Retinoblastoma Expression and Targeting by CDK4/6 Inhibitors in Small Cell Lung Cancer.

The canonical model of "small cell lung cancer" (SCLC) depicts tumors arising from dual inactivation of TP53 and RB1. However, many genomic studies have persistently identified tumors with no RB1 mutations. Here, we examined RB1 protein expression and function in SCLC. RB1 expression was examined by IHC analysis of 62 human SCLC tumors. These studies showed that ∼14% of SCLC tumors expressed abundant RB1 protein, which is associated with neuroendocrine gene expression and is enriched in YAP1 expression, but no other lineage proteins that stratify SCLC. SCLC cells and xenograft tumors with RB1 protein expression were sensitive to growth inhibition by the CDK4/6 inhibitor palbociclib, and this inhibition was shown to be dependent on RB1 expression by CRISPR knockout. Furthermore, a patient with biopsy-validated wild-type RB1 SCLC who received the CDK4/6 inhibitor abemaciclib demonstrated a dramatic decrease in mutant TP53 ctDNA allelic fraction from 62.1% to 0.4% and decreased tumor mass on CT scans. Importantly, IHC of the diagnostic biopsy specimen showed RB1 positivity. Finally, we identified a transcriptomics-based RB1 loss-of-function signature that discriminates between SCLC cells with or without RB1 protein expression and validated it in the patient who was responsive to abemaciclib, suggesting its potential use to predict CDK4/6 inhibitor response in patients with SCLC. Our study demonstrates that RB1 protein is an actionable target in a subgroup of SCLC, a cancer that exhibits no currently targetable mutations.

A non-canonical role for pyruvate kinase M2 as a functional modulator of Ca2+ signalling through IP3 receptors.

Pyruvate kinase isoform M2 (PKM2) is a rate-limiting glycolytic enzyme that is widely expressed in embryonic tissues. The expression of PKM2 declines in some tissues following embryogenesis, while other pyruvate kinase isozymes are upregulated. However, PKM2 is highly expressed in cancer cells and is believed to play a role in supporting anabolic processes during tumour formation. In this study, PKM2 was identified as an inositol 1,4,5-trisphosphate receptor (IP3R)-interacting protein by mass spectrometry. The PKM2:IP3R interaction was further characterized by pull-down and co-immunoprecipitation assays, which showed that PKM2 interacted with all three IP3R isoforms. Moreover, fluorescence microscopy indicated that both IP3R and PKM2 localized at the endoplasmic reticulum. PKM2 binds to IP3R at a highly conserved 21-amino acid site (corresponding to amino acids 2078-2098 in mouse type 1 IP3R isoform). Synthetic peptides (denoted 'TAT-D5SD' and 'D5SD'), based on the amino acid sequence at this site, disrupted the PKM2:IP3R interaction and potentiated IP3R-mediated Ca2+ release both in intact cells (TAT-D5SD peptide) and in a unidirectional 45Ca2+ flux assay on permeabilized cells (D5SD peptide). The TAT-D5SD peptide did not affect the enzymatic activity of PKM2. Reducing PKM2 protein expression using siRNA increased IP3R-mediated Ca2+ signalling in intact cells without altering the ER Ca2+ content. These data identify PKM2 as an IP3R-interacting protein that inhibits intracellular Ca2+ signalling. The elevated expression of PKM2 in cancer cells is therefore not solely connected to its canonical role in glycolytic metabolism, rather PKM2 also has a novel non-canonical role in regulating intracellular signalling.

A synthetic peptide targeting the BH4 domain of Bcl-2 induces apoptosis in multiple myeloma and follicular lymphoma cells alone or in combination with agents targeting the BH3-binding pocket of Bcl-2.

Bcl-2 inhibits apoptosis by two distinct mechanisms but only one is targeted to treat Bcl-2-positive malignancies. In this mechanism, the BH1-3 domains of Bcl-2 form a hydrophobic pocket, binding and inhibiting pro-apoptotic proteins, including Bim. In the other mechanism, the BH4 domain mediates interaction of Bcl-2 with inositol 1,4, 5-trisphosphate receptors (IP3Rs), inhibiting pro-apoptotic Ca2+ signals. The current anti-Bcl-2 agents, ABT-263 (Navitoclax) and ABT-199 (Venetoclax), induce apoptosis by displacing pro-apoptotic proteins from the hydrophobic pocket, but do not inhibit Bcl-2-IP3R interaction. Therefore, to target this interaction we developed BIRD-2 (Bcl-2 IP3 Receptor Disruptor-2), a decoy peptide that binds to the BH4 domain, blocking Bcl-2-IP3R interaction and thus inducing Ca2+-mediated apoptosis in chronic lymphocytic leukemia, multiple myeloma, and follicular lymphoma cells, including cells resistant to ABT-263, ABT-199, or the Bruton's tyrosine kinase inhibitor Ibrutinib. Moreover, combining BIRD-2 with ABT-263 or ABT-199 enhances apoptosis induction compared to single agent treatment. Overall, these findings provide strong rationale for developing novel therapeutic agents that mimic the action of BIRD-2 in targeting the BH4 domain of Bcl-2 and disrupting Bcl-2-IP3R interaction.

PIAS3 expression in squamous cell lung cancer is low and predicts overall survival.

Unlike lung adenocarcinoma, little progress has been made in the treatment of squamous cell lung carcinoma (SCC). The Cancer Genome Atlas (TCGA) has recently reported that receptor tyrosine kinase signaling pathways are altered in 26% of SCC tumors, validating the importance of downstream Signal Transducers and Activators of Transcription 3 (STAT3) activity as a prime therapeutic target in this cancer. In the present report we examine the status of an endogenous inhibitor of STAT3, called Protein Inhibitor of Activated STAT3 (PIAS3), in SCC and its potential role in this disease. We examine PIAS3 expression in SCC tumors and cell lines by immunohistochemistry of a tissue microarray and western blotting. PIAS3 mRNA expression and survival data are analyzed in the TCGA data set. SCC cell lines are treated with curcumin to regulate PIAS3 expression and cell growth. PIAS3 protein expression is decreased in a majority of lung SCC tumors and cell lines. Analysis of PIAS3 mRNA transcript levels demonstrated that low PIAS3 levels predicted poor survival; Cox regression analysis revealed a hazard ratio of 0.57 (95% CI: 0.37-0.87), indicating a decrease in the risk of death by 43% for every unit elevation in PIAS3 gene expression. Curcumin treatment increased endogenous PIAS3 expression and decreased cell growth and viability in Calu-1 cells, a model of SCC. Our results implicate PIAS3 loss in the pathology of lung SCC and raise the therapeutic possibility of upregulating PIAS3 expression as a single target that can suppress signaling from the multiple receptor tyrosine kinase receptors found to be amplified in SCC.

Acidosis Sensing Receptor GPR65 Correlates with Anti-Apoptotic Bcl-2 Family Member Expression in CLL Cells: Potential Implications for the CLL Microenvironment.

The tumor microenvironment is generally an acidic environment, yet the effect of extracellular acidosis on chronic lymphocytic leukemia (CLL) is not well established. Here we are the first to report that the extracellular acid sensing G-protein coupled receptor, GPR65, is expressed in primary CLL cells where its level correlate strongly with anti-apoptotic Bcl-2 family member levels. GPR65 expression is found normally within the lymphoid lineage and has not been previously reported in CLL. We demonstrate a wide range of GPR65 mRNA expression among CLL 87 patient samples. The correlation between GPR65 mRNA levels and Bcl-2 mRNA levels is particularly strong (r=0.8063, p= <0.001). The correlation extends to other anti-apoptotic Bcl-2 family members, Mcl-1 (r=0.4847, p=0.0010) and Bcl-xl (r=0.3411, p=0.0252), although at lower levels of significance. No correlation is detected between GPR65 and levels of the pro-apoptotic proteins BIM, PUMA or NOXA. GPR65 expression also correlates with the favorable prognostic marker of 13q deletion. The present findings suggest the acid sensing receptor GPR65 may be of significance to allow CLL tolerance of extracellular acidosis. The correlation of GPR65 with Bcl-2 suggests a novel cytoprotective mechanism that enables CLL cell adaptation to acidic extracellular conditions. These findings suggest the potential value of targeting GPR65 therapeutically.

Glucocorticoid elevation of dexamethasone-induced gene 2 (Dig2/RTP801/REDD1) protein mediates autophagy in lymphocytes.

Glucocorticoid hormones, including dexamethasone, induce apoptosis in lymphocytes and consequently are used clinically as chemotherapeutic agents in many hematologic malignancies. Dexamethasone also induces autophagy in lymphocytes, although the mechanism is not fully elucidated. Through gene expression analysis, we found that dexamethasone induces the expression of a gene encoding a stress response protein variously referred to as Dig2, RTP801, or REDD1. This protein is reported to inhibit mammalian target of rapamycin (mTOR) signaling. Because autophagy is one outcome of mTOR inhibition, we investigated the hypothesis that Dig2/RTP801/REDD1 elevation contributes to autophagy induction in dexamethasone-treated lymphocytes. In support of this hypothesis, RNAi-mediated suppression of Dig2/RTP801/REDD1 reduces mTOR inhibition and autophagy in glucocorticoid-treated lymphocytes. We observed similar results in Dig2/Rtp801/Redd1 knock-out murine thymocytes treated with dexamethasone. Dig2/RTP801/REDD1 knockdown also leads to increased levels of dexamethasone-induced cell death, suggesting that Dig2/RTP801/REDD1-mediated autophagy promotes cell survival. Collectively, these findings demonstrate for the first time that elevation of Dig2/RTP801/REDD1 contributes to the induction of autophagy.

Glucocorticoid-mediated repression of the oncogenic microRNA cluster miR-17~92 contributes to the induction of Bim and initiation of apoptosis.

Synthetic glucocorticoids were one of the first effective treatments for lymphoid malignancies because of their ability to induce apoptosis and are still used in combination with other chemotherapeutic agents. Up-regulation of Bim, a proapoptotic member of the B-cell lymphoma-2 family, is an important mediator of glucocorticoid-induced apoptosis. Although glucocorticoids are known to elevate Bim mRNA and protein, little is known about the mechanism. Here, we report that glucocorticoids repress the expression of the microRNA cluster miR-17∼92, which results in elevated Bim protein expression as a mechanism by which glucocorticoids induce Bim. Using a luciferase-Bim 3' untranslated region construct, we demonstrate that glucocorticoids mediate Bim induction posttranscriptionally after miR-17∼92 repression, resulting in increased Bim protein expression. Overexpression of miR-17∼92 microRNAs decreases Bim induction and attenuates glucocorticoid-mediated apoptosis. Conversely, knockdown of miR-17∼92 increases Bim protein expression and glucocorticoid-mediated apoptosis. These findings indicate that endogenous levels of miR-17∼92 repress Bim expression in T-cell lymphoid malignancies and that glucocorticoids induce Bim expression via down-regulation of the miR-17∼92 microRNA cluster. Our findings present a novel mechanism that contributes to the up-regulation of Bim and induction of apoptosis in lymphocytes after glucocorticoid treatment. Furthermore, our work demonstrating that inhibition of miR-17∼92 increases glucocorticoid-induced apoptosis highlights the potential importance of miR-17∼92 as a therapeutic target in leukemias and lymphomas.

Glucocorticoids downregulate Fyn and inhibit IP(3)-mediated calcium signaling to promote autophagy in T lymphocytes.

T cell receptor activation induces inositol 1,4,5 trisphosphate (IP(3))-mediated calcium signaling that is essential for cell metabolism and survival. Moreover, inhibitors of IP(3) or pharmacological agents that disrupt calcium homeostasis readily induce autophagy. Using a glucocorticoid-sensitive CD4/CD8 positive T cell line, we found that dexamethasone prevented both IP(3)-mediated and spontaneous calcium signals within a timeframe that correlated with the induction of autophagy. We determined that this loss in IP(3)-mediated calcium signaling was dependent upon the downregulation of the Src kinase Fyn at the mRNA and protein level. Because it has previously been shown that Fyn positively regulates IP(3)-mediated calcium release by phosphorylating Type I IP(3) receptors (IP(3)R1), we investigated the effect of glucocorticoids on IP(3)R1 phosphorylation at Tyr353. Accordingly, glucocorticoid-mediated downregulation of Fyn prevented IP(3)R1 phosphorylation at Tyr353. Moreover, selective knockdown of Fyn or treatment with a Src inhibitor also attenuated IP(3)-mediated calcium release and induced autophagy. Collectively, these data indicate that glucocorticoids promote autophagy by inhibiting IP(3)-dependent calcium signals. These findings carry important therapeutic implications given the widespread use of dexamethasone as both a chemotherapeutic and immunosuppressive agent.

Dexamethasone-induced inositol 1,4,5-trisphosphate receptor elevation in murine lymphoma cells is not required for dexamethasone-mediated calcium elevation and apoptosis.

Glucocorticosteroid hormones, including dexamethasone, have diverse effects on immature lymphocyte function that ultimately lead to cell death. Previous studies established that glucocorticoid-induced alterations in intracellular calcium homeostasis promote apoptosis, but the mechanism by which glucocorticoids disrupt calcium homeostasis is unknown. Through gene expression array analysis, we found that dexamethasone induces a striking elevation of inositol 1,4,5-trisphosphate receptor (IP(3)R) levels in two murine lymphoma cell lines, WEHI7.2 and S49.A2. IP(3)R elevation was confirmed at both mRNA and protein levels. However, there was not a strong correlation between IP(3)R elevation and altered calcium homeostasis in terms of either kinetics or dose response. Moreover, IP(3)R knockdown, by either antisense or small interfering RNA, did not prevent either calcium disruption or apoptosis. Finally, DT40 lymphoma cells lacking all three IP(3)R isoforms were just as sensitive to dexamethasone-induced apoptosis as wild-type DT40 cells expressing all three IP(3)R isoforms. Thus, although alterations in intracellular calcium homeostasis contribute to glucocorticoid-induced apoptosis, these calcium alterations are not directly attributable to IP(3)R elevation.

Bcl-2 differentially regulates Ca2+ signals according to the strength of T cell receptor activation.

To investigate the effect of Bcl-2 on Ca2+ signaling in T cells, we continuously monitored Ca2+ concentration in Bcl-2-positive and -negative clones of the WEHI7.2 T cell line after T cell receptor (TCR) activation by anti-CD3 antibody. In Bcl-2-negative cells, high concentrations of anti-CD3 antibody induced a transient Ca2+ elevation, triggering apoptosis. In contrast, low concentrations of anti-CD3 antibody induced Ca2+ oscillations, activating the nuclear factor of activated T cells (NFAT), a prosurvival transcription factor. Bcl-2 blocked the transient Ca2+ elevation induced by high anti-CD3, thereby inhibiting apoptosis, but did not inhibit Ca2+ oscillations and NFAT activation induced by low anti-CD3. Reduction in the level of all three inositol 1,4,5-trisphosphate (InsP(3)) receptor subtypes by small interfering RNA inhibited the Ca2+ elevation induced by high but not low anti-CD3, suggesting that Ca2+ responses to high and low anti-CD3 may have different requirements for the InsP(3) receptor. Therefore, Bcl-2 selectively inhibits proapoptotic Ca2+ elevation induced by strong TCR activation without hindering prosurvival Ca2+ signals induced by weak TCR activation.

Bcl-2 functionally interacts with inositol 1,4,5-trisphosphate receptors to regulate calcium release from the ER in response to inositol 1,4,5-trisphosphate.

Inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are channels responsible for calcium release from the endoplasmic reticulum (ER). We show that the anti-apoptotic protein Bcl-2 (either wild type or selectively localized to the ER) significantly inhibited InsP3-mediated calcium release and elevation of cytosolic calcium in WEHI7.2 T cells. This inhibition was due to an effect of Bcl-2 at the level of InsP3Rs because responses to both anti-CD3 antibody and a cell-permeant InsP3 ester were decreased. Bcl-2 inhibited the extent of calcium release from the ER of permeabilized WEHI7.2 cells, even at saturating concentrations of InsP3, without decreasing luminal calcium concentration. Furthermore, Bcl-2 reduced the open probability of purified InsP3Rs reconstituted into lipid bilayers. Bcl-2 and InsP3Rs were detected together in macromolecular complexes by coimmunoprecipitation and blue native gel electrophoresis. We suggest that this functional interaction of Bcl-2 with InsP3Rs inhibits InsP3R activation and thereby regulates InsP3-induced calcium release from the ER.

Microarray analysis uncovers the induction of the proapoptotic BH3-only protein Bim in multiple models of glucocorticoid-induced apoptosis.

Despite being one of the earliest recognized and most clinically relevant forms of apoptosis, little is known about the transcriptional events that mediate glucocorticoid-induced apoptosis. Therefore, we used oligonucleotide microarrays to identify the pattern of dexamethasone-induced changes in gene expression in two well characterized models of glucocorticoid-induced apoptosis, the murine lymphoma cell lines S49.A2 and WEHI7.2. Dexamethasone treatment induced a diverse set of gene changes that evolved over a 24-h period preceding the onset of cell death. These include previously reported changes in the expression of genes regulating prosurvival signals mediated by c-Myc and NFkappaB. Unexpectedly, we discovered that glucocorticoid treatment increases expression of the gene encoding Bim, a BH3-only member of the Bcl-2 family that is capable of directly activating the apoptotic cascade. Induction of Bim was confirmed by immunoblotting not only in S49.A2 and WEHI7.2 cells but also in the human leukemia cell line CEM-C7 and in primary murine thymocytes. All three prototypical isoforms of Bim (BimEL, BimL, and BimS) were induced by dexamethasone. Because elevated expression of Bim initiates the execution phase of cell death, this report that Bim is induced by dexamethasone provides novel insight into the mechanism through which glucocorticoid-mediated changes in gene expression induce apoptosis in lymphoid cells.