Her-2 overexpression increases the metastatic outgrowth of breast cancer cells in the brain.

Retrospective studies of breast cancer patients suggest that primary tumor Her-2 overexpression or trastuzumab therapy is associated with a devastating complication: the development of central nervous system (brain) metastases. Herein, we present Her-2 expression trends from resected human brain metastases and data from an experimental brain metastasis assay, both indicative of a functional contribution of Her-2 to brain metastatic colonization. Of 124 archival resected brain metastases from breast cancer patients, 36.2% overexpressed Her-2, indicating an enrichment in the frequency of tumor Her-2 overexpression at this metastatic site. Using quantitative real-time PCR of laser capture microdissected epithelial cells, Her-2 and epidermal growth factor receptor (EGFR) mRNA levels in a cohort of 12 frozen brain metastases were increased up to 5- and 9-fold, respectively, over those of Her-2-amplified primary tumors. Co-overexpression of Her-2 and EGFR was also observed in a subset of brain metastases. We then tested the hypothesis that overexpression of Her-2 increases the colonization of breast cancer cells in the brain in vivo. A subclone of MDA-MB-231 human breast carcinoma cells that selectively metastasizes to brain (231-BR) overexpressed EGFR; 231-BR cells were transfected with low (4- to 8-fold) or high (22- to 28-fold) levels of Her-2. In vivo, in a model of brain metastasis, low or high Her-2-overexpressing 231-BR clones produced comparable numbers of micrometastases in the brain as control transfectants; however, the Her-2 transfectants yielded 3-fold greater large metastases (>50 microm(2); P < 0.001). Our data indicate that Her-2 overexpression increases the outgrowth of metastatic tumor cells in the brain in this model system.

Antifolates targeting purine synthesis allow entry of tumor cells into S phase regardless of p53 function.

The class of folate antimetabolites typified by (6R)-dideazatetrahydrofolate (lometrexol, DDATHF) are specific inhibitors of de novo purine synthesis because of potent inhibition of glycinamide ribonucleotide formyltransferase (GART) but do not induce detectable levels of DNA strand breaks. As such, they are a test case of the concept that ribonucleotide depletion can be sensed by p53, resulting in a G(1) cell cycle block. The GART inhibitors have been proposed previously to be cytotoxic in tumor cells lacking p53 function but only cytostatic in p53 wild-type tumor cells. We have investigated this concept. Cell cycle progression into and through S phase was slowed by DDATHF, but both p53 +/+ and -/- human colon carcinoma cells entered and completed one S phase in the presence of drug. This inability of p53 to initiate a G(1) arrest after DDATHF treatment was mirrored by an independence of the cytotoxicity of DDATHF on p53 function. We conclude that carcinoma cells are killed equally well by DDATHF and related compounds whether or not the p53 pathway is intact and that the utility of GART inhibitors would not be limited to p53-negative tumors.

Analyses of resected human brain metastases of breast cancer reveal the association between up-regulation of hexokinase 2 and poor prognosis.

Brain metastases of breast cancer seem to be increasingin incidence as systemic therapy improves. Metastatic disease in the brain is associated with high morbidity and mortality. We present the first gene expression analysis of laser-captured epithelial cells from resected human brain metastases of breast cancer compared with unlinked primary breast tumors. The tumors were matched for histology, tumor-node-metastasis stage, and hormone receptor status. Most differentially expressed genes were down-regulated in the brain metastases, which included, surprisingly, many genes associated with metastasis. Quantitative real-time PCR analysis confirmed statistically significant differences or strong trends in the expression of six genes: BMP1, PEDF, LAMgamma3, SIAH, STHMN3, and TSPD2. Hexokinase 2 (HK2) was also of interest because of its increased expression in brain metastases. HK2 is important in glucose metabolism and apoptosis. In agreement with our microarray results, HK2 levels (both mRNA and protein) were elevated in a brain metastatic derivative (231-BR) of the human breast carcinoma cell line MDA-MB-231 relative to the parental cell line (231-P) in vitro. Knockdown of HK2 expression in 231-BR cells using short hairpin RNA reduced cell proliferation when cultures were maintained in glucose-limiting conditions. Finally, HK2 expression was analyzed in a cohort of 123 resected brain metastases of breast cancer. High HK2 expression was significantly associated with poor patient survival after craniotomy (P = 0.028). The data suggest that HK2 overexpression is associated with metastasis to the brain in breast cancer and it may be a therapeutic target.

Vorinostat inhibits brain metastatic colonization in a model of triple-negative breast cancer and induces DNA double-strand breaks.

PURPOSE: As chemotherapy and molecular therapy improve the systemic survival of breast cancer patients, the incidence of brain metastases increases. Few therapeutic strategies exist for the treatment of brain metastases because the blood-brain barrier severely limits drug access. We report the pharmacokinetic, efficacy, and mechanism of action studies for the histone deactylase inhibitor vorinostat (suberoylanilide hydroxamic acid) in a preclinical model of brain metastasis of triple-negative breast cancer. EXPERIMENTAL DESIGN: The 231-BR brain trophic subline of the MDA-MB-231 human breast cancer cell line was injected into immunocompromised mice for pharmacokinetic and metastasis studies. Pharmacodynamic studies compared histone acetylation, apoptosis, proliferation, and DNA damage in vitro and in vivo. RESULTS: Following systemic administration, uptake of [(14)C]vorinostat was significant into normal rodent brain and accumulation was up to 3-fold higher in a proportion of metastases formed by 231-BR cells. Vorinostat prevented the development of 231-BR micrometastases by 28% (P = 0.017) and large metastases by 62% (P < 0.0001) compared with vehicle-treated mice when treatment was initiated on day 3 post-injection. The inhibitory activity of vorinostat as a single agent was linked to a novel function in vivo: induction of DNA double-strand breaks associated with the down-regulation of the DNA repair gene Rad52. CONCLUSIONS: We report the first preclinical data for the prevention of brain metastasis of triple-negative breast cancer. Vorinostat is brain permeable and can prevent the formation of brain metastases by 62%. Its mechanism of action involves the induction of DNA double-strand breaks, suggesting rational combinations with DNA active drugs or radiation.

Effect of lapatinib on the outgrowth of metastatic breast cancer cells to the brain.

BACKGROUND: The brain is increasingly being recognized as a sanctuary site for metastatic tumor cells in women with HER2-overexpressing breast cancer who receive trastuzumab therapy. There are no approved or widely accepted treatments for brain metastases other than steroids, cranial radiotherapy, and surgical resection. We examined the efficacy of lapatinib, an inhibitor of the epidermal growth factor receptor (EGFR) and HER2 kinases, for preventing the outgrowth of breast cancer cells in the brain in a mouse xenograft model of brain metastasis. METHODS: EGFR-overexpressing MDA-MB-231-BR (231-BR) brain-seeking breast cancer cells were transfected with an expression vector that contained or lacked the HER2 cDNA and used to examine the effect of lapatinib on the activation (ie, phosphorylation) of cell signaling proteins by immunoblotting, on cell growth by the tetrazolium salt 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay, and on cell migration using a Boyden chamber assay. The outgrowth of large (ie, >50 microm(2)) and micrometastases was counted in brain sections from nude mice that had been injected into the left cardiac ventricle with 231-BR cells and, beginning 5 days later, treated by oral gavage with lapatinib or vehicle (n = 22-26 mice per treatment group). All statistical tests were two-sided. RESULTS: In vitro, lapatinib inhibited the phosphorylation of EGFR, HER2, and downstream signaling proteins; cell proliferation; and migration in 231-BR cells (both with and without HER2). Among mice injected with 231-BR-vector cells, those treated with 100 mg lapatinib/kg body weight had 54% fewer large metastases 24 days after starting treatment than those treated with vehicle (mean number of large metastases per brain section: 1.56 vs 3.36, difference = 1.80, 95% confidence interval [CI] = 0.92 to 2.68, P < .001), whereas treatment with 30 mg lapatinib/kg body weight had no effect. Among mice injected with 231-BR-HER2 cells, those treated with either dose of lapatinib had 50%-53% fewer large metastases than those treated with vehicle (mean number of large metastases per brain section, 30 mg/kg vs vehicle: 3.21 vs 6.83, difference = 3.62, 95% CI = 2.30 to 4.94, P < .001; 100 mg/kg vs vehicle: 3.44 vs 6.83, difference = 3.39, 95% CI = 2.08 to 4.70, P < .001). Immunohistochemical analysis revealed reduced phosphorylation of HER2 in 231-BR-HER2 cell-derived brain metastases from mice treated with the higher dose of lapatinib compared with 231-BR-HER2 cell-derived brain metastases from vehicle-treated mice (P < .001). CONCLUSIONS: Lapatinib is the first HER2-directed drug to be validated in a preclinical model for activity against brain metastases of breast cancer.

In vivo MRI of cancer cell fate at the single-cell level in a mouse model of breast cancer metastasis to the brain.

Metastasis (the spread of cancer from a primary tumor to secondary organs) is responsible for most cancer deaths. The ability to follow the fate of a population of tumor cells over time in an experimental animal would provide a powerful new way to monitor the metastatic process. Here we describe a magnetic resonance imaging (MRI) technique that permits the tracking of breast cancer cells in a mouse model of brain metastasis at the single-cell level. Cancer cells that were injected into the left ventricle of the mouse heart and then delivered to the brain were detectable on MR images. This allowed the visualization of the initial delivery and distribution of cells, as well as the growth of tumors from a subset of these cells within the whole intact brain volume. The ability to follow the metastatic process from the single-cell stage through metastatic growth, and to quantify and monitor the presence of solitary undivided cells will facilitate progress in understanding the mechanisms of brain metastasis and tumor dormancy, and the development of therapeutics to treat this disease.

Breast cancer metastasis to the central nervous system.

Clinically symptomatic metastases to the central nervous system (CNS) occur in approximately 10 to 15% of patients with metastatic beast cancer. CNS metastases are traditionally viewed as a late complication of systemic disease, for which few effective treatment options exist. Recently, patients with Her-2-positive breast tumors who were treated with trastuzumab have been reported to develop CNS metastases at higher rates, often while responding favorably to treatment. The blood:brain barrier and the unique brain microenvironment are hypothesized to promote distinct molecular features in CNS metastases that may require tailored therapeutic approaches. New research approaches using cell lines that reliably and preferentially metastasize in vivo to the brain have been reported. Using such model systems, as well as in vitro analogs of blood-brain barrier penetration and tissue-based studies, new molecular leads into this disease are unfolding.

A defect in the p53 response pathway induced by de novo purine synthesis inhibition.

p53 is believed to sense cellular ribonucleotide depletion in the absence of DNA strand breaks and to respond by imposition of a p21-dependent G1 cell cycle arrest. We now report that the p53-dependent G1 checkpoint is blocked in human carcinoma cell lines after inhibition of de novo purine synthesis by folate analogs inhibitory to glycinamide ribonucleotide formyltransferase (GART). p53 accumulated in HCT116, MCF7, or A549 carcinoma cells upon GART inhibition, but, surprisingly, transcription of several p53 targets, including p21cip1/waf1, was impaired. The mechanism of this defect was examined. The p53 accumulating in these cells was nuclear but was not phosphorylated at serines 6, 15, and 20, nor was it acetylated at lysines 373 or 382. The DDATHF-stabilized p53 bound to the p21 promoter in vitro and in vivo but did not activate histone acetylation over the p53 binding sites in the p21 promoter that is an integral part of the transcriptional response mediated by the DNA damage pathway. We concluded that the robust initial response of the p53 pathway to GART inhibitors is not transcriptionally propagated to target genes due to a defect in p53 post-translational modifications and a failure to open chromatin structure despite promoter binding of this unmodified p53.