Targeting bone metastases starting from the preneoplastic niche: home sweet home

Can the microRNA expression profile help to identify novel targets for zoledronic acid in breast cancer?

Zoledronic acid (ZOL), belonging to third generation bisphosphonate family, is a potent inhibitor of osteoclast-mediated bone resorption, widely used to effectively prevent osteolysis in breast cancer patients who develop bone metastases. Low doses of ZOL have been shown to exhibit a direct anticancer role, by inhibiting cell adhesion, invasion, cytoskeleton remodelling and proliferation in MCF-7 breast cancer cells. In order to identify the molecular mechanisms and signaling pathways underlying the anticancer activity exerted by ZOL, we analyzed for the first time the microRNA expression profile in breast cancer cells. A large-scale microarray analysis of 377 miRNAs was performed on MCF7 cells treated with 10 μM ZOL for 24 h compared to untreated cells. Furthermore, the expression of specific ZOL-induced miRNAs was analyzed in MCF-7 and SkBr3 cells through Real-time PCR. Low-dose treatment with ZOL significantly altered expression of 54 miRNAs. Nine upregulated and twelve downregulated miRNAs have been identified after 24 h of treatment. Also, ZOL induced expression of 11 specific miRNAs and silenced expression of 22 miRNAs. MiRNA data analysis revealed the involvement of differentially expressed miRNAs in PI3K/Akt, MAPK, Wnt, TGF-β, Jak-STAT and mTOR signaling pathways, and regulation of actin cytoskeleton. Our results have been shown to be perfectly coherent with the recent findings reported in literature concerning changes in expression of some miRNAs involved in bone metastasis formation, progression, therapy resistance in breast cancer. In conclusion, this data supports the hypothesis that ZOL-induced modification of the miRNA expression profile contributes to the anticancer efficacy of this agent.

Impact of breast cancer subtypes and patterns of metastasis on outcome

Clinical outcome of patients with stage IV breast cancer is dependent on tumor biology, extent, and localization of metastases. Routine imaging diagnostics for distant metastasis is not recommended by the national guidelines for breast cancer follow-up. In this study, we evaluated different patterns of metastases of cancer subtypes in order to generate hypotheses on individualization of follow-up after breast cancer in the adjuvant setting. Patients of the Regional Breast Cancer Center Dresden diagnosed within the years 2006-2011 were classified into the five intrinsic subtypes luminal A (ER+, Her2-, G1/2), luminal B/Her2 negative (ER+, Her2-, G3), triple positive (ER+, PR+, Her2+), Her2-enriched (ER-, Her2+), and triple negative (ER-, PR-, Her2-) and with a median follow-up of 45 months. Tumor stage at time of first diagnosis of breast cancer as well as time and site of metastasis at first diagnosis of distant metastatic disease was analyzed. Tumor specimen of 2284 female patients with primary breast cancer was classified into five subtypes. Distant recurrence-free survival at 3 years was most unfavorable in Her2-enriched (66.8 %), triple negative (75.9 %), and triple-positive breast cancer (81.7 %). The same subtypes most frequently presented with visceral metastases only at first presentation: Her2-enriched 46.9 %, triple negative 45.5 %, and triple-positive breast cancer 37.5 %. Longest median survival of 2.3 years was seen in luminal A and in Her2-enriched metastatic disease, respectively. Median survival was significantly better in the luminal A, Her2-enriched, and triple-positive subtype compared to triple-negative breast cancer (p < 0.005). Differences in time to metastatic disease, first localization of metastases, and overall survival after diagnosis of metastatic disease were shown. Considering new targeted therapies and the option of surgery of oligometastases, screening for visceral metastases might be reasonable after diagnosis of Her2-positive subtypes.

Intratumor heterogeneity: nature and biological significance

Intratumor heterogeneity inherent in the majority of human cancers is a major obstacle for a highly efficient diagnosis and successful prognosis and treatment of these diseases. Being a result of clonal diversity within the same tumor, intratumor heterogeneity can be manifested in variability of genetic and epigenetic status, gene and protein expression, morphological structure, and other features of the tumor. It is most likely that the appearance of this diversity is a source for the adaptation of the tumor to changes in microenvironmental conditions and/or a tool for changing its malignant potential. In any case, both processes result in the appearance of cell clones with different undetermined sets of hallmarks. In this review, we describe the heterogeneity of molecular disorders in various human tumors and consider modern viewpoints of its development including genetic and non-genetic factors of heterogeneity origin and the role of cancer stem cells and clonal evolution. We also systematize data on the contribution of tumor diversity to progression of various tumors and the efficiency of their treatment. The main problems are indicated in the diagnosis and therapy of malignant tumors caused by intratumor heterogeneity and possible pathways for their solution. Moreover, we also suggest the key goals whose achievement promises to minimize the problem of intratumor heterogeneity and to identify new prognostic, predictive, and target markers for adequate and effective treatment of cancer.

A single administration of combination therapy inhibits breast tumour progression in bone and modifies both osteoblasts and osteoclasts

We have previously shown that repeated sequential administration of doxorubicin, followed 24 h later by zoledronic acid, inhibits tumour growth in models of established breast cancer bone metastasis. As breast cancer patients only receive zoledronic acid every 3–4 weeks, the aim of the current study was to establish the anti-tumour and bone effects of a single administration of doxorubicin/zoledronic acid combination therapy in a bone metastasis model.

MDA-MB-231-GFP cells were injected i.c. in 6-week-old nude mice. On day 2, animals received PBS, doxorubicin (2 mg/kg i.v.), zoledronic acid (100 μg/kg s.c.) or doxorubicin followed 24 h later by zoledronic acid. Anti-tumour effects were assessed on days 15/23 by quantification of apoptotic and proliferating cells and changes in expression of genes implicated in apoptosis, proliferation and bone turnover. Bone effects were assessed by μCT analysis, bone histomorphometry and measurement of serum markers. A tumour-free control group was included.

Combination treatment reduced bone tumour burden compared to single agent or PBS control and increased levels of tumour cell apoptosis on day 15, but this was no longer detectable on day 23. Animals receiving zoledronic acid had increased bone density, without evidence of tumour-induced lesions. Bone histomorphometry showed that zoledronic acid caused a decrease in osteoblast and osteoclast numbers and an increase in osteoclast size, in both tumour-free and tumour-bearing animals.

Our data show that although zoledronic acid modifies the bone microenvironment through effects on both osteoblasts and osteoclasts, this does not result in a significant anti-tumour effect in the absence of doxorubicin.