Abstract P6-17-15: Evaluating preclinical efficacy of anti-HER2 drug combinations using ER+/HER2 mutant models

Background

Until recently, HER2 gene amplification was the only mechanism of HER2 activation recognized. However, activating HER2 mutations have been noted in different cancer types. A trials of HER2 mutant breast cancer and the subsequent SUMMIT trial data have shown that monotherapy with the pan-HER drug neratinib as showed clinical efficacy, but with poor response durability. This study therefore investigates the preclinical efficacy of anti HER2 agents alone or in combination with endocrine therapy agents or in combination with CDK4/6 inhibitors using ER+/HER2 mutant cell lines and ex vivo HER2 mutant patient derived xenograft (PDX) model to define a more effective treatment approach.

Methods

ER+ breast cancer cell lines (T47D and MCF7) stably expressing HER2V777L, and ER+/HER2 mutant PDX model (HER2G778_P780 dup) were used to examine HER2 signaling and drug responses. Signaling downstream mutant HER2 was examined by immunoblot analysis. Effects of neratinib alone, neratinib + fulvestrant, and neratinib + abemaciclib on cell growth were examined in ER+/HER2 mutant cell lines and in an ex vivo HER2G778_P780 dup.

Results

We found that MCF7/T47D cells expressing HER2V777L and HER2G778_P780 dup PDX tumors showed strongly activated autophosphorylation of HER2 and increased expression of CDK4, CDK6, phospho-Rb, and cyclin D1 as compared to MCF7/T47D cells expressing HER2WT or ER+/non-HER2mut PDX modes, suggesting that HER2 mutations preferentially depend on CDK4/6 signaling for cell growth. Additionally, we showed that activating MCF7 HER2 V777L cause resistance to endocrine therapy treatment (fulvestrant IC50 >5μM). Further, we show that neratinib alone is effective at higher concentrations (IC50 < 2μM) in MCF7/HER2 V777L cells. We also demonstrate that abemaciclib alone exhibited moderate activity against MCF7 HER2 V777L cells (IC50 < 0.4μM) and additional activity in combination with neratinib (IC50 < 0.06μM) was seen. Moreover, ex vivo HER2 G778_P780 dup cells are relatively resistant to fulvestrant alone (IC50 < 0.2μM), neratinib alone (IC50 < 0.006μM), abemaciclib alone (IC50 < 0.04μM), and neratinib in combination with abemaciclib (IC50 < 0.005μM), suggesting that patients harboring ER+/HER2-mutant tumors may benefit from neratinib in combination with abemaciclib.

Conclusion

These preclinical data suggest that neratinib monotherapy may not be effective to treat ER+/HER2 mutant patients and we propose that simultaneous targeting of both HER2 and the CDK4/6 axis will be required for effective treatment of ER+ breast cancers harboring HER2 activating mutations.

Citation Format: Kavuri SM, Devarakonda V, Williams LC, Seker S, Lei JT, Singh P, Han A, Anurag M, Holloway KR, Welm AL, Ellis MJ. Evaluating preclinical efficacy of anti-HER2 drug combinations using ER+/HER2 mutant models [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-17-15.

Abstract P5-09-03: Endocrine response in invasive lobular carcinoma is characterized by unique estrogen-mediated gene expression and de novo tamoxifen resistance

Invasive lobular carcinoma (ILC) represents ∼10% of newly diagnosed breast tumors, or ∼30,000 cases annually in the US. However, ILC-specific signaling and endocrine responsiveness are not well characterized. Retrospective analyses suggest that ILC patients treated with endocrine therapy have poorer outcomes than invasive ductal carcinoma (IDC) patients with similar biomarkers, and that ILC patients may not benefit from adjuvant tamoxifen. We hypothesize that estrogen receptor-alpha (ER) regulated gene expression is unique in ILC cells and drives endocrine resistance.

The ER-positive ILC cell lines MDA MB 134VI and SUM44PE were used as in vitro models of cell growth and ER-regulated gene expression in response to estradiol (E2). To examine the ER-regulated transcriptome, we performed gene expression microarray analyses and ER ChIP-Seq following E2 treatment. In parallel, E2 response was assessed in vivo in the primary ILC xenograft HCI-013. Response to endocrine therapies, tamoxifen (Tam), 4-hydroxytamoxifen (4OHT), endoxifen (Bx), and fulvestrant (ICI), were also examined in ILC cell lines.

We observed that E2 induced growth and ER target gene expression in MDA MB 134VI and SUM44PE. We compared our ILC microarray data to published data from ER-positive IDC cell lines (MCF-7, T47D, BT474), and identified 254 genes that were E2-regulated in all 5 cell lines (e.g. GREB1, MYC). 915 genes were E2-regulated only in both ILC cell lines. Consistent with this, roughly half of ER binding sites identified in MDA MB 134VI ChIP-Seq were unique versus published MCF-7 data. We chose a subset of ILC-specific and common E2-regulated genes (n = 107) to assess in vivo using Nanostring gene expression analyses of HCI-013. E2-regulation was observed for 32/107 target genes (30%), suggesting that these genes may be E2-regulated in vivo in ILC patient tumors.

Consistent with clinical data, both ILC cell lines presented de novo tamoxifen resistance. SUM44PE were growth-inhibited by ICI, but unaffected by Tam, 4OHT, or Bx. Similarly, ICI blocked E2-induced growth in MDA MB 134VI, but Tam, 4OHT, and Bx acted as partial agonists, inducing ∼25% growth. Partial agonism was not limited to tamoxifen, as other SERMs (e.g. raloxifene) also induced growth. We then measured ER-regulated gene expression in MDA MB 134VI following tamoxifen treatment. Tam, 4OHT, and Bx acted as agonists for 38/107 genes, whereas ICI acted as an antagonist. All 38 genes were E2-repressed targets (e.g. CCNG2), suggesting that ER-mediated gene repression may be critical to tamoxifen-resistance in ILC. Finally, we observed that FGFR1, frequently amplified in ILC, may be critical for ILC cell survival in the presence of tamoxifen.

These data support the hypothesis that unique ER-mediated gene expression in ILC cells drives endocrine resistance. The de novo tamoxifen resistance observed in ILC cells may correlate with the worse outcomes in ILC patients recently reported. We hypothesize that genes regulated by tamoxifen as an agonist may play a role in tamoxifen-induced growth or serve as biomarkers of resistance. Targeting growth factor signaling using FGFR1 inhibitors may block survival pathways required by ILC cells and reverse tamoxifen resistance.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-09-03.

Abstract P6-04-20: Endocrine resistance in invasive lobular carcinoma cells parallels unique estrogen-mediated gene expression

Invasive lobular carcinoma (ILC) represents ∼10% of newly diagnosed breast tumors, accounting for ∼30,000 cases annually in the US. However, ILC has been understudied as a breast cancer subtype. ILC-specific signaling pathways and responses to endocrine therapies are not well characterized. Recent retrospective analyses of luminal-type breast cancers suggest that ILC patients treated with endocrine therapy have poorer disease-free survival and overall survival than invasive ductal carcinoma (IDC) patients with similar biomarkers. We hypothesize that unique transcriptional control of estrogen receptor-alpha (ERα) by estrogens and anti-estrogens in ILC cells drive a differential response of ILC tumors to endocrine therapies.

The ILC cell lines MDA MB 134VI and SUM44PE were used as in vitro models of ILC tumors to examine responses to estradiol (E2) and the anti-estrogens tamoxifen (Tam), 4-hydroxytamoxifen (4OHT), endoxifen (Bx), and fulvestrant (ICI). We examined cell growth and expression of canonical ERα-regulated genes in response to E2. Cells were also treated with anti-estrogens +/− E2 to examine their ability to inhibit E2-induced growth. To establish a genome-wide profile of ERα-mediated gene regulation in ILC cells, ILC cells were subjected to gene expression microarray analysis following 0–24hrs treatment with 1nM E2. In parallel to these in vitro studies, in vivo models of ILC are being assessed for endocrine responsiveness, including MDA MB 134VI xenografts and the primary human tumor xenograft HCI-013.

We observed that both MDA MB 134VI and SUM44PE are growth-induced by E2 treatment. However, both cell lines also present de novo resistance to Tam, 4OHT, and Bx. While ICI treatment completely blocked E2-induced growth in MDA MB 134VI, Tam, 4OHT, and Bx acted as partial agonists. Treatment with these compounds alone induced ∼25% growth relative to E2, and E2-induced growth could only be inhibited ∼75%. Similarly, SUM44PE cells are strongly growth-inhibited by ICI treatment, whereas growth is not reduced by Tam, 4OHT, or Bx treatment. Consistent with these observations, novel patterns of gene expression are induced by E2 treatment in ILC cells. E2-treatment induced canonical targets (e.g. GREB1, IGFBP4) in both ILC cell lines. However, in MDA MB 134VI cells only ∼35% of genes regulated by E2 at 24hrs overlap with those regulated in the IDC cell line MCF-7. Further, a subset of the overlapping genes is differentially regulated between cell types, including ESR1 (1.25-fold and 0.67-fold versus control in MDA MB 134VI and MCF-7, respectively), HOXC6, PMP22, and L1CAM. Examination of these observations in in vivo models is currently ongoing.

These data are consistent with the hypothesis that E2 and anti-estrogens differentially regulate ERα-mediated gene expression in ILC cells versus IDC cells. The de novo resistance to tamoxifen observed in ILC cells may correlate with the worse outcomes in ILC patients observed in retrospective analyses. We hypothesize that elucidation of the mechanisms responsible for differential ERα regulation may reveal novel therapeutic targets for treating ILC patients and overcoming endocrine resistance.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-04-20.

RNA CUG repeats sequester CUGBP1 and alter protein levels and activity of CUGBP1

An RNA CUG triplet repeat binding protein, CUGBP1, regulates splicing and translation of various RNAs. Expansion of RNA CUG repeats in the 3'-untranslated repeat of the mutant myotonin protein kinase (DMPK) mRNA in myotonic dystrophy (DM) is associated with alterations in binding activity of CUGBP1. To investigate whether CUGBP1 is directly affected by expansion of CUG repeats in DM tissues, we examined the intracellular status of CUGBP1 in DM patients as well as in cultured cells over expressing RNA CUG repeats. The analysis of RNA-protein complexes showed that, in control tissues, the majority of CUGBP1 is free of RNA, whereas in DM patients the majority of CUGBP1 is associated with RNA containing CUG repeats. Similarly to DM patients, overexpression of RNA CUG repeats in cultured cells results in the re-allocation of CUGBP1 from a free state to the RNA.protein complexes containing CUG repeats. CUG repeat-dependent translocation of CUGBP1 into RNA-protein complexes is associated with increased levels of CUGBP1 protein and its binding activity. Experiments with cyclohexamide-dependent block of protein synthesis showed that the half-life of CUGBP1 is increased in cells expressing CUG repeats. Alteration of CUGBP1 in DM is accompanied by alteration in translation of a transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta), which has been previously described to be a target of CUGBP1. Analysis of C/EBPbeta isoforms in DM patients with altered levels of CUGBP1 showed that translation of a dominant negative isoform, LIP, is induced by CUGBP1. Results of this paper demonstrate that the expansion of CUG repeats in DM affects RNA-binding proteins and leads to alteration in RNA processing.

Translational induction of liver-enriched transcriptional inhibitory protein during acute phase response leads to repression of CCAAT/enhancer binding protein alpha mRNA

Lipopolysacharide (LPS) induced acute phase response (APR) in mouse liver leads to elevation of the low molecular weight CCAAT/Enhancer binding protein (C/EBP) beta isoform, liver-enriched transcriptional inhibitory protein (LIP). In this paper, we investigate the pathway for LIP induction during APR and the role of LIP in regulation of the C/EBPalpha promoter. The 5' region of C/EBPbeta mRNA has been shown to be involved in the regulation of LIP translation. Our data demonstrate that binding of cytoplasmic proteins to the 5' region of C/EBPbeta mRNA is altered in response to LPS administration. One of the major changes is induced binding of a cytoplasmic protein that is immunologically identical to the previously characterized RNA-binding protein CUGBP1. Induction of CUGBP1 binding activity in liver cytoplasm during APR is accompanied by the elevation of CUGBP1 binding activity on polysomes. CUGBP1 immunoprecipitated from livers of LPS-treated mice, but not from normal animals, is capable of inducing LIP translation in a cell-free translation system. The ability of CUGBP1 to induce LIP translation during APR depends on phosphorylation of CUGBP1. We show that elevation of LIP during APR and after partial hepatectomy leads to increased binding of LIP to the C/EBP consensus site found within the mouse C/EBPalpha promoter. This binding correlates with reduction of C/EBPalpha mRNA levels in both biological situations. Co-transfection experiments showed that full-length C/EBPbeta activates the C/EBPalpha promoter, while LIP blocks this activation. Our data suggest that the dominant negative isoform of C/EBPbeta, LIP, down-regulates the C/EBPalpha promoter in liver and in cultured hepatocytes. Because full-length C/EBPalpha and C/EBPbeta proteins regulate liver proliferation, this function of LIP may be important in liver growth and differentiation.

CUG repeat binding protein (CUGBP1) interacts with the 5' region of C/EBPbeta mRNA and regulates translation of C/EBPbeta isoforms

The transcription factor CCAAT/enhancer binding protein beta, C/EBPbeta, plays a significant role in the regulation of hepatocyte growth and differentiation. A single mRNA coding for C/EBPbeta produces several protein isoforms. Two pathways for generation of low molecular weight C/EBPbeta isoforms have been described: specific proteolytic cleavage and initiation of translation from different AUG codons of C/EBPbeta mRNA. A truncated C/EBPbeta isoform, LIP, is induced in rat livers in response to partial hepatectomy (PH) via the alternative translation mechanism. Here we present evidence that CUG repeat binding protein, CUGBP1, interacts with the 5' region of C/EBPbeta mRNA and regulates translation of C/EBPbeta isoforms. Two binding sites for CUGBP1 are located side by side between the first and second AUG codons of C/EBPbeta mRNA. One binding site is observed in an out of frame short open reading frame (sORF) that has been previously shown to regulate initiation of translation from different AUG codons of C/EBPbeta mRNA. Analysis of cytoplasmic and polysomal proteins from rat liver after PH showed that CUGBP1 is associated with polysomes that translate low molecular weight isoforms of C/EBPbeta. The binding activity of CUGBP1 to the 5' region of C/EBPbeta mRNA shows increased association with these polysomal fractions after PH. Addition of CUGBP1 into a cell-free translation system leads to increased translation of low molecular weight isoforms of C/EBPbeta. Our data demonstrate that CUGBP1 protein is an important component for the regulation of initiation from different AUG codons of C/EBPbeta mRNA.

C/EBPalpha regulates generation of C/EBPbeta isoforms through activation of specific proteolytic cleavage

C/EBPalpha and C/EBPbeta are intronless genes that can produce several N-terminally truncated isoforms through the process of alternative translation initiation at downstream AUG codons. C/EBPbeta has been reported to produce four isoforms: full-length 38-kDa C/EBPbeta, 35-kDa LAP (liver-enriched transcriptional activator protein), 21-kDa LIP (liver-enriched transcriptional inhibitory protein), and a 14-kDa isoform. In this report, we investigated the mechanisms by which C/EBPbeta isoforms are generated in the liver and in cultured cells. Using an in vitro translation system, we found that LIP can be generated by two mechanisms: alternative translation and a novel mechanism-specific proteolytic cleavage of full-length C/EBPbeta. Studies of mice in which the C/EBPalpha gene had been deleted (C/EBPalpha-/-) showed that the regulation of C/EBPbeta proteolysis is dependent on C/EBPalpha. The induction of C/EBPalpha in cultured cells leads to induced cleavage of C/EBPbeta to generate the LIP isoform. We characterized the cleavage activity in mouse liver extracts and found that the proteolytic cleavage activity is specific to prenatal and newborn livers, is sensitive to chymostatin, and is completely abolished in C/EBPalpha-/- animals. The lack of cleavage activity in the livers of C/EBPalpha-/- mice correlates with the decreased levels of LIP in the livers of these animals. Analysis of LIP production during liver regeneration showed that, in this system, the transient induction of LIP is dependent on the third AUG codon and most likely involves translational control. We propose that there are two mechanisms by which C/EBPbeta isoforms might be generated in the liver and in cultured cells: one that is determined by translation and a second that involves C/EBPalpha-dependent, specific proteolytic cleavage of full-length C/EBPbeta. The latter mechanism implicates C/EBPalpha in the regulation of posttranslational generation of the dominant negative C/EBPbeta isoform, LIP.