Interleukin-1β induces posttranslational carboxymethylation and alterations in subnuclear distribution of lamin B in insulin-secreting RINm5F cells

We examined the effects of interleukin-1beta (IL-1beta) treatment on the distribution and degradation of lamin B in the nuclear fraction from insulin-secreting RINm5F cells. Western blot analysis indicated that IL-1beta treatment caused significant alterations in the redistribution of lamin B, specifically between the Triton X-100-soluble ( membrane) and - insoluble ( matrix) fractions of the nucleus. IL-1beta treatment also increased the lamin carboxymethyltransferase activity and the relative abundance of the carboxymethylated lamin in the nuclear fraction. A significant increase in the relative abundance of lamin B degradation products was also observed in the nuclear fraction from the IL-1beta-treated cells. These findings are compatible with a measurable increase in the lamin-degrading caspase-6 activity in IL-1beta-treated cells. Confocal microscopic observation of IL-1beta-treated cells suggested a significant dissociation of lamin B from the nuclear lamina and its subsequent association with the DNA-rich elements within the nucleus. N-G-monomethyl-L-arginine, a known inhibitor of inducible nitric oxide synthetase ( iNOS), markedly inhibited IL-1beta-induced iNOS gene expression, NO release, caspase-3 and caspase-6 activation, lamin B degradation, and loss of metabolic cell viability, indicating that the observed IL-1beta-induced effects on nuclear lamin B involve the intermediacy of NO. Together, our data support the hypothesis that IL-1beta treatment results in significant increase in the carboxymethylation of lamin B, which would place lamin B in a strategic location for its degradation mediated by caspases. This could possibly lead to dissolution of the nuclear envelope, culminating in the demise of the effete beta-cell.