Shengliang Zhang

Apoptosis is a form of programmed cell death that is mediated by intrinsic and extrinsic pathways. Dysregulation of and resistance to cell death are hallmarks of cancer. For over three decades, the development of therapies to promote treatment of cancer by inducing various cell death modalities, including apoptosis, has been a main goal of clinical oncology. Apoptosis pathways also interact with other signaling mechanisms, such as the p53 signaling pathway and the integrated stress response (ISR) pathway. In addition to agents directly targeting the intrinsic and extrinsic pathway components, anticancer drugs that target the p53 and ISR signaling pathways are actively being developed. In this Review, we discuss selected and promising anticancer therapies in various stages of development, including drug targets, mechanisms, and resistance to related treatments, focusing especially on B cell lymphoma 2 (BCL-2) inhibitors, TRAIL analogues, DR5 antibodies, and strategies that target p53, mutant p53, and the ISR.

The tumor suppressor p73 is a member of the p53 family and transcriptionally activates multiple p53-targets involved in cell cycle regulation and apoptosis. In addition to pro- apoptotic signaling, outcomes of p73 activation include cell survival signals. Thus, p73 activity and targets may provide insight in cell fate outcomes between cell survival and apoptosis following cellular stress. We report that cellular FLICE inhibitory protein ( c- FLIP ), a master antiapoptotic factor, is a transcriptional target of p73. The activation of p73 (α and β isoforms) transcriptionally upregulates c-FLIP-L/S expression in cancer cells. The cell fate decision following p73 activation is determined by the adjustment of the balance of outcomes of p73 activation between p73-induced pro-apoptotic signaling and c-FLIP-L/S expression in cancer cells. p73 primes extrinsic apoptosis via an autocrine death ligand-DR5 axis, and the priming appears to be titrated at the level of c-FLIP-L/S. The p73-upregulation of c-FLIP-L/S increases the threshold of extrinsic apoptosis. Cells with poor priming levels convert to cell cycle arrest and survival. Depletion of c-FLIP-L/S increases the p73-priming levels towards extrinsic apoptosis and sensitizes cancer cells to p73-primed extrinsic apoptosis. We further identified a small-molecule CB-7587351 (“switcher compound”) that alters p73 activation outcomes through c-FLIP-L/S protein degradation. Therapeutic activation of p73 can restore p53- signaling in mutant p53-expressing cancer cells effectively bypassing the p53 deficiency in cancer cells. Our discovery of p73 transcriptional upregulation of c-FLIP provides a promising strategy for depleting c-FLIP to improve antitumor efficacy of p73-targeting cancer therapy for p53-mutant tumors.

TP53 is a tumor suppressor gene that encodes a sequence-specific DNA-binding transcription factor activated by stressful stimuli; it upregulates target genes involved in growth suppression, cell death, DNA repair, metabolism, among others. TP53 is the most frequently mutated gene in tumors, with mutations not only leading to loss-of-function (LOF), but also gain-of-function (GOF) that promotes tumor progression, and metastasis. The tumor-specific status of mutant p53 protein has suggested it is a promising target for cancer therapy. We summarize the current progress of targeting wild-type and mutant p53 for cancer therapy through biotherapeutic and biopharmaceutical methods for (1) boosting p53 activity in cancer, (2) p53-dependent and p53-independent strategies for targeting p53 pathway functional restoration in p53-mutated cancer, (3) targeting p53 in immunotherapy, and (4) combination therapies targeting p53, p53 checkpoints, or mutant p53 for cancer therapy.

A long-term goal in the cancer-field has been to develop strategies for treating p53-mutated tumors. A novel small-molecule, PG3-Oc, restores p53 pathway-signaling in tumor cells with mutant-p53, independently of p53/p73. PG3-Oc partially upregulates the p53-transcriptome (13.7% of public p53 target-gene dataset; 15.2% of in-house dataset) and p53-proteome (18%, HT29; 16%, HCT116-p53 ^-/- ). Bioinformatic analysis indicates critical p53-effectors of growth-arrest (p21), apoptosis (PUMA, DR5, Noxa), autophagy (DRAM1), and metastasis-suppression (NDRG1) are induced by PG3-Oc. ERK1/2- and CDK9-kinases are required to upregulate ATF4 by PG3-Oc which restores p53 transcriptomic-targets in cells without functional-p53. PG3-Oc represses MYC (ATF4-independent), and upregulates PUMA (ATF4-dependent) in mediating cell death. With largely nonoverlapping transcriptomes, induced-ATF4 restores p53 transcriptomic targets in drug-treated cells including functionally important mediators such as PUMA and DR5. Our results demonstrate novel p53-independent drug-induced molecular reprogramming involving ERK1/2, CDK9, and ATF4 to restore upregulation of p53 effector genes required for cell death and tumor suppression.

Most cancers harbor intra-tumoral hypoxia which promotes tumor progression and therapy resistance. Hypoxia-inducible factor 1α (HIF1α) mediates an adaptive response to hypoxia and contributes to multiple cancer hallmarks. We describe cancer therapeutic targeting of HIF1α by combination of CDK4/6 inhibitors (CDK4/6i) and heat-shock protein 90 inhibitors (HSP90i). CDK1 contributes to HSP90-mediated HIF1α stabilization whereas CDK1-knockdown enhances HIF1α reduction by HSP90i. Dual CDK1- and HSP90-inhibition increases apoptosis and synergistically inhibits cancer cell viability. To translate our findings, we use FDA-approved CDK4/6i in combination with HSP90i to reduce HIF1α expression and suppress viability of multiple cancer cell types, including Rb-deficient cancer cells. Overexpression of HIF1α ^668E partially rescues the cell viability inhibition by combination CDK4/6i and HSP90i treatment under hypoxia. CDK4/6i and HSP90i suppresses tumor growth in vivo . Thus, combined targeting of CDK4/6 and HSP90, through a drug class effect, inhibits HIF1α and shows preclinical anti-cancer therapeutic efficacy, including with Rb-deficiency.

COVID-19 affects vulnerable populations including elderly individuals and patients with cancer. Natural Killer (NK) cells and innate-immune TRAIL suppress transformed and virally-infected cells. ACE2, and TMPRSS2 protease promote SARS-CoV-2 infectivity, while inflammatory cytokines IL-6, or G-CSF worsen COVID-19 severity. We show MEK inhibitors (MEKi) VS-6766, trametinib and selumetinib reduce ACE2 expression in human cells. In some human cells, remdesivir increases ACE2-promoter luciferase-reporter expression, ACE2 mRNA and protein, and ACE2 expression is attenuated by MEKi. In serum-deprived and stimulated cells treated with remdesivir and MEKi we observed correlations between pRB, pERK, and ACE2 expression further supporting role of proliferative state and MAPK pathway in ACE2 regulation. We show elevated cytokines in COVID-19-(+) patient plasma (N=9) versus control (N=11). TMPRSS2, inflammatory cytokines G-CSF, M-CSF, IL-1α, IL-6 and MCP-1 are suppressed by MEKi alone or with remdesivir. We observed MEKi stimulation of NK-cell killing of target-cells, without suppressing TRAIL-mediated cytotoxicity. Pseudotyped SARS-CoV-2 virus with a lentiviral core and SARS-CoV-2 D614 or G614 SPIKE (S) protein on its envelope infected human bronchial epithelial cells, small airway epithelial cells, or lung cancer cells and MEKi suppressed infectivity of the pseudovirus. We show a drug class-effect with MEKi to stimulate NK cells, inhibit inflammatory cytokines and block host-factors for SARS-CoV-2 infection leading also to suppression of SARS-CoV-2-S pseudovirus infection of human cells. MEKi may attenuate SARS-CoV-2 infection to allow immune responses and antiviral agents to control disease progression.