Ethanol and urea affect insulin secretion from islets and insulinoma cells by different mechanisms

Abstract

Secretion of insulin could be stimulated by several ways. Comparison of glucose-and swelling-induced mechanisms in pancreatic islets revealed the involvement of a novel signal transduction pathway with specific features of osmotically stimulated peptide hormone release including Ca 2+ independence and resistance to noradrenalin (NA) inhibition. Cell swelling can be induced by hypotonicity or small permeant molecules (e.g. ethanol, urea). Our experiments were aimed to compare the effect of these permeants on insulin secretion from natural system -freshly isolated pancreatic islets and rat insulinoma cell lines INS-1 and INS-1E. As expected glucose and both permeants (80 mM ethanol and urea in isosmotic medium) induced insulin release from islets and NA did not inhibit permeant-induced secretion. Although ethanol and urea induced similar swelling of tumor cells, they produced opposite effect on insulin secretion; while exposure to ethanol led to stimulation of insulin secretion, exposure to urea led to suppression in both types of neoplastic cells. Surprisingly, stimulating effect of ethanol was completely suppressed by NA in both tumor cell lines. Ethanol in hyperosmotic medium failed to stimulate and even inhibited insulin release from both tumor cell lines in present study indicating thus involvement of an osmotic component. In conclusion, the opposite effect of ethanol and urea on insulin secretion from insulinoma cells and sensitivity of ethanol stimulation to NA indicate utilization of different cellular signaling pathways in tumor cells as compared to natural β-cells. Participation of permeant effect in the mechanism of ethanol stimulation remains to be clarified.

Mechanism of Ethanol-Induced Insulin Secretion from INS-1 and INS-1E Tumor Cell Lines

“…Although permeants and hypotonicity have been believed to induce exocytosis via the same mechanismcell-swelling (for review see [23,70]), sensitivity of ethanol induced insulin secretion to noradrenalin in both tumor cell lines in our recent study [20] indicates different properties of the mechanism of ethanol-induced insulin secretion in these cell lines compared to hypotonicity; therefore its mediation by permeant effect, at least in tumor cells, has been questioned [20]. Moreover, INS-1E cells do respond to ethanol but not to hypotonicity at basal conditions [9].…”

“…Direct measurements of cell size revealed cell swelling in both cell lines after exposure to the ethanol [20]. The independence of cell swelling-induced insulin release from the extracellular Ca 2+ has been repeatedly demonstrated [4,7,11], here we tested also importance of Ca 2+ recruited from intracellular stores in ethanol induced insulin secretion from insulinoma cell lines.…”

“…Resulting cell swelling evokes an immediate secretory burst of hormones stored in secretory vesicles. Cell swelling induced in vitro by permeant molecules in isotonic medium stimulate prompt high amplitude hormone secretion from various type of cells [4,[13][14][15][16][17][18][19][20], but mechanism of its effect inducing exocytosis remains unclear [8,9]. Recently Best et al [21] showed that glycerol and urea can activate beta-cells via their rapid uptake across the beta-cell plasma membrane, possibly via AQP7.…”

“…It has been believed, that ethanol and hypotonicity induce exocytosis via the same mechanism (for review see [23]). Our recent experiments indicate, however, that the mechanism of ethanol-induced insulin secretion in the tumor cells INS-1E differs from that induced by hypotonicity; INS-1E cells are able to release insulin in response to ethanol [20] but not to hypotonicity [9]. This is of special interest since hypotonicity induces membrane depolarization and electrical activity in these cells [24].…”

Cell Swelling-induced Insulin Secretion from INS-1E Cells is Inhibited by Extracellular Ca²⁺ and is Tetanus Toxin Resistant

“…Recently Madsen et al [22] found that calcineurin and calcium/calmodulin suppress exocytosis of renin from juxtaglomerular cells independent of PKA. It is of interest that ethanol effect which in many aspects shares common mechanisms and properties with hypotonic stimulation [18,23,24] on insulin secretion from INS-1E cell line is not hindered by the presence of calcium [25]. It is likely that swelling and secretion-inducing effect of various permeants utilizes different pathways.…”