IDO (indoleamine 2,3-dioxygenase) enzyme inhibitors have entered clinical trials for cancer treatment based on preclinical studies indicating that they can defeat immune escape and broadly enhance other therapeutic modalities. However, clear genetic evidence of IDO’s impact on tumorigenesis in physiologic models of primary or metastatic disease is lacking. Investigating the impact of Ido1 gene disruption in mouse models of oncogenic KRAS-induced lung carcinoma and breast carcinoma-derived pulmonary metastasis, we have found that IDO-deficiency resulted in reduced lung tumor burden and improved survival in both models. Micro-CT imaging further revealed that the density of the underlying pulmonary blood vessels was significantly reduced in Ido1-nullizygous mice. During lung tumor and metastasis outgrowth, IL6 induction was greatly attenuated in conjunction with the loss of IDO. Biologically, this resulted in a consequential impairment of pro-tumorigenic MDSCs (myeloid-derived suppressor cells), as restoration of IL6 recovered both MDSC suppressor function and metastasis susceptibility in Ido1-nullizygous mice. Together, our findings define IDO as a prototypical integrative modifier that bridges inflammation, vascularization and immune escape to license primary and metastatic tumor outgrowth.

Serotonin (5-HT) modulates neural responses to socioaffective cues and can bias approach or avoidance behavioral decisions, yet the cellular mechanisms underlying its contribution to the regulation of social experiences remain poorly understood. We hypothesized that GABAergic neurons in the dorsal raphe nucleus (DRN) may participate in socioaffective regulation by controlling serotonergic tone during social interaction. We tested this hypothesis using whole-cell recording techniques in genetically identified DRN GABA and 5-HT neurons in mice exposed to social defeat, a model that induces long-lasting avoidance behaviors in a subset of mice responsive to serotonergic antidepressants. Our results revealed that social defeat engaged DRN GABA neurons and drove GABAergic sensitization that strengthened inhibition of 5-HT neurons in mice that were susceptible, but not resilient to social defeat. Furthermore, optogenetic silencing of DRN GABA neurons disinhibited neighboring 5-HT neurons and prevented the acquisition of social avoidance in mice exposed to a social threat, but did not affect a previously acquired avoidance phenotype. We provide the first characterization of GABA neurons in the DRN that monosynaptically inhibit 5-HT neurons and reveal their key role in neuroplastic processes underlying the development of social avoidance.

HDAC inhibitors have been reported to produce antidepressant and pro-cognitive effects in animal models, however, poor brain bioavailability or lack of isoform selectivity of current probes has limited our understanding of their mode of action. We report the characterization of novel pyrimidine hydroxyl amide small molecule inhibitors of HDAC6, brain bioavailable upon systemic administration. We show that two compounds in this family, ACY-738 and ACY-775, inhibit HDAC6 with low nanomolar potency and a selectivity of 60-to 1500-fold over class I HDACs. In contrast to tubastatin A, a reference HDAC6 inhibitor with similar potency and peripheral activity, but more limited brain bioavailability, ACY-738 and ACY-775 induce dramatic increases in a-tubulin acetylation in brain and stimulate mouse exploratory behaviors in novel, but not familiar environments. Interestingly, despite a lack of detectable effect on histone acetylation, we show that ACY-738 and ACY-775 share the antidepressant-like properties of other HDAC inhibitors, such as SAHA and MS-275, in the tail suspension test and social defeat paradigm. These effects of ACY-738 and ACY-775 are directly attributable to the inhibition of HDAC6 expressed centrally, as they are fully abrogated in mice with a neural-specific loss of function of HDAC6. Furthermore, administered in combination, a behaviorally inactive dose of ACY-738 markedly potentiates the anti-immobility activity of a subactive dose of the selective serotonin reuptake inhibitor citalopram. Our results validate new isoform-selective probes for in vivo pharmacological studies of HDAC6 in the CNS and reinforce the viability of this HDAC isoform as a potential target for antidepressant development.

Genetic variations in certain components of the glucocorticoid receptor (GR) chaperone complex have been associated with the development of stress-related affective disorders and individual variability in therapeutic responses to antidepressants. Mechanisms that link GR chaperoning and stress susceptibility are not well understood. Here, we show that the effects of glucocorticoid hormones on socioaffective behaviors are critically regulated via reversible acetylation of Hsp90, a key component of the GR chaperone complex. We provide pharmacological and genetic evidence indicating that the cytoplasmic lysine deacetylase HDAC6 controls Hsp90 acetylation in the brain, and thereby modulates Hsp90-GR protein-protein interactions, as well as hormone- and stress-induced GR translocation, with a critical impact on GR downstream signaling and behavior. Pet1-Cre driven deletion of HDAC6 in serotonin neurons, the densest HDAC6-expressing cell group in the mouse brain, dramatically reduced acute anxiogenic effects of the glucocorticoid hormone corticosterone in the open field, elevated plus maze, and social interaction tests. Serotonin-selective depletion of HDAC6 also blocked the expression of social avoidance in mice exposed to chronic social defeat and concurrently prevented the electrophysiological and morphological changes induced, in serotonin neurons, by this murine model of traumatic stress. Together, these results identify HDAC6 inhibition as a potential new strategy for pro-resilience and antidepressant interventions through regulation of the Hsp90-GR heterocomplex and focal prevention of GR signaling in serotonin pathways. Our data thus uncover an alternate mechanism by which pan-HDAC inhibitors may regulate stress-related behaviors independently of their action on histones.

Age is the major risk factor for cancer, but few genetic pathways that modify cancer incidence during aging have been described. Bin1 is a prototypic member of the BAR adapter gene family that functions in vesicle dynamics and nuclear processes. Bin1 limits oncogenesis and is often attenuated in human cancers, but its role in cancer suppression has yet to be evaluated fully in vivo. In the mouse, homozygous deletion of Bin1 causes developmental lethality, so to assess this role, we examined cancer incidence in mosaic null mice generated by a modified Cre-lox technology. During study of these animals, one notable phenotype was an extended period of female fecundity during aging, with mosaic null animals retaining reproductive capability until the age of 17.3 F 1.1 months. Through 1 year of age, cancer incidence was unaffected by Bin1 ablation; however, by 18 to 20 months of age, f50% of mosaic mice presented with lung adenocarcinoma and f10% with hepatocarcinoma. Aging mosaic mice also displayed a higher incidence of inflammation and/or premalignant lesions, especially in the heart and prostate. In mice where colon tumors were initiated by a ras-activating carcinogen, Bin1 ablation facilitated progression to more aggressive invasive status. In cases of human lung and colon cancers, immunohistochemical analyses evidenced frequent attenuation of Bin1 expression, paralleling observations in other solid tumors. Taken together, our findings highlight an important role for Bin1 as a negative modifier of inflammation and cancer susceptibility during aging. [Cancer Res 2007;67(16):7605-12]