Metformin-induced downregulation of c-Met is a determinant of sensitivity in MDA-MB-468 breast cancer cells

Metformin, the widely used anti-diabetic drug, is emerging as a promising anti-cancer agent. However, response variation among different tumors remains a significant challenge. Hence, identification of the factors that determine metformin sensitivity is of greatest significance for its clinical implementation. In this study, we showed that MDA-MB-468 cells were most sensitive among the five breast cancer cell lines tested. We found that metformin-induced inhibition of MDA-MB-468 cells was correlated with downregulation of c-Met at both protein and mRNA levels. To understand the functional significance of c-Met downregulation in metformin-mediated tumor inhibition, we established control and c-Met overexpressing sublines of MDA-MB-468 cells (468/C and 468/Met) using lentiviral expression system. We demonstrated that overexpression of c-Met significantly attenuated metformin induced inhibition of MDA-MB-468 cells. Metformin-induced inhibition of ALDH1+ cells, which are enriched with cancer stem cells, was also abrogated in 468/Met cells as compared to 468/C cells. Signal transduction analysis of the paired cell lines indicated that c-Met-induced activation of STAT3 and AKT1, and upregulation of Gab1 are related to c-Met-modulated metformin responsiveness. These findings highlight c-Met as a potential key regulator of metformin-mediated inhibition of proliferation and stemness of breast cancer cells, indicating that c-Met overexpression may be a critical factor contributing to metformin resistance. The data also suggest that combination of metformin with c-Met inhibitors could be a useful strategy to improve metformin-mediated anti-cancer efficacies in breast cancer treatment.

Abstract 1123: c-Met downregulation contributes to Metformin-induced inhibition of MDA-MB-468 triple negative breast cancer cells

Triple negative breast cancer (TNBC) is the breast cancer subtype that is most refractory to treatment. Thus, identification of novel molecular targets and drug treatments are needed to improve prognosis for patients. The widely used anti-diabetic drug Metformin has been reported to possess anti-cancer properties. Although it is believed that Metformin may exert its anti-cancer effects through indirect reduction of glucose and insulin levels, and directly activating the AMPK kinase and inhibiting downstream cancer-related pathways, the molecular mechanisms underlying its anti-cancer function in TNBC have yet to be fully elucidated. In this study, we investigated the role and mechanism of Metformin-mediated downregulation of oncogenic c-Met in MDA-MB-468 TNBC cells. Our results demonstrated greater sensitivity of MDA-MB-468 cells to Metformin compared to other breast cancer cell lines. Metformin-induced inhibition of MDA-MB-468 cell proliferation was correlated with the downregulation of c-Met at both protein and mRNA levels. Overexpression of c-Met in MDA-MB-468 cells using a lentiviral vector system rendered the cells resistant to Metformin, which was accompanied by the promotion of cancer stem cell stemness. Functionally, we demonstrated that c-Met induced activation of receptor tyrosine kinase (RTK) signaling pathways play a critical role in c-Met-modulated Metformin responsiveness. These novel findings highlight c-Met as a key regulator of Metformin-mediated inhibition of TNBC cell growth, suggesting that c-Met overexpression may be a critical factor contributing to metformin resistance. To this end, combination of Metformin with c-Met inhibitors could be a useful strategy to improve Metformin-mediated anti-cancer efficacies in TNBC.

Citation Format: Dana M. Gant, Zhikun Ma, Amanda B. Parris, Xiaohe Yang. c-Met downregulation contributes to Metformin-induced inhibition of MDA-MB-468 triple negative breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1123.

FGFR1 overexpression renders breast cancer cells resistant to metformin through activation of IRS1/ERK signaling

Metformin has been suggested as an anti-cancer agent. However, increasing reports show that some tumors are resistant to metformin. Identification of factors affecting metformin mediated cancer therapy is of great significance. FGFR1 is a receptor-tyrosine-kinase that is frequently overexpressed in breast cancer, which is associated with poor-prognosis. To investigate the effect of FGFR1 overexpression on metformin-induced inhibition of breast cancer cells, we demonstrated that FGFR1 overexpression rendered MCF-7 and T47D cells resistant to metformin. In particular, we found that, in addition to AKT and ERK1/2 activation, FGFR1-induced activation of IRS1 and IGF1R, key regulators connecting metabolism and cancer, was associated with metformin resistance. Targeting IRS with IRS1 KO or IRS inhibitor NT157 significantly sensitized FGFR1 overexpressing cells to metformin. Combination of NT157 with metformin induced enhanced inhibition of p-IGF1R, p-ERK1/2 and p-mTOR. Moreover, we demonstrated that IRS1 functions as a critical mediator of the crosstalk between FGFR1 and IGF1R pathways, which involves a feedback loop between IRS1 and MAPK/ERK. Our study highlights the significance of FGFR1 status and IRS1 activation in metformin-resistance, which will facilitate the development of strategies targeting FGFR overexpression-associated metformin resistance.

Abstract A27: Bisphenol S induces proestrogenic effect in vitro and modifies mammary development dynamics after in utero exposure

Due to the growing health concern of bisphenol A (BPA), a popular endocrine-disrupting compound (EDC), the FDA has banned its use in the production of baby bottles. Bisphenol S (BPS) is a substitute of BPA and was believed to be less estrogenic/toxic, which has been widely used in the production of many household products. However, recent studies suggest that BPS exhibits proestrogenic/proliferative effects similar to BPA. Whether/how BPS exposure impacts human health at various stages, especially in children/adolescents, is emerging as a new public health concern. In this study, we investigated the in vitro and in vivo effect of BPS on breast cancer cell proliferation and signaling and mouse mammary development. Results from MTT and clonogenic assays using MCF-7 and T47D cells, two ER+ breast cancer cell lines, indicated that BPS at 0.5–5 μM promoted cell proliferation. Cell cycle analysis showed that BPS at 1 and 5 μM significantly increased the percentage of cells in S phase and decreased cells in G0/G1 phase. Luciferase reporter assays also indicated that BPS potently activated estrogen response element (ERE)-mediated transcription. Analysis of signal transduction demonstrated that PBS induced the upregulation and activation/phosphorylation of ERα, erbB3, Akt, and Erk/12. Examination of the mRNA levels of key regulators in ER signaling and growth regulation showed that BPS induced significant upregulation of AREG, MYC, IGF1R/2R, and CCND1. We also demonstrated that BPS at 1 μM attenuated tamoxifen-mediated growth inhibition of MCF-7 and T47D cells. These data demonstrate that BPS is a potent endocrine disruptor. To test the in vivo effect of in utero exposure to BPS on pubertal mammary development, MMTV-erbB2 transgenic mice were exposed to BPS via drinking water at doses of 0, 15, and 30 μM BPS/L between gestation day 10 to day 19. Mammary glands of female offspring at week 5 and week 10 of age were analyzed for morphogenesis, colony formation cell (CFC) assay, and mammary epithelial subpopulations. BPS-treated mammary tissues at 5 weeks of age displayed decreased ductal elongation and CFC numbers. At 10s week of age, mammary tissues with 30 μM BPS/L exposure displayed more complex ductal trees, CFC numbers, and increased CD24high/CD49fhigh and CD61high/CD49fmid cells, which are enriched with putative mammary stem/progenitor subpopulations. Overall, we demonstrated that BPS has a proestrogenic effect that may promote the growth of ER+ breast cancer cells. In vivo results demonstrated that in utero exposure to BPS modifies the dynamics of mammary development. The health impact of BPS exposure on mammary development warrants further investigation.

Citation Format: Qingxia Zhao, Amanda B. Parris, Zhikun Ma, Xiaohe Yang. Bisphenol S induces proestrogenic effect in vitro and modifies mammary development dynamics after in utero exposure [abstract]. In: Proceedings of the AACR Special Conference on Environmental Carcinogenesis: Potential Pathway to Cancer Prevention; 2019 Jun 22-24; Charlotte, NC. Philadelphia (PA): AACR; Can Prev Res 2020;13(7 Suppl): Abstract nr A27.

In Utero Exposure to Bisphenol a Promotes Mammary Tumor Risk in MMTV-Erbb2 Transgenic Mice Through the Induction of ER-erbB2 Crosstalk

Bisphenol A (BPA) is the most common environmental endocrine disrupting chemical. Studies suggest a link between perinatal BPA exposure and increased breast cancer risk, but the underlying mechanisms remain unclear. This study aims to investigate the effects of in utero BPA exposure on mammary tumorigenesis in MMTV-erbB2 transgenic mice. Pregnant mice were subcutaneously injected with BPA (0, 50, 500 ng/kg and 250 µg/kg BW) daily between gestational days 11–19. Female offspring were examined for mammary tumorigenesis, puberty onset, mammary morphogenesis, and signaling in ER and erbB2 pathways. In utero exposure to low dose BPA (500 ng/kg) induced mammary tumorigenesis, earlier puberty onset, increased terminal end buds, and prolonged estrus phase, which was accompanied by proliferative mammary morphogenesis. CD24/49f-based FACS analysis showed that in utero exposure to 500 ng/kg BPA induced expansion of luminal and basal/myoepithelial cell subpopulations at PND 35. Molecular analysis of mammary tissues at PND 70 showed that in utero exposure to low doses of BPA induced upregulation of ERα, p-ERα, cyclin D1, and c-myc, concurrent activation of erbB2, EGFR, erbB-3, Erk1/2, and Akt, and upregulation of growth factors/ligands. Our results demonstrate that in utero exposure to low dose BPA promotes mammary tumorigenesis in MMTV-erbB2 mice through induction of ER-erbB2 crosstalk and mammary epithelial reprogramming, which advance our understanding of the mechanism associated with in utero exposure to BPA-induced breast cancer risk. The studies also support using MMTV-erbB2 mouse model for relevant studies.

Abstract 5071: Palbociclib inhibits the stemness of mammary epithelial cells in premalignant tissues of MMTV-erbB2 transgenic mice

Palbociclib is the first CDK4/6-Cyclin D inhibitor approved by the FDA for ER+/Her2(erbB2)-breast cancer treatment. With the success of the drug in hormone-positive breast cancer, it is encouraging to explore more options of Palbociclib as an anticancer agent. erbB2 is a receptor tyrosine kinase (RTK) frequently overexpressed in breast cancer. The molecular hallmarks of erbB2+ breast cancer include the upregulation of cyclin D. The aim of this study was to explore the potential of Palbociclib as a preventive agent using MMTV-erbB2 transgenic mice, with a focus on the stemness of mammary epithelial cells (MECs) in the premalignant tissues. We first tested the in vivo effect of Palbociclib on tumor cells on MMTV-erbB2 mammary tumors using syngeneic tumor graft models. The animals bearing grafted tumors were treated with saline (control), low (75 mg/kg/day) and high (150 mg/kg/day) Palbociclib via oral gavage every 3 days for 29 days. We found that Palbociclib significantly inhibited tumor growth in a concentration dependent manner. To test the effect of Palbociclib on MEC stemness in premalignant mammary tissues, the animals at 9 wks of age were treated with saline, low and high (0, 75, 150 mg/kg/day) doses of Palbociclib every 3 days for 4 wks. We found the efficiency of both primary and secondary mammosphere formation of MECs from drug treated mice was significantly inhibited, suggesting the inhibition of mammary stemness in the premalignant tissues. Consistently, results from 3D cultures showed that colony numbers of the cells from drug treated mice were also lower than the control. Using CD24 and CD49f as markers, flow cytometry was performed to analyze the effect of Palbociclib on MEC subpopulations, including luminal, basal, and stromal populations. We showed that the percentage of luminal cells and putative mammary reconstitution units (MRUs) in drug treated mice was significantly inhibited. We also found that the percentage of CD61+/CD49+ cells, which are enriched with luminal progenitor cells and the origin of tumor initiation cells of this model, in Palbociclib treated tissues were also inhibited. The data suggest that Palbociclib induces MEC reprogramming in the premalignant tissues. Moreover, we found that the protein levels of pRb, E2F1, cyclin D1, c-myc, and cdc2 in cell cycle regulation, and pAKT, pERK, pmTOR, and p4EBP1 in the RTK pathway, pER, and DVL2 and LRP6 in the Wnt pathway were significantly downregulated in Palbociclib treated tissues, which was more evident in the high dose group. Taken together, our data indicate that Palbociclib inhibits MEC proliferation and stemness in the premalignant mammary tissues. Downregulation of cell cycle regulators and the signaling in ER, RTK, and Wnt pathway plays a critical role in this process. Our data support further investigation of Palbociclib in the prevention of ER+/ErbB2+ breast cancer.

Citation Format: Amanda B. Parris, Yongxuan Liu, Zhikun Ma, Erin W. Howard, Xiaoshan Feng, Xiaohe Yang. Palbociclib inhibits the stemness of mammary epithelial cells in premalignant tissues of MMTV-erbB2 transgenic mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5071.

Abstract 3009: FGFR1 overexpression ER positive breast cancer cells confer the cells resistant to Palbociclib via upregulation of RTK-ER signaling

Palbociclib is the first CDK4/6 inhibitor approved by the FDA for treating ER+/HER2- breast cancer. Palbociclib resistance is emerging as a clinical challenge. Identification of the factors that determine the sensitivity to Palbociclib is of pivotal significance. Fibroblast Growth Factor Receptor (FGFR) overexpression is frequently detected in breast cancer, which has been associated with poor prognosis. To investigate the impact of FGFR overexpression on Palbociclib resistance, we first established stable sublines of MCF7 and T47D cells that overexpress FGFR1, and examined their responses to Palbociclib. Data from MTT and clonogenic assays demonstrated that overexpression of FGFR1 rendered ER+ MCF7 and T47D cells resistant to Palbociclib significantly. Cell cycle analysis showed that the percentage of cells in S phase in Palbociclib treated MCF7/FGFR1 and T47D/FGFR1 cells was significantly higher than that of the control. FGFR overexpression also significantly attenuated Palbociclib-associated inhibition of cancer cell invasion. We further found that Palbociclib-induced inhibition of cancer cell stemness, as indicated by colony formation in soft agar and mammosphere assays, was significantly reduced in MCF7/FGFR1 and T47D/FGFR1 cells, as compared to control. The data indicate that FGFR1 overexpression in ER+ breast cancer cells are resistant to Palbociclib. We next examined the signaling of the key pathways in the paired cell lines treated with Palbociclib. We found that Palbociclib-induced inhibition of phospho-Rb, E2F, cyclin D3, cyclin B1, cdc-2, c-myc, and cdc-25 was significantly attenuated in MCF-7/FGFR1 and T47D/FGFR1 cells. The protein levels of p-ERK, p-AKT, and p-p38 in MCF-7/FGFR1 and T47D/FGFR1 cells were significantly higher than the control cells with/without Palbociclib treatment. The results also showed that ER alpha phosphorylation, estrogen response element (ERE)-luciferase activity and mRNA levels of ER target genes, including PS2, c-myc, E2F, FGF2 and FGFR2, in MCF-7/FGFR1 and T47D/FGFR1 cells were maintained at higher levels post Palbociclib, as compared to control. We then treated control and FGFR1 overexpressing MCF-7 and T47D cells with Palbociclib in combination with FGFR inhibitor, AZD 4547. The combined drug treatment resulted in remarkably synergistic effect on MCF-7/FGFR1 and T47D/FGFR1 cells, as indicated by MTT and clonogenic assays. Key markers in cell cycle regulation, RTK signaling and ER pathway were significantly inhibited in the cells treated with both Palbociclib and AZD 4547. Taken together, our results demonstrated that FGFR1 overexpression is a critical factor affecting Palbociclib sensitivity. The underlying mechanisms involve FGFR1 overexpression-associated RTK and ER signaling. Combination of Palbociclib with FGFR targeting agents may overcome FGFR-associated Palbociclib resistance.

Citation Format: Yujie Shi, Zhikun Ma, Yudan Wu, Amanda B. Parris, Qiong Cheng, Lingfei Kong, Xiaohe Yang. FGFR1 overexpression ER positive breast cancer cells confer the cells resistant to Palbociclib via upregulation of RTK-ER signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3009.

Bisphenol AF promotes estrogen receptor-positive breast cancer cell proliferation through amphiregulin-mediated crosstalk with receptor tyrosine kinase signaling

Exposure to bisphenol A (BPA), an endocrine-disrupting compound, is associated with increased risk of estrogen-related diseases, including estrogen receptor-positive (ER⁺) breast cancer. Although bisphenol analogs, i.e. bisphenol AF (BPAF), have replaced BPA in industrial settings, increasing data indicate that these alternatives may have similar or even more potent estrogenic effects. As such, BPAF exhibits increased ER binding affinities than BPA in biochemical assays. However, preclinical studies exploring the effects of BPAF on ER⁺ breast cancer are missing mechanistic data. Thus, we aimed to characterize the effects of BPAF on MCF-7 and T47D ER⁺ breast cancer cells with mechanistic insight. We found that BPAF promoted cell growth and cell cycle progression concurrently with BPAF-induced ERα transcriptional activity and ER-RTK signaling activation. ER signaling blockage revealed that BPAF-induced cell proliferation and ER-RTK crosstalk were ER-dependent. Gene expression data demonstrated that AREG is a sensitive target of BPAF in our in vitro models. Importantly, we determined that AREG upregulation is necessary for BPAF-induced cellular responses. Ultimately, our novel finding that AREG mediates BPAF-induced ER-RTK crosstalk in ER⁺ breast cancer cells supports future studies to characterize the impact of BPAF on human ER⁺ breast cancer risk and to assess the safety profile of BPAF.

Caloric restriction inhibits mammary tumorigenesis in MMTV-ErbB2 transgenic mice through the suppression of ER and ErbB2 pathways and inhibition of epithelial cell stemness in premalignant mammary tissues

Caloric intake influences the onset of many diseases, including cancer. In particular, caloric restriction (CR) has been reported to suppress mammary tumorigenesis in various models. However, the underlying cancer preventive mechanisms have not been fully explored. To this end, we aimed to characterize the anticancer mechanisms of CR using MMTV-ErbB2 transgenic mice, a well-established spontaneous ErbB2-overexpressing mammary tumor model, by focusing on cellular and molecular changes in premalignant tissues. In MMTV-ErbB2 mice with 30% CR beginning at 8 weeks of age, mammary tumor development was dramatically inhibited, as exhibited by reduced tumor incidence and increased tumor latency. Morphogenic mammary gland analyses in 15- and 20-week-old mice indicated that CR significantly decreased mammary epithelial cell (MEC) density and proliferative index. To understand the underlying mechanisms, we analyzed the effects of CR on mammary stem/progenitor cells. Results from fluorescence-activated cell sorting analyses showed that CR modified mammary tissue hierarchy dynamics, as evidenced by decreased luminal cells (CD24highCD49flow), putative mammary reconstituting unit subpopulation (CD24highCD49fhigh) and luminal progenitor cells (CD61highCD49fhigh). Mammosphere and colony-forming cell assays demonstrated that CR significantly inhibited mammary stem cell self-renewal and progenitor cell numbers. Molecular analyses indicated that CR concurrently inhibited estrogen receptor (ER) and ErbB2 signaling. These molecular changes were accompanied by decreased mRNA levels of ER-targeted genes and epidermal growth factor receptor/ErbB2 family members and ligands, suggesting ER-ErbB2 signaling cross-talk. Collectively, our data demonstrate that CR significantly impacts ER and ErbB2 signaling, which induces profound changes in MEC reprogramming, and mammary stem/progenitor cell inhibition is a critical mechanism of CR-mediated breast cancer prevention.

DMBA promotes ErbB2‑mediated carcinogenesis via ErbB2 and estrogen receptor pathway activation and genomic instability

Environmental factors, including 7,12‑dimethylbenz[a]anthracene (DMBA) exposure, and genetic predisposition, including ErbB2 overexpression/amplification, have been demonstrated to increase breast cancer susceptibility. Although DMBA‑ and ErbB2‑mediated breast cancers are well‑studied in their respective models, key interactions between environmental and genetic factors on breast cancer risk remain unclear. Therefore, the present study aimed to investigate the effect of DMBA exposure on ErbB2‑mediated mammary tumorigenesis. MMTV‑ErbB2 transgenic mice exposed to DMBA (1 mg) via weekly oral gavage for 6 weeks exhibited significantly enhanced mammary tumor development, as indicated by reduced tumor latency and increased tumor multiplicity compared with control mice. Whole mount analysis of premalignant mammary tissues from 15‑week‑old mice revealed increased ductal elongation and proliferative index in DMBA‑exposed mice. Molecular analyses of premalignant mammary tissues further indicated that DMBA exposure enhanced epidermal growth factor receptor (EGFR)/ErbB2 and estrogen receptor (ER) signaling, which was associated with increased mRNA levels of EGFR/ErbB2 family members and ER‑targeted genes. Furthermore, analysis of tumor karyotypes revealed that DMBA‑exposed tumors displayed more chromosomal alterations compared with control tumors, implicating DMBA‑induced chromosomal instability in tumor promotion in this model. Together, the data suggested that DMBA‑induced deregulation of EGFR/ErbB2‑ER pathways plays a critical role in the enhanced chromosomal instability and promotion of ErbB2‑mediated mammary tumorigenesis. The study highlighted gene‑environment interactions that may increase risk of breast cancer, which is a critical clinical issue.

Abstract 903: Upregulation of DARPP32/PPP1R1B is a critical mediator of acquired lapatinib resistance in ErbB2-overexpressing breast cancer cells

ErbB2/Her2 overexpression is detected in approximately 30% of human breast cancers and is associated with poor prognosis. Lapatinib, a small molecule dual inhibitor targeting ErbB2 and EGFR, is FDA-approved for the treatment of ErbB2-positive (ErbB2+) advanced or metastatic breast cancer. However, lapatinib resistance (LR) is emerging as a critical issue in clinical oncology. To understand the molecular mechanisms of LR, we first developed a lapatinib-resistant cell line from ErbB2-overexpressing BT474 breast cancer cells by prolonged exposure of parental BT474 cells to gradually increasing concentrations of lapatinib (up to 8 µM). We performed microarray analyses on parental and LR cells to identify novel factors/pathways that contribute to LR. Our microarray data indicated that the gene expression profile of the BT474/LR cells is significantly different from the parental BT474 cells. We found that DARPP32/PPP1R1B is one of the most significantly upregulated genes in the BT474/LR cells. To investigate the role of DARPP32 in LR development, we examined the effect of DARPP32 knockdown via lentiviral shRNA on BT474/LR cell proliferation and apoptosis. Results from MTT, clonogenic, cell cycle, and cleaved caspase-3 and PARP analyses indicated that DARPP32 knockdown significantly sensitized BT474/LR cells to lapatinib. We also found that DARPP32 knockdown renders MDA-MB-361 breast cancer cells, which express high endogenous levels of DARPP32, more sensitive to lapatinib. These data demonstrate that DARPP32 overexpression contributes to LR. To understand the mechanism of DARPP32 overexpression-mediated LR, we focused on the regulation of transcription factor cAMP response element-binding protein (CREB) in this process. As such, DARPP32 overexpression was generally associated with CREB upregulation. We also found that CREB-mediated transcription was increased in BT474/LR cells, and DARPP32 knockdown in BT474/LR and MDA-MB-361 cells significantly downregulated CREB protein levels and CREB-mediated transcription, as indicated by luciferase reporter assays. CREB overexpression rendered BT474 cells resistant to lapatinib and reversed DARPP32 knockdown-induced sensitization in BT474/LR cells. In contrast, CREB knockdown sensitized the cells to lapatinib. Further analysis with MG132 blockage suggests that DARPP32 may regulate CREB protein levels through the proteasomal degradation pathway. Taken together, we have identified DARPP32 overexpression as a critical mediator of acquired lapatinib resistance. The underlying mechanisms involve the deregulation of CREB protein and activity levels. These novel findings advance our understanding of the molecular mechanisms of LR and provide fundamental support for testing the DARPP32-CREB axis in clinical settings for the management of LR breast cancers, which is of significant translational value.

Citation Format: Zhikun Ma, Erin W. Howard, Amanda B. Parris, Xiaohe Yang. Upregulation of DARPP32/PPP1R1B is a critical mediator of acquired lapatinib resistance in ErbB2-overexpressing breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 903.

Abstract P6-07-04: Rapamycin inhibits the stemness of mammary epithelial cells in the premalignant tissues of MMTV-ErbB2 transgenic mice

Rapamycin, a well-studied mTOR inhibitor, has been demonstrated to inhibit mammary carcinogenesis at multiple stages, including initiation, invasion, and metastasis, in preclinical animal models. Nevertheless, the cancer preventative potential and underlying mechanisms remain unclear, especially in individual breast cancer subtypes like ErbB2/Her2-positive breast cancers. ErbB2 amplification/overexpression is a particular clinical concern because it occurs in approximately one-third of human breast cancers and is associated with poor prognosis. Therefore, we used MMTV-ErbB2 transgenic mice as our model system to test the efficacy of rapamycin in the prevention of ErbB2-mediated mammary tumor development. Our initial data provided proof of concept regarding the anti-cancer effects of rapamycin in vivo. Indeed, rapamycin (1.5 mg/kg/day for 12 days) significantly reduced the volume and weight of syngeneic 78617 cell-derived mammary tumors in MMTV-ErbB2 mice, despite observed decreases in CD4+ and CD8+ immune cells. Since advanced mammary gland development can serve as an indicator of breast cancer risk, we investigated the effects of rapamycin on mammary gland development in MMTV-ErbB2 mice that were treated with low-dose rapamycin (1 mg/kg/day) between weeks 10 and 20 of age. As such, rapamycin significantly attenuated mammary morphogenesis at 20 weeks of age, as indicated by decreased branching density, ductal elongation, and proliferative index of the premalignant mammary glands. Flow cytometric analysis of isolated primary mammary epithelial cells (MECs) was performed using CD24 and CD49f markers to identify MEC populations. We found that rapamycin has a significant impact on MEC stemness based on changes in luminal (CD24highCD49flow), mammary stem cell (MaSC)-enriched (CD24highCD49fhigh), and myoepithelial/basal (CD24low/highCD49fhigh) MEC populations. We also used CD61 and CD49f markers to identify a population enriched with luminal progenitor cells (CD61highCD49fhigh) that was selectively inhibited by rapamycin. Consistent with our flow cytometric analyses, rapamycin inhibited the luminal progenitor cell-enriched population, self-renewal, and anchorage-independent cell growth of primary MECs, as demonstrated by colony-forming cell, mammosphere, and 3D culture assays, respectively. These functional stem cell assays further corroborate that rapamycin suppresses the stemness of primary MECs. Molecular analysis of MECs demonstrated that rapamycin inhibited mTOR signaling, as expected. Importantly, rapamycin also significantly suppressed the receptor tyrosine kinase/ErbB2, estrogen receptor, Wnt/β-catenin, and TGFβ/Smad3 signaling pathways prior to malignant transformation. Collectively, our study provides evidence that rapamycin has potential cancer preventative effects in the mammary glands of MMTV-ErbB2 mice during the premalignant risk window. These rapamycin-induced anti-cancer effects ultimately highlight the promising clinical significance of rapamycin for the prevention and treatment of human ErbB2-overexpressing breast cancers.

Citation Format: Yang X, Zhao Q, Parris AB, Howard EW, Zhao M, Guo Z, Xing Y, Ma Z. Rapamycin inhibits the stemness of mammary epithelial cells in the premalignant tissues of MMTV-ErbB2 transgenic mice [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-07-04.

FGFR inhibitor, AZD4547, impedes the stemness of mammary epithelial cells in the premalignant tissues of MMTV-ErbB2 transgenic mice

The fibroblast growth factor receptor (FGFR) family of receptor tyrosine kinases (RTKs) regulates signaling pathways involved in cell proliferation and differentiation. Currently, the anti-tumor properties of FGFR inhibitors are being tested in preclinical and clinical studies. Nevertheless, reports on FGFR inhibitor-mediated breast cancer prevention are sparse. In this study, we investigated the anti-cancer benefits of AZD4547, an FGFR1-3 inhibitor, in ErbB2-overexpressing breast cancer models. AZD4547 (1-5 µM) demonstrated potent anti-proliferative effects, inhibition of stemness, and suppression of FGFR/RTK signaling in ErbB2-overexpressing human breast cancer cells. To study the in vivo effects of AZD4547 on mammary development, mammary epithelial cell (MEC) populations, and oncogenic signaling, MMTV-ErbB2 transgenic mice were administered AZD4547 (2-6 mg/kg/day) for 10 weeks during the 'risk window' for mammary tumor development. AZD4547 significantly inhibited ductal branching and MEC proliferation in vivo, which corroborated the in vitro anti-proliferative properties. AZD4547 also depleted CD24/CD49f-sorted MEC populations, as well as the CD61^highCD49f^high tumor-initiating cell-enriched population. Importantly, AZD4547 impaired stem cell-like characteristics in primary MECs and spontaneous tumor cells. Moreover, AZD4547 downregulated RTK, mTOR, and Wnt/β-catenin signaling pathways in premalignant mammary tissues. Collectively, our data provide critical preclinical evidence for AZD4547 as a potential breast cancer preventative and therapeutic agent.

Phenformin inhibits growth and epithelial-mesenchymal transition of ErbB2-overexpressing breast cancer cells through targeting the IGF1R pathway

Reports suggest that metformin, a popular anti-diabetes drug, prevents breast cancer through various systemic effects, including insulin-like growth factor receptor (IGFR) regulation. Although the anti-cancer properties of metformin have been well-studied, reports on a more bioavailable/potent biguanide, phenformin, remain sparse. Phenformin exerts similar functional activity to metformin and has been reported to impede mammary carcinogenesis in rats. Since the effects of phenformin on specific breast cancer subtypes have not been fully explored, we used ErbB2-overexpressing breast cancer cell and animal models to test the anti-cancer potential of phenformin. We report that phenformin (25-75 μM) decreased cell proliferation and impaired cell cycle progression in SKBR3 and 78617 breast cancer cells. Reduced tumor size after phenformin treatment (30 mg/kg/day) was demonstrated in an MMTV-ErbB2 transgenic mouse syngeneic tumor model. Phenformin also blocked epithelial-mesenchymal transition, decreased the invasive phenotype, and suppressed receptor tyrosine kinase signaling, including insulin receptor substrate 1 and IGF1R, in ErbB2-overexpressing breast cancer cells and mouse mammary tumor-derived tissues. Moreover, phenformin suppressed IGF1-stimulated proliferation, receptor tyrosine kinase signaling, and epithelial-mesenchymal transition markers in vitro. Together, our study implicates phenformin-mediated IGF1/IGF1R regulation as a potential anti-cancer mechanism and supports the development of phenformin and other biguanides as breast cancer therapeutics.

Activation of cancerous inhibitor of PP2A (CIP2A) contributes to lapatinib resistance through induction of CIP2A-Akt feedback loop in ErbB2-positive breast cancer cells

Lapatinib, a small molecule ErbB2/EGFR inhibitor, is FDA-approved for the treatment of metastatic ErbB2-overexpressing breast cancer; however, lapatinib resistance is an emerging clinical challenge. Understanding the molecular mechanisms of lapatinib-mediated anti-cancer activities and identifying relevant resistance factors are of pivotal significance. Cancerous inhibitor of protein phosphatase 2A (CIP2A) is a recently identified oncoprotein that is overexpressed in breast cancer. Our study investigated the role of CIP2A in the anti-cancer efficacy of lapatinib in ErbB2-overexpressing breast cancer cells. We found that lapatinib concurrently downregulated CIP2A and receptor tyrosine kinase signaling in ErbB2-overexpressing SKBR3 and 78617 cells; however, these effects were attenuated in lapatinib-resistant (LR) cells. CIP2A overexpression rendered SKBR3 and 78617 cells resistant to lapatinib-induced apoptosis and growth inhibition. Conversely, CIP2A knockdown via lentiviral shRNA enhanced cell sensitivity to lapatinib-induced growth inhibition and apoptosis. Results also suggested that lapatinib downregulated CIP2A through regulation of protein stability. We further demonstrated that lapatinib-induced CIP2A downregulation can be recapitulated by LY294002, suggesting that Akt mediates CIP2A upregulation. Importantly, lapatinib induced differential CIP2A downregulation between parental BT474 and BT474/LR cell lines. Moreover, CIP2A shRNA knockdown significantly sensitized the BT474/LR cells to lapatinib. Collectively, our results demonstrate that CIP2A is a molecular target and resistance factor of lapatinib with a critical role in lapatinib-induced cellular responses, including the inhibition of the CIP2A-Akt feedback loop. Further investigation of lapatinib-mediated CIP2A regulation will advance our understanding of lapatinib-associated anti-tumor activities and drug resistance.

Ganetespib induces G2/M cell cycle arrest and apoptosis in gastric cancer cells through targeting of receptor tyrosine kinase signaling

Heat shock protein 90 (HSP90) regulates several important cellular processes via its repertoire of 'client proteins'. These client proteins have been found to play fundamental roles in signal transduction, cell proliferation, cell cycle progression and survival, as well as other features of malignant cells, such as invasion, tumor angiogenesis and metastasis. Thus, HSP90 is an emerging target for cancer therapy. To this end, we evaluated ganetespib (STA-9090), a novel and potent HSP90 inhibitor, for its activity in gastric cancer cell lines. Ganetespib significantly inhibited the proliferation of AGS and N87 human gastric cancer cell lines and potently induced G2/M cell cycle arrest and apoptosis. Upregulation of cleaved poly(ADP-ribose) polymerase (c-PARP), c-caspase-3, c-caspase-8 and c-caspase-9 and suppression of gastric cancer‑associated HSP90 client proteins, including ErbB2, Erk, Akt, mTOR, GSK3 and Src, were observed in ganetespib-treated cells. These findings demonstrate that the ganetespib-induced mechanism of cell growth inhibition involves the activation of death receptor and mitochondrial pathways and the inhibition of receptor tyrosine kinase signaling pathways. Our study implicates ganetespib as a potential strategy for gastric cancer treatment, which warrants further preclinical and clinical investigation.

Abstract 1908: FGFR inhibitor, AZD4547, impedes the stemness of mammary epithelial cells in the premalignant tissues of MMTV-ErbB2 transgenic mice

The fibroblast growth factor receptor (FGFR) family (FGFR1-4) of receptor tyrosine kinases (RTKs) regulates signaling pathways involved in cell proliferation and differentiation. In particular, FGFR1 and FGFR2, which are found in the terminal end buds of developing mammary ducts, play a role in mammary development and glandular morphogenesis involving the regulation of mammary stem cells (MaSCs) in mice. As such, a number of FGFR inhibitors are being tested in preclinical studies and clinical trials for anti-tumor properties. Nevertheless, reports on FGFR inhibitor-mediated breast cancer prevention are sparse. In this study, we aimed to investigate the anti-cancer benefits of AZD4547, a small molecule inhibitor of FGFR1-3, on ErbB2-overexpressing breast cancer models. We particularly focus on the effects of AZD4547 on MaSCs and tumor-initiating cells (TICs) in the premalignant tissues of MMTV-ErbB2 transgenic mice. We first demonstrated the anti-proliferative effects of AZD4547 (1-5 µM) on human ErbB2-overexpressing breast cancer cell lines. We further showed that AZD4547 confers potent inhibition of the stemness of these breast cancer cells, as indicated by significant depletion of ALDH+ cells and impaired tumorsphere formation. To study the in vivo effects of AZD4547 on the stemness of mammary epithelial cells (MECs), MMTV-ErbB2 transgenic mice were administered AZD4547 (2-6 mg/kg/day) for 10 weeks (weeks 8-18 of age) during the ‘risk window’ for mammary tumor development. Histopathological analysis indicated that AZD4547 significantly inhibited ductal branching and MEC proliferation. To examine the effect of AZD4547 on MEC subpopulations and tissue hierarchy dynamics in the premalignant mammary tissues of this model, we performed flow cytometry analyses on the primary MECs using CD24/CD49f and CD61/CD49f cell surface markers. The results showed that AZD4547 treatment substantially reduced MaSC-derived luminal and myoepithelial cell populations. AZD4547 also selectively suppressed the CD61highCD49fhigh cell population, which is enriched with luminal progenitor cells that give rise to TICs during MMTV-ErbB2 mammary tumor development. Mammosphere and colony-forming cell (CFC) assays on primary MECs demonstrated that the stemness of these cells was also blocked by AZD4547 prior to malignant transformation. Consistently, AZD4547 inhibited the anchorage-independent growth of cells from spontaneous tumors. Moreover, we demonstrated that AZD4547 downregulated multiple pathways, including the inactivation of FGFR, EGFR, and Wnt/β-catenin signaling. Collectively, the morphogenic, MaSC/TIC, and signaling regulation associated with AZD4547 treatment provides critical evidence for AZD4547 as a breast cancer preventative and therapeutic agent, which ultimately reveals clues for more effective eradication of refractory mammary tumors.

Citation Format: Qingxia Zhao, Amanda B. Parris, Erin W. Howard, Ming Zhao, Ying Xing, Zhikun Ma, Xiaohe Yang. FGFR inhibitor, AZD4547, impedes the stemness of mammary epithelial cells in the premalignant tissues of MMTV-ErbB2 transgenic mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1908. doi:10.1158/1538-7445.AM2017-1908

Abstract 2223: Rapamycin suppresses ErbB2-overexpressing mammary tumors through selective inhibition of luminal progenitor cells and tumor-initiating cells

Rapamycin is a potent inhibitor of the mTOR pathway and has demonstrated anti-cancer properties in various cancer models. Although rapamycin was reported to inhibit mammary carcinogenesis at multiple stages, including initiation, invasion, and metastasis, the underlying mechanisms remain unclear, especially in individual breast cancer subtypes. Specifically, ErbB2/Her2 amplification occurs in approximately 25% of invasive human breast cancers, and is characterized by Akt-mTOR pathway deregulation. Recent reports also suggest that cancer stem cells (CSCs) play a critical role in tumor development and metastasis. Therefore, we investigated the efficacy of rapamycin in the prevention of mammary tumor development in MMTV-ErbB2 transgenic mice by focusing on its effects on CSCs/tumor-initiating cells (TICs). In this study, MMTV-ErbB2 mice were inoculated with syngeneic 78617 cells, which are derived from MMTV-ErbB2 mammary tumors, followed by rapamycin treatment (1.5 mg/kg/day) for 12 days. We found that rapamycin elicited strong anti-tumorigenic effects on the syngeneic tumors, as indicated by substantially reduced tumor volume and weight. To determine the effect of rapamycin on mammary epithelial cells (MECs) in premalignant tissues, MMTV-ErbB2 mice were treated with low-dose rapamycin (1 mg/kg/day) between weeks 10 and 20 of age. Rapamycin significantly inhibited mammary morphogenesis at 20 weeks of age as compared to the control mice. As such, the branching density, ductal elongation, and proliferative index of the mammary glands from the rapamycin-treated mice were markedly decreased. Flow cytometric analysis of primary MECs demonstrated that rapamycin selectively inhibited the CD61highCD49fhigh subpopulation, which are enriched with luminal progenitor cells and TICs. 3D culture of FACS-sorted MEC subpopulations indicated that CD61highCD49fhigh cells are the major subpopulation that gives rise to 3D colonies. We further demonstrated that rapamycin significantly suppressed colony-forming cell (CFC) number and primary/secondary sphere formation of primary MECs and spontaneous MMTV-ErbB2 tumor cells, indicating rapamycin-induced inhibition of mammary progenitor cells and CSC self-renewal, respectively. Furthermore, molecular analysis of MECs demonstrated that rapamycin inhibited signaling associated with ER, ErbB2, and mTOR. In particular, rapamycin strikingly inhibited Wnt/β-catenin and TGFβ/Smad3 signaling. Taken together, our results demonstrate that rapamycin-mediated prevention of MMTV-ErbB2 mammary tumor development involves selective targeting of mammary progenitor cells and potential TICs in the premalignant mammary tissues through regulation of MEC stemness and multiple signaling pathways, which contributes to its potent anti-tumor effects on ErbB2-overexpressing breast cancer.

Citation Format: Qingxia Zhao, Amanda B. Parris, Erin W. Howard, Ming Zhao, Zhiying Guo, Ying Xing, Zhikun Ma, Xiaohe Yang. Rapamycin suppresses ErbB2-overexpressing mammary tumors through selective inhibition of luminal progenitor cells and tumor-initiating cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2223. doi:10.1158/1538-7445.AM2017-2223

p53 pathway determines the cellular response to alcohol-induced DNA damage in MCF-7 breast cancer cells

Alcohol consumption is associated with increased breast cancer risk; however, the underlying mechanisms that contribute to mammary tumor initiation and progression are unclear. Alcohol is known to induce oxidative stress and DNA damage; likewise, p53 is a critical modulator of the DNA repair pathway and ensures genomic integrity. p53 mutations are frequently detected in breast and other tumors. The impact of alcohol on p53 is recognized, yet the role of p53 in alcohol-induced mammary carcinogenesis remains poorly defined. In our study, we measured alcohol-mediated oxidative DNA damage in MCF-7 cells using 8-OHdG and p-H2AX foci formation assays. p53 activity and target gene expression after alcohol exposure were determined using p53 luciferase reporter assay, qPCR, and Western blotting. A mechanistic study delineating the role of p53 in DNA damage response and cell cycle arrest was based on isogenic MCF-7 cells stably transfected with control (MCF-7/Con) or p53-targeting siRNA (MCF-7/sip53), and MCF-7 cells that were pretreated with Nutlin-3 (Mdm2 inhibitor) to stabilize p53. Alcohol treatment resulted in significant DNA damage in MCF-7 cells, as indicated by increased levels of 8-OHdG and p-H2AX foci number. A p53-dependent signaling cascade was stimulated by alcohol-induced DNA damage. Moderate to high concentrations of alcohol (0.1-0.8% v/v) induced p53 activation, as indicated by increased p53 phosphorylation, reporter gene activity, and p21/Bax gene expression, which led to G0/G1 cell cycle arrest. Importantly, compared to MCF-7/Con cells, alcohol-induced DNA damage was significantly enhanced, while alcohol-induced p21/Bax expression and cell cycle arrest were attenuated in MCF-7/sip53 cells. In contrast, inhibition of p53 degradation via Nutlin-3 reinforced G0/G1 cell cycle arrest in MCF-7 control cells. Our study suggests that functional p53 plays a critical role in cellular responses to alcohol-induced DNA damage, which protects the cells from DNA damage associated with breast cancer risk.

Up-Regulation of Follistatin-Like 1 By the Androgen Receptor and Melanoma Antigen-A11 in Prostate Cancer

BACKGROUND

High affinity androgen binding to the androgen receptor (AR) activates genes required for male sex differentiation and promotes the development and progression of prostate cancer. Human AR transcriptional activity involves interactions with coregulatory proteins that include primate-specific melanoma antigen-A11 (MAGE-A11), a coactivator that increases AR transcriptional activity during prostate cancer progression to castration-resistant/recurrent prostate cancer (CRPC).

METHODS

Microarray analysis and quantitative RT-PCR were performed to identify androgen-regulated MAGE-A11-dependent genes in LAPC-4 prostate cancer cells after lentivirus shRNA knockdown of MAGE-A11. Chromatin immunoprecipitation was used to assess androgen-dependent AR recruitment, and immunocytochemistry to localize an androgen-dependent protein in prostate cancer cells and tissue and in the CWR22 human prostate cancer xenograft.

RESULTS

Microarray analysis of androgen-treated LAPC-4 prostate cancer cells indicated follistatin-like 1 (FSTL1) is up-regulated by MAGE-A11. Androgen-dependent up-regulation of FSTL1 was inhibited in LAPC-4 cells by lentivirus shRNA knockdown of AR or MAGE-A11. Chromatin immunoprecipitation demonstrated AR recruitment to intron 10 of the FSTL1 gene that contains a classical consensus androgen response element. Increased levels of FSTL1 protein in LAPC-4 cells correlated with higher levels of MAGE-A11 relative to other prostate cancer cells. FSTL1 mRNA levels increased in CRPC and castration-recurrent CWR22 xenografts in association with predominantly nuclear FSTL1. Increased nuclear localization of FSTL1 in prostate cancer was suggested by predominantly cytoplasmic FSTL1 in benign prostate epithelial cells and predominantly nuclear FSTL1 in epithelial cells in CRPC tissue and the castration-recurrent CWR22 xenograft. AR expression studies showed nuclear colocalization of AR and endogenous FSTL1 in response to androgen.

CONCLUSION

AR and MAGE-A11 cooperate in the up-regulation of FSTL1 to promote growth and progression of CRPC. Prostate 77:505-516, 2017. © 2016 Wiley Periodicals, Inc.

Androgen receptor regulation by histone methyltransferase Suppressor of variegation 3-9 homolog 2 and Melanoma antigen-A11

Androgen receptor (AR) transcriptional activity depends on interactions between the AR NH₂-terminal region and transcriptional coregulators. A yeast two-hybrid screen of a human testis library using predicted α-helical NH₂-terminal fragment AR-(370-420) as bait identified suppressor of variegation 3-9 homolog 2 (SUV39H2) histone methyltransferase as an AR interacting protein. SUV39H2 interaction with AR and the AR coregulator, melanoma antigen-A11 (MAGE-A11), was verified in two-hybrid, in vitro glutathione S-transferase affinity matrix and coimmunoprecipitation assays. Fluorescent immunocytochemistry colocalized SUV39H2 and AR in the cytoplasm without androgen, in the nucleus with androgen, and with MAGE-A11 in the nucleus independent of androgen. Chromatin immunoprecipitation using antibodies raised against SUV39H2 demonstrated androgen-dependent recruitment of AR and SUV39H2 to the androgen-responsive upstream enhancer of the prostate-specific antigen gene. SUV39H2 functioned cooperatively with MAGE-A11 to increase androgen-dependent AR transcriptional activity. SUV39H2 histone methyltransferase is an AR coactivator that increases androgen-dependent transcriptional activity through interactions with AR and MAGE-A11.

Buformin inhibits the stemness of erbB-2-overexpressing breast cancer cells and premalignant mammary tissues of MMTV-erbB-2 transgenic mice

BACKGROUND

Metformin, an FDA-approved drug for the treatment of Type II diabetes, has emerged as a promising anti-cancer agent. Other biguanide analogs, including buformin and phenformin, are suggested to have similar properties. Although buformin was shown to reduce mammary tumor burden in carcinogen models, the anti-cancer effects of buformin on different breast cancer subtypes and the underlying mechanisms remain unclear. Therefore, we aimed to investigate the effects of buformin on erbB-2-overexpressing breast cancer with in vitro and in vivo models.

METHODS

MTT, cell cycle, clonogenic/CFC, ALDEFLUOR, tumorsphere, and Western blot analyses were used to determine the effects of buformin on cell growth, stem cell populations, stem cell-like properties, and signaling pathways in SKBR3 and BT474 erbB-2-overexpressing breast cancer cell lines. A syngeneic tumor cell transplantation model inoculating MMTV-erbB-2 mice with 78617 mouse mammary tumor cells was used to study the effects of buformin (1.2 g buformin/kg chow) on tumor growth in vivo. MMTV-erbB-2 mice were also fed buformin for 10 weeks, followed by analysis of premalignant mammary tissues for changes in morphological development, mammary epithelial cell (MEC) populations, and signaling pathways.

RESULTS

Buformin significantly inhibited SKBR3 and BT474 cell growth, and in vivo activity was demonstrated by considerable growth inhibition of syngeneic tumors derived from MMTV-erbB-2 mice. In particular, buformin suppressed stem cell populations and self-renewal in vitro, which was associated with inhibited receptor tyrosine kinase (RTK) and mTOR signaling. Consistent with in vitro data, buformin suppressed mammary morphogenesis and reduced cell proliferation in MMTV-erbB-2 mice. Importantly, buformin decreased MEC populations enriched with mammary reconstitution units (MRUs) and tumor-initiating cells (TICs) from MMTV-erbB-2 mice, as supported by impaired clonogenic and mammosphere formation in primary MECs. We further demonstrated that buformin-mediated in vivo inhibition of MEC stemness is associated with suppressed activation of mTOR, RTK, ER, and β-catenin signaling pathways.

CONCLUSIONS

Overall, our results provide evidence for buformin as an effective anti-cancer drug that selectively targets TICs, and present a novel prevention and/or treatment strategy for patients who are genetically predisposed to erbB-2-overexpressing breast cancer.

Short-term early exposure to lapatinib confers lifelong protection from mammary tumor development in MMTV-erbB-2 transgenic mice

BACKGROUND

Although chemopreventative agents targeting the estrogen/estrogen receptor (ER) pathway have been effective for ER⁺ breast cancers, prevention of hormone receptor-negative breast cancers, such as Her2/erbB-2⁺ breast cancers, remains a significant issue. Previous studies have demonstrated that administration of EGFR/erbB-2-targeting lapatinib to MMTV-erbB-2 transgenic mice inhibited mammary tumor development. The prevention, however, was achieved by prolonged high dose exposure. The tolerance to high dose/long-term drug administration may hinder its potential in clinical settings. Therefore, we aimed to test a novel, short-term chemopreventative strategy using lapatinib during the premalignant risk window in MMTV-erbB-2 mice.

METHODS

We initially treated cultured cells with lapatinib to explore the anti-proliferative effects of lapatinib in vitro. We used a syngeneic tumor graft model to begin exploring the in vivo anti-tumorigenic effects of lapatinib in MMTV-erbB-2 mice. Then, we tested the efficacy of brief exposure to lapatinib (100 mg/kg/day for 8 weeks), beginning at 16 weeks of age, in the prevention of mammary tumor development in MMTV-erbB-2 mice.

RESULTS

In the syngeneic tumor transplant model, we determined that lapatinib significantly inhibited tumor cell proliferation. Furthermore, we demonstrated that short-term lapatinib exposure resulted in life-long protective effects, as supported by increased tumor latency in lapatinib-treated mice compared to the control mice. We further established that delayed tumor development in the treated mice was preceded by decreased BrdU nuclear incorporation and inhibited mammary morphogenesis. Molecular analysis indicated that lapatinib inhibited phosphorylation and expression of EGFR, erbB-3, erbB-2, Akt1, and Erk1/2 in premalignant mammary tissues. Also, lapatinib drastically inhibited the phosphorylation and expression of ERα and the transcription of ER target genes in premalignant mammary tissues. We also determined that lapatinib suppressed the stemness of breast cancer cell lines, as evidenced by decreased tumorsphere formation and ALDH⁺ cell populations.

CONCLUSIONS

Taken together, these data demonstrate that brief treatment with EGFR/erbB-2-targeting agents before the onset of tumors may provide lifelong protection from mammary tumors, through the concurrent inhibition of erbB-2 and ER signaling pathways and consequential reprogramming. Our findings support further clinical testing to explore the benefit of shorter lapatinib exposure in the prevention of erbB-2-mediated carcinogenesis.

Alcohol promotes migration and invasion of triple-negative breast cancer cells through activation of p38 MAPK and JNK

Although alcohol is an established breast cancer risk factor, the underlying mechanisms remain unclear. Previous studies examined the general association between alcohol consumption and breast cancer risk; however, the risk for different breast cancer subtypes has been rarely reported. Triple-negative breast cancer (TNBC) is a subtype of breast cancer lacking hormone receptors and HER2 expression, and having poor prognosis. Understanding the molecular mechanisms of TNBC etiology remains a significant challenge. In this study, we investigated cellular responses to alcohol in two TNBC cell lines, MDA-MB-231 and MDA-MB-468. Our results showed that alcohol at low concentrations (0.025-0.1% v/v) induced cell proliferation, migration, and invasion in 1% FBS-containing medium. Molecular analysis indicated that these phenotypic changes were associated with alcohol-induced reactive oxygen species production and increased p38 and JNK phosphorylation. Likewise, p38 or JNK inhibition attenuated alcohol-induced cell migration and invasion. We revealed that alcohol treatment activated/phosphorylated NF-κB regulators and increased transcription of NF-κB-targeted genes. While examining the role of acetaldehyde, the major alcohol metabolite, in alcohol-associated responses in TNBC cells, we saw that acetaldehyde induced cell migration, invasion, and increased phospho-p38, phospho-JNK, and phospho-IκBα in a pattern similar to alcohol treatment. Taken together, we established that alcohol promotes TNBC cell proliferation, migration, and invasion in vitro. The underlying mechanisms involve the induction of oxidative stress and the activation of NF-κB signaling. In particular, the activation of p38 and JNK plays a pivotal role in alcohol-induced cellular responses. These results will advance our understanding of alcohol-mediated development and promotion of TNBC. © 2016 Wiley Periodicals, Inc.

Genistein targets the cancerous inhibitor of PP2A to induce growth inhibition and apoptosis in breast cancer cells

Genistein is a soy isoflavone with phytoestrogen and tyrosine kinase inhibitory properties. High intake of soy/genistein has been associated with reduced breast cancer risk. Despite the advances in genistein-mediated antitumor studies, the underlying mechanisms remain unclear. In the present study, we investigated genistein-induced regulation of the cancerous inhibitor of protein phosphatase 2A (CIP2A), a novel oncogene frequently overexpressed in breast cancer, and its functional impact on genistein-induced growth inhibition and apoptosis. We demonstrated that genistein induced downregulation of CIP2A in MCF-7-C3 and T47D breast cancer cells, which was correlated with its growth inhibition and apoptotic activities. Overexpression of CIP2A attenuated, whereas CIP2A knockdown sensitized, genistein-induced growth inhibition and apoptosis. We further showed that genistein-induced downregulation of CIP2A involved both transcriptional suppression and proteasomal degradation. In particular, genistein at higher concentrations induced concurrent downregulation of E2F1 and CIP2A. Overexpression of E2F1 attenuated genistein-induced downregulation of CIP2A mRNA, indicating the role of E2F1 in genistein-induced transcriptional suppression of CIP2A. Taken together, our results identified CIP2A as a functional target of genistein and demonstrated that modulation of E2F1-mediated transcriptional regulation of CIP2A contributes to its downregulation. These data advance our understanding of genistein-induced growth inhibition and apoptosis, and support further investigation on CIP2A as a therapeutic target of relevant anticancer agents.

p53 is required for metformin-induced growth inhibition, senescence and apoptosis in breast cancer cells

The p53 tumor repressor gene is commonly mutated in human cancers. The tumor inhibitory effect of metformin on p53-mutated breast cancer cells remains unclear. Data from the present study demonstrated that p53 knockdown or mutation has a negative effect on metformin or phenformin-induced growth inhibition, senescence and apoptosis in breast cancer cells. We also found that p53 reactivating agent nutlin-3α and CP/31398 promoted metformin-induced growth inhibition, senescence and apoptosis in MCF-7 (wt p53) and MDA-MB-231 (mt p53) cells, respectively. Treatment of MCF-7 cells with metformin or phenformin induced increase in p53 protein levels and the transcription of its downstream target genes, Bax and p21, in a dose-dependent manner. Moreover, we demonstrated that AMPK-mTOR signaling played a role in metformin-induced p53 up-regulation. The present study showed that p53 is required for metformin or phenformin-induced growth inhibition, senescence and apoptosis in breast cancer cells. The combination of metformin with p53 reactivating agents, like nutlin-3α and CP/31398, is a promising strategy for improving metformin-mediated anti-cancer therapy, especially for tumors with p53 mutations.