Regulation of intracellular calcium release and PP1alpha in a mechanism for 4-hydroxytamoxifen-induced cytotoxicity.

Treatment with tamoxifen, or its metabolite 4-hydroxytamoxifen (4OHT), has cytostatic and cytotoxic effects on breast cancer cells in vivo and in culture. Although the effectiveness of 4OHT as an anti-breast cancer agent is due to its action as an estrogen receptor-alpha (ERalpha) antagonist, evidences show that 4OHT is also cytotoxic for ERalpha-negative breast cancer cells and can be effective therapy against tumors that lack estrogen receptors. These findings underscore 4OHT signaling complexities and belie the most basic understandings of 4OHT action and resistance. Here, we have investigated the effects of 4OHT on Ca2+ homeostasis and cell death in breast cancer cells in culture. Measurement of Ca2+ signaling in breast cancer cells showed that 4OHT treatment altered Ca2+ homeostasis and was cytotoxic for both an ERalpha+ and an ERalpha- cell line, MCF-7 and MDA-MB-231, respectively. Further investigation lead us to the novel discovery that 4OHT-induced increase of ATP-dependent Ca2+ release from the endoplasmic reticulum correlated with 4OHT-induced upregulation of protein phosphatase 1alpha (PP1alpha) and the inositol 1,4,5-trisphosphate receptor (IP3R). Blocking 4OHT-induced PP1alpha upregulation by siRNA strategy reduced the effects of 4OHT on both Ca2+ signaling and cytotoxicity. Results from these investigations strongly suggest a role for PP1alpha upregulation in a mechanism for 4OHT-induced changes to Ca2+ signaling that ultimately contribute to the cytotoxic effects of 4OHT.

Suppression of IP3-mediated calcium release and apoptosis by Bcl-2 involves the participation of protein phosphatase 1.

The involvement and potential interdependence of inositol trisphosphate (IP3) receptors and Bcl-2 in the regulation of Ca2+ signaling is not clear. Here, we have explored the mechanism(s) of how Bcl-2 suppresses the IP3-sensitive Ca2+ release in MCF-7 cells focusing on the possible role of protein phosphatase 1 (PP1). We found that through influences on protein-protein interaction, Bcl-2 may alter the balance between the effects of phosphatase (PP1) and kinase (PKA) on the IP3 R1 signaling complex. Using various experimental approaches including phosphatase inhibition and RNAi, we show that Bcl-2 by competing with IP3R1 for the binding of PP1 can reduce the IP3-mediated calcium signal and protect cells from mitochondrial dysfunction and cell death.

Regulation of Ca2+-induced permeability transition by Bcl-2 is antagonized by Drpl and hFis1.

The regulation of mitochondrial permeability transition (MPT) is essential for cell survival. Un-controlled opening of the MPT pore is often associated with cell death. Anti-death protein Bcl-2 can block MPT as assessed by the enhanced capacity of mitochondria to accumulate and retain Ca2+. We report here that two proteins of the mitochondrial fission machinery, dynamin-related protein (Drp1) and human mitochondrial fission protein (hFis1), have an antagonistic effect on Bcl-2. Drp1, with the assistance of hFis1, sensitizes cells to MPT by reducing the mitochondrial Ca2+ retention capacity (CRC). While the reduction of CRC by Drp1/hFis1 is linked to mitochondrial fission, the antagonism between Bcl-2 and Drp1 appears to be mediated by mutually exclusive interactions of the two proteins with hFis1 . The complexity of protein-protein interactions demonstrated in the present study suggests that in addition to the previously described role of Bcl-2 in the control of apoptosis, Bcl-2 may also participate directly or indirectly in the regulation of mitochondrial fission.

Altered Ca2+ homeostasis and impaired mitochondrial function in cardiomyopathy.

Altered Ca2+ homeostasis and myocyte death is a major characteristic of the hereditary cardiomyopathy in the dystrophic hamster. Despite numerous studies, the mechanisms that link calcium dysregulation and cell death in this animal model remain unclear. We have shown previously that the maintenance of mitochondrial Ca2+ homeostasis is essential for cell survival, and that loss of mitochondrial Ca2+ is closely correlated with cell death in cultured cells. Here, we have further investigated the role of mitochondrial Na+-Ca2+ exchange (NCE) in the development of cardiomyopathy in the dystrophic hamster. We found that the myocyte death was associated with elevated NCE activity together with a reduced level of matrix Ca2+, and impaired mitochondrial energetics. The upregulation of NCE activity in myopathic heart was also accompanied by enhanced expression of the sarcolemmal Na+-Ca2+ exchange (NCX) without alterations in the L-type Ca2+ channel expression. Treatment of dystrophic hamsters with diltiazem (a potent inhibitor of NCE and Ca2+ channels) prevented the occurrence of cell death and restored the normal expression of NCX. Our findings implicate the dysregulation of both sarcolemmal and mitochondrial Na+-Ca2+ exchange in cell death in the myopathic heart.

Role of mitochondria in ciprofloxacin induced apoptosis in bladder cancer cells.

PURPOSE: In our earlier series we showed that ciprofloxacin inhibits bladder tumor cell growth with concomitant S/G2M cell cycle arrest and reported an increased Bax-to-Bcl-2 ratio in cells undergoing cell death. In the current series we elucidated the molecular mechanisms by which ciprofloxacin induces apoptotic processes. MATERIALS AND METHODS: Ciprofloxacin mediated mitochondrial depolarization was detected by flow cytometry in HTB9 cells. Mitochondrial permeability transition was measured by spectrophotometry in isolated mitochondria treated with ciprofloxacin in the presence and absence of cyclosporin. The consequential decrease in mitochondrial calcium, cytochrome c release and Bax translocation to mitochondria, which resulted in the activation of caspase 3 leading to apoptotic cell death, was measured by biochemical and confocal microscopy. RESULTS: Mitochondrial depolarization was observed during ciprofloxacin induced apoptotic processes. Cyclosporin A, a known inhibitor of the mitochondrial permeability transition pore, protected cells against decreased mitochondrial potential. Also, ciprofloxacin induced an alteration of mitochondrial calcium as early as 5 minutes and this disruption of intracellular calcium homeostasis was prevented by cyclosporin. Ciprofloxacin also had a direct effect on swelling of isolated mitochondria, which was absent in the presence of cyclosporin. Mitochondrial changes were accompanied by cytochrome c release and caspase 3 activation. Our findings also showed Bcl-2 dependent subcellular redistribution of Bax to the mitochondrial membrane in ciprofloxacin treated bladder tumor cells. CONCLUSIONS: The disruption of calcium homeostasis, mitochondrial swelling and redistribution of Bax to the mitochondrial membrane are key events in the initiation of apoptotic processes in ciprofloxacin treated bladder cancer cells.