MiR-301a-3p Suppresses Estrogen Signaling by Directly Inhibiting ESR1 in ERα Positive Breast Cancer.

BACKGROUND/AIMS: MiRNA-301a-3p is an oncogenic miRNA whose expression is associated with tumor development, metastases and overall poor prognosis. Estrogen receptor α (ERα) is one of the estrogen hormone-activated transcription factors, which regulates a large number of genes and is involved in the mammary gland development. Expression of ERα is considered to be a good indicator for endocrine therapy and breast cancer survival. Loss of ERα in breast cancer patients indicates invasiveness and poor prognosis. In this study, we focus on the regulation of ERα by miR-301a and its role in transition from estrogen-dependent to estrogen-independent breast cancer. METHODS: Expression of miR-301a-3p was measured by qRT-PCR in tumor tissue samples from 111 patients with primary breast carcinoma and in mammospheres representing in vitro model of cancer stem-like cells. Dual reporter luciferase assay and complementary experiments were performed to validate ESR1 as a direct target of miR-301a-3p. The effect of miR-301a-3p on estrogen signaling was evaluated on the level of gene and protein expression and growth response to estrogens. Finally, the effect of miR-301a-3p expression on tumor growth was studied in nude mice. RESULTS: We identified ESR1 as a direct target of miR-301a-3p. Ectopic miR-301a-3p causes a decrease in ESR1 mRNA and protein level and modulates the expression of ERα target genes in ERα positive breast cancer cells. Consistently, miR-301a-3p causes a decrease in sensitivity of MCF7 cells to 17ß-estradiol and inhibits the growth of estrogen dependent tumor in nude mice. Yet, the mice tumors have significantly increased expression of genes related to cancer stem-like cells and epithelial to mesenchymal transition suggesting enrichment of the population of cells with more invasive properties, in line with our observation that miR-301a-3p expression is highly increased in mammospheres which show a decrease in estrogenic signaling. Importantly, miR-301a-3P level is also increased in primary breast cancer samples exhibiting an ER/PR negative phenotype. CONCLUSION: Our results confirm ESR1 as a direct target of miR-301a-3p and suggest that miR-301a-3p likely contributes to development of estrogen independence, which leads to a more invasive phenotype of breast cancer.

Selective Disruption of Respiratory Supercomplexes as a New Strategy to Suppress Her2high Breast Cancer.

AIMS: Expression of the HER2 oncogene in breast cancer is associated with resistance to treatment, and Her2 may regulate bioenergetics. Therefore, we investigated whether disruption of the electron transport chain (ETC) is a viable strategy to eliminate Her2high disease. RESULTS: We demonstrate that Her2high cells and tumors have increased assembly of respiratory supercomplexes (SCs) and increased complex I-driven respiration in vitro and in vivo. They are also highly sensitive to MitoTam, a novel mitochondrial-targeted derivative of tamoxifen. Unlike tamoxifen, MitoTam efficiently suppresses experimental Her2high tumors without systemic toxicity. Mechanistically, MitoTam inhibits complex I-driven respiration and disrupts respiratory SCs in Her2high background in vitro and in vivo, leading to elevated reactive oxygen species production and cell death. Intriguingly, higher sensitivity of Her2high cells to MitoTam is dependent on the mitochondrial fraction of Her2. INNOVATION: Oncogenes such as HER2 can restructure ETC, creating a previously unrecognized therapeutic vulnerability exploitable by SC-disrupting agents such as MitoTam. CONCLUSION: We propose that the ETC is a suitable therapeutic target in Her2high disease. Antioxid. Redox Signal. 26, 84-103.

The effect of mitochondrially targeted anticancer agents on mitochondrial (super)complexes.

The mitochondrial respiratory chain is organized into dynamic high molecular weight complexes that associate to form supercomplexes. The function of these SCs is to minimize the production of reactive oxygen species (ROS) generated during electron transfer within them and to efficiently transfer electrons to complex IV. These supra-molecular structures as well as whole mitochondria are stress-responsive and respond to mitochondrially targeted anti-cancer agent by destabilization and induction of massive production of ROS leading to apoptosis. We have recently developed mitochondrially targeted anti-cancer agents epitomized by the mitochondrially targeted analogue of the redox-silent compound vitamin E succinate, which belongs to the group of agents that kill cancer cells via their mitochondria-destabilizing activity, referred to as mitocans. To understand the molecular mechanism of the effect of such agents, the use of native blue gel electrophoresis and clear native electrophoresis coupled with in-gel activity assays, are methods of choice. The relevant methodology is described in this chapter.

Mitochondrially targeted vitamin E succinate modulates expression of mitochondrial DNA transcripts and mitochondrial biogenesis.

AIMS: To assess the effect of mitochondrially targeted vitamin E (VE) analogs on mitochondrial function and biogenesis. RESULTS: Mitochondrially targeted vitamin E succinate (MitoVES) is an efficient inducer of apoptosis in cancer cells. Here, we show that unlike its untargeted counterpart α-tocopheryl succinate, MitoVES suppresses proliferation of cancer cells at sub-apoptotic doses by way of affecting the mitochondrial DNA (mtDNA) transcripts. We found that MitoVES strongly suppresses the level of the displacement loop transcript followed by those of mtDNA genes coding for subunits of mitochondrial complexes. This process is coupled to the inhibition of mitochondrial respiration, dissipation of the mitochondrial membrane potential, and generation of reactive oxygen species. In addition, exposure of cancer cells to MitoVES led to decreased expression of TFAM and diminished mitochondrial biogenesis. The inhibition of mitochondrial transcription was replicated in vivo in a mouse model of HER2(high) breast cancer, where MitoVES lowered the level of mtDNA transcripts in cancer cells but not in normal tissue. INNOVATION: Our data show that mitochondrially targeted VE analogs represent a novel class of mitocans that not only induce apoptosis at higher concentrations but also block proliferation and suppress normal mitochondrial function and transcription at low, non-apoptogenic doses. CONCLUSIONS: Our data indicate a novel, selective anti-cancer activity of compounds that act by targeting mitochondria of cancer cells, inducing significant alterations in mitochondrial function associated with transcription of mtDNA-coded genes. These changes subsequently result in the arrest of cell proliferation.

Mitochondrial genome acquisition restores respiratory function and tumorigenic potential of cancer cells without mitochondrial DNA.

We report that tumor cells without mitochondrial DNA (mtDNA) show delayed tumor growth, and that tumor formation is associated with acquisition of mtDNA from host cells. This leads to partial recovery of mitochondrial function in cells derived from primary tumors grown from cells without mtDNA and a shorter lag in tumor growth. Cell lines from circulating tumor cells showed further recovery of mitochondrial respiration and an intermediate lag to tumor growth, while cells from lung metastases exhibited full restoration of respiratory function and no lag in tumor growth. Stepwise assembly of mitochondrial respiratory (super)complexes was correlated with acquisition of respiratory function. Our findings indicate horizontal transfer of mtDNA from host cells in the tumor microenvironment to tumor cells with compromised respiratory function to re-establish respiration and tumor-initiating efficacy. These results suggest pathophysiological processes for overcoming mtDNA damage and support the notion of high plasticity of malignant cells.

Mitochondrial targeting of α-tocopheryl succinate enhances its anti-mesothelioma efficacy.

UNLABELLED: Malignant mesothelioma (MM) is a fatal neoplastic disease with no therapeutic option. Therefore, the search for novel therapies is of paramount importance. METHODS: Since mitochondrial targeting of α-tocopheryl succinate (α-TOS) by its tagging with triphenylphosphonium enhances its cytotoxic effects to cancer cells, we tested its effect on MM cells and experimental mesotheliomas. RESULTS: Mitochondrially targeted vitamin E succinate (MitoVES) was more efficient in killing MM cells than α-TOS with IC50 lower by up to two orders of magnitude. Mitochondrial association of MitoVES in MM cells was documented using its fluorescently tagged analogue. MitoVES caused apoptosis in MM cells by mitochondrial destabilization, resulting in the loss of mitochondrial membrane potential, generation of reactive oxygen species, and destabilization of respiratory supercomplexes. The role of the mitochondrial complex II in the activity of MitoVES was confirmed by the finding that MM cells with suppressed succinate quinone reductase were resistant to MitoVES. MitoVES suppressed mesothelioma growth in nude mice with high efficacy. DISCUSSION: MitoVES is more efficient in killing MM cells and suppressing experimental mesotheliomas compared with the non-targeted α-TOS, giving it a potential clinical benefit.

Antiproliferative and antiangiogenic effects of flavone eupatorin, an active constituent of chloroform extract of Orthosiphon stamineus leaves.

Flavone eupatorin is one of the constituents of Orthosiphon stamineus, a medicinal herb used in folk medicine in South East Asia for treatment of various disorders. In our study, we investigated the antiproliferative properties of a chloroform extract of the leaves of O. stamineus and of pure eupatorin. The compound was able to reduce the number of viable cancer cells to the same extent as the extract, with IC(50) values in micromolar range. Moreover, both the eupatorin standard and the extract caused cells to arrest in the G2/M phase of the cell cycle. This clearly demonstrates that eupatorin contributes significantly to the overall extract activity. Induction of mitotic catastrophe, accompanied by key molecular events defining apoptosis, is the mechanism of eupatorin-induced cell death. Importantly, eupatorin (at the doses cytotoxic to cancer cells) did not kill normal cells; it only limited migration of HUVEC endothelial cells and their ability to create tubes. The ability of eupatorin to nonspecifically inhibit many protein kinases was proven and is the probable cause of its cellular effects. In summary, eupatorin emerges as a promising agent in anticancer research.