Increasing incidence of inflammatory bowel disorders demands a better understanding of the molecular mechanisms underlying its multifactorial aetiology. Here we demonstrate that mice deficient for REGγ, a proteasome activator, show significantly attenuated intestinal inflammation and colitis-associated cancer in dextran sodium sulfate model. Bone marrow transplantation experiments suggest that REGγ's function in non-haematopoietic cells primarily contributes to the phenotype. Elevated expression of REGγ exacerbates local inflammation and promotes a reciprocal regulatory loop with NFκB involving ubiquitin-independent degradation of IκBɛ. Additional deletion of IκBɛ restored colitis phenotypes and inflammatory gene expression in REGγ -deficient mice. In sum, this study identifies REGγ-mediated control of IκBɛ as a molecular mechanism that contributes to NFκB activation and promotes bowel inflammation and associated tumour formation in response to chronic injury.

The magnitude, duration and oscillation of cellular signalling pathway responses are often limited by negative feedback loops, defined as an 'activator-induced inhibitor' regulatory motif. Within the NFkB signalling pathway, a key negative feedback regulator is IkBa. We show here that, contrary to current understanding, NFkB-inducible expression is not sufficient for providing effective negative feedback. We then employ computational simulations of NFkB signalling to identify IkBa molecular properties that are critical for proper negative feedback control and test the resulting predictions in biochemical and single-cell live-imaging studies. We identified nuclear import and nuclear export of IkBa and the IkBa-NFkB complex, as well as the free IkBa half-life, as key determinants of post-induction repression of NFkB and the potential for subsequent reactivation. Our work emphasizes that negative feedback is an emergent systems property determined by multiple molecular and biophysical properties in addition to the required 'activator-induced inhibitor' relationship.

Whereas ubiquitin-dependent degrons have been characterized in some detail, how proteins may be targeted to ubiquitin-independent proteasomal degradation remains unclear. Here we show that IκBα contains a ubiquitin-independent degron whose activity is portable to heterologous proteins such as the globular protein GFP via a proteasome-dependent, ubiquitin-independent, non-lysosomal pathway. The ubiquitin-independent degradation signal resides in a 11 amino acid sequence, which is not only sufficient but also required for IκBα’s short half-life. Finally, we show that this degron’s activity is regulated by the interaction with NFκB, which controls its solvent exposure, and we demonstrate that this regulation of the degron’s activity is critical for IκBα’s signaling functions.