Hypo-osmotic stress inhibits doxorubicin-induced apoptosis via a protein kinase A-dependent mechanism in cardiomyocytes

Forskolin improves sensitivity to doxorubicin of triple negative breast cancer cells via Protein Kinase A-mediated ERK1/2 inhibition

Triple negative breast cancer (TNBC) is an invasive, metastatic, highly aggressive tumor. Cytotoxic chemotherapy represents the current treatment for TNBC. However, relapse and chemo-resistance are very frequent. Therefore, new therapeutic approaches that are able to increase the sensitivity to cytotoxic drugs are needed. Forskolin, a natural cAMP elevating agent, has been used for several centuries in medicine and its safeness has also been demonstrated in modern studies. Recently, forskolin is emerging as a possible novel molecule for cancer therapy. Here, we investigate the effects of forskolin on the sensitivity of MDA-MB-231 and MDA-MB-468 TNBC cells to doxorubicin through MTT assay, flow cytometry-based assays (cell-cycle progression and cell death), cell number counting and immunoblotting experiments. We demonstrate that forskolin strongly enhances doxorubicin-induced antiproliferative effects by cell death induction. Similar effects are observed with IBMX and isoproterenol cAMP elevating agents and 8-Br-cAMP analog, but not by using 8-pCPT-2'-O-Me-cAMP Epac activator. It is important to note that the forskolin-induced potentiation of sensitivity to doxorubicin is accompanied by a strong inhibition of ERK1/2 phosphorylation, is mimicked by ERK inhibitor PD98059 and is prevented by pre-treatment with Protein Kinase A (PKA) and adenylate cyclase inhibitors. Altogether, our data indicate that forskolin sensitizes TNBC cells to doxorubicin via a mechanism depending on the cAMP/PKA-mediated ERK inhibition. Our findings sustain the evidence of anticancer activity mediated by forskolin and encourage the design of future in-vivo/clinical studies in order to explore forskolin as a doxorubicin sensitizer for possible use in TNBC patients.

Involvement of intracellular cAMP in epirubicin-induced vascular endothelial cell injury

We investigated the involvement of intracellular cAMP in endothelial cell injury induced by epirubicin. Epirubicin-induced decrease in cell viability and increase in caspase-3/7 activity were reversed by a cAMP analog dibutyryl cAMP (DBcAMP) or an activator of adenylate cyclase forskolin concomitant with a phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. Moreover, epirubicin-induced elevation of lipid peroxide levels was attenuated by DBcAMP. Interestingly, the exposure of epirubicin decreased intracellular cAMP levels before the onset of epirubicin-induced production of lipid peroxidation. These results suggest that intracellular cAMP plays an important role in epirubicin-induced endothelial cell injury.

miR-8 microRNAs regulate the response to osmotic stress in zebrafish embryos

MicroRNAs (miRNAs) are highly conserved small RNAs that act as translational regulators of gene expression, exerting their influence by selectively targeting mRNAs bearing complementary sequence elements. These RNAs function in diverse aspects of animal development and physiology. Because of an ability to act as rapid responders at the level of translation, miRNAs may also influence stress response. In this study, we show that the miR-8 family of miRNAs regulates osmoregulation in zebrafish embryos. Ionocytes, which are a specialized cell type scattered throughout the epidermis, are responsible for pH and ion homeostasis during early development before gill formation. The highly conserved miR-8 family is expressed in ionocytes and enables precise control of ion transport by modulating the expression of Nherf1, which is a regulator of apical trafficking of transmembrane ion transporters. Ultimately, disruption of miR-8 family member function leads to an inability to respond to osmotic stress and blocks the ability to properly traffic and/or cluster transmembrane glycoproteins at the apical surface of ionocytes.

The volume-sensitive chloride channel inhibitors prevent both contractile dysfunction and apoptosis induced by doxorubicin through PI3kinase, Akt and Erk 1/2

Contractile dysfunction and cardiomyopathies secondary to apoptotic cell death are limiting factors for treating cancer with doxorubicin. Inhibition of volume-sensitive chloride currents (I(Cl,vol)) has been reported to blunt doxorubicin-induced apoptosis in cardiomyocytes. To investigate cellular contractility during acute induction of apoptosis by doxorubicin and to determine whether I(Cl,vol) inhibitors are able to prevent the subsequent contractile dysfunction, electrically paced ventricular myocytes freshly isolated from adult rabbits were acutely exposed to doxorubicin in the presence and absence of I(Cl,vol) inhibitors IAA-94 or DIDS. Doxorubicin induced increases in both annexin V labelling and caspase-3 activity and decreases in cell volume. Alteration in cardiac contractility was observed after doxorubicin exposure. Both IAA-94 and DIDS abolished the doxorubicin-induced decreases in peak shortening and cell volume as well as the increases in caspase-3 activity and annexin V labelling. These protective effects of I(Cl,vol) inhibitors were abolished by previous inhibition of PI(3)kinase, Akt and Erk 1/2. Thus, I(Cl,vol) inhibitors prevent doxorubicin-induced apoptosis and subsequent contractile dysfunction through PI(3)kinase/Akt and Erk 1/2. Inhibition of I(Cl,vol) may represent a new pharmacological strategy for developing cytoprotective drugs against apoptotic cell death and contractile dysfunction.

Cytoplasmic condensation is both necessary and sufficient to induce apoptotic cell death

Programmed cell death (apoptosis) is important in tissue maintenance. Hallmarks of apoptosis include caspase activation, DNA fragmentation and an overall reduction in cell volume. Whether this apoptotic volume decrease (AVD) is a mere response to initiators of apoptosis or whether it is functionally significant is not clear. In this study, we sought to answer this question using human malignant glioma cells as a model system. In vivo, high grade gliomas demonstrate an increased percentage of apoptotic cells as well as upregulation of death ligand receptors. By dynamically monitoring cell volume, we show that the induction of apoptosis, via activation of either the intrinsic or extrinsic pathways with staurosporine or TRAIL, respectively, resulted in a rapid AVD in D54-MG human glioma cells. This decrease in cell volume could be prevented by inhibiting the efflux of Cl(-) through channels. Such suppression of AVD also reduced the activation of caspases 3, 8 and 9 and suppressed DNA fragmentation. Importantly, experimental manipulations that reduce the cell volume to 70% of the original volume for periods of at least 3 hours were sufficient to initiate apoptosis even in the absence of death ligands. Hence, this data suggests that cell condensation is both necessary and sufficient for the induction of apoptosis.

ClC-3 chloride channel prevents apoptosis induced by thapsigargin in PC12 cells

Cell volume can be altered by two different ways, swelling and shrinkage. Cell swelling is regulated by volume-regulated Cl- channel (VRC). It is not well understood whether shrinkage is regulated by VRC. We previously found that antisense oligonucleotide specific to ClC-3 (ClC-3 antisense) prevented cell proliferation, which was related to cell swell volume regulation. In the present study, we further studied the role of ClC-3 Cl- channel in cell apoptosis which was related to cell shrinkage volume regulation by using antisense oligonucleotide specific to ClC-3 (ClC-3 antisense) and ClC-3 cDNA transfection techniques. We found that thapsigargin (TG), a specific inhibitor of the endoplasmic reticulum calcium ATPase, evoked apoptotic morphological changes (including cytoplasmic blebbing, condensation of nuclear chromatin, and the formation of apoptotic bodies), DNA laddering, and caspase-3 activation in PC12 cells (Pheochromocytoma-derived cell line). TG increased the cell apoptotic population with a decrease in cell viability. These effects were consistent with the decrease in endogenous ClC-3 protein expression, which was also induced by TG. Overexpression of ClC-3 significantly inhibited TG effect on PC12 cell apoptosis, whereas the ClC-3 antisense produced opposite effects and facilitated apoptosis induced by TG. Our data strongly suggest that ClC-3 channel in PC12 cells mediates TG-induced apoptotic process through inhibitory mechanism. Thus, it appears that ClC-3 Cl- channel mediates both cell proliferation and apoptosis through accelerative and inhibitory fashions, respectively.

The ClC-3 Cl− channel in cell volume regulation, proliferation and apoptosis in vascular smooth muscle cells

The volume-regulated Cl(-) current (I(Cl.vol)) is responsible for the transmembrane Cl(-) transport that is involved in cell volume regulatory mechanisms. Although the regulation of cell volume is a fundamental function of healthy cells for maintaining constant size, the molecular genetic identification of I(Cl.vol) is still being debated. Recent studies in vascular smooth muscle support the idea that ClC-3, a member of the voltage-gated ClC Cl(-) channel family, is the molecular component involved in the activation or regulation of I(Cl.vol). Moreover, gene-targeting studies in vascular smooth muscle cells (VSMCs) and other cell types indicate emerging roles of ClC-3 in cell proliferation and apoptosis. These findings indicate that ClC-3 might be involved in modulating vascular remodeling in hypertension and arteriosclerosis.