Cereblon-based Bifunctional Degrader of SOS1, BTX-6654, Targets Multiple KRAS Mutations and Inhibits Tumor Growth

Mutations within the oncogene KRAS drive an estimated 25% of all cancers. Only allele-specific KRAS G12C inhibitors are currently available and are associated with the emergence of acquired resistance, partly due to upstream pathway reactivation. Given its upstream role in the activation of KRAS, son of sevenless homolog 1 (SOS1), has emerged as an attractive therapeutic target. Agents that target SOS1 for degradation could represent a potential pan-KRAS modality that may be capable of circumventing certain acquired resistance mechanisms. Here, we report the development of two SOS1 cereblon-based bifunctional degraders, BTX-6654 and BTX-7312, cereblon-based bifunctional SOS1 degraders. Both compounds exhibited potent target-dependent and -specific SOS1 degradation. BTX-6654 and BTX-7312 reduced downstream signaling markers, pERK and pS6, and displayed antiproliferative activity in cells harboring various KRAS mutations. In two KRAS G12C xenograft models, BTX-6654 degraded SOS1 in a dose-dependent manner correlating with tumor growth inhibition, additionally exhibiting synergy with KRAS and MEK inhibitors. Altogether, BTX-6654 provided preclinical proof of concept for single-agent and combination use of bifunctional SOS1 degraders in KRAS-driven cancers.

BNIP3-dependent mitophagy promotes cytosolic localization of LC3B and metabolic homeostasis in the liver

Mitophagy formed the basis of the original description of autophagy by Christian de Duve when he demonstrated that GCG (glucagon) induced macroautophagic/autophagic turnover of mitochondria in the liver. However, the molecular basis of liver-specific activation of mitophagy by GCG, or its significance for metabolic stress responses in the liver is not understood. Here we show that BNIP3 is required for GCG-induced mitophagy in the liver through interaction with processed LC3B; an interaction that is also necessary to localize LC3B out of the nucleus to cytosolic mitophagosomes in response to nutrient deprivation. Loss of BNIP3-dependent mitophagy caused excess mitochondria to accumulate in the liver, disrupting metabolic zonation within the liver parenchyma, with expansion of zone 1 metabolism at the expense of zone 3 metabolism. These results identify BNIP3 as a regulator of metabolic homeostasis in the liver through its effect on mitophagy and mitochondrial mass distribution.

The novel protein homeostatic modulator BTX306 is active in myeloma and overcomes bortezomib and lenalidomide resistance

Small molecules targeting the cereblon-containing E3 ubiquitin ligase including thalidomide, lenalidomide, and pomalidomide modulate turnover of downstream client proteins and demonstrate pre-clinical and clinical anti-myeloma activity. Different drugs that engage with cereblon hold the potential of unique phenotypic effects, and we therefore studied the novel protein homeostatic modulator (PHM™) BTX306 with a unique thiophene-fused scaffold bearing a substituted phenylurea and glutarimide. This agent much more potently reduced human-derived myeloma cell line viability, with median inhibitory concentrations in the single nanomolar range versus micromolar values for lenalidomide or pomalidomide, and more potently activated caspases 3/8/9. While lenalidomide and pomalidomide induced greater degradation of Ikaros and Aiolos in myeloma cells, BTX306 more potently reduced levels of GSPT1, eRF1, CK1α, MCL-1, and c-MYC. Suppression of cereblon or overexpression of Aiolos or Ikaros induced relative resistance to BTX306, and this agent did not impact viability of murine hematopoietic cells in an in vivo model, demonstrating its specificity for human cereblon. Interestingly, BTX306 did show some reduced activity in lenalidomide-resistant cell line models but nonetheless retained its nanomolar potency in vitro, overcame bortezomib resistance, and was equipotent against otherwise isogenic cell line models with either wild-type or knockout TP53. Finally, BTX306 demonstrated strong activity against primary CD138-positive plasma cells, showed enhanced anti-proliferative activity in combination with bortezomib and dexamethasone, and was effective in an in vivo systemic model of multiple myeloma. Taken together, the data support further translational studies of BTX306 and its derivatives to the clinic for patients with relapsed and/or refractory myeloma. KEY MESSAGES: BTX306 has a unique thiophene-fused scaffold bearing phenylurea and glutarimide. BTX306 is more potent against myeloma cells than lenalidomide or pomalidomide. BTX306 overcomes myeloma cell resistance to lenalidomide or bortezomib in vitro. BTX306 is active against primary myeloma cells, and shows efficacy in vivo.

Dia1-dependent adhesions are required by epithelial tissues to initiate invasion

Developing tissues change shape and tumors initiate spreading through collective cell motility. Conserved mechanisms by which tissues initiate motility into their surroundings are not known. We investigated cytoskeletal regulators during collective invasion by mouse tumor organoids and epithelial Madin-Darby canine kidney (MDCK) acini undergoing branching morphogenesis in collagen. Use of the broad-spectrum formin inhibitor SMIFH2 prevented the formation of migrating cell fronts in both cell types. Focusing on the role of the formin Dia1 in branching morphogenesis, we found that its depletion in MDCK cells does not alter planar cell motility either within the acinus or in two-dimensional scattering assays. However, Dia1 was required to stabilize protrusions extending into the collagen matrix. Live imaging of actin, myosin, and collagen in control acini revealed adhesions that deformed individual collagen fibrils and generated large traction forces, whereas Dia1-depleted acini exhibited unstable adhesions with minimal collagen deformation and lower force generation. This work identifies Dia1 as an essential regulator of tissue shape changes through its role in stabilizing focal adhesions.

Tumor suppressor functions of BNIP3 and mitophagy

There is a growing realization that tumor cells rely on healthy mitochondria to promote their growth under changing microenvironmental stresses and do so by dynamically modulating both their mitochondrial mass and state of mitochondrial fusion. Our recent work adds to this appreciation by showing that the mitophagy receptor BNIP3 functions as a tumor suppressor in mammary tumorigenesis and also as a prognostic indicator of progression to metastasis in certain sub-types of human breast cancer.

Abstract 228: Tumor suppressor functions of BNIP3 and mitophagy in pancreatic ductal adenocarcinoma

The role of mitochondrial dysfunction in cancer is an emerging research field fuelled by the growing realization that mitochondrial dynamics are subject to oncogenic control, with mitochondrial biogenesis and mitophagy in particular modulated directly or indirectly by key oncogenes (c-Myc, B-Raf, K-ras) and tumor suppressors (p53, pRB). Work from our lab has focused on understanding how defects in mitophagy influence tumorigenesis with a particular focus on how BNIP3 loss during tumor progression influences disease outcome. BNIP3 functions as a dimer at the outer mitochondrial membrane to target mitochondria for degradation at the autophagolysosome through its interaction with processed LC3 at nascent phagophores. BNIP3 is a transcriptional target of HIF-1, E2F and FoxO3A transcription factors and is induced by activated Ras and suppressed by the RB and p53 tumor suppressors. Various studies have shown BNIP3 expression to be induced at early stages of human cancers and then lost as cancers progress to malignancy. In particular, epigenetic silencing of BNIP3 in human pancreas cancer has been correlated with reduced median survival. However, the mechanisms by which loss of BNIP3 may be contributing to cancer progression are not well delineated and a better understanding of how BNIP3-dependent mitophagy modulates pancreatic cancer in particular could provide new insight to the development of this deadly disease. We set out to examine how loss of BNIP3 affects the progression of pancreatic ductal adenocarcinoma using human cell lines and genetically engineered mouse models. We show that loss of BNip3 markedly accelerates tumor initiation and progression to invasive PDAC in the Pdx1-Cre;LSL-KRasG12D mouse model with metastases to the liver and lungs evident in BNip3 null mice but not in age-matched wild-type mice. Supporting a role for BNIP3 in suppressing pancreatic tumorigenesis, we also showed that re-expression of exogenous BNIP3 in Miapaca-2 cells (in which endogenous BNIP3 is silenced) resulted in reduced growth while knockdown of BNIP3 in CFPAC1 cells (that express BNIP3) promotes cell growth. Work described here examines the underlying mechanisms explaining these tumor suppressor functions of BNIP3 in pancreatic ductal adenocarcinoma.

Citation Format: Aparajita H. Chourasia, Maya Z. Springer, Kay F. Macleod. Tumor suppressor functions of BNIP3 and mitophagy in pancreatic ductal adenocarcinoma. [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 228.

Mitophagy defects arising from BNip3 loss promote mammary tumor progression to metastasis

BNip3 is a hypoxia-inducible protein that targets mitochondria for autophagosomal degradation. We report a novel tumor suppressor role for BNip3 in a clinically relevant mouse model of mammary tumorigenesis. BNip3 delays primary mammary tumor growth and progression by preventing the accumulation of dysfunctional mitochondria and resultant excess ROS production. In the absence of BNip3, mammary tumor cells are unable to reduce mitochondrial mass effectively and elevated mitochondrial ROS increases the expression of Hif-1α and Hif target genes, including those involved in glycolysis and angiogenesis—two processes that are also markedly increased in BNip3-null tumors. Glycolysis inhibition attenuates the growth of BNip3-null tumor cells, revealing an increased dependence on autophagy for survival. We also demonstrate that BNIP3 deletion can be used as a prognostic marker of tumor progression to metastasis in human triple-negative breast cancer (TNBC). These studies show that mitochondrial dysfunction—caused by defects in mitophagy—can promote the Warburg effect and tumor progression, and suggest better approaches to stratifying TNBC for treatment.

Mitophagy and cancer

Mitophagy is a selective form of macro-autophagy in which mitochondria are selectively targeted for degradation in autophagolysosomes. Mitophagy can have the beneficial effect of eliminating old and/or damaged mitochondria, thus maintaining the integrity of the mitochondrial pool. However, mitophagy is not only limited to the turnover of dysfunctional mitochondria but also promotes reduction of overall mitochondrial mass in response to certain stresses, such as hypoxia and nutrient starvation. This prevents generation of reactive oxygen species and conserves valuable nutrients (such as oxygen) from being consumed inefficiently, thereby promoting cellular survival under conditions of energetic stress. The failure to properly modulate mitochondrial turnover in response to oncogenic stresses has been implicated both positively and negatively in tumorigenesis, while the potential of targeting mitophagy specifically as opposed to autophagy in general as a therapeutic strategy remains to be explored. The challenges and opportunities that come with our heightened understanding of the role of mitophagy in cancer are reviewed here.

Mitochondrial dysfunction in cancer

A mechanistic understanding of how mitochondrial dysfunction contributes to cell growth and tumorigenesis is emerging beyond Warburg as an area of research that is under-explored in terms of its significance for clinical management of cancer. Work discussed in this review focuses less on the Warburg effect and more on mitochondria and how dysfunctional mitochondria modulate cell cycle, gene expression, metabolism, cell viability, and other established aspects of cell growth and stress responses. There is increasing evidence that key oncogenes and tumor suppressors modulate mitochondrial dynamics through important signaling pathways and that mitochondrial mass and function vary between tumors and individuals but the significance of these events for cancer are not fully appreciated. We explore the interplay between key molecules involved in mitochondrial fission and fusion and in apoptosis, as well as in mitophagy, biogenesis, and spatial dynamics of mitochondria and consider how these distinct mechanisms are coordinated in response to physiological stresses such as hypoxia and nutrient deprivation. Importantly, we examine how deregulation of these processes in cancer has knock on effects for cell proliferation and growth. We define major forms of mitochondrial dysfunction and address the extent to which the functional consequences of such dysfunction can be determined and exploited for cancer diagnosis and treatment.

Characterization of circulating endothelial cells in acute myocardial infarction

Acute myocardial infarction (MI), which involves the rupture of existing atheromatous plaque, remains highly unpredictable despite recent advances in the diagnosis and treatment of coronary artery disease. Accordingly, a clinical measurement that can predict an impending MI is desperately needed. Here, we characterize circulating endothelial cells (CECs) using an automated and clinically feasible CEC three-channel fluorescence microscopy assay in 50 consecutive patients with ST-segment elevation MI and 44 consecutive healthy controls. CEC counts were significantly elevated in MI cases versus controls, with median numbers of 19 and 4 cells/ml, respectively (P = 1.1 × 10(-10)). A receiver-operating characteristic (ROC) curve analysis demonstrated an area under the ROC curve of 0.95, suggesting near-dichotomization of MI cases versus controls. We observed no correlation between CECs and typical markers of myocardial necrosis (ρ = 0.02, creatine kinase-myocardial band; ρ = -0.03, troponin). Morphological analysis of the microscopy images of CECs revealed a 2.5-fold increase (P < 0.0001) in cellular area and a twofold increase (P < 0.0001) in nuclear area of MI CECs versus healthy controls, age-matched CECs, as well as CECs obtained from patients with preexisting peripheral vascular disease. The distribution of CEC images that contained from 2 to 10 nuclei demonstrates that MI patients were the only subject group to contain more than 3 nuclei per image, indicating that multicellular and multinuclear clusters are specific for acute MI. These data indicate that CEC counts may serve as a promising clinical measure for the prediction of atherosclerotic plaque rupture events.