Interleukin-1 receptor-dependent and -independent caspase-1 activity in retinal cells mediated by receptor...

Inflammation and cell death play an important role in the pathogenesis of diabetic retinopathy. Previously we observed sustained activation of pro-inflammatory caspase-1 in retinas of diabetic animals and patients. In this study, we aimed to look at mechanisms underlying chronic caspase-1 activation in vitro and in vivo.

Non-diabetic and diabetic wild type and IL-1 receptor (IL-1R1) knockout mice were used for in vivo experiments. Diabetes was induced using STZ (streptozotocin). Human Müller cells were used for in vitro studies. Cells were treated with either 5 mM or 25 mM glucose or interleukin-1beta (IL-1β) in the presence or absence of IL-1 receptor antagonist (IL-1ra) or siRNA against RIP2 (receptor interacting protein-2) for up to 96 h. Outcome measurements to assess Müller cell functions included measurements of caspase-1 activity using a fluorescence peptide substrate, production of IL-1β by Elisa, and cell death using trypan blue exclusion assays.

Our in vivo results demonstrate that caspase-1 activation progresses from an IL-1R1 independent mechanism at 10 weeks of diabetes to an IL-1R1 dependent mechanism at 20 weeks indicating that feedback through IL-1R1 is crucial for sustained caspase-1 activity in retinas of mice. A similar hyperglycemia-mediated caspase-1/IL-1β/IL-1R1 feedback signaling was detected in vitro in human Müller cells which was prevented by treatment with IL-1ra. Our data also indicate that hyperglycemia induces caspase-1 activation initially but IL-1β sustains caspase-1 activation via caspase-1/IL-1β/IL-1R1 feedback and we identified RIP2 as mediator for both hyperglycemia- and IL-1β-induced caspase-1 activation. Activation of caspase-1/IL-1β/IL-1R1 feedback signaling caused Müller cell death which was prevented by RIP2 knockdown.

We conclude that any intervention in caspase-1/IL-1β/IL-1R1 feedback signaling presents novel therapeutic options for the treatment of diabetic retinopathy.