Radiation Therapy, Immune Therapy, and Breast Cancer
Nearly two thirds of cancer patients receive radiotherapy at some stage of their treatment. Concomitant or sequential immunotherapy may boost recognition of tumor antigens and any antitumor innate and adaptive immune responses that occur after radiotherapy-induced tissue damage. However, preclinical experiments with such combinations have had mixed success to date, possibly due to tumor-induced immune suppression and immune tolerance. Verbrugge and colleagues describe an approach in which radiotherapy increases the ability of immunomodulatory therapeutic antibodies to reject established triple-negative mouse mammary cancers. To achieve this result, the authors combined monoclonal antibodies (mAb) that target multiple immune-stimulatory/inhibitory pathways (anti-CD137, anti-CD40, and anti-PD-1) with single- and low-dose fractionated radiotherapy. All mice bearing established orthotopic AT-3 mammary tumors were cured when anti-CD137 and anti-PD-1 mAbs were combined with radiotherapy. CD8+ T cells were essential for these curative responses. CD137 expression on tumor-associated CD8+ T cells was largely restricted to a subset that highly expressed the immune-inhibitory PD-1. In addition, the authors found that radiotherapy did not deplete, but rather enriched tumors of these effector cells. This study shows the efficacy and safety of combining radiotherapy with immunostimulatory and checkpoint-inhibitory mAbs. In terms of triple-negative breast cancer, these results are especially encouraging, as these malignancies do not respond to endocrine treatment and other currently available targeted agents. However, this approach could also work in other cancers that are treated with radiotherapy.
Haploinsufficiency in the Polyamine–Hypusine Axis as a Driver of Lymphoma
Scuoppo and colleagues describe an intriguing mechanism for disabling a tumor suppressor network that contributes to tumor development. In a canonical view of cancer etiology, tumor suppressors are ablated by point mutations or genomic deletions in both alleles. Scuoppo and colleagues show that the haploinsufficiency of 2 distinct tumor suppressors that work in the polyamine–hypusine axis cooperate in Myc-induced B-cell lymphomas. They developed a short-hairpin RNA (shRNA) library that targets mouse orthologs of deleted genes in human B-cell non-Hodgkin lymphoma and used this library to identify tumor-suppressor genes whose decreased expression would accelerate B-cell lymphomagenesis in Eμ-myc transgenic mice. This approach led them to identify specific drivers of tumor development. Importantly, some of these genes are also altered in human B-cell lymphomas. Scuoppo and colleagues focus on 2 new candidate genes: adenosylmethionine decarboxylase 1 (AMD1) and the eukaryotic initiation factor 5A (eIF5A). AMD1 is an important enzyme that controls the synthesis of polyamines such as spermidine, which in turn is incorporated into the unique amino acid hypusine present in eIF5A. Mutations in p53 are linked to underexpression of eIF5A in lymphoma patients, which is associated with decreased rates of event-free survival. However, these authors show that lymphomas associated with loss of function of AMD1 or eIF5A arise independent of mutations in p53, supporting the idea that the 2 tumor suppressors cooperate in cancer etiology independent of known mechanisms that contribute to lymphoma (i.e., loss of the p53 tumor suppressor). The authors further demonstrate that targeting 3 major enzymes in the polyamine pathway [SRM (spermidine synthase); HDPS (deoxyhypusine synthase); and AMD1] also promotes tumorigenic activity in vivo. They also show that lymphomas derived from the knockdown of these enzymes exhibit alterations in hypusination of eIF5A. Scuoppo and colleagues present interesting data that highlight the role of the polyamine–hypusine axis as a new tumor suppressor pathway in lymphoma.
One big question that remains to be addressed is the role of eIF5A in tumor suppression. eIF5A is an initiation factor of translation that promotes formation of the first peptide bond. It would be significant to determine whether eIF5A works as a tumor suppressor through its role as a translation initiation factor or through as-yet-undiscovered additional functions. The authors show at a cellular level that reduction of polyamine enzymes and eIF5A expression leads to apoptosis. To identify which specific apoptotic factors were associated with this cellular outcome, the authors employed an unbiased mass spectrometry approach using premalignant B cells transduced with shRNAs targeting eIF5A and AMD1. They observed that proapoptotic Bax proteins were underexpressed in these samples, and it would be important in the future to unravel the mechanisms by which eIF5A hypusination controls Bax expression. The authors analyzed the already established comparative genomic hybridization data of diffuse large B-cell lymphomas and show codeletion of eIF5A and AMD1 in human lymphomas. They provide a final functional in vivo demonstration that eIF5A and AMD1 can cooperate in Myc-induced B-cell lymphomagenesis by undertaking a co-knockdown of these 2 genes, which resulted in accelerated tumor development compared with knockdown of each individual shRNA. Significantly, they show that the polyamine–hypusine pathway, which has been relegated historically to more of a housekeeping function in the cells, can be regulated and that reduction of this pathway's activity can lead to a specific step in cancer. Another value of this article resides in the fact that the authors' approach can discriminate between driver (causal) and passenger (noncausal) lesions in tumor etiology. Finally, this study underscores the importance of the fact that although we are approaching a genomic era in sequencing human tumors, it is essential to compare and integrate this genomic analysis with functional data, as reduction in expression of distinct genes that work in the same pathway can have additive effects that cannot be revealed by genomic sequencing alone. (Image courtesy of Wikimedia Commons.)
TGFβ, the Microenvironment, and Radioresistance in Glioblastoma
Radiation represents standard-of-care therapy for patients with glioblastoma (GBM) despite the fact that these tumors are relatively radio resistant. While some studies have implicated glioma-initiating cells in radioresistance, other studies suggest that the tumor microenvironment also plays a role. GBM is known to produce abundant TGFβ, which has been shown previously to affect the DNA damage response. Hardee and colleagues show that GBM cells become sensitized to radiation upon treatment with a small-molecule inhibitor of the TGFβ type I receptor kinase. Although inhibition of TGFβ did not affect the capacity to form neurospheres, radiation decreased colony forming efficiency, and addition of the drug further increased the radiosensitivity of these cells. Addition of the drug in neurosphere culture reduced the increased γH2AX foci induced in response to radiation, indicating that inhibition of TGFβ compromised the molecular recognition of DNA damage. Interestingly, mouse glioma neurospheres produced more TGFβ per cell compared with traditional monolayer culture, suggesting that TGFβ production in neurosphere culture promotes microenvironment-mediated resistance. The authors examined CXCR4 and Notch1, which have been implicated in self-renewal pathways in glioma-initiating cells. Irradiation of primary neurospheres significantly induced both markers, which were blocked by TGFβ inhibition. The authors controlled for off-target effects of the drug by replicating findings using an anti-TGFβ antibody. They speculate that secretion of TGFβ by glioma-initiating cells creates a microenvironment that promotes radioresistance in glioma cells, and they suggest inhibition of TGFβ as a therapeutic strategy to sensitize gliomas to radiation. (Image adapted from Fig. 6 of cited article.)
Bax Inhibitor-1: A Novel Autophagy Regulator That Links Ca2 Signaling between the Endoplasmic Reticulum and Mitochondria
Autophagy is a double-edged sword, providing a protective or a destructive role in a cell context– and environment-specific manner. In cancer, autophagy has been implicated in diverse processes, including tumor development, progression, and responses to therapy. Possibly one of the most significant roles of autophagy is to enhance cancer cell survival under conditions of stress, including nutrient and oxygen deprivation. Under conditions frequently encountered in the microenvironment, such as nutrient depravation, autophagy can activate a lysosomal degradation pathway resulting in the conversion of cellular macromolecules into substrates for energy production. Cellular bioenergetics, a process regulated by constitutive uptake of endoplasmic reticulum (ER) Ca2+ through inositol triphosphate receptors (IP3R), can suppress autophagy. Sano and colleagues show that Bax inhibitor-1 (BI-1), an antiapoptotic protein originally discovered by functional screening of cDNA libraries for inhibitors of yeast cell death induced by ectopic expression of mammalian Bax, promotes autophagy in an IP3R-dependent manner. BI-1 regulates mitochondrial bioenergetics, reducing oxygen consumption, influencing ATP levels, and stimulating autophagy through reduction of the steady-state levels of ER Ca2+ through IP3Rs. The importance of BI-1 in regulating bioenergetics is further supported by demonstrating reduced autophagy in BI-1-deficient mice and inhibition of tumor xenograft growth in vivo by reducing BI-1 expression. The present study provides new insights into the role of the antiapoptotic protein BI-1 in regulating autophagy, uncovering its role as a bridge between Ca2+ signaling in the ER and mitochondria culminating in decreased oxygen utilization and causing cellular resilience when confronted with metabolic stress. Further studies of this interesting molecule could lead to ways of exploiting this discovery for development of novel cancer therapeutics targeting BI-1. (Image from Clarke R, Cook K, Hu R, Facey C, Schwartz J, Baumann W, et al. Cancer Res 2012;72:61321–31; courtesy of publisher.)
Note: Breaking Advances are written by Cancer Research Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.
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