C2-Ceramide increases cytoplasmic calcium concentrations in human parathyroid cells

Ceramide induces a multicomponent intracellular calcium increase triggering the acrosome secretion in human sperm

Exocytosis of spermatozoon's secretory vesicle, named acrosome reaction (AR), is a regulated event that plays a central role in fertilization. It is coupled to a complex calcium signaling. Ceramide is a multitasking lipid involved in exocytosis. Nevertheless, its effect on secretion is controversial and the underlying cellular and molecular mechanisms remain unknown. Human spermatozoa are useful to dissect the role of ceramide in secretion given that the gamete is not capable to undergo any trafficking mechanisms other than exocytosis. We report for the first time, the presence of sphingolipid metabolism enzymes such as neutral-sphingomyelinase and ceramide synthase in sperm. Ceramidases are also present and active. Both the addition of cell-permeable ceramide and the rise of the endogenous one, increase intracellular calcium acting as potent inducers of exocytosis. Ceramide triggers AR in capacitated spermatozoa and enhances the gamete response to progesterone. The lipid induces physiological ultrastructural changes in the acrosome and triggers an exocytosis-signaling cascade involving protein tyrosine phosphatase 1B and VAMP2. Real-time imaging showed an increment of calcium in the cytosol upon ceramide treatment either in the absence or in the presence of extracellular calcium. Pharmacological experiments demonstrate that at early stages the process involves ryanodine receptors, CatSper (calcium channel of sperm), and store-operated calcium channels. We set out the signaling sequence of events that connect ceramide to internal calcium mobilization and external calcium signals during secretion. These results allow the coordination of lipids and proteins in a pathway that accomplishes secretion. Our findings contribute to the understanding of ceramide's role in regulated exocytosis and fertilization.

Assessment of cytosolic free calcium changes during ceramide-induced cell death in MDA-MB-231 breast cancer cells expressing the calcium sensor GCaMP6m

Alterations in Ca²⁺ signaling can regulate key cancer hallmarks such as proliferation, invasiveness and resistance to cell death. Changes in the regulation of intracellular Ca²⁺ and specific components of Ca²⁺ influx are a feature of several cancers and/or cancer subtypes, including the basal-like breast cancer subtype, which has a poor prognosis. The development of genetically encoded calcium indicators, such as GCaMP6, represents an opportunity to measure changes in intracellular free Ca²⁺ during processes relevant to breast cancer progression that occur over long periods (e.g. hours), such as cell death. This study describes the development of a MDA-MB-231 breast cancer cell line stably expressing GCaMP6m. The cell line retained the key features of this aggressive basal-like breast cancer cell line. Using this model, we defined alterations in relative cytosolic free Ca²⁺ ([Ca²⁺]_CYT) when the cells were treated with C2-ceramide. Cell death was measured simultaneously via assessment of propidium iodide permeability. Treatment with ceramide produced delayed and heterogeneous sustained increases in [Ca²⁺]_CYT. Where cell death occurred, [Ca²⁺]_CYT increases preceded cell death. The sustained increases in [Ca²⁺]_CYT were not related to the rapid morphological changes induced by ceramide. Silencing of the plasma membrane Ca²⁺ ATPase isoform 1 (PMCA1) was associated with an augmentation in ceramide-induced increases in [Ca²⁺]_CYT and also cell death. This work demonstrates the utility of GCaMP6 Ca²⁺ indicators for investigating [Ca²⁺]_CYT changes in breast cancer cells during events relevant to tumor progression, which occur over hours rather than minutes.

Sphingosine kinase 1 (Sphk1) negatively regulates platelet activation and thrombus formation

Sphingosine 1-phosphate (S1P) is a powerful regulator of platelet formation. Enzymes generating S1P include sphingosine kinase 1. The present study thus explored the role of sphingosine kinase 1 in platelet formation and function. Activation-dependent platelet integrin αIIbβ3 activation and secretion of platelets lacking functional sphingosine kinase 1 (sphk1(-/-)) and of wild-type platelets (sphk1(+/+)) were determined utilizing flow cytometry and chronolume luciferin assay. Cytosolic Ca(2+) activity ([Ca(2+)]i) and aggregation were measured using fura-2 fluorescence and aggregometry, respectively. In vitro platelet adhesion and thrombus formation were evaluated using a flow chamber with shear rates of 1,700 s(-1). Activation-dependent increase of [Ca(2+)]i, degranulation (release of alpha and dense granules), integrin αIIbβ3 activation, and aggregation were all significantly increased in sphk1(-/-) platelets compared with sphk1(+/+) platelets. Moreover, while platelet adhesion and thrombus formation under arterial shear rates were significantly augmented in Sphk1-deficient platelets, bleeding time and blood count were unaffected in sphk1(-/-) mice. In conclusion, sphingosine kinase 1 is a powerful negative regulator of platelet function counteracting degranulation, aggregation, and thrombus formation.

Alkaline ceramidase 2 (ACER2) and its product dihydrosphingosine mediate the cytotoxicity of N-(4-hydroxyphenyl)retinamide in tumor cells

Increased generation of dihydrosphingosine (DHS), a bioactive sphingolipid, has been implicated in the cytotoxicity of the synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) in tumor cells. However, how 4-HPR increases DHS remains unclear. Here we demonstrate that 4-HPR increases the expression of ACER2, which catalyzes the hydrolysis of dihydroceramides to generate DHS, and that ACER2 up-regulation plays a key role in mediating the 4-HPR-induced generation of DHS as well as the cytotoxicity of 4-HPR in tumor cells. Treatment with 4-HPR induced the accumulation of dihydroceramides (DHCs) in tumor cells by inhibiting dihydroceramide desaturase (DES) activity, which catalyzes the conversion of DHCs to ceramides. Treatment with 4-HPR also increased ACER2 expression through a retinoic acid receptor-independent and caspase-dependent manner. Overexpression of ACER2 augmented the 4-HPR-induced generation of DHS as well as 4-HPR cytotoxicity, and 4-HPR-induced death in tumor cells, whereas knocking down ACER2 had the opposite effects. ACER2 overexpression, along with treatment with GT11, another DES inhibitor, markedly increased cellular DHS, leading to tumor cell death, whereas ACER2 overexpression or GT11 treatment alone failed to do so, suggesting that both ACER2 up-regulation and DES inhibition are necessary and sufficient to mediate 4-HPR-induced DHS accumulation, cytotoxicity, and death in tumor cells. Taken together, these results suggest that up-regulation of the ACER2/DHS pathway mediates the cytotoxicity of 4-HPR in tumor cells and that up-regulating or activating ACER2 may improve the anti-cancer activity of 4-HRR and other DHC-inducing agents.

The calcium sensing receptor: from understanding parathyroid calcium homeostasis to bone metastases

The cloning of the calcium sensing receptor (CaR) confirmed that parathyroid cells monitor extracellular calcium concentration ([Ca2+]ext) via a receptor-type mechanism. This lead to the hypothesis that abnormalities in the expression and/or function of the CaR could explain the biochemical abnormalities in primary hyperparathyroidism (PHPT). Cultured cells from parathyroid adenomas of patients operated for PHPT were used to monitor real-time changes in intracellular calcium concentration ([Ca2+]i) as measured by fluorescent microscopy using the Fura-2/AM dye. We found that CaR agonists trigger release of intracellular calcium pools and such responses are amplified by increasing the affinity of IP3 receptors. Using confocal microscopy to monitor membrane trafficking in living parathyroid cells labelled with the fluorescent dye FM1-43, we found that a decrease in [Ca2+]i rather than an absolute change in [Ca2+]ext is the main stimulus for exocytosis from human parathyroid cells. These data suggest that, in PHPT, a defective signalling mechanism from the CaR allows cells from parathyroid adenomas to maintain low [Ca2+]i with uninhibited PTH secretion in the face of hypercalcaemia. Over longer periods of time, CaR controls parathyroid proliferation via changes in tyrosine phosphorylation. We found that multiple proteins of molecular weight 20-65 kDa are phosphorylated within 10-60 min in response to CaR agonists. Further work demonstrated that high [Ca2+]i stimulates the expression of bcl-2 oncoprotein in cultured human parathyroid cells and that, in parathyroid adenomas, predominant expression of bcl-2 rather than bax oncoprotein might prevent apoptosis and explain the slow growth rate of these tumours. More recently, it became apparent that CaR stimulates cell proliferation in several cell types not involved in calcium homeostasis. Using archived histological material from 65 patients who died with metastatic breast cancer, we identified CaR expression predominantly in tumours from patients who developed bone rather than visceral metastases (35 of 49 versus 7 of 16; P < 0.01, chi-squared test). These data suggest that CaR expression has the potential to become a new biological marker predicting the risk of bone metastases in patients with breast cancer. A prospective study should investigate if patients with CaR-positive tumours are more likely to develop bone metastases and whether they could benefit more from prophylactic treatment with bisphosphonates or the newly developed CaR antagonists.

Ceramides induce programmed cell death in Arabidopsis cells in a calcium-dependent manner

While the role of C2-ceramide in the induction of programmed cell death (PCD) in animal systems has been well documented, little is known of its role in plant cells. Here we show that C2-ceramide induces PCD in Arabidopsis suspension cultures, which is preceded by the generation of a calcium transient and an increase in reactive oxygen species (ROS). Inhibition of the calcium transient prevented cell death, whereas inhibition of ROS had no effect on cell survival. These observations suggest that calcium signalling plays a role in ceramide-induced PCD but is independent of the generation of ROS.

Ceramide 1-phosphate enhances calcium entry through voltage-operated calcium channels by a protein kinase C-dependent mechanism in GH4C1 rat pituitary cells

Sphingomyelin derivatives modulate a multitude of cellular processes, including the regulation of [Ca2+]i (the intracellular free calcium concentration). Previous studies have shown that these metabolites often inhibit calcium entry through VOCCs (voltage-operated calcium channels). In the present study, we show that, in pituitary GH4C1 cells, C1P (C2-ceramide 1-phosphate) enhances calcium entry in a dose-dependent manner. The phospholipase C inhibitor U73122 attenuated the response. C1P invoked a small, but significant, increase in the formation of inositol phosphates. Pre-treatment of the cells with pertussis toxin was without an effect on the C1P-evoked increase in [Ca2+]i. The effect of C1P was critically dependent on extracellular calcium, since no increase in [Ca2+]i was observed when cells in a calcium-free buffer were stimulated with C1P. Furthermore, if the cells were retreated with 300 nM of the VOCC inhibitor nimodipine, the effect of C1P was almost totally abolished. In addition, ceramide C8-1-phosphate evoked an increase in [Ca2+]i, but the onset of the response was slow compared with that of C1P. In cells treated with 1 mM thapsigargin for 15 min, C1P still evoked an increase in [Ca2+]i. In patch-clamp experiments in the whole-cell mode, C1P enhanced calcium entry through the VOCCs compared with vehicle-treated cells. Dialysis of the cells with C1P did not enhance the calcium current. On-cell patch-clamp experiments showed an enhanced probability of the VOCCs being open (P(open)) in the presence of C1P. Inhibition of PKC (protein kinase C) with GF109203X and down-regulation of PKC with PMA attenuated the C1P-evoked increase in [Ca2+]i. Furthermore, down-regulation of PKC abolished the effect of C1P on P(open). This is the first report showing that a sphingomyelin derivative enhances calcium entry through VOCCs.

Acute exposure of ceramide enhances cardiac contractile function in isolated ventricular myocytes

1. The sphingolipid ceramide, a primary building block for all other sphingolipids, is associated with growth arrest, apoptosis, and lipotoxic dysfunction. Interestingly, ceramide may attenuate high glucose-induced myocyte dysfunction, produce Ca2+ influx, and augment smooth muscle contraction. To determine the role of ceramide on cardiac excitation-contraction (E-C) coupling, electrically paced adult rat ventricular myocytes were acutely exposed to a cell-permeable ceramide analog (10 pm-100 microM) and the following indices were determined: peak shortening (PS), time-to-PS, time-to-90% relengthening, and the maximal velocity of shortening and relengthening (+/-dLdt). Intracellular Ca2+ properties were assessed using fura-2AM fluorescent microscopy. 2. Our results revealed a concentration- and time-dependent increase of PS in ventricular myocytes in response to ceramide associated with an increase in +/-dLdt. The maximal increase in PS was approximately 35% from control value and was maintained throughout the first 20 min of ceramide exposure. However, the ceramide-induced increase in PS was not maintained once the exposure time was beyond 20 min. Acute exposure of ceramide significantly enhanced intracellular Ca2+ release, although at a much lower concentration range. The ceramide-induced augmentation of PS was not significantly affected by inhibition of phosphatidylinositol (PI)-3-kinase, protein kinase C (PKC), ceramide-activated protein phosphatase (CAPP), and nitric oxide (NO) synthase. 3. Our data suggest that ceramide acutely augments the contractile function of cardiac myocytes through an alternative mechanism(s) rather than PI-3-kinase, PKC, CAPP, or NO.

Differential roles of 2, 6, and 8 carbon ceramides on the modulation of gap junctional communication and apoptosis during carcinogenesis

The inhibition of apoptosis and gap junctional intercellular communication (GJIC) has been implicated in tumor promotion. Ionizing radiation and oxidative toxicants activate sphingomyelinases resulting in the release of ceramides that control cell proliferation and apoptosis. A rat liver epithelial cell line treated with ceramides containing a 6 (C6) or 8 (C8) carbon acyl-group were potent inhibitors of GJIC and apoptosis, whereas a C2-ceramide was only a weak inhibitor of GJIC and strong inducer of apoptosis. Apoptosis induced by either serum deprivation or C2-ceramide was inhibited by the GJIC inhibitory C8-ceramide. In conclusion, these results suggest that a chronic release of ceramides with acyl groups larger than C6 might act as tumor promoters.

Ceramide 1-phosphate increases intracellular free calcium concentrations in thyroid FRTL-5 cells: evidence for an effect mediated by inositol 1,4,5-trisphosphate and intracellular sphingosine 1-phosphate

Sphingolipid (SP) derivatives have diverse effects on the regulation of intracellular free calcium concentrations ([Ca2+]i) in a multitude of non-excitable cells. In the present investigation, the effect of C2-ceramide 1-phosphate (C1P) on [Ca2+]i was investigated in thyroid FRTL-5 cells. C1P evoked a concentration-dependent increase in [Ca2+]i, both in a calcium-containing and a calcium-free buffer. A substantial part of the C1P-evoked increase in [Ca2+]i was due to calcium entry. The effect of C1P was attenuated by overnight pretreatment of the cells with pertussis toxin. Similar results were obtained with C8-ceramide 1-phosphate, although the magnitude of the responses was smaller than with C1P. The phospholipase C inhibitor U73122 attenuated the effect of C1P. C1P invoked a small, but significant, increase in inositol 1,4,5-trisphosphate (IP3). However, the effect of C1P on [Ca2+]i was inhibited by neither Xestospongin C, 2-aminoethoxydiphenylborate nor neomycin. C1P mobilized calcium from an IP3-sensitive calcium store, as C1P did not increase [Ca2+]i in cells pretreated with thapsigargin. The effect of C1P on [Ca2+]i was potently attenuated by dihydrosphingosine and dimethylsphingosine, two inhibitors of sphingosine kinase, but not by the inactive SP-derivative N -acetyl sphingosine. Stimulating the cells with C1P evoked an increase in the production of intracellular sphingosine 1-phosphate. C1P did not modulate DNA synthesis or the forskolin-evoked production of cAMP. The results indicate that C1P may be an important SP participating in cellular signalling.