening and Selection Toward Co-compatible cJun and cFos Antagonists

Basic leucine-zipper (bZIP) proteins represent difficult, yet compelling, oncogenic targets since numerous cell-signaling cascades converge upon them, where they function to modulate the transcription of specific gene targets. bZIPs are widely recognized as important regulators of cellular processes that include cell proliferation, apoptosis, and differentiation. Once such validated transcriptional regulator, activator protein-1, is typically composed of heterodimers of Fos and Jun family members, with cFos-cJun being the best described. It has been shown to be key in the progression and development of a number of different diseases. As a proof-of-principle for our approach, we describe the first use of a novel combined in silico/in cellulo peptide-library screening platform that facilitates the derivation of a sequence that displays high selectivity for cJun relative to cFos, while also avoiding homodimerization. In particular, >60 million peptides were computationally screened and all potential on/off targets ranked according to predicted stability, leading to a reduced size library that was further refined by intracellular selection. The derived sequence is predicted to have limited cross-talk with a second previously derived peptide antagonist that is selective for cFos in the presence of cJun. The study provides new insight into the use of multistate screening with the ability to combine computational and intracellular approaches in evolving multiple cocompatible peptides that are capable of satisfying conflicting design requirements.

Computational Competitive and Negative Design To Derive a Specific cJun Antagonist

Basic leucine zipper (bZIP) proteins reside at the end of cell-signaling cascades and function to modulate transcription of specific gene targets. bZIPs are recognized as important regulators of cellular processes such as cell growth, apoptosis, and cell differentiation. One such validated transcriptional regulator, activator protein-1, is typically comprised of heterodimers of Jun and Fos family members and is key in the progression and development of a number of different diseases. The best described component, cJun, is upregulated in a variety of diseases such as cancer, osteoporosis, and psoriasis. Toward our goal of inhibiting bZIP proteins implicated in disease pathways, we here describe the first use of a novel in silico peptide library screening platform that facilitates the derivation of sequences exhibiting a high affinity for cJun while disfavoring homodimer formation or formation of heterodimers with other closely related Fos sequences. In particular, using Fos as a template, we have computationally screened a peptide library of more than 60 million members and ranked hypothetical on/off target complexes according to predicted stability. This resulted in the identification of a sequence that bound cJun but displayed little homomeric stability or preference for cFos. The computationally selected sequence maintains an interaction stability similar to that of a previous experimentally derived cJun antagonist while providing much improved specificity. Our study provides new insight into the use of tandem in silico screening/ in vitro validation and the ability to create a peptide that is capable of satisfying conflicting design requirements.