Evidence of functional ryanodine receptor involved in apoptosis of prostate cancer (LNCaP) cells

BACKGROUND

Very little is known about the functional expression and the physiological role of ryanodine receptors in nonexcitable cells, and in prostate cancer cells in particular. Nonetheless, different studies have demonstrated that calcium is a major factor involved in apoptosis. Therefore, the calcium-regulatory mechanisms, such as ryanodine-mediated calcium release, may play a substantial role in the regulation of apoptosis.

METHODS

We assessed the presence of such functional receptors in LNCaP prostate cancer cells, using fluorimetric measurements of intracellular calcium and expression assays of mRNA encoding ryanodine receptors.

RESULTS

We show here that LNCaP cells responded to caffeine, a ryanodine receptor agonist, by mobilizing calcium. Another ryanodine receptor agonist, 4-chloro-m-cresol, had a similar effect and promoted calcium release. These effects were inhibited by pretreatment with ryanodine or thapsigargin. In addition to a calcium release, caffeine was able to produce a calcium entry blocked by nickel. We used a reverse transcription-polymerase chain reaction assay to investigate the expression of ryanodine receptors in LNCaP cells. Two types of ryanodine receptor mRNAs were expressed in LNCaP cells: RyR1 and RyR2 mRNAs. Finally, we show that ryanodine receptor activation by caffeine slightly stimulates apoptosis of prostate cancer cells, and that the inhibition of these receptors by ryanodine protects the cells against apoptosis.

CONCLUSIONS

The combination of results showed that LNCaP cells, derived from a human prostate cancer, express functional RyRs able to mobilize Ca(2+) from intracellular stores and which might control apoptosis.

Arachidonic acid-induced hormone release in somatotropes: involvement of calcium

Arachidonic acid (AA) has been implicated in signaling actions in several cell types including endocrine cells. In the present study, we investigated the effect of exogenous AA on GH release from dispersed pituitary cells and tried to elucidate the mechanism involved in this process. We show that AA stimulates GH release in a dose- and extracellular calcium-dependent manner. The effects of AA on cytosolic calcium concentration ([Ca2+]i) were studied using dual-emission microspectrofluorimetry in identified somatotropes. AA (1 microM) induced an increase in intracellular calcium concentration ([Ca2+]i) by stimulating Ca2+ influx through dihydropyridine-sensitive, voltage-dependent calcium channels. In these cells, the effects of AA were only reduced by the inhibition of protein kinase C (PKC) activity, suggesting that the fatty acid may act by both PKC-dependent and PKC-independent pathways. In order to determine whether AA metabolites were involved in the effects attributed to AA, and, if so, which ones, we inhibited the three arachidonate metabolic pathways: cyclo-oxygenase by indomethacin (50 microM), lipoxygenase by nordihydroguaiaretic acid (NGDA, 50 microM), and epoxygenase by 5,8,11, 14-eicosatetraynoic acid (ETYA, 10 microM). NGDA and ETYA reduced the effects of AA on GH release (50 and 74%, respectively) and inhibited the [Ca2+]i response, whereas indomethacin slightly potentiated both AA-induced GH release and [Ca2+]i increase. As these results suggested that lipoxygenase metabolites may be responsible for AA-induced Ca2+ influx and GH release, we tested the effects of 5-, 12- and 15-hydroperoxyeicosatetraenoic acids (5-, 12- and 15-HpETE) on [Ca2+]i and GH release. They all stimulated calcium influx and GH release in a dose-dependent manner, 12-HpETE being more potent than 5- and 15-HpETE. We conclude that lipoxygenase metabolites of arachidonic acid, particularly 12-HpETE, may be involved in the GH secretion mechanism, probably by facilitating Ca2+ influx via L-type Ca2+ channels.

Arachidonic acid increases cytosolic calcium and stimulates hormone release in rat lactotrophs

Arachidonic acid (AA) released from membrane phospholipids after activation of surface receptors causes cellular signaling actions in neurons and endocrine cells, including stimulation of prolactin (PRL) release from dissociated rat pituitary cells and clonal cells of the GH3 pituitary tumor line. In the present study, we investigated the effect of exogenous AA on PRL release from dispersed pituitary cells and tried to elucidate the mechanism involved in this process. The effects of AA on cytosolic Ca2+ concentration ([Ca2+]i) were studied using dual-emission microspectrofluorometry in identification lactotrophs and on PRL release in dispersed pituitary cell populations. AA had a dose-dependent effect on [Ca2+]i. At 1 microM, the Ca2+ increase was biphasic: a mobilization of intracellular Ca2+ from intracellular stores was followed by stimulation of Ca2+ influx. For lower concentrations (10 and 100 nM), only the stimulation of Ca2+ influx was observed. AA-induced Ca2+ influx and PRL release were not due to the stimulation of a phorbol 12-myristate 13-acetate-sensitive protein kinase C. In the same way, AA-stimulated PRL release and intracellular Ca2+ increase were independent of intracellular thapsigargin-sensitive Ca2+ pools. Furthermore, blockade of Ca2+ channels suppressed AA-induced PRL release. We hypothesize that Ca2+ influx plays a major role in AA-induced PRL release.