MicroRNAs and toxicology: A love marriage

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Circulating microRNAs can serve as novel toxicological biomarkers.

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MicroRNAs are non-invasive biomarkers for early detection of tissue injury.

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MicroRNAs regulate gene activity in tissues exposed to toxic substances.

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They are novel tools for identifying and monitoring safety risks in drug development.

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MicroRNA are highly-conserved and potentially useful in preclinical animal studies.

With the dawn of personalized medicine, secreted microRNAs (miRNAs) have come into the very focus of biomarker development for various diseases. MiRNAs fulfil key requirements of diagnostic tools such as i) non or minimally invasive accessibility, ii) robust, standardized and non-expensive quantitative analysis, iii) rapid turnaround of the test result and iv) most importantly because they provide a comprehensive snapshot of the ongoing physiologic processes in cells and tissues that package and release miRNAs into cell-free space. These characteristics have also established circulating miRNAs as promising biomarker candidates for toxicological studies, where they are used as biomarkers of drug-, or chemical-induced tissue injury for safety assessment. The tissue-specificity and early release of circulating miRNAs upon tissue injury, when damage is still reversible, are main factors for their clinical utility in toxicology. Here we summarize in brief, current knowledge of this field.

Secreted microvesicular miR-31 inhibits osteogenic differentiation of mesenchymal stem cells

Damage to cells and tissues is one of the driving forces of aging and age-related diseases. Various repair systems are in place to counteract this functional decline. In particular, the property of adult stem cells to self-renew and differentiate is essential for tissue homeostasis and regeneration. However, their functionality declines with age (Rando, 2006). One organ that is notably affected by the reduced differentiation capacity of stem cells with age is the skeleton. Here, we found that circulating microvesicles impact on the osteogenic differentiation capacity of mesenchymal stem cells in a donor-age-dependent way. While searching for factors mediating the inhibitory effect of elderly derived microvesicles on osteogenesis, we identified miR-31 as a crucial component. We demonstrated that miR-31 is present at elevated levels in the plasma of elderly and of osteoporosis patients. As a potential source of its secretion, we identified senescent endothelial cells, which are known to increase during aging in vivo (Erusalimsky, 2009). Endothelial miR-31 is secreted within senescent cell-derived microvesicles and taken up by mesenchymal stem cells where it inhibits osteogenic differentiation by knocking down its target Frizzled-3. Therefore, we suggest that microvesicular miR-31 in the plasma of elderly might play a role in the pathogenesis of age-related impaired bone formation and that miR-31 might be a valuable plasma-based biomarker for aging and for a systemic environment that does not favor cell-based therapies whenever osteogenesis is a limiting factor.

Secretion of microvesicular miRNAs in cellular and organismal aging

Changes of factors circulating in the systemic environment during human aging have been investigated for a long time. Only recently however, miRNAs have been found to be secreted into the systemic and tissue environments where they are protected from RNAses by either carrier proteins or by being packaged into microvesicles. These miRNAs are then taken up by recipient cells, changing the cellular behavior by the classical miRNA induced silencing of target mRNAs. The origin of circulating miRNAs, however, is in most instances unclear, but senescent cells emerge as a possible source of such secreted miRNAs. Since differences in the circulating miRNAs have been found in a variety of age-associated diseases, and accumulation of senescent cells in the elderly emerges as a possible detrimental factor in aging, it is well conceivable that these miRNAs might contribute to the functional decline observed during aging of organisms.

Therefore, we here give an overview on current knowledge on microvesicular secretion of miRNAs, changes of the systemic and tissue environments during aging of cells and organisms. Finally, we summarize current knowledge on miRNAs that are found to be specific for age-associated diseases.

From cellular senescence to age-associated diseases: the miRNA connection

Cellular senescence has evolved from an in-vitro model system to study aging in vitro to a multifaceted phenomenon of in-vivo importance as senescent cells in vivo have been identified and their removal delays the onset of age-associated diseases in a mouse model system. From the large emerging class of non-coding RNAs, miRNAs have only recently been functionally implied in the regulatory networks that are modified during the aging process. Here we summarize examples of similarities between the differential expression of miRNAs during senescence and age-associated diseases and suggest that these similarities might emphasize the importance of senescence for the pathogenesis of age-associated diseases. Understanding such a connection on the level of miRNAs might offer valuable opportunities for designing novel diagnostic and therapeutic strategies.

α1-adrenergic drugs exhibit affinity to a thapsigargin-sensitive binding site and interfere with the intracellular Ca2+ homeostasis in human erythroleukemia cells

Even though the erythroleukemia cell lines K562 and HEL do not express α1-adrenoceptors, some α1-adrenergic drugs influence both survival and differentiation of these cell lines. Since Ca2+ is closely related to cellular homeostasis, we examined the capacity of α1-adrenergic drugs to modulate the intracellular Ca2+ content in K562 cells. Because of morphological alterations of mitochondria following α1-adrenergic agonist treatment, we also scrutinized mitochondrial functions. In order to visualize the non-adrenoceptor binding site(s) of α1-adrenergic drugs in erythroleukemia cells, we evaluated the application of the fluorescent α1-adrenergic antagonist BODIPY® FL-Prazosin. We discovered that the α1-adrenergic agonists naphazoline, oxymetazoline and also the α1-adrenergic antagonist benoxathian are able to raise the intracellular Ca2+-content in K562 cells. Furthermore, we demonstrate that naphazoline treatment induces ROS-formation as well as an increase in Δψm in K562 cells. Using BODIPY® FL-Prazosin we were able to visualize the non-adrenoceptor binding site(s) of α1-adrenergic drugs in erythroleukemia cells. Interestingly, the SERCA-inhibitor thapsigargin appears to interfere with the binding of BODIPY® FL-Prazosin. Our data suggest that the effects of α1-adrenergic drugs on erythroleukemia cells are mediated by a thapsigargin sensitive binding site, which controls the fate of erythroleukemia cells towards differentiation, senescence and cell death through modulation of intracellular Ca2+.

Induction of autophagy by spermidine promotes longevity

Ageing results from complex genetically and epigenetically programmed processes that are elicited in part by noxious or stressful events that cause programmed cell death. Here, we report that administration of spermidine, a natural polyamine whose intracellular concentration declines during human ageing, markedly extended the lifespan of yeast, flies and worms, and human immune cells. In addition, spermidine administration potently inhibited oxidative stress in ageing mice. In ageing yeast, spermidine treatment triggered epigenetic deacetylation of histone H3 through inhibition of histone acetyltransferases (HAT), suppressing oxidative stress and necrosis. Conversely, depletion of endogenous polyamines led to hyperacetylation, generation of reactive oxygen species, early necrotic death and decreased lifespan. The altered acetylation status of the chromatin led to significant upregulation of various autophagy-related transcripts, triggering autophagy in yeast, flies, worms and human cells. Finally, we found that enhanced autophagy is crucial for polyamine-induced suppression of necrosis and enhanced longevity.

Acute adrenergic stress inhibits proliferation of murine hematopoietic progenitor cells via p38/MAPK signaling

Acute adrenergic stress is a cause of hematopoietic failure that accompanies severe injury. Although the communication between neuronal and immune system is well documented and catecholamines are known as important regulators of homeostasis, the molecular mechanisms of hematopoietic failure are not well understood. To study the influence of adrenergic stress on hematopoietic progenitor cells (HPCs), which recently have been found to express adrenergic receptors, Lin(-),Sca(+), cells were isolated and treated with alpha- and beta-adrenergic agonists in vitro. Indeed, this stimulation resulted in significantly decreased colony formation capacity using granulocyte/macrophage colony-forming unit assays. This decline was dependent on the formation of reactive oxygen species (ROS) and activation of the p38/mitogen-activated protein kinase (MAPK) pathway, since the addition of antioxidants or a p38 inhibitor restored CFU formation. DNA damage by adrenergically induced ROS, however, does not seem to account for the reduction of colonies. Thus, catecholamine/p38/MAPK is identified as a key signal transduction pathway in HPCs besides those dependent on Wnt, Notch, and sonic hedgehog. Furthermore, a well-known target of p38 signaling, p16 is transcriptionally activated after adrenergic stimulation, suggesting that cell cycle arrest might importantly contribute to hematopoietic failure and immune dysfunctions after severe injury. Since increased levels of catecholamines are also observed in other conditions, such as during aging which is linked with decline of immune functions, adrenergic stress might as well contribute to the lowered immune defence in the elderly.

lin-Sca-1+ cells and age-dependent changes of their proliferation potential are reliant on mesenchymal stromal cells and are leukemia inhibitory factor dependent

Aging as a process is paralleled by a variety of hematological alterations. Characteristic features are a diminished homeostatic control of blood cell production and a decline in immune functions. It is generally accepted that stromal cells play a basal role in hematopoiesis by providing survival and differentiation signals, by secreting cytokines, or through direct contact with hematopoietic stem cells, thereby supporting the generation and replenishment of hematopoi- etic progenitor cells (HPC). Here we demonstrated that HPC-related colony formation is positively influenced by mesenchymal stromal cells (MSCs) when grown in co-culture, in particular regarding the number of primary granulocyte/macrophage colony-forming units as well as with respect to the average size of the formed colonies. These effects were more pronounced when the MSCs originated from young donors than from old ones. Because leukemia inhibitory factor (LIF) plays an important role during hematopoiesis, properties of lin- Sca-1+ cells and MSCs derived from LIF-deficient mice (LIF-/-) were determined both ex vivo and in vitro. LIF-/- animals contain a significantly reduced number of lin- Sca-1+ cells, nevertheless the replating capacity of LIF-/- HPCs was found to be generally unchanged when compared to those from LIF+/+ animals. However, when cocultured with MSCs, LIF-/- lin- Sca-1+ cells exhibited comparable characteristics to HPCs derived from old wild-type animals.

Anticancer activity of novel extracts from Cautleya gracilis (Smith) Dandy: apoptosis in human medullary thyroid carcinoma cells

BACKGROUND

Medullary thyroid carcinoma (MTC) is a calcitonin-producing tumor of the thyroid arising from the parafollicular C-cells. MTC is poorly responsive to chemotherapy and radiotherapy, hence the only effective therapy is surgery. Based on this fact, alternative strategies have been sought.

MATERIALS AND METHODS

The effects of Cautleya gracilis (Smith) Dandy were investigated for the first time in three human MTC cell lines and in MTC-transplanted mice. Proliferation and viability were quantified by cell counting, WST-1 tests, and ATP luminescent cell viability assays. Apoptosis was studied by DAPI staining, flow cytometry and luminescent assays for caspases 3/7, 8 and 9.

RESULTS

A dose-dependent reduction of proliferation and an induction of apoptosis were found in all MTC cell lines, while normal fibroblasts were not impaired. Similar tumor inhibition was seen in heterotransplanted mice.

CONCLUSION

Our in vitro and in vivo findings suggest a new potential clinical effect of Cautleya.

The dichloromethane fraction of Stemona tuberosa Lour inhibits tumor cell growth and induces apoptosis of human medullary thyroid carcinoma cells

Medullary thyroid carcinoma (MTC), a neuroendocrine tumor arising from the thyroid gland, is known to be poorly responsive to conventional chemotherapy. The root of Stemona tuberosa Lour, also called Bai Bu, is a commonly used traditional Chinese anti-tussive medicine. The present study investigated this medicinal herb for the first time with respect to its anticancer activity in human medullary thyroid carcinoma cells. Four extracts of Stemona tuberosa Lour, including the n-hexane fraction, (ST-1), dichloromethane (DCM) fraction, (ST-2), ethyl acetate (EtOAc) fraction, (ST-3), and methanol fraction, (ST-4) were examined for antiproliferative effects in two MTC cell lines. We observed that only the DCM fraction ST-2 inhibited cell growth and viability in a dose-dependent manner. Furthermore, we found that ST-2 also induced the apoptosis of MTC-SK cells. Caspase-3/7 was activated, while caspase-9 was not, implying that at least a caspase-dependent apoptotic pathway was involved in this process. In addition, the multicellular spheroids of MTC-SK were destroyed and the cell morphology was changed by ST-2. Our results show the strong apoptotic effects of the DCM fraction of Stemona tuberosa Lour on human medullary thyroid carcinomas, so suggesting a new candidate for chemotherapy of the so far chemo-resistant medullary thyroid carcinoma.

Norepinephrine treatment and aging lead to systemic and intracellular oxidative stress in rats

Reactive oxygen species (ROS) play important roles in cellular senescence and organismic aging. Furthermore, they have been implicated in some of the adverse effects of chronic stress due to elevated peripheral levels of catecholamines. Here, we applied three different techniques to individually compare the systemic and intracellular oxidative stress in aged (23 months) and young (5 months) Sprague-Dawley rats, and in young rats treated for 12 or 24 h with norepinephrine (NE). Thiol groups of blood serum proteins (RSH) were determined by means of Ellman's reaction. Intracellular ROS were assessed in spleen cells and peripheral blood lymphocytes (PBL) by carbonylation of cellular (spleen) proteins as determined by immunoblotting (Oxyblot) and/or by means of 2',7'-dichlorofluorescein (DCF) fluorescence. As compared to the young, untreated controls, both old rats and NE treated young rats showed similarly lowered RSH values paralleled by elevated intracellular ROS levels or enhanced Oxyblot signals. Individual RSH values were highly significantly, negatively correlated with respective Oxyblot data as well as with DCF fluorescence. The results confirm the roles of ROS in aging and adrenergic stress in the rat model, and suggest that the decrease in RSH of blood serum may be taken as a valid indicator for the enhanced oxidative stress in lymphocytes.

The non-competitive metabotropic glutamate receptor-1 antagonist CPCCOEt inhibits the in vitro growth of human melanoma

Five decades ago, the dicarboxylic amino acid glutamate became recognized as the major excitatory neurotransmitter in the central nervous system. In recent years, the expression of glutamate receptors was detected also in peripheral, non-neuronal tissues. Furthermore, it was found that glutamate stimulated the proliferation and migration of several peripheral tumor cells, and that glutamate receptor antagonists limited tumor growth. Most of these studies, however, used broad spectrum compounds and/or group-specific antagonists. Here we report that a selective, non-competitive metabotropic glutamate receptor-1 antagonist, CPCCOEt (7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester), significantly inhibited the proliferation and modified the morphology of two human melanoma cell lines. These effects were independent of the external glutamate level in the culture medium. In addition, CPCCOEt significantly enhanced the tumoricidal effects of cytostatic drugs. Thus, selective non-competitive metabotropic glutamate receptor antagonists may be used alone and/or with the synergistic effects of chemotherapy, thus enhancing existing therapies of melanoma and possibly other malignancies.

T-cadherin mediates low-density lipoprotein-initiated cell proliferation via the Ca(2+)-tyrosine kinase-Erk1/2 pathway

The GPI-anchored protein T-cadherin was found to be an atypical LDL binding site that is expressed in various types of cells, including endothelial cells, smooth muscle cells, and neurons. Notably, the expression of T-cadherin was reduced in numerous types of cancers, although it was up-regulated in tumor-penetrating blood vessels, atherosclerotic lesions, and during neointima formation. Despite these intriguing findings, our knowledge of the physiological role and the signal transduction pathways associated with this protein is limited. Therefore, T-cadherin was overexpressed in the human umbilical vein-derived endothelial cell line EA.hy926, the human embryonic kidney cell line HEK293, and LDL-initiated signal transduction, and its consequences were elucidated. Our data revealed that T-cadherin serves as a receptor specifically for LDL. Following LDL binding to T-cadherin, mitogenic signal transduction was initiated that involved activation of PLC and IP3 formation, which subsequently yielded intracellular Ca2+ mobilization. Downstream to these early phenomena, activation of tyrosine kinase(s) Erk 1/2 kinase, and the translocation of NF kappa B toward the nucleus were found. Finally, overexpression of T-cadherin in HEK293 cells resulted in accelerated cell proliferation in an LDL-dependent manner, although cell viability was not influenced. Because LDL uptake was not facilitated by T-cadherin, our data suggest that T-cadherin serves as a signaling receptor for LDL that facilitates an LDL-dependent mitogenic signal in the vasculature.

Restoration of sterol-regulatory-element-binding protein-1c gene expression in HepG2 cells by peroxisome-proliferator-activated receptor-gamma co-activator-1alpha

The expression of SREBP-1 (sterol-regulatory-element-binding protein-1) isoforms differs between tissues and cultured cell lines in that SREBP-1a is the major isoform in established cell lines, whereas SREBP-1c predominates in liver and most other human tissues. SREBP-1c is transcriptionally less active than SREBP-1a, but is a main mediator of hepatic insulin action and is selectively up-regulated by LXR (liver X receptor) agonists. LXR-mediated transactivation is co-activated by PGC-1alpha (peroxisome-proliferator-activated receptor-gamma co-activator-1alpha), which displays deficient expression in skeletal-muscle-derived cell lines. In the present paper, we show that PGC-1alpha expression is also deficient in HepG2 cells and in a human brown adipocyte cell line (PAZ6). In transient transfection studies, PGC-1alpha selectively amplified the LXR-mediated transcription from the human SREBP-1c promoter in HepG2 and PAZ6 cells via two LXR-response elements with extensive similarity to the respective murine sequence. Mutational analysis showed that the human LXR-response element-1 (hLXRE-1) was essential for co-activation of LXR-mediated SREBP-1c gene transcription by PGC-1alpha. Ectopic overexpression of PGC-1alpha in HepG2 cells enhanced basal SREBP-1c and, to a lesser extent, -1a mRNA expression, but only SREBP-1c expression was augmented further in an LXR/RXR (retinoic X receptor)-dependent fashion, thereby inducing mRNA abundance levels of SREBP-1c target genes, fatty acid synthase and acetyl-CoA carboxylase. These results indicate that PGC-1alpha contributes to the regulation of SREBP-1 gene expression, and can restore the SREBP-1 isoform expression pattern of HepG2 cells to that of human liver.

Potentiation of liver X receptor transcriptional activity by peroxisome-proliferator-activated receptor gamma co-activator 1 alpha

Peroxisome-proliferator-activated receptor (PPAR) gamma co-activator 1 alpha (PGC-1 alpha/PPARGC1) plays an important role in energy metabolism by co-ordinating transcriptional programmes of mitochondrial biogenesis, adaptive thermogenesis and fatty acid beta-oxidation. PGC-1 alpha has also been identified to play a role in the intermediary metabolism by co-activating key transcription factors of hepatic gluconeogenesis and glucose uptake in muscles. In the present study, we show that PGC-1 alpha serves as a co-activator for the liver X receptor (LXR) alpha, known to contribute to the regulation of cellular cholesterol homoeostasis. In transient transfection studies, PGC-1 alpha amplified the LXR-mediated autoregulation of the LXR alpha promoter in a human brown adipocyte line and in 3T3-L1 cells via an LXR response element described previously. LXR-mediated transactivation via a natural LXR response element from the cholesteryl ester transfer-protein gene promoter was also enhanced by PGC-1 alpha in a ligand-dependent manner. Mutational analysis showed that the LXXLL signature motif (L2) of PGC-1 alpha was essential for co-activation of LXR-mediated transcriptional responses. This motif is located in the vicinity of the binding region for a putative repressor described previously. The repressor sequesters PGC-1 alpha from PPAR alpha and the glucocorticoid receptor, and this repressor did not interfere with PGC-1 alpha-mediated co-activation of LXR-dependent gene transcription. Moreover, inhibition of p38 mitogen-activated protein kinase signalling, shown to abolish the co-activation of PPAR alpha by PGC-1 alpha, had only a moderate inhibitory effect on the co-activation of LXR. These results identify PGC-1 alpha as a bona fide LXR co-activator and implicate distinct interfaces of PGC-1 alpha and/or additional cofactors in the modulation of LXR and PPAR alpha transcriptional activities.