Circadian desynchronization desensitizes insulin-producing cells to cytokine-mediated transcriptomic remodeling and cell death: a novel beta-cell anti-apoptotic response to inflammation
Perturbation of the circadian clock is a risk factor for metabolic diseases. {beta}-Cell specific clock disruption causes glucose intolerance in mice, associated with oxidative stress and secretory failure in {beta}-cells. Proinflammatory cytokines alter the expression of core-clock machinery in human and rodent {beta}-cells, but the molecular mechanisms and consequences for cell viability are unclear. We hypothesized that cytokine-mediated clock perturbation in {beta}-cells is NF-{kappa}B driven, concomitant with cytokine-induced apoptosis, and depends on the cellular synchronization status. Cytokine-mediated changes of core-clock mRNA expression observed in non-synchronized INS-1 cells were potentiated in synchronized cells. These transcriptional changes differentially translated into alterations in core-clock protein levels. Interestingly, synchronization also sensitized INS-1 cells to cytokine-mediated cytotoxicity, associated with potentiation in the expression of inducible (ind) proteasomal catalytic subunits, ER stress markers, NF-{kappa}B activity, and activation of the intrinsic apoptotic pathway. Small-molecule NF- {kappa}B inhibition abrogated cytokine-mediated regulation of clock gene expression in both synchronized and non-synchronized INS-1 cells and reversed cytokine-mediated alterations in circadian parameters in INS-1 reporter cells at non-cytotoxic concentrations. However, at cytotoxic cytokine concentrations, NF-{kappa}B inhibition caused a loss of circadian rhythmicity while still reducing the cytotoxic effects of cytokines, indicating a differential effect of NF-{kappa}B signaling in controlling {beta}-cell viability and clock regulation. We propose that in synchronized cells, the proinflammatory transcriptional activity of NF-{kappa}B is enhanced by interaction with clock transcription factors, as has been suggested for the clock activator Brain and muscle Arnt-like protein-1 (Bmal1). Thus, desynchronization provides a novel anti-apoptotic defense mechanism in response to cytokine assault, similar to that provided by {beta}-cell phenotypic de-differentiation.