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Tumor cells exploit immune checkpoints by expressing checkpoint ligands such as PD-L1, effectively masking them from immune destruction. Programmed cell death- 1 (PD-1), plays a major role in tumor immune escape. The interaction of PD-1/PD-L1 inhibits cytotoxic T lymphocyte (CTL) proliferation, induces apoptosis of tumor-specific T cells, and promotes the resistance of tumor cells to CTL attack. Monoclonal antibodies to these proteins have been developed and approved for use in the clinic. Theoretically, peptide mimics, which have the benefit of lower half-life and significantly smaller size, are able to penetrate into solid tumors and tissues better than antibodies, and can function as inhibitors for the PD1/PD-L1 interaction. Here we report for the first time the rational design and validation of a peptide inhibitor to the PD-1/PD-L1 immune checkpoint as a viable alternative to current inhibitory antibodies. We demonstrated, by biolayer interferometry and in silico docking simulations, that a PD-L1 peptide mimetic (PL120131) can interfere with the PD-1/PD-L1 interaction by binding to PD-1. We established the binding constant for the interaction and PL120131 reduces association of PD-L1 in a dose dependent manner. We show that PL120131 is capable of inhibiting PD-1 mediated apoptotic signaling pathway and rescuing Jurkat cells and primary lymphocytes from apoptosis. Additionally, we show that PL120131 treatment allows for CTL anti-tumor activity. Furthermore, PL120131 can maintain co-culture survivability and activity of T Cells in a 3D co-culture model. Together, the characterization of this PD-1/PD-L1 inhibiting peptide provides insight regarding the ability to inhibit PD-L1 binding while maintaining CTL viability and activity. Understanding the mechanism(s) of action of this peptide has yielded information on the pathways that are drivers of immune evasion in a tumor microenvironment, which may improve and synergize with other forms of immunotherapy, thus increasing efficacy of cancer treatment. Outcomes from this study have provided a path forward to further development of small molecule inhibitors to the PD-1/PD-L1 interaction, and outlines a strategy for the design and development of small molecules to additional cell-surface checkpoint proteins. Citation Format: Rebecca J. Boohaker, Vijaya Sambandam, Mark Suto, Bo Xu. The rational design and evaluation of a peptide inhibitor of the PD-1/PD-L1 interaction [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5668.

The Ataxia-Telangiectasia Mutated (ATM) Kinase, mutation of which is responsible for the autosomal recessive disorder Ataxia-Telangiectasia, is an essential element for cells to deal with DNA double-strand breaks. In the absence of DNA damage, it has also been recognized that ATM has a role in optimizing the spindle assembly checkpoint during mitosis. In the process, ATM is activated at a lower level (compared to the DNA damage response) which requires phosphorylation by the Aurora-B kinase. Known ATM targets in mitosis include kinetochore proteins Bub1 and Mad1. However, it has become clear that ATM-mediated phosphorylation in mitosis is distinct from that in the DNA damage response. Therefore, we performed an unbiased assay to identify ATM substrates in mitosis in order to study the functional significance of mitotic ATM activation. To achieve our goal, we utilized a stable isotope labeling by amino acid (SILAC) /Peptide IP with phosphor-specific SQ/TQ antibodies and mass spectrometry (MS). About 18 proteins were identified as substrates of mitotically activated ATM in the assay. We then conducted bioinformatic analyses for the identified proteins using PANTHER and Ingenuity programs. With many proteins having their known functions in mitosis, we found a much smaller number, but highly interconnected, pool of proteins in the ATM substrate list as compared to that of the DNA damage response. Among them, we validated several phosphorylation events using an in vitro kinase assay. Furthermore we present data to demonstrate the functional significance of ATM-mediated phosphorylation. Taken together, our data provide a comprehensive picture of how mitotically activated ATM signals to downstream targets to fine-tune the mitotic checkpoint. Citation Format: Rebecca J. Boohaker, Qinghua Zeng, Bo Xu. Identification and characterization of ATM substrates in mitosis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 180.

The Speckle type Poz Protein (SPOP), an E3 ubiquitin ligase adaptor, has recently been identified as the gene that has the most common somatic point mutations in prostate cancer. SPOP mutations are associated with genomic alterations, indicating a role for SPOP in the maintenance of genome stability. We, and others, have recently demonstrated a critical role of SPOP in the DNA damage response (DDR), suggesting SPOP mutants may represent a subgroup of patients that have hyper sensitivity to DNA damaging therapies. However, how SPOP mutations might impact its function and their roles in the progress of prostate tumorigenesis remain to be extensively studied. Genomic studies have found that SPOP mutations are clustered within its substrate binding pocket. Of a particular interest, Serine119 is found to be frequently mutated to Asparagine (S119N). Using computational modeling, we found that Ser119 resides in the SBC-MATH binding interface and is in close contact with the non-polar residue of the SPOP-binding consensus motif. Therefore we hypothesized that phosphorylation of Serine 119 might directly affect substrate binding. First we characterized prostate cancer cells expressing this mutation and we found that they displayed a suboptimal DNA damage response with prolonged DNA repair. Using an in situ proximity ligation assay, we demonstrate that Serine 119 is essential for SPOP ionizing irradiation induced interaction with the Ataxia Telangiectasia Mutated kinase (ATM), a major hub protein in the DDR. With the evidence of SPOP being a phospho-protein, we further provide in vitro evidence that ATM phosphorylates SPOP on Serine 119 in response to DNA damage. Characterization of the functional significance of ATM-mediated SPOP phosphorylation indicates a wide range of downstream targets regulating cell cycle progression and DNA repair. Taken together, our data reveal a critical pathway linking ATM and SPOP in regulation of prostate cancer initiation and therapeutic responses to DNA damage. This also provides the first evidence of a pathophysiological relevant mutation linked to ATM phosphorylation in the DDR. Citation Format: Joshua Fried, Rebecca Boohaker, Qinghua Zeng, Wei Zhang, Bo Xu. Characterization of the ATM-SPOP pathway in prostate cancer tumorigenesis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2752.

To cope with constant challenges toward the fidelity of genome by DNA damaging insults (both endogenous and exogenous), activation of the DNA damage response (DDR) is an essential step in preventing genomic instability. Studies of mammalian mechanisms have revealed that there are comprehensive, coordinated pathways that, when activated, serve to regulate the DDR. Recently, we have shown that hyperactive DDR mediated by the Ataxia-Telangiectasia Mutated (ATM) kinase is linked to activation of the Zinc finger protein SNAI1 (Snail). Activated Snail in metastasis is well-characterized by its role as transcriptional repressor of E-cadherin thereby inducing an epithelial-to-mesenchymal transition (EMT). Here we report that, in response to DNA damage, activation of the ATM-Snail pathway, mediated by Snail Serine 100 phosphorylation, also contributes to the regulation of the cellular sensitivity to ionizing radiation. Expression of a hyperactive Snail led to radioresistance. More interestingly, the Serine 100 to Glutamic Acid mutant variant of Snail displayed an enhanced invasion index after low dose radiation, independent of survival. Functional genomics and proteomics studies revealed downstream targets and interacting partners of Snail in the DDR. Together, we highlight the functional significance of the ATM-Snail pathway in the DDR. Citation Format: Rebecca J. Boohaker, Joshua Fried, Xiaoli Cui, Bo Xu. Delineation of the ATM-Snail pathway in the DNA damage response. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3939. doi:10.1158/1538-7445.AM2014-3939

BACKGROUND: The marked difference in metabolism observed between tumor and normal cells could contribute to the development of invasive and metastatic forms of breast cancer. The problem is that while patients diagnosed with invasive forms of breast cancer may be initially responsive to treatment, a significant number develop relapsing and even metastatic disease. There is a critical unmet need to develop new therapeutic approaches for patients diagnosed with invasive forms of breast cancer that are effective given the unique metabolism of tumor cells. METHODS: We examined the cytotoxic properties of a novel peptide, CT20p, derived from the C-terminus of Bax. For delivery to cells, the amphipathic nature of CT20p allowed it to be encapsulated in polymeric nanoparticles (NPs). NPs were made using aliphatic hyperbranched polyester (HBPE) that incorporated surface carboxylic groups and interior hydrophobic cavities for encapsulation of CT20p. To examine the cytotoxic potential and targeting capacity of CT20p-HBPE-NPs, we treated MDA-MB-231 breast cancer cells and MCF-10A breast epithelial cells with the peptide-nanoparticle combination and measured changes in mitochondrial function, cell metabolism and induction of apoptotic and non-apoptotic cell death. The ability of CT20p-NP-HBPE to cause tumor regression was examined by subcutaneously implanting MDA-MB-231 cells in nude mice. RESULTS: Initial studies showed that CT20p caused the release of calcein from mitochondrial-like lipid vesicles, without disrupting vesicle integrity, and, when expressed as a fusion protein in cells, localized to mitochondria. While the peptide alone had little effect upon intact cells, likely not penetrating the plasma membrane, when encapsulated and delivered by nanoparticles, CT20p-HBPE-NPs proved an effective killer of breast cancer cells. CT20p-HBPE-NPs initiated non-apoptotic cell death within 3 hours of treatment by targeting mitochondria and deregulating cellular metabolism. Nanoparticles alone or nanoparticles encapsulating a control peptide had minimal effects. The cytotoxicity of CT20p-HPBE-NPs was most pronounced in breast cancer cells, sparing normal, epithelial cells. In implanted breast tumors, CT20p-HBPE-NPs accumulated in tumors within 24 hours and reduced tumor burden by 50-80%. CONCLUSION: These results reveal the innovative features of CT20p that allow nanoparticle-mediated delivery to tumors and the potential application in combination therapies that target the unique metabolism of cancer cells. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P6-04-13.

The Bcl-2 protein family modulates the balance between cellular life and death. Bax, a pro-apoptotic member of this family, participates in mitochondria-mediated apoptosis but also supports mitochondrial bioenergetics. In previous work, we showed that a peptide derived from the alpha 9-helix of Bax is cytotoxic, specifically when used to treat breast cancer cells. Such structural and biochemical studies are useful for identifying functional domains within Bax that can be developed into therapeutic peptides, but are limiting when ascribing function to domains at a molecular level. Computational modeling and molecular dynamics simulations of Bax provides a high-power analysis of the subtle changes that modulate the regulatory and effector activities of the protein. This study seeks to elucidate the consequence of alterations in micro-domains that confer activity, apoptotic or otherwise, to the protein. Molecular dynamics simulation was used to determine how the trans-membrane region of Bax conferred stability to the protein, identifying regions of high flexibility and differential exposure. Computationally rotating the Ψ angle at Pro168 altered the position of the α9-helix of Bax, mimicking the “active” and “inactive” forms of Bax. We found that “active” Bax, but not “inactive” Bax, presented a more relaxed structure which altered the conformation and exposure of the BH3 domain, potentially enhancing interactions among the family of Bcl-2 proteins. Since dysregulation of Bcl-2 proteins is implicated in a number of cancers, examining the basic structural dynamics at a molecular level yielded information that can aid in the design novel Bcl-2 family-targeting chemotherapeutics. Citation Format: Annette Khaled, Rebecca Boohaker, Craig Finch, Sergio Tafur. Structural insights into the dynamic functionality of Bax through molecular dynamics simulations. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5138. doi:10.1158/1538-7445.AM2013-5138

The Bcl-2 family proteins maintain the balance of life and death in a cell. Bax, a pro-apoptotic member, translocates to the mitochondrial membrane in response to diverse death stimuli, initiating a cascade of programmed events that lead to cell death. Tumor progression involves deregulation of this process, in part, through mutations that cause aberrant expression of anti-apoptotic members of the Bcl-2 family that normally sequester and inhibit Bax. This study seeks to identify functional domains of Bax that can be developed into therapeutic peptides to promote the death of cancer cells resistant to existing chemotherapeutics. Our goal was to generate a peptide, much like antimicrobial peptides, that could target and disrupt mitochondrial membranes. To this end, we discovered that the C-terminal, alpha-9 helix of Bax (CT20), when tagged to EGFP or a destabilization domain (DD), could bind mitochondria and cause cell death, mimicking the activity of full-length Bax. Cell death was measured by morphological changes, such as membrane blebbing, as well as DNA fragmentation. Targeted mutagenesis of two adjacent lysines at the carboxyl end of the CT20 peptide demonstrated that these residues enabled mitochondrial membrane association. Expression of the CT20 peptide in the presence or absence of endogenous Bax resulted in cytotoxicity, indicating that the death mechanism engaged could function independently of the Bcl-2 family. These findings suggest the CT20 peptide has the potential to be developed into a viable therapeutic agent in the treatment of a broad range of cancer types. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2010. doi:1538-7445.AM2012-2010

The dual functionality of the tumor suppressor, BAX, is implied by the non-apoptotic functions of other pro-apoptotic BCL-2 family proteins. To explore this, the mitochondrial morphology and ATP producing capability in both BAX-containing and BAX-deficient HCT-116 colon cancer cells were examined. While BAX deficient cells maintain the same mitochondrial mass as their BAX containing counterparts, these mitochondria were visibly smaller, more compact, and unable to maintain an organized network. Oxygen consumption and ATP levels were also reduced in these cells, a phenomenon that was replicated upon treatment of BAX-containing cells with the uncoupling agent, FCCP. These results, coupled with a marked decrease in citrate synthase activity, suggested that cells lacking BAX have a deficiency in the ability to generate ATP through oxidative phosphorylation. Inhibition of endogenous BAX through treatment with small interfering RNAs reinforced the necessity of BAX activity in the maintenance of aerobic respiration and the production of ATP. An examination of the localization of full length and C-terminal truncated BAX in non-apoptotic, BAX deficient cells, indicated that a small fraction of the existing cytosolic BAX is needed at the mitochondrial membrane to maintain ATP production. However, this function is not dependent upon the C-terminal helix. Co-expression of BAX and BCL-2 resulted in a subsequent loss of ATP production, implying that, even under non-apoptotic conditions, an antagonistic interaction exists between the two proteins. These findings infer that a basal amount of BAX associated with the mitochondria is necessary to maintain mitochondrial bioenergetics. The discovery of this novel, non-apoptotic role for BAX in the maintenance of ATP synthesis and suggests that complete abrogation of BAX would be detrimental to cancer cells dependent on energy production via aerobic respiration. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3794. doi:10.1158/1538-7445.AM2011-3794