Diadenosine polyphosphates in insulin-secreting cells: interaction with specific receptors and degradation.

Abstract

A role of diadenosine polyphosphates as second messengers was suggested for insulin-secreting cells. It has not yet been investigated whether specific receptors for these compounds exist and how these extracellular compounds and their degradation products may contribute to insulin release. Specific saturable binding sites for diadenosine polyphosphates exist in INS-1 cells and rat pancreatic islets. In INS-1 cells, the rank order of diadenosine polyphosphates displacing [3H]Ap4A from binding sites was Ap4A = Ap5A >Ap3A = Ap6A. Binding was specific, since suramin was not able to displace the binding; adenosine, ATP, UTP, alpha,beta-methylene ATP, beta,gamma-methylene ATP, ADP-betaS, 2-methylthio ATP, and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) were able to displace [3H]Ap4A from its binding sites. Insulin release was investigated in INS-1 cells. Perifusion experiments showed an increase in insulin release stimulated by the diadenosine polyphosphates in the presence of 8.3 mmol/l glucose; in static incubations (90 min), however, insulin release was inhibited dose dependently by the four diadenosine polyphosphates. This discrepancy might be due to the instability of the compounds. [3H]Ap4A was degraded in the extracellular medium to mainly adenosine and low concentrations of ATP, ADP, AMP, and inosine (half-maximal degradation after 25 min). The insulin stimulatory effect is due to the original compounds (acute perifusion experiments), and the insulin inhibitory effect (static incubation experiments) is due to the production of inhibitory compounds, such as adenosine, in the medium. Small amounts of intact [3H]Ap4A, but mainly [3H]ATP, accumulated in the cells within 20 min. The uptake of labeled compounds is dependent on an intact metabolism and intact receptor internalization. This data indicates that 1) specific bindings sites for diadenosine polyphosphates exist in INS-1 cells and rat pancreatic islets mediating insulin release; 2) the receptors involved in INS-1 cells may be diadenosine polyphosphate receptors, albeit others, such as P2X-receptors, cannot be ruled out; and 3) diadenosine polyphosphates, and mainly their degradation products in the extracellular space, are to a high degree accumulated within cells with unknown function. Thus, diadenosine polyphosphates are worth being investigated more closely in physiological and pathophysiological terms.

Stimulatory effect of exogenous diadenosine tetraphosphate on insulin and glucagon secretion in the perfused rat pancreas

“…Given that glucose alone induces an elevation of AP4A in B‐cells (Ripoll et al ., 1996), it might be thought that such an increment represents the maximal insulinotropic activity of this nucleotide, and that a further increase in its concentration does not enhance the response. In agreement with our findings, Verspohl & Johannwille (1998) recently reported the stimulation of insulin output by AP4A in a preparation of perfused rat INS‐1 cells.…”

Influence of diadenosine tetraphosphate (Ap4A) on lipid metabolism

“…One possible explanation could be that the diadenosine polyphosphate is partly d. rü sing and e. j. verspohl degraded during this long incubation period. 7 Since in neither the in vivo experiments with rats nor in the in vitro investigations with HepG2 cells the sterol synthesis is increased in response to Ap 4 A, these results do not corroborate the hypothesis of a link between diadenosine tetraphosphate and an elevation of cholesterol biosynthesis or for increased plasma cholesterol levels typical of diabetes mellitus. The disturbed cholesterol metabolism often observed in patients with NIDDM does not seem to result from elevated levels of Ap 4 A.…”

“…Diadenosine polyphosphates have been proposed to be involved in glucose and insulin metabolism 7 and in the pathophysiology of diabetes mellitus. 5,6,8 Since NIDDM is often associated with obesity and altered levels of plasma lipids we investigated the influence of diadenosine tetraphosphate (Ap 4 A) on lipid metabolism.…”

“…5,6 In static incubation experiments with INS-1 cells Ap [3][4][5][6] A inhibit glucose-induced insulin release. 7 There is evidence for a correlation between the long-term development of diabetes mellitus and increased levels of diadenosine polyphosphates. 8 Since non-insulin dependent diabetes mellitus (NIDDM) is often accompanied by insulin resistance, obesity and dyslipidemia the hypothesis was tested of whether there may be a link between disturbed lipid metabolism and an increased level of diadenosine tetraphosphates (Ap 4 A).…”

Mining metabolic pathways through gene expression

“…Both AppppA and ApppA are known to have a role in insulin release (Verspohl and Johannwille, 1998) and have more recently been linked to diabetes (Rüsing and Verspohl, 2004; Verspohl et al , 2003). Additionally, cyclic AMP (cAMP) has recently been indicated in the production of GLP-1 that is a known factor in insulin secretion and Type II diabetes (Yu and Jin, 2010).…”