PLTP is a p53 target gene with roles in cancer growth suppression and ferroptosis

The tumor suppressor protein p53 suppresses cancer by regulating processes such as apoptosis, cell cycle arrest, senescence, and ferroptosis, which is an iron-mediated and lipid peroxide-induced cell death pathway. Whereas numerous p53 target genes have been identified, only a few appear to be critical for the suppression of tumor growth. Additionally, while ferroptosis is clearly implicated in tumor suppression by p53, few p53 target genes with roles in ferroptosis have been identified. We have previously studied germline missense p53 variants that are hypomorphic or display reduced activity. These hypomorphic variants are associated with increased risk for cancer, but they retain the majority of p53 transcriptional function; as such, study of the transcriptional targets of these hypomorphs has the potential to reveal the identity of other genes important for p53-mediated tumor suppression. Here, using RNA-seq in lymphoblastoid cell lines, we identify PLTP (phospholipid transfer protein) as a p53 target gene that shows impaired transactivation by three different cancer-associated p53 hypomorphs: P47S (Pro47Ser, rs1800371), Y107H (Tyr107His, rs368771578), and G334R (Gly334Arg, rs78378222). We show that enforced expression of PLTP potently suppresses colony formation in human tumor cell lines. We also demonstrate that PLTP regulates the sensitivity of cells to ferroptosis. Taken together, our findings reveal PLTP to be a p53 target gene that is extremely sensitive to p53 transcriptional function and which has roles in growth suppression and ferroptosis.

Abstract PR08: African-centric p53 hypomorphs play significant roles in cancer risk and efficacy of therapy

The p53 tumor suppressor gene plays a central role in human cancer. In addition to mutation, there are genetic coding region variants of p53 that exist in different ethnic groups. In some cases, these variants have incomplete or impaired function and confer increased cancer risk. We call such variants ‘hypomorphs’. Two p53 hypomorphs exist in populations of African descent: P47S and Y107H. Our research on the P47S hypomorph of p53 revealed that this variant confers increased risk for cancer in humans and in mouse models (Jennis, Genes Dev 2016; Murphy, NPJ Breast Cancer 2017). We find that P47S protein is selectively impaired for the ability to confer sensitivity to ferroptosis (Leu, PNAS 2019); consequently, we find that human and mouse cells with this variant accumulate iron in certain tissues, and that the P47S variant is associated with iron overload in African Americans (Singh, Nat Comm 2020). We find that tumor cells containing the P47S variant are resistant to most genotoxic chemotherapies, but we have discovered that P47S tumor cells are markedly more sensitive to other therapies, including cisplatin and BRD2/4 inhibitors (Barnoud, Can Res 2018). Our current focus is on another African-centric p53 hypomorph, Y107H. We show that this variant is impaired for p53-mediated apoptosis, and is associated with cancer risk in a mouse model. We also show that Y107H is impaired for the transactivation of only a dozen p53 target genes. Notably, half of these genes are known tumor suppressor genes. Our biophysical and NMR structure of the Y107H protein explains the mechanistic basis for the impaired function of this hypomorph. Our analysis of the P47S and Y107H hypomorphs of p53 has significant potential to understand cancer risk and improve cancer therapy in populations of African descent.

Citation Format: Tim Barnoud, Jessica Leung, Keerthana Gnanapradeepan, Lei Ke, John Karanicolas, Maureen E. Murphy. African-centric p53 hypomorphs play significant roles in cancer risk and efficacy of therapy [abstract]. In: Proceedings of the AACR Virtual Conference: Thirteenth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2020 Oct 2-4. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(12 Suppl):Abstract nr PR08.

Increased mTOR activity and metabolic efficiency in mouse and human cells containing the African-centric tumor-predisposing p53 variant Pro47Ser

The Pro47Ser variant of p53 (S47) exists in African-descent populations and is associated with increased cancer risk in humans and mice. Due to impaired repression of the cystine importer Slc7a11, S47 cells show increased glutathione (GSH) accumulation compared to cells with wild -type p53. We show that mice containing the S47 variant display increased mTOR activity and oxidative metabolism, as well as larger size, improved metabolic efficiency, and signs of superior fitness. Mechanistically, we show that mTOR and its positive regulator Rheb display increased association in S47 cells; this is due to an altered redox state of GAPDH in S47 cells that inhibits its ability to bind and sequester Rheb. Compounds that decrease glutathione normalize GAPDH-Rheb complexes and mTOR activity in S47 cells. This study reveals a novel layer of regulation of mTOR by p53, and raises the possibility that this variant may have been selected for in early Africa.

A Novel Inhibitor of HSP70 Induces Mitochondrial Toxicity and Immune Cell Recruitment in Tumors

The protein chaperone HSP70 is overexpressed in many cancers including colorectal cancer, where overexpression is associated with poor survival. We report here the creation of a uniquely acting HSP70 inhibitor (HSP70i) that targets multiple compartments in the cancer cell, including mitochondria. This inhibitor was mitochondria toxic and cytotoxic to colorectal cancer cells, but not to normal colon epithelial cells. Inhibition of HSP70 was efficacious as a single agent in primary and metastatic models of colorectal cancer and enabled identification of novel mitochondrial client proteins for HSP70. In a syngeneic colorectal cancer model, the inhibitor increased immune cell recruitment into tumors. Cells treated with the inhibitor secreted danger-associated molecular patterns (DAMP), including ATP and HMGB1, and functioned effectively as a tumor vaccine. Interestingly, the unique properties of this HSP70i in the disruption of mitochondrial function and the inhibition of proteostasis both contributed to DAMP release. This HSP70i constitutes a promising therapeutic opportunity in colorectal cancer and may exhibit antitumor activity against other tumor types. SIGNIFICANCE: These findings describe a novel HSP70i that disrupts mitochondrial proteostasis, demonstrating single-agent efficacy that induces immunogenic cell death in treated tumors.

African-centric TP53 variant increases iron accumulation and bacterial pathogenesis but improves response to malaria toxin

A variant at amino acid 47 in human TP53 exists predominantly in individuals of African descent. P47S human and mouse cells show increased cancer risk due to defective ferroptosis. Here, we show that this ferroptotic defect causes iron accumulation in P47S macrophages. This high iron content alters macrophage cytokine profiles, leads to higher arginase level and activity, and decreased nitric oxide synthase activity. This leads to more productive intracellular bacterial infections but is protective against malarial toxin hemozoin. Proteomics of macrophages reveal decreased liver X receptor (LXR) activation, inflammation and antibacterial defense in P47S macrophages. Both iron chelators and LXR agonists improve the response of P47S mice to bacterial infection. African Americans with elevated saturated transferrin and serum ferritin show higher prevalence of the P47S variant (OR = 1.68 (95%CI 1.07–2.65) p = 0.023), suggestive of its role in iron accumulation in humans. This altered macrophage phenotype may confer an advantage in malaria-endemic sub-Saharan Africa.

A polymorphism in human TP53 (P47S) that predominantly exists in individuals of African descent affects ferroptosis. Here, the authors show that this results in iron accumulation in macrophages leading to more productive infection by intracellular bacteria but improved anti-inflammatory response to the malarial toxin hemozoin.

Paradoxical Role for Wild-Type p53 in Driving Therapy Resistance in Melanoma

Metastatic melanoma is an aggressive disease, despite recent improvements in therapy. Eradicating all melanoma cells even in drug-sensitive tumors is unsuccessful in patients because a subset of cells can transition to a slow-cycling state, rendering them resistant to most targeted therapy. It is still unclear what pathways define these subpopulations and promote this resistant phenotype. In the current study, we show that Wnt5A, a non-canonical Wnt ligand that drives a metastatic, therapy-resistant phenotype, stabilizes the half-life of p53 and uses p53 to initiate a slow-cycling state following stress (DNA damage, targeted therapy, and aging). Inhibiting p53 blocks the slow-cycling phenotype and sensitizes melanoma cells to BRAF/MEK inhibition. In vivo, this can be accomplished with a single dose of p53 inhibitor at the commencement of BRAF/MEK inhibitor therapy. These data suggest that taking the paradoxical approach of inhibiting rather than activating wild-type p53 may sensitize previously resistant metastatic melanoma cells to therapy.

Abstract 3483: The African-specific S47 variant of the p53 tumor suppressor gene alters cell metabolism

The TP53 gene, often referred to as the guardian of the genome, is the most frequently mutated gene in human cancer. This gene encodes the tumor suppressor p53, a master regulator of various processes such as programmed cell death, growth arrest and senescence. However, there is increasing evidence that highlights the role of p53 in tumor suppression with regards to maintaining metabolic homeostasis. Our lab has identified a polymorphic variant of p53 that encodes a serine residue instead of a proline at amino acid 47 (hereafter S47). This variant is most prevalent in African and African-American individuals, and mice and humans carrying this variant have increased cancer risk and impaired response to therapy. Using both mouse models and human cells, we have found that S47 cells have significantly altered metabolism compared to the WT counterpart. Specifically, S47 mice have increased weight and increased lean content compared to WT mice. These mice also show increased fitness in treadmill studies. Analyses of cell metabolism in S47 murine and human cells indicate that S47 cells show increased glycolysis and glycolytic flux, along with increased oxygen consumption rate on a Seahorse analyzer. The combined data support the premise that the S47 variant confers increased cancer risk but also increased metabolic fitness; the latter may explain the high rate of this variant in certain regions of Africa. By clearly elucidating the roles of WT p53 and the S47 variant in the context of metabolism, we hope to uncover new therapeutic avenues and ultimately enable more personalized medicine approaches for individuals who carry this variant.

Citation Format: Keerthana Gnanapradeepan, Subhasree Basu, Che-Pei Kung, Thibaut Barnoud, Madeline Good, William Quinn, Joyce Lee, Kathryn Wellen, Zachary Schug, Joseph Baur, Donna George, Maureen Murphy. The African-specific S47 variant of the p53 tumor suppressor gene alters cell metabolism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3483.

Mutant p53 controls tumor metabolism and metastasis by regulating PGC-1α

Basu et al. show that mutant p53 enhances migration and metastasis of tumors through the ability to bind and regulate PGC-1α and that this regulation is markedly impacted by the codon 72 polymorphism.

Mutant forms of p53 protein often possess protumorigenic functions, conferring increased survival and migration to tumor cells via their “gain-of-function” activity. Whether and how a common polymorphism in TP53 at amino acid 72 (Pro72Arg; referred to here as P72 and R72) impacts this gain of function has not been determined. We show that mutant p53 enhances migration and metastasis of tumors through the ability to bind and regulate PGC-1α and that this regulation is markedly impacted by the codon 72 polymorphism. Tumor cells with the R72 variant of mutant p53 show increased PGC-1α function along with greatly increased mitochondrial function and metastatic capability. Breast cancers containing mutant p53 and the R72 variant show poorer prognosis compared with P72. The combined results reveal PGC-1α as a novel “gain-of-function” partner of mutant p53 and indicate that the codon 72 polymorphism influences the impact of mutant p53 on metabolism and metastasis.

The p53 Tumor Suppressor in the Control of Metabolism and Ferroptosis

The p53 tumor suppressor continues to be distinguished as the most frequently mutated gene in human cancer. It is widely believed that the ability of p53 to induce senescence and programmed cell death underlies the tumor suppressor functions of p53. However, p53 has a number of other functions that recent data strongly implicate in tumor suppression, particularly with regard to the control of metabolism and ferroptosis (iron- and lipid-peroxide-mediated cell death) by p53. As reviewed here, the roles of p53 in the control of metabolism and ferroptosis are complex. Wild-type (WT) p53 negatively regulates lipid synthesis and glycolysis in normal and tumor cells, and positively regulates oxidative phosphorylation and lipid catabolism. Mutant p53 in tumor cells does the converse, positively regulating lipid synthesis and glycolysis. The role of p53 in ferroptosis is even more complex: in normal tissues, WT p53 appears to positively regulate ferroptosis, and this pathway appears to play a role in the ability of basal, unstressed p53 to suppress tumor initiation and development. In tumors, other regulators of ferroptosis supersede p53's role, and WT p53 appears to play a limited role; instead, mutant p53 sensitizes tumor cells to ferroptosis. By clearly elucidating the roles of WT and mutant p53 in metabolism and ferroptosis, and establishing these roles in tumor suppression, emerging research promises to yield new therapeutic avenues for cancer and metabolic diseases.

Prospective functional classification of all possible missense variants in PPARG

Clinical exome sequencing routinely identifies missense variants in disease-related genes, but functional characterization is rarely undertaken, leading to diagnostic uncertainty1,2. For example, mutations in PPARG cause Mendelian lipodystrophy3,4 and increase risk of type 2 diabetes (T2D)5. While approximately one in 500 people harbor missense variants in PPARG, most are of unknown consequence. To prospectively characterize PPARγ variants we used highly parallel oligonucleotide synthesis to construct a library encoding all 9,595 possible single amino acid substitutions. We developed a pooled functional assay in human macrophages, experimentally evaluated all protein variants, and used the experimental data to train a variant classifier by supervised machine learning (http://miter.broadinstitute.org). When applied to 55 novel missense variants identified in population-based and clinical sequencing, the classifier annotated six as pathogenic; these were subsequently validated by single-variant assays. Saturation mutagenesis and prospective experimental characterization can support immediate diagnostic interpretation of newly discovered missense variants in disease-related genes.