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Conditional loss of ErbB3 delays mammary gland hyperplasias induced by mutant PIK3CA but does not affect mammary tumor latency, gene expression or signaling

ABSTRACT: Mutations in PIK3CA, the gene encoding the p110α subunit of PI3K, induce transformation of mammary epithelial cells (MEC). Studies suggest the transforming action of mutant PIK3CA requires binding to activated receptor tyrosine kinases or adaptors. We examined in transgenic mice if ErbB3, a powerful activator of PI3K, is required for mutant PIK3CA-mediated MEC transformation. ErbB3 loss delayed PIK3CAH1047R-dependent mammary gland hyperplasia, but tumor latency, gene expression and markers of PI3K signaling were unaffected. In ErbB3-deficient tumors, mutant PI3K remained associated with other tyrosine phosphorylated adaptors, potentially explaining the dispensability of ErbB3 for tumorigenicity and PI3K activity. Combined inhibition of ErbB RTKs and PI3K with lapatinib and the p110α inhibitor BYL719, respectively, reduced PIK3CAH1047R-dependent tumor growth and PI3K signaling more potently than the p110α inhibitor alone. In human breast cancer cells harboring PIK3CAH1047R, the combination with BYL719 and an ErbB3 inhibitor synergistically inhibited tumor growth and P-Akt. These data suggest that basal tumor growth and PI3K signaling do not depend on ErbB RTKs in PIK3CAH1047R-expressing tumors. However, upon inhibition of p110α, these receptors limit the action of PI3K inhibitors potentially explaining the more potent effect of the combination against PI3K-mutant cancers.

ORGANISM(S): Mus musculus

PROVIDER: GSE43037 | GEO | 2013/06/20

SECONDARY ACCESSION(S): PRJNA184147

REPOSITORIES: GEO

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Project description:Activating mutations of PIK3CA are the most frequent genomic alterations in estrogen receptor (ER)-positive breast tumors and selective PI3Kα inhibitors are in clinical development. The activity of these agents, however, is not homogenous and only a fraction of patients bearing PIK3CA-mutant ER-positive tumors benefit from single agent administration. Searching for mechanisms of resistance, we observed that suppression of PI3K signaling with different agents results in induction of ER-dependent transcriptional activity as demonstrated by changes in expression in genes containing ER binding sites, enhanced ER transcription and increased occupancy by the ER of promoter regions of upregulated genes. Furthermore, expression of ESR1 mRNA and ER protein levels themselves were also increased upon PI3K inhibition. These changes in gene expression were confirmed in vivo in xenograft and patient derived models and in tumors from patients undergoing treatment with the PI3Kα inhibitor BYL719. The observed effects on transcription were enhanced by the addition of estradiol and suppressed by the anti-ER therapies fulvestrant and tamoxifen. Fulvestrant markedly sensitized ER-positive tumors to PI3Kα inhibition. We propose that increased ER transcriptional activity may be a compensatory mechanism that limits the activity of PI3K inhibitors and that combined PI3K and ER inhibition is a rational approach to target these tumors. The aim of our study was to explore the mechanism by which combination of PI3K pathway inhibitors and estrogen receptor function blockade results in superior antitumor activity. We aimed to evaluate whether changes in ER function were influencing the clinical response to anti-PI3K therapy in ER-positive breast tumors that harbor PI3K pathway activation. For this purpose, we planned to use various specific PI3K inhibitors, namely: BYL719 (p110α specific catalytic inhibitor), GDC0941 (pan-PI3K inhibitor), GDC0032 and BAY80-6946 (p110sparing PI3K inhibitors) in a panel of ER-positive breast cancer cell lines and xenografts that harbor PIK3CA activating mutations. We also used MK2206 (pan-AKT allosteric inhibitor) to inhibit the PI3K pathway in ER-positive cell lines which activate this pathway through PTEN loss. Finally, in order to evaluate the role of ER up-regulation as a pro-survival signal in our in vitro and in vivo models, we planned to use the selective ER modulator 4-hydroxy-tamoxifen (4-OHT) and degrader fulvestrant. For the in vivo experiments, the number of animals in each group was calculated to measure a 25% difference between the means of placebo and treatment groups with a power of 80% and a p value of 0.01. Host mice carrying xenografts were randomly and equally assigned to either control or treatment groups. Animal experiments were conducted in a controlled and non-blinded manner. Moreover, we evaluated by means of RNAseq gene expression changes breast cancer patients that underwent BYL719 based therapy to validate our in vitro findings in terms of ER expression. In vitro experiments were performed at least two times and at least in triplicate for each replica.

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Conditional loss of ErbB3 delays mammary gland hyperplasias induced by mutant PIK3CA but does not affect mammary tumor latency, gene expression or signaling

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