Effects of imidazoline binding site ligands on the growth and viability of clonal pancreatic beta-cells

Antineoplastic activity of idazoxan hydrochloride

PURPOSE

Idazoxan hydrochloride (IDA) is a 241 molecular weight imidazoline and adrenoreceptor ligand. It binds to mitochondrial membranes and promotes apoptosis of pancreatic beta cells. Since IDA has not been tested against tumor cells, the purpose of our study was to determine if IDA has antineoplastic activity.

METHODS

We used the conversion of a soluble tetrazolium salt to an insoluble formazan precipitate and differential staining cytotoxicity assays to determine if IDA was cytotoxic to cell lines of murine lung cancer and human prostate cancer, as well as to a variety of fresh human tumor samples. We used flow cytometry to analyze cell death and calreticulin expression.

RESULTS

IDA is cytotoxic to both cell lines and against aliquots of specimens of breast, gastric, lung, ovarian and prostate cancers as well as non-Hodgkin's lymphoma. It produces apoptotic cell death and promotes calreticulin expression, suggesting that IDA might be immunomodulatory in vivo.

CONCLUSION

We anticipate that IDA will be clinically useful in cancer treatment.

Desensitization of insulin secretion by depolarizing insulin secretagogues

Prolonged stimulation of insulin secretion by depolarization and Ca2+ influx regularly leads to a reversible state of decreased secretory responsiveness to nutrient and nonnutrient stimuli. This state is termed "desensitization." The onset of desensitization may occur within 1 h of exposure to depolarizing stimuli. Desensitization by exposure to sulfonylureas, imidazolines, or quinine produces a marked cross-desensitization against other ATP-sensitive K+ channel (KATP channel)-blocking secretagogues. However, desensitized beta-cells do not necessarily show changes in KATP channel activity or Ca2+ handling. Care has to be taken to distinguish desensitization-induced changes in signaling from effects due to the persisting presence of secretagogues. The desensitization by depolarizing secretagogues is mostly accompanied by a reduced content of immunoreactive insulin and a marked reduction of secretory granules in the beta-cells. In vitro recovery from a desensitization by the imidazoline efaroxan was nearly complete after 4 h. At this time point the depletion of the granule content was partially reversed. Apparently, recovery from desensitization affects the whole lifespan of a granule from biogenesis to exocytosis. There is, however, no direct relation between the beta-cell granule content and the secretory responsiveness. Even though a prolonged exposure of isolated islets to depolarizing secretagogues is often associated with the occurrence of ultrastructural damage to beta-cells, we could not find a cogent link between depolarization and Ca2+ influx and apoptotic or necrotic beta-cell death.

β-Cell toxicity of ATP-sensitive K+ channel-blocking insulin secretagogues

A prolonged exposure of isolated pancreatic islets to insulin secretagogues, the imidazolines phentolamine, alinidine and idazoxan (100microM each), the sulfonylurea tolbutamide (500microM), or the alkaloid quinine (100microM) resulted in morphological damage of 4-18% of beta-cells compared to less than 2% in controls. Thus, the question arose whether K(ATP) channel-blocking insulin secretagogues are beta-cell toxic as has already been suggested for sulfonylureas. The concentration- and time-dependency of the secretagogue-associated toxicity was documented by viability assays in insulin-secreting HIT T15 cells. Treatment for 24h with idazoxan reduced MTT conversion by 50% at 100microM and by 98% at 1000microM. Phentolamine and quinine reduced viability comparably at 1000microM, but were less toxic at 100microM. On the other hand, the imidazoline alinidine and the sulfonylurea tolbutamide were only moderately toxic (less than 40% viability loss at 1000microM). The imidazoline efaroxan appeared even to be non-toxic. Apoptotic DNA fragmentation and DEVD-caspase activation was observed at 100microM of idazoxan and phentolamine, whereas at 1000microM signs of necrosis predominated. Alinidine, tolbutamide and quinine treatment did not increase markers of apoptotic cell death. Blocking Ca(2+) influx by D600 did not diminish secretagogue-associated toxicity. Electron microscopy confirmed the validity of these observations for beta-cells in intact mouse islets. In summary, beta-cell toxicity of the tested insulin secretagogues varied widely and did not depend on a prolonged Ca(2+) influx via L-type Ca(2+) channels. Thus, secretagogue-mediated closure of K(ATP) channels is apparently not per se beta-cell toxic.