Cytokine-induced beta-cell apoptosis and its regulation by SOCS-1 and imidazoline compounds

<p>A selective destruction of pancreatic β-cells as a consequence of inflammation in the islets of Langerhans is a feature of type 1 diabetes. Pro-inflammatory cytokines secreted by T lymphocytes and macrophages infiltrating the pancreatic islets participate in the development of this autoimmune disease by acting directly on the β-cell. The aim of this thesis was to investigate mechanisms of β-cell dysfunction and death induced by the mixture of proinflammatory cytokines IL-1β, IFNγ and TNFα, i.e., under conditions modelling those during inflammation in type 1 diabetes. Furthermore, we aimed to study whether imidazoline compounds RX871024 and efaroxan can affect pancreatic β-cell death under these conditions and if so, to explore underlying molecular mechanisms.</p><p>Some imidazoline compounds can promote insulin secretion and have been discussed as potential therapeutic drugs in type 2 diabetes. Among those compounds are insulinotropic imidazolines RX871024 and efaroxan. It was previously shown that these imidazolines do not induce apoptosis in mouse pancreatic β-cells but even protect against IL-1β-induced primary β-cell apoptosis. The protective effect of RX871024 on IL-1β-induced β-cell apoptosis has been accompanied by inhibition of IL-1β-induced NO production. However, in a first study we have shown that the imidazoline compounds cannot protect pancreatic β-cells against death induced by a combination of pro-inflammatory cytokines IL-1β, IFNγ and TNFα, despite RX871024 decreases the cytokine-induced NO production both in islets and in β-cells. RX871024-induced decrease in p38 MAPK phosphorylation may explain the partial inhibitory effect of RX871024 on cytokine-induced NO production. Thus pancreatic β-cell death triggered by a mixture of pro-inflammatory cytokines IL-1β, IFNγ and TNFα, conditions resembling those that take place in type 1 diabetes, does not directly correlate with NO production and rather relies on other players which cannot be counteracted with agents such as imidazoline compounds.</p><p>Malignant insulinoma is an uncommon tumour, however, it has a poor prognosis. Chemotherapy to this tumour is not very effective. Therefore, search for effective and specific chemotherapeutical drugs for patients with malignant insulinomas is of utmost importance. Unlike primary β-cells where RX871024 was without any effect, the imidazoline compound selectively destructs insulinoma MIN6 cells and potentiates cytokine-induced insulinoma cell death. The cytotoxic effects of RX871024 does not include changes in NO production but involve increase in basal and cytokine-induced JNK activation associated with stimulation of initiator caspases-1, -8 and -9 and executor caspase-3. In contrast to primary mouse β-cells, there was no effect of cytokines or imidazolines on p38 activation in MIN6 cells. It has been shown that expression of SOCS-1, an endogenous inhibitor of IFNγ-induced signalling, in pancreatic β-cells protects NOD mice against diabetes. In a third study we investigated how signaling via JAK/STAT pathway controls cytokine-induced β-cell death. SOCS-1 overexpression diminishes activation of both caspase-8 and -9 in primary mouse β-cells leading to inhibition of cytokine-induced β-cell death. This finding in association with the observation that SOCS-1 does not affect glucose stimulated insulin release and islet cell death in the absence of cytokines indicates the possibility to use an elevation of SOCS-1 expression in the treatment of type 1 diabetes.</p><p>In conclusion, results of this thesis implicate that pancreatic β-cell death induced by mixture of pro-inflammatory cytokines IL-1β, IFNγ and TNFα cannot be counteracted with agents such as imidazoline compounds, but can be suppressed by inhibition of IFNγ-induced signalling. We have also found that RX871024 exerts selective cytotoxic effect towards insulinoma cells.</p><h3>List of scientific papers</h3><p>I. Zaitseva II, Sharoyko V, Storling J, Efendic S, Guerin C, Mandrup-Poulsen T, Nicotera P, Berggren PO, and Zaitsev SV. RX871024 reduces NO production but does not protect against pancreatic beta-cell death induced by proinflammatory cytokines. Biochem Biophys Res Commun. 2006;347(4):1121-8. <br><a href="https://doi.org/10.1016/j.bbrc.2006.06.197">https://doi.org/10.1016/j.bbrc.2006.06.197</a><br><br> </p><p>II. Zaitseva II, Storling J, Mandrup-Poulsen T, Berggren PO, and Zaitsev SV. The imidazoline RX871024 induces death of proliferating insulin-secreting cells by activation of c-jun N-terminal kinase. Cell Mol Life Sci. 2008;65(7-8):1248-55. <br><a href="https://doi.org/10.1007/s00018-008-7564-x">https://doi.org/10.1007/s00018-008-7564-x</a><br><br> </p><p>III. Zaitseva II, Hultcrantz M, Sharoyko V, Flodstrom-Tullberg M, Zaitsev SV, and Berggren PO. Suppressor of cytokine signaling-1 inhibits caspase activation and protects from cytokine-induced beta cell death. Cell Mol Life Sci. 2009;66(23):3787-95. <br><a href="https://doi.org/10.1007/s00018-009-0151-y">https://doi.org/10.1007/s00018-009-0151-y</a><br><br> </p>