Dual inhibition of Type I and Type III PI3 kinases increases tumor cell apoptosis in HER2+ breast cancers

Introduction

Human epidermal growth factor receptor-2 (HER2) gene amplification (HER2+) drives tumor cell growth and survival in ~25 % of breast cancers. HER2 signaling activates the type I phosphoinositide 3-kinase (PI3K), upon which these tumors rely. Consequently, inhibitors of HER2 and type I PI3K block growth and increase apoptosis in HER2+ breast cancers, especially when used in combination. However, the impact of type III PI3K inhibition, particularly in combination with HER2 blockade or type I PI3K inhibition, remains less clear.

Methods

We utilized small molecule kinase inhibitors, locked nucleic acid antisense oligonucleotides (LNA-ASOs), and siRNA to assess proliferation, autophagy, apoptosis, and protein expression in cell culture models of HER2+ breast cancers.

Results

Treatment of HER2+ breast cancer cells with HER2 inhibitors or type I PI3K kinase inhibitors, alone or in combination, blocked type I PI3K signaling, reduced tumor cell growth, and induced autophagy. Knockdown of the type I PI3K, p110α, using an LNA-ASO termed EZN4150 inhibited PI3K-mediated Akt phosphorylation. However, in contrast to catalytic inhibitors of type I PI3Ks, EZN4150 did not induce autophagy, and blocked autophagy in response to inhibitors of HER2 or type I PI3Ks in a dominant fashion. Sequence analysis of EZN4150 revealed significant homology to the gene encoding the type III PI3K, Vps34, a key component for autophagy induction. EZN4150 simultaneously reduced expression of both p110α and Vps34. Combined inhibition of PI3K signaling and autophagy using individual siRNAs against p110α and Vps34 or using pharmacological type I and type III PI3K inhibitors recapitulated what was seen with EZN4150, and robustly enhanced tumor cell killing.

Conclusions

These studies highlight the important role of Vps34-mediated autophagy in limiting the anti-tumor response to inhibitors of HER2 or type I PI3K in HER2+ breast cancers. The type III PI3K Vps34 represents a potential therapeutic target to block treatment-induced autophagy and enhance tumor cell killing.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0656-2) contains supplementary material, which is available to authorized users.

Abstract LB-012: Inhibition of PI3K induces paracrine factors, which promote growth and survival of human breast cancer cells

Phosphoinositide 3-kinase (PI3K) is aberrantly activated in many human cancers. To blunt the mitogenic action of this oncogenic pathway, PI3K inhibitors are currently in clinical development. However, inhibition of PI3K results in feedback activation of receptor tyrosine kinases (RTKs) and cap-independent translation of pro-survival proteins, thus diminishing the net antitumor effect of PI3K inhibitors. Therefore, characterization of pathways potentially activated by PI3K inhibitors is necessary in order to identify drug combinations which will better eradicate tumors. We demonstrate herein that inhibition of PI3K in breast cancer cells resulted in the increased expression of EGFR ligands and activation of EGFR/ERK signaling. FoxO transcription factors are repressed by PI3K and previous studies have shown that inhibition of PI3K results in nuclear localization of FoxO and activation of FoxO-mediated transcription of RTKs and IGF-I/II. However, RNAi-mediated knockdown of FoxO3A only partially attenuated the activation of EGFR induced by PI3K inhibition. Using a panel of transcription factor luciferase reporters, we identified 10 transcription factors (including FoxO) which are activated upon PI3K inhibition in three breast cancer cell lines from different intrinsic subtypes (MCF7, BT20 and SUM159). In addition, the serum-free media conditioned by MCF7 or BT20 cells in the presence of the pan-PI3K inhibitor BKM120 induced the survival and proliferation of recipient cancer cells which otherwise undergo apoptosis in serum-free conditions. Thus, we hypothesized that inhibition of PI3K results in

activation of paracrine factors, including EGFR ligands, which may promote the survival of a heterogeneous tumor cell population. Indeed, evaluation of media conditioned by MCF7 and BT20 cells with cytokine/growth factor antibody arrays demonstrated over 100 proteins to be induced by PI3K inhibition, including ligands for receptors in the EGF, FGF, cytokine, chemokine and TGFβ families. We are currently performing SILAC-based mass spectrometry profiling of media conditioned by breast cancer cells ± PI3K inhibitors to determine modulation of secreted factors upon inhibition of PI3K. We are also

determining whether inhibition of PI3K results in increased ADAM protease activity, inducing the shedding of EGFR ligands from the cell membrane. The results of these experiments will identify extracellular factors activated by the inhibition of PI3K and suggest which pathways need to be simultaneously inhibited to maximize the clinical activity of PI3K inhibitors. Further, these secreted proteins may serve as circulating pharmacodynamics biomarkers indicative of effective blockade of PI3K in patients.

Citation Format: Christian D. Young, James P. Koch, Rebecca S. Cook, Carlos L. Arteaga. Inhibition of PI3K induces paracrine factors, which promote growth and survival of human breast cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-012. doi:10.1158/1538-7445.AM2015-LB-012

Small molecule inhibitor of the bone morphogenetic protein pathway DMH1 reduces ovarian cancer cell growth

The bone morphogenetic protein (BMP) pathway belonging to the Transforming Growth Factor beta (TGFβ) family of secreted cytokines/growth factors is an important regulator of cancer. BMP ligands have been shown to play both tumor suppressive and promoting roles in human cancers. We have found that BMP ligands are amplified in human ovarian cancers and that BMP receptor expression correlates with poor progression-free-survival (PFS). Furthermore, active BMP signaling has been observed in human ovarian cancer tissue. We also determined that ovarian cancer cell lines have active BMP signaling in a cell autonomous fashion. Inhibition of BMP signaling with a small molecule receptor kinase antagonist is effective at reducing ovarian tumor sphere growth. Furthermore, BMP inhibition can enhance sensitivity to Cisplatin treatment and regulates gene expression involved in platinum resistance in ovarian cancer. Overall, these studies suggest targeting the BMP pathway as a novel source to enhance chemo-sensitivity in ovarian cancer.

Abstract B49: mTORC2 directs breast morphogenesis through Rictor-dependent PKCα/Rac1 signaling independent of Akt

Akt phosphorylation is a major driver of cell survival, motility, and proliferation in development and disease, causing increased interest in upstream regulators of Akt like mTOR complex 2 (mTORC2). We used genetic disruption of Rictor to impair mTORC2 activity in mouse mammary epithelia, which decreased Akt phosphorylation, ductal length, secondary branching, cell motility, and cell survival. These effects were recapitulated with a pharmacological dual inhibitor of mTORC1/mTORC2, but not upon genetic disruption of mTORC1 function via Raptor knock-out. Surprisingly, Akt re-activation was insufficient to rescue survival, branching, or motility of mTORC2-impaired mammary epithelial cells (MECs). However, activation of the mTORC2 substrate protein kinase C (PKC)-α fully rescued branching, invasion, and survival of MECs after genetic or pharmacological mTORC2 inhibition. PKCα-mediated signaling through the small GTPase Rac1 was necessary for mTORC2-dependent mammary morphogenesis, revealing a novel role for Rictor/mTORC2 in survival and motility of untransformed MECs through an Akt-independent, PKCα/Rac1-dependent mechanism.

Citation Format: Meghan M. Morrison, Christian D. Young, Shan Wang, Violeta M. Sanchez, Cook S. Rebecca, Donna J. Hicks, Dana M. Brantley-Sieders. mTORC2 directs breast morphogenesis through Rictor-dependent PKCα/Rac1 signaling independent of Akt. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr B49.

mTOR Directs Breast Morphogenesis through the PKC-alpha-Rac1 Signaling Axis

Akt phosphorylation is a major driver of cell survival, motility, and proliferation in development and disease, causing increased interest in upstream regulators of Akt like mTOR complex 2 (mTORC2). We used genetic disruption of Rictor to impair mTORC2 activity in mouse mammary epithelia, which decreased Akt phosphorylation, ductal length, secondary branching, cell motility, and cell survival. These effects were recapitulated with a pharmacological dual inhibitor of mTORC1/mTORC2, but not upon genetic disruption of mTORC1 function via Raptor deletion. Surprisingly, Akt re-activation was not sufficient to rescue cell survival or invasion, and modestly increased branching of mTORC2-impaired mammary epithelial cells (MECs) in culture and in vivo. However, another mTORC2 substrate, protein kinase C (PKC)-alpha, fully rescued mTORC2-impaired MEC branching, invasion, and survival, as well as branching morphogenesis in vivo. PKC-alpha-mediated signaling through the small GTPase Rac1 was necessary for mTORC2-dependent mammary epithelial development during puberty, revealing a novel role for Rictor/mTORC2 in MEC survival and motility during branching morphogenesis through a PKC-alpha/Rac1-dependent mechanism.

The protein kinase mTOR is frequently activated in breast cancers, where it enhances cancer cell growth, survival, and metastastic spread to distant organs. Thus, mTOR is an attractive, clinically relevant molecular target for drugs designed to treat metastatic breast cancers. However, mTOR exists in two distinct complexes, mTORC1 and mTORC2, and the relative roles of each complex have not been elucidated. Moreover, as pathways that regulate normal tissue growth and development are often highjacked to promote cancer, understanding mTOR function in normal mammary epithelial development will likely provide insight into its role in tumor progression. In this study, we assessed the role of mTORC1 and mTORC2 complexes in normal mammary epithelial cell branching, survival, and invasion. Interestingly, while mTORC1 was not required for branching, survival and invasion of mammary epithelial cells, mTORC2 was necessary for these processes in both mouse and human models. Furthermore, we found that mTORC2 exerts its effects primarily through downstream activation of a PKC-alpha-Rac1 signaling axis rather than the more well-studied Akt signaling pathway. Our studies identify a novel role for the mTORC2 complex in mammary morphogenesis, including cell survival and motility, which are relevant to breast cancer progression.

Activating PIK3CA Mutations Induce an Epidermal Growth Factor Receptor (EGFR)/Extracellular Signal-regulated Kinase (ERK) Paracrine Signaling Axis in Basal-like Breast Cancer

Mutations in PIK3CA, the gene encoding the p110α catalytic subunit of phosphoinositide 3-kinase (PI3K) have been shown to transform human mammary epithelial cells (MECs). These mutations are present in all breast cancer subtypes, including basal-like breast cancer (BLBC). Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we identified 72 protein expression changes in human basal-like MECs with knock-in E545K or H1047R PIK3CA mutations versus isogenic MECs with wild-type PIK3CA. Several of these were secreted proteins, cell surface receptors or ECM interacting molecules and were required for growth of PIK3CA mutant cells as well as adjacent cells with wild-type PIK3CA. The proteins identified by MS were enriched among human BLBC cell lines and pointed to a PI3K-dependent amphiregulin/EGFR/ERK signaling axis that is activated in BLBC. Proteins induced by PIK3CA mutations correlated with EGFR signaling and reduced relapse-free survival in BLBC. Treatment with EGFR inhibitors reduced growth of PIK3CA mutant BLBC cell lines and murine mammary tumors driven by a PIK3CA mutant transgene, all together suggesting that PIK3CA mutations promote tumor growth in part by inducing protein changes that activate EGFR.

Abstract PD5-8: HER2/PIK3CAH1047R transgenic mammary tumors develop acquired resistance to triple therapy with trastuzumab, pertuzumab and PI3K inhibitors via multiple mechanisms

HER2 amplification and activating mutations in PIK3CA, the gene encoding the p110α subunit of PI3K, often co-occur in breast cancer. We generated a transgenic mouse model of HER2-overexpressing (HER2+), PIK3CAH1047R-mutant breast cancer. In these mice, PIK3CAH1047R accelerates HER2-mediated mammary epithelial transformation and metastatic progression, confers stem cell-like properties to HER2-overexpressing cancers and generates resistance to the combination of trastuzumab and pertuzumab (Hanker et al. PNAS 2013). HER2+/PIK3CA tumor growth was inhibited by treatment with the HER2 antibodies trastuzumab and pertuzumab in combination with the pan-PI3K inhibitor BKM120 (TPB). We sought to discover mechanisms of acquired resistance to the triple therapy by long-term treatment of established HER2+/PIK3CA tumors. We used tumor transplants derived from two HER2+/PIK3CA transgenic mice, #564 and #635. Tumor transplants from model 564 were initially growth inhibited by TPB, but did not regress. A subset of 564 transplants (3/11) resumed growth in the presence of continuous TPB therapy. All transplants (n=9) from model 635 regressed to a volume of <100 mm3 within 6 weeks of treatment. All tumors recurred and 2 tumors continued growth when re-treated with TPB. Resistance was maintained following passaging in mice and tumors were cross-resistant to trastuzumab/pertuzumab/BYL719, a p110α-specific inhibitor. TPB-resistant tumor 635-2 expressed p95 HER2, which was not detected in untreated tumors. In contrast, HER2 expression was significantly reduced in TPB-resistant tumor 635-3. P-AKT remained suppressed in some resistant tumors, but was restored in others. Short-term TPB treatment strongly suppressed P-S6 in sensitive tumors, whereas P-S6 was no longer inhibited in all TPB-resistant tumors from both models. We are currently performing whole-exome sequencing and RNA-sequencing on TPB-resistant vs. untreated tumors in order to identify additional mechanisms of resistance. In parallel, we established human HER2+, PIK3CA-mutant cell lines (MDA-MB 453, UACC893, and HCC1954) resistant to TPB by long-term treatment (>5 months) in the presence of the three drugs. Similar to the TPB-resistant tumors, P-S6 was no longer inhibited following TPB treatment in the resistant cell lines. Treatment with the TORC1/2 inhibitor MLN0128 abolished levels of P-S6 in HER2+/PIK3CAH1047R tumors. Combined treatment with MLN0128 and TPB inhibited growth of the drug-resistant tumors. Interestingly, Both TPB-resistant HER2+/PIK3CAH1047R tumor lines displayed resistance to the antibody-drug conjugate trastuzumab-DM1 (T-DM1) in vitro and in vivo, despite maintenance of HER2 overexpression. In addition, HCC1954 cells selected for resistance to TPB in culture were 66-fold less sensitive to T-DM1 than parental cells, despite maintaining equal levels of HER2 by western blot. These data suggest that multiple mechanisms may contribute to resistance to dual HER2 and PI3K blockade, including re-activation of mTOR signaling. We speculate that a similar heterogeneity of resistance mechanisms may occur in HER2+/PIK3CA-mutant metastases in patients.

Citation Format: Ariella B Hanker, Benjamin Bulen, Monica Red Brewer, Christian D Young, Kirsten M Farrar, Rebecca S Cook, Thomas P Stricker, Carlos L Arteaga. HER2/PIK3CAH1047R transgenic mammary tumors develop acquired resistance to triple therapy with trastuzumab, pertuzumab and PI3K inhibitors via multiple mechanisms [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr PD5-8.

Abstract 1822: HER2/PIK3CAH1047R transgenic tumors develop acquired resistance to triple therapy with trastuzumab, pertuzumab, and PI3K inhibitors via multiple mechanisms

The HER2 (ERBB2) oncogene is amplified in 20-25% of breast cancers and is associated with poor patient outcome. Activating mutations in PIK3CA, the gene encoding the p110α catalytic subunit of phosphatidylinositol 3-kinase (PI3K), occur in ∼30% of breast cancers. HER2 amplification and PIK3CA mutations often co-occur in breast cancer. Aberrant activation of the PI3K pathway correlates with a diminished response to HER2-directed therapies. We previously generated a conditional transgenic mouse model of HER2-overexpressing (HER2+), PIK3CAH1047R-mutant breast cancer. We showed that PIK3CAH1047R accelerates HER2-mediated breast epithelial transformation and metastatic progression, alters the intrinsic phenotype of HER2-overexpressing cancers and generates resistance to FDA-approved combinations of anti-HER2 therapies (Hanker et al. PNAS 2013). HER2+/PIK3CA tumor growth was inhibited by treatment with the HER2 antibodies trastuzumab and pertuzumab in combination with the pan-PI3K inhibitor BKM120 (TPB). We sought to discover mechanisms of acquired resistance to the triple therapy by long-term treatment of established HER2+/PIK3CA tumors. We utilized tumor transplants derived from two HER2+/PIK3CA transgenic mice, #564 and #635. Tumor transplants from model 564 were initially growth inhibited by TPB, but did not regress. A subset of 564 transplants (3/11) resumed growth in the presence of continuous TPB therapy. Resistance was maintained following passaging in mice and tumors were cross-resistant to trastuzumab/pertuzumab/BYL719, a p110α-specific inhibitor. P-AKT remained suppressed in resistant tumors, whereas P-ERK was elevated.

All transplants (n=9) from model 635 regressed to a volume of <100 mm3 within 6 weeks of treatment. All tumors recurred within 2 months; 2 tumors continued growth when re-treated with TPB. Unlike the 564 resistant tumors, P-AKT was restored in the 635 resistant tumors, while short-term TPB treatment strongly inhibited P-AKT in the 635 tumors. TPB-resistant tumor 635-2 expressed p95 HER2, which was not detected in untreated tumors. In contrast, HER2 expression was significantly reduced in TPB-resistant tumor 635-3. We are currently performing whole-exome sequencing on TPB-resistant vs. untreated tumors in order to determine the mechanisms of resistance. Both 564 and 635 TPB-resistant tumor transplants displayed resistance to the antibody-drug conjugate trastuzumab-DM1, despite maintenance of HER2 overexpression. These early data suggest that multiple mechanisms may contribute to resistance to dual HER2 blockade in combination with PI3K inhibitors. In parallel, we are currently establishing human HER2+, PIK3CA-mutant cell lines resistant to TPB. We speculate that a similar heterogeneity of mechanisms of acquired resistance may occur in different HER2+/PIK3CA-mutant metastases in patients.

Citation Format: Ariella B. Hanker, Christian D. Young, Thomas P. Stricker, Rebecca S. Cook, Carlos L. Arteaga. HER2/PIK3CAH1047R transgenic tumors develop acquired resistance to triple therapy with trastuzumab, pertuzumab, and PI3K inhibitors via multiple mechanisms. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1822. doi:10.1158/1538-7445.AM2014-1822

Molecular profiling of the residual disease of triple-negative breast cancers after neoadjuvant chemotherapy identifies actionable therapeutic targets

Neoadjuvant chemotherapy (NAC) induces a pathologic complete response (pCR) in approximately 30% of patients with triple-negative breast cancers (TNBC). In patients lacking a pCR, NAC selects a subpopulation of chemotherapy-resistant tumor cells. To understand the molecular underpinnings driving treatment-resistant TNBCs, we performed comprehensive molecular analyses on the residual disease of 74 clinically defined TNBCs after NAC, including next-generation sequencing (NGS) on 20 matched pretreatment biopsies. Combined NGS and digital RNA expression analysis identified diverse molecular lesions and pathway activation in drug-resistant tumor cells. Ninety percent of the tumors contained a genetic alteration potentially treatable with a currently available targeted therapy. Thus, profiling residual TNBCs after NAC identifies targetable molecular lesions in the chemotherapy-resistant component of the tumor, which may mirror micrometastases destined to recur clinically. These data can guide biomarker-driven adjuvant studies targeting these micrometastases to improve the outcome of patients with TNBC who do not respond completely to NAC.

SIGNIFICANCE

This study demonstrates the spectrum of genomic alterations present in residual TNBC after NAC. Because TNBCs that do not achieve a CR after NAC are likely to recur as metastatic disease at variable times after surgery, these alterations may guide the selection of targeted therapies immediately after mastectomy before these metastases become evident.

Abstract B015: Knock-in of PIK3CA mutations in MCF10A mammary epithelial cells modifies their proteomic profile to resemble basal-like breast cancer and stimulate EGFR-dependent cell proliferation

PIK3CA, the gene encoding the p110α catalytic subunit of phosphatidylinositol-3 kinase (PI3K), is frequently mutated in breast cancer. Activating mutations in PIK3CA are known to transform mammary epithelial cells (MECs). Genomic knock-in of the two most frequent hot-spot PIK3CA mutations (E545K or H1047R) into MCF10A MECs resulted in growth factor-independent proliferation. Shotgun LC-MS/MS mass spectrometry analysis of wild type, E545K and H1047R MCF10A cell lysates revealed 73 proteins uniquely altered in the both mutant cell lines compared to wild type cells. KEGG pathway analysis demonstrated that PIK3CA mutant cells have elevated levels of proteins involved in focal adhesion, ECM-receptor interactions and actin cytoskeleton regulation. Nearly half the proteins upregulated in the mutant cells are secreted or involved in extracellular matrix (ECM) processing or signaling. The EGFR ligand amphiregulin was five-fold higher in the conditioned media harvested from PIK3CA mutant cells as compared to wild type cells. The conditioned media of PIK3CA mutant cells, but not that of wild-type cells, was sufficient to stimulate proliferation and EGFR phosphorylation in wild type MCF10A cells. Proliferation and EGFR activation were inhibited with an amphiregulin-neutralizing antibody or with EGFR neutralizing antibodies and kinase inhibitors. PIK3CA mutant cells downregulated PTPRF, a receptor tyrosine phosphatase. EGFR signaling and proliferation were stimulated in wild type cells when PTPRF was downregulated with siRNA, suggesting that PIK3CA mutant MCF10A cells activate EGFR-dependent proliferation by suppression of a negative regulatory phosphatase and increased secretion of amphiregulin.

The expression of transglutaminase 2, peroxidasin, fibronectin, integrin α5, laminin β3, laminin γ2, thrombospondin and EphA2, eight proteins upregulated in PIK3CA mutant MCF10A cells, was evaluated in a panel of breast cancer cell lines. All eight proteins were more highly expressed in basal-like compared to luminal-like breast cancer cell lines. The expression of PTPRF, which is decreased in PIK3CA mutant MCF10A cells, was lower in basal-like cell lines. Interrogation of microarray data demonstrated that the RNA signal of proteins upregulated in mutant PIK3CA MCF10A cells correlated with decreased relapse-free survival in basal-like, but not in luminal-like breast cancer. siRNA-mediated silencing of peroxidasin, laminin γ2, EphA2, integrin β1 or amphiregulin reduced the proliferation of PIK3CA mutant MCF10A cells, suggesting that these proteins are necessary for maintenance of the transformed phenotype. siRNA-mediated silencing of peroxidasin and EphA2 proteins also reduces the proliferation of basal-like breast cancer cells. Our proteomic analysis of PIK3CA mutant MCF10A cells revealed mechanisms of autocrine and paracrine induced proliferation and alterations mostly limited to basal-like breast cancer cells. These may serve as therapeutic targets in this subtype of breast cancer.

Citation Format: Christian D. Young, Lisa J. Zimmerman, Corbin A. Whitwell, Ariella B. Hanker, Thomas Stricker, Dana M. Brantley-Sieders, Ben Ho Park, Daniel C. Liebler, Rebecca S. Cook, Carlos L. Arteaga. Knock-in of PIK3CA mutations in MCF10A mammary epithelial cells modifies their proteomic profile to resemble basal-like breast cancer and stimulate EGFR-dependent cell proliferation. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr B015.

Abstract A007: Mutant PIK3CA accelerates HER2-driven transgenic mammary tumor progression, enhances cancer stem cell features, and induces resistance to combinations of anti-HER2 therapies

The HER2 (ERBB2) oncogene is amplified in 20-25% of breast cancers and is associated with poor patient outcome. Mutational activation of PIK3CA, the gene encoding the p110α catalytic subunit of phosphatidylinositol 3-kinase (PI3K), occurs in ~30% of breast cancers. HER2 amplification and PIK3CA mutations often co-occur in breast cancer. Aberrant activation of the PI3K pathway has been shown to correlate with a diminished response to HER2-directed therapies. We sought to directly determine whether mutant PI3K enhances HER2-induced tumor progression and promotes resistance to HER2 inhibition in vivo. For this purpose, we generated a conditional transgenic mouse model of HER2-overexpressing (HER2+), PIK3CAH1047R-mutant breast cancer. Mice expressing both human HER2 and mutant PIK3CA in the mammary epithelium developed tumors with shorter latencies compared to mice expressing PIK3CA alone (p=0.0012) or HER2 alone (p<0.0001). HER2 and mutant PIK3CA also strongly cooperated to promote lung metastases following tail vein injection of cells derived from transgenic tumors (p<0.01). PIK3CA and HER2+/PIK3CA tumors were histologically diverse (i.e., adenocarcinoma, adenomyoepithelioma, adenocarcinoma with squamous metaplasia, and metaplastic carcinoma), expressed luminal and basal cytokeratins by IHC, and were associated with the claudin-low subtype of human breast cancers by microarray analysis. Accordingly, HER2+/PIK3CA tumors expressed elevated transcripts encoding EMT and stem cell markers, such as Snail, Slug, Twist, Bmi1, ITGB1 (CD29), and Procr. Further, single cells from HER2+/PIK3CA tumors formed more and larger mammospheres compared to cells from HER2-driven tumors. HER2+/PIK3CA mammospheres and tumors were completely resistant to the HER2 monocolonal antibody trastuzumab, alone and in combination with the HER2 tyrosine kinase inhibitor lapatinib or with pertuzumab, an antibody that inhibits ligand-induced HER2 dimerization. Both drug resistance and enhanced mammosphere formation were reversed by treatment with the pan-PI3K inhibitor BKM120. BKM120 treatment strongly inhibited T308 and S473 pAKT, whereas pAKT levels were unaffected by trastuzumab in combination with lapatinib or with pertuzumab. We further generated a model of resistance to the combination of trastuzumab, pertuzumab, and BKM120 by long-term treatment of HER2+/PIK3CA tumor transplants. Tumors initially responded, but a subset (3/11) resumed growth in the presence of drugs within 2 weeks of treatment initiation. Resistance was maintained following passaging in mice. We are currently performing whole-exome sequencing on resistant vs. untreated tumors in order to determine the mechanism(s) of resistance. In sum, PIK3CAH1047R accelerates HER2-mediated breast epithelial transformation and metastatic progression, alters the intrinsic phenotype of HER2-overexpressing cancers and generates resistance to FDA-approved combinations of anti-HER2 therapies. We propose the mouse model presented herein will be a valuable tool to investigate the underlying biology of HER2+/PI3K-mutant tumors and for the preclinical testing of therapeutic strategies against this subtype of breast cancer.

Citation Format: Ariella B. Hanker, Adam Pfefferle, Justin M. Balko, Maria Gabriela Kuba, Christian D. Young, Violeta Sanchez, Cammie R. Sutton, Hailing Cheng, Charles M. Perou, Jean J. Zhao, Rebecca S. Cook, Carlos L. Arteaga. Mutant PIK3CA accelerates HER2-driven transgenic mammary tumor progression, enhances cancer stem cell features, and induces resistance to combinations of anti-HER2 therapies. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A007.

Conditional loss of ErbB3 delays mammary gland hyperplasia induced by mutant PIK3CA without affecting mammary tumor latency, gene expression, or signaling

Mutations in PIK3CA, the gene encoding the p110α catalytic subunit of phosphoinositide 3-kinase (PI3K), have been shown to transform mammary epithelial cells (MEC). Studies suggest this transforming activity requires binding of mutant p110α via p85 to phosphorylated YXXM motifs in activated receptor tyrosine kinases (RTK) or adaptors. Using transgenic mice, we examined if ErbB3, a potent activator of PI3K, is required for mutant PIK3CA-mediated transformation of MECs. Conditional loss of ErbB3 in mammary epithelium resulted in a delay of PIK3CA(H1047R)-dependent mammary gland hyperplasia, but tumor latency, gene expression, and PI3K signaling were unaffected. In ErbB3-deficient tumors, mutant PI3K remained associated with several tyrosyl phosphoproteins, potentially explaining the dispensability of ErbB3 for tumorigenicity and PI3K activity. Similarly, inhibition of ErbB RTKs with lapatinib did not affect PI3K signaling in PIK3CA(H1047R)-expressing tumors. However, the p110α-specific inhibitor BYL719 in combination with lapatinib impaired mammary tumor growth and PI3K signaling more potently than BYL719 alone. Furthermore, coinhibition of p110α and ErbB3 potently suppressed proliferation and PI3K signaling in human breast cancer cells harboring PIK3CA(H1047R). These data suggest that PIK3CA(H1047R)-driven tumor growth and PI3K signaling can occur independently of ErbB RTKs. However, simultaneous blockade of p110α and ErbB RTKs results in superior inhibition of PI3K and mammary tumor growth, suggesting a rational therapeutic combination against breast cancers harboring PIK3CA activating mutations.

Abstract LB-103: Conditional loss of ErbB3 delays mammary gland hyperplasias induced by mutant PIK3CA in transgenic mice, but does not affect mammary tumor latency, gene expression or signaling

The phosphatidylinositol-3-kinase (PI3K) pathway is the most frequently mutated pathway in breast cancer. Signaling through the ErbB family of receptor tyrosine kinases (RTKs) strongly activates PI3K. Six YXXM PI3K binding motifs in ErbB3 make this RTK a strong upstream activator of PI3K. Mutations in PIK3CA, the gene encoding the p110α catalytic subunit of PI3K, are associated with increased PI3K activity, enhanced transformation, and drug resistance. We hypothesized that upstream receptors, such as ErbB3, are still required to localize PI3K to the plasma membrane and maximally drive tumor formation induced by mutant PI3K. To test this hypothesis, we generated a doxycycline (Dox)-inducible system in triple transgenic mice (MMTV-rtTA, Tet-Op-HA-PIK3CAH1047R-IRES-Luc and Tet-Op-Cre) which are homozygous or heterozygous for floxed ErbB3 alleles. In this model, referred to as iPI3K.iCre.ErbB3FL/FL, Dox induces the expression of PIK3CAH1047R and Cre recombinase. This results in Cre-mediated deletion of floxed ErbB3 alleles in mammary cells expressing PIK3CAH1047R. Loss of ErbB3 delayed mammary hyperplasias induced by PIK3CAH1047R in 12-week old animals. The mean mammary tumor latency of iPI3K.iCre.ErbB3FL/+ animals was 398 days compared to 419 days in iPI3K.iCre.ErbB3FL/FL animals (p=0.93). Both ErbB3FL/Fl and ErbB3+ tumors showed mixed pathologies of solid, papillary or cribiform adenocarcinomas (with and without squamous metaplasia). The heterogeneity of PI3K mutant tumors was also shown by the mixed expression of cytokeratin 8-positive (luminal-like) and cytokeratin 5-positive (basal-like) epithelial cells, which was not altered by loss of ErbB3. Microarray analysis of tumors revealed similar gene expression patterns in tumors with or without ErbB3. Immunoblot analysis of tumor lysates confirmed the absence of ErbB3 protein in iPI3K.iCre.ErbB3FL/FL tumors. However, downstream effectors of PI3K such as Akt, PDK1, SGK1, GSK3, 4EBP1 and S6 were similarly phosphorylated in tumors with and without ErbB3 expression. RTKs other than ErbB3, such as ErbB2, PDGFR, EGFR and MSPR, were phosphorylated in both tumor types. In ErbB3+ tumors, P-ErbB3 associated with p85, the regulatory subunit of PI3K, and coprecipitated with p85 antibodies. In ErbB3-deficient tumors and in breast cancer cell lines where ErbB3 was knocked down with RNAi, several P-Tyr proteins remained bound to p85, including the adaptor molecules IRS-1 and Gab2. We speculate these molecules circumvent a potential requirement for ErbB3 in cancers induced and driven by mutant PIK3CA. However, ErbB3 is required for the early mouse mammary hyperplasias induced by mutant PI3K.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-103. doi:1538-7445.AM2012-LB-103

Modulation of glucose transporter 1 (GLUT1) expression levels alters mouse mammary tumor cell growth in vitro and in vivo

Tumor cells exhibit an altered metabolism characterized by elevated aerobic glycolysis and lactate secretion which is supported by an increase in glucose transport and consumption. We hypothesized that reducing or eliminating the expression of the most prominently expressed glucose transporter(s) would decrease the amount of glucose available to breast cancer cells thereby decreasing their metabolic capacity and proliferative potential.

Of the 12 GLUT family glucose transporters expressed in mice, GLUT1 was the most abundantly expressed at the RNA level in the mouse mammary tumors from MMTV-c-ErbB2 mice and cell lines examined. Reducing GLUT1 expression in mouse mammary tumor cell lines using shRNA or Cre/Lox technology reduced glucose transport, glucose consumption, lactate secretion and lipid synthesis in vitro without altering the concentration of ATP, as well as reduced growth on plastic and in soft agar. The growth of tumor cells with reduced GLUT1 expression was impaired when transplanted into the mammary fat pad of athymic nude mice in vivo. Overexpression of GLUT1 in a cell line with low levels of endogenous GLUT1 increased glucose transport in vitro and enhanced growth in nude mice in vivo as compared to the control cells with very low levels of GLUT1.

These studies demonstrate that GLUT1 is the major glucose transporter in mouse mammary carcinoma models overexpressing ErbB2 or PyVMT and that modulation of the level of GLUT1 has an effect upon the growth of mouse mammary tumor cell lines in vivo.

A vitamin D receptor-alkylating derivative of 1α,25-dihydroxyvitamin D3 inhibits growth of human kidney cancer cells and suppresses tumor growth

1,25-Dihydroxyvitamin D₃ [1,25(OH)₂D₃] has shown strong promise as an antiproliferative agent in several malignancies, yet its therapeutic use has been limited by its toxicity leading to search for analogues with antitumor property and low toxicity. In this study, we evaluated the in vitro and in vivo properties of 1,25-dihydroxyvitamin D₃-3-bromoacetate [1,25(OH)₂D₃-3-BE], an alkylating derivative of 1,25(OH)₂D₃, as a potential therapeutic agent for renal cancer. Dose response of 1,25(OH)₂D₃-3-BE in 2 kidney cancer cell lines was evaluated for its antiproliferative and apoptotic properties, and mechanisms were evaluated by Western blot and FACS analyses. Therapeutic potential of 1,25(OH)₂D₃-3-BE was assessed both by determining its stability in human serum and by evaluating its efficacy in a mouse xenograft model of human renal tumor. We observed that 1,25(OH)₂D₃-3-BE is significantly more potent than an equivalent concentration of 1,25(OH)₂D₃ in inhibiting growth of A498 and Caki 1 human kidney cancer cells. 1,25(OH)₂D₃-3-BE-mediated growth inhibition was promoted through inhibition of cell-cycle progression by downregulating cyclin A and induction of apoptosis by stimulating caspase activity. Moreover, 1,25(OH)₂D₃-3-BE strongly inhibited Akt phosphorylation and phosphorylation of its downstream target, caspase-9. 1,25(OH)₂D₃-3-BE seemed to be stable in human serum. In xenograft mouse model of human renal tumor, 1,25(OH)₂D₃-3-BE was more potent at reducing tumor size than 1,25(OH)₂D₃, which was accompanied by an increase in apopotosis and reduction of cyclin A staining in the tumors. These results suggest a translational potential of this compound as a therapeutic agent in renal cell carcinoma. Data from this study and extensive studies of vitamin D for the prevention of many malignancies support the potential of 1,25(OH)₂D₃-3-BE for preventing renal cancer and the development of relevant in vivo prevention models for assessing this potential, which do not exist at present.

AKT regulates BRCA1 stability in response to hormone signaling

The observation that inherited mutations within BRCA1 result in breast and ovarian cancers suggests a functional relationship may exist between hormone signaling and BRCA1 function. We demonstrate that AKT activation promotes the expression of BRCA1 in response to estrogen and IGF-1 receptor signaling, and the rapid increase in BRCA1 protein levels appears to occur independently of new protein synthesis. Further, we identify a novel AKT phosphorylation site in BRCA1 at S694 which is responsive to activation of these signaling pathways. These data suggest AKT phosphorylation of BRCA1 increases total protein expression by preventing proteasomal degradation. AKT activation also appears to support nuclear localization of BRCA1, and co-expression of activated AKT with BRCA1 decreases radiation sensitivity, suggesting this interaction has functional consequences for BRCA1's role in DNA repair. Targets within this pathway could provide strategies for modulation of BRCA1 protein, which may prove therapeutically beneficial for breast and ovarian cancer treatment.

Rah, rah, ROS: metabolic changes caused by loss of adhesion induce cell death

The high rate of glucose utilization by cancer cells has been well characterized. Recent data suggest that when normal mammary epithelial cells are cultured under nonadherent conditions, glucose consumption decreases, ATP levels fall, and concentrations of reactive oxygen species rise. The rise in reactive oxygen species causes death of nonadherent cells, which can be suppressed with antioxidants. Nonadherent ErbB2-transformed mammary epithelial cells maintain glucose transport and antioxidant production; however, antioxidants appear to enhance anchorage-independent growth. These findings integrate aspects of glucose metabolism, anoikis suppression and antioxidant production in tumor cell biology and suggest that antioxidant therapy could stimulate tumor survival.

Activated Akt1 accelerates MMTV-c-ErbB2 mammary tumourigenesis in mice without activation of ErbB3

Introduction

ErbB2, a member of the epidermal growth factor receptor (EGFR) family, is overexpressed in 20% to 30% of human breast cancer cases and forms oncogenic signalling complexes when dimerised to ErbB3 or other EGFR family members.

Methods

We crossed mouse mammary tumour virus (MMTV)-myr-Akt1 transgenic mice (which express constitutively active Akt1 in the mammary gland) with MMTV-c-ErbB2 transgenic mice to evaluate the role of Akt1 activation in ErbB2-induced mammary carcinoma using immunoblot analysis, magnetic resonance spectroscopy and histological analyses.

Results

Bitransgenic MMTV-c-ErbB2, MMTV-myr-Akt1 mice develop mammary tumours twice as fast as MMTV-c-ErbB2 mice. The bitransgenic tumours were less organised, had more mitotic figures and fewer apoptotic cells. However, many bitransgenic tumours displayed areas of extensive necrosis compared with tumours from MMTV-c-ErbB2 mice. The two tumour types demonstrate dramatically different expression and activation of EGFR family members, as well as different metabolic profiles. c-ErbB2 tumours demonstrate overexpression of EGFR, ErbB2, ErbB3 and ErbB4, and activation/phosphorylation of both ErbB2 and ErbB3, underscoring the importance of the entire EGFR family in ErbB2-induced tumourigenesis. Tumours from bitransgenic mice overexpress the myr-Akt1 and ErbB2 transgenes, but there was dramatically less overexpression and phosphorylation of ErbB3, diminished phosphorylation of ErbB2, decreased level of EGFR protein and undetectable ErbB4 protein. There was also an observable attenuation in a subset of tyrosine-phosphorylated secondary signalling molecules in the bitransgenic tumours compared with c-ErbB2 tumours, but Erk was activated/phosphorylated in both tumour types. Finally, the bitransgenic tumours were metabolically more active as indicated by increased glucose transporter 1 (GLUT1) expression, elevated lactate production and decreased intracellular glucose (suggesting increased glycolysis).

Conclusion

Expression of activated Akt1 in MMTV-c-ErbB2 mice accelerates tumourigenesis with a reduced requirement for signalling through the EGFR family, as well as a reduced requirement for a subset of downstream signaling molecules with a metabolic shift in the tumours from bitransgenic mice. The reduction in signalling downstream of ErbB2 when Akt is activated suggest a possible mechanism by which tumour cells can become resistant to ErbB2-targeted therapies, necessitating therapies that target oncogenic signalling events downstream of ErbB2.

Sugar and fat - that's where it's at: metabolic changes in tumors

Tumor cells exhibit an altered metabolism, characterized by increased glucose uptake and elevated glycolysis, which was first recognized by Otto Warburg 70 years ago. Warburg originally hypothesized that these metabolic changes reflected damage to mitochondrial oxidative phosphorylation. Although hypoxia and hypoxia inducible factor can induce transcriptional changes that stimulate glucose transport and glycolysis, it is clear that these changes can occur in cultured tumor or transformed cells cultured under normoxic conditions, and thus there must be genetic alterations independent of hypoxia that can stimulate aerobic glycolysis. In recent years it has become clear that loss of p53 and activation of Akt can induce all or part of the metabolic changes reflected in the Warburg effect. Likewise, changes in expression of lactate dehydrogenase and other glycolytic control enzymes can contribute to increased or altered glycolysis. It is also clear that changes in lipid biosynthesis occur in tumor cells to support increased membrane biosynthesis and perhaps the altered energy needs of the cells. Changes in fatty acid synthase, Spot 14, Akt, and DecR1 (2,4-dienoylcoenzyme A reductase) may underlie altered lipid metabolism in tumor cells and contribute to the ability of tumor cells to proliferate or metastasize. Although these advances provide new therapeutic targets that merit exploration, there remain critical questions to be explored at the mechanistic level; this work may yield insights into tumor cell biology and identify additional therapeutic targets.

Mechanistic and pharmacodynamic studies of a 25-hydroxyvitamin D3 derivative in prostate cancer cells

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the biologically active form of vitamin D has strong antiproliferative effects in cancer cells. But it is highly toxic at therapeutic doses. We have observed that 25-hydroxyvitamin D(3)-3-bromoacetate (25-OH-D(3)-3-BE), a derivative of 25-hydroxyvitamin D(3), the pro-hormonal form of 1,25(OH)(2)D(3) has strong growth-inhibitory and proapoptotic properties in hormone-sensitive and hormone-refractory prostate cancer cells. In the present investigation we demonstrate that the antiproliferative effect of 25-OH-D(3)-3-BE is predominantly mediated by VDR in ALVA-31 prostate cancer cells. In other mechanistic studies we show that the proapoptotic property of 25-OH-D(3)-3-BE is related to the inhibition of phosphorylation of Akt, a pro-survival protein. Furthermore, we carried out cellular uptake and serum stability studies of 25-OH-D(3)-3-BE to demonstrate potential therapeutic applicability of 25-OH-D(3)-3-BE in hormone-sensitive and hormone-insensitive prostate cancer.

Extragenic suppressors of the nimX2(cdc2) mutation of Aspergillus nidulans affect nuclear division, septation and conidiation

The Aspergillus nidulans NIMX(CDC2) protein kinase has been shown to be required for both the G(2)/M and G(1)/S transitions, and recent evidence has implicated a role for NIMX(CDC2) in septation and conidiation. While much is understood of its G(2)/M function, little is known about the functions of NIMX(CDC2) during G(1)/S, septation, and conidiophore development. In an attempt to better understand how NIMX(CDC2) is involved in these processes, we have isolated four extragenic suppressors of the A. nidulans nimX2(cdc2) temperature-sensitive mutation. Mutation of these suppressor genes, designated snxA-snxD for suppressor of nimX, affects nuclear division, septation, and conidiation. The cold-sensitive snxA1 mutation leads to arrest of nuclear division during G(1) or early S. snxB1 causes hyperseptation in the hyphae and sensitivity to hydroxyurea, while snxC1 causes septation in the conidiophore stalk and aberrant conidiophore structure. snxD1 leads to slight septation defects and hydroxyurea sensitivity. The additional phenotypes that result from the suppressor mutations provide genetic evidence that NIMX(CDC2) affects septation and conidiation in addition to nuclear division, and cloning and biochemical analysis of these will allow a better understanding of the role of NIMX(CDC2) in these processes.

The distribution and origin of the calretinin-containing innervation of the nucleus accumbens of the rat

The nucleus accumbens of the rat consists of several subregions that can be distinguished on the basis of histochemical markers. For example, the calcium-binding protein calbindin D28k is a useful marker of the core compartment of the nucleus accumbens. Calretinin, another calcium-binding protein, is found in a dense fibre plexus in the accumbal shell and septal pole regions. The source of the accumbal calretinin innervation is not known. We examined the distribution of calretinin in the nucleus accumbens and used tract-tracing and lesion methods to determine the source of this calretinin innervation. Intense calretinin immunoreactivity was present in the medial shell, but the density of calretinin axons diminished sharply in the ventrolateral shell. Regions of dense calretinin immunostaining and those areas with calbindin-like immunoreactive cell bodies were generally segregated in the nucleus accumbens, although some overlap in the transition region between the core and shell was seen. Small clusters of calretinin-immunoreactive fibres were seen in the core, where they were restricted to calbindin-negative patches. Injections of the anterograde tracer biotinylated dextran amine into the paraventricular thalamic nucleus labelled fibres in calretinin-rich regions of the accumbens. Conversely, injections of Fluoro-gold into the accumbal shell retrogradely labelled numerous cells in the paraventricular thalamic nucleus that were calretinin-immunoreactive. Electrolytic lesions of the paraventricular thalamic nucleus reduced calretinin levels in the shell by approximately 80%. These data indicate that the calretinin innervation of the nucleus accumbens is derived primarily from the thalamic paraventricular nucleus, and marks accumbal territories that are largely complementary to those defined by calbindin.

The impact of race on freedom from prostate-specific antigen failure in prostate cancer patients treated with definitive radiation therapy

Many studies have reported that African-American men have the highest incidence and mortality rates for prostate cancer in the United States. A retrospective analysis of 607 patients treated with definitive radiation therapy was performed at the University of California San Francisco and its affiliated hospitals between 1987 and 1995. The patient population analyzed included African-American, Caucasian, and Asian men with AJCC T1-T3 disease. Race, Gleason score, pretreatment prostate-specific antigen levels, stage, and treatment delivery were all evaluated. The percent free from PSA failure at 48 months for African-American, Caucasian, and Asian men were 53%, 59%, and 53%, respectively. There was no difference among the three races or for any of the pairwise comparisons. Gleason score and stage of disease were each independent predictors of outcome, but race was not associated with remaining free from PSA failure. These results are similar to those recently reported in the literature from centers of excellence across the United States.

Clozapine pretreatment modifies haloperidol-elicited forebrain Fos induction: a regionally-specific double dissociation

RATIONALE

Acute administration of typical antipsychotic drugs, such as haloperidol, results in the induction of the immediate-early gene c-fos in the dorsolateral striatum. In contrast, the atypical antipsychotic drug clozapine, which lacks significant extrapyramidal side effect liability, does not induce Fos protein in the dorsal striatum. Several studies have attempted to define the mechanisms through which typical antipsychotic drugs induce striatal Fos, often by pretreating animals with specific receptor antagonists. Despite the broad receptor profile of clozapine, there has been no study of the effect of clozapine pretreatment on haloperidol-elicited striatal Fos expression.

METHODS

We examined the effects of clozapine pretreatment of rats on haloperidol-elicited forebrain Fos expression, using both immunoblot and immunohistochemical methods. The effects of clozapine pretreatment were assessed in the dorsal striatum and in the different nucleus accumbens compartments, the septum, and the prefrontal cortex.

RESULTS

Clozapine pretreatment markedly decreased haloperidol-elicited striatal Fos induction and blocked haloperidol-induced catalepsy. Clozapine also attenuated haloperidol-elicited Fos expression in the nucleus accumbens, but in the prefrontal cortex and ventrolateral septum the effects of haloperidol and clozapine were additive.

CONCLUSIONS

An emerging body of literature suggests a high incidence of rapid relapse in schizophrenic patients when clozapine treatment is discontinued. This psychosis is relatively resistant to haloperidol and other neuroleptics, even in patients who had previously responded well to neuroleptics. The present data may shed light on the central sites associated with and perhaps model certain aspects of the relapse associated with clozapine discontinuation.

Psychostimulant-induced Fos protein expression in the thalamic paraventricular nucleus

Lesions of glutamatergic afferents to the nucleus accumbens have been reported to block psychostimulant-induced behavioral sensitization. However, thalamic glutamatergic projections to the nucleus accumbens have received little attention in the context of psychostimulant actions. We examined the effects of acute amphetamine and cocaine administration on expression of Fos protein in the thalamic paraventricular nucleus (PVT), which provides glutamatergic inputs to the nucleus accumbens and also receives dopaminergic afferents. Immunoblot and immunohistochemical studies revealed that both psychostimulants dose-dependently increased PVT Fos expression. PVT neurons retrogradely labeled from the nucleus accumbens were among the PVT cells that showed a Fos response to amphetamine. D2 family dopamine agonists, including low doses of the D3-preferring agonist 7-OH-DPAT, increased the numbers of Fos-like-immunoreactive neurons in the PVT. Conversely, the effects of cocaine and amphetamine on PVT Fos expression were blocked by pretreatment with the dopamine D2/3 antagonist raclopride. Because PVT neurons express D3 but not other dopamine receptor transcripts, it appears that psychostimulants induce Fos in PVT neurons through a D3 dopamine receptor. We suggest that the PVT may be an important part of an extended circuit subserving both the arousing properties and reinforcing aspects of psychostimulants.