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

Abstract

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.

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.

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.

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.