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Induction of the Ubiquitin–Proteasome Pathway during the Keratocyte Transition to the Repair Fibroblast Phenotype

¹ From the Vision Research Laboratories, New England Eye Center, Tufts University School of Medicine, the Tufts University Sackler School of Graduate Biomedical Sciences; and the ² Laboratory for Nutrition and Vision Research, Jean Mayer United States Department of Agriculture-Human Nutrition Research Center on Aging, Tufts University, Boston, Massachusetts.

PURPOSE. To examine dynamics and function of the ubiquitin (Ub)-proteasome pathway (UPP) during corneal stromal cell acquisition of the repair fibroblast phenotype.

METHODS. An established cell culture model was used in which freshly isolated rabbit corneal stromal cells acquire a repair fibroblast phenotype, thereby mimicking injury-induced stromal cell activation.

RESULTS. Transition to the repair fibroblast phenotype during the 72 hours after initial plating was coincident with progressive UPP induction. Levels of Ub, Ub-conjugated proteins, ubiquitinylating enzymes E1 and E2-25K, and 26 S proteasome increased two- to fivefold in activated stromal cells. These increases were associated with enhanced (>10-fold) capacity for Ub-dependent proteolysis of ¹²⁵I-labeled H2A and with progressive (>6-fold) increases in the UPP substrate, inhibitor of B (IB). Because IB expression is induced by nuclear factor (NF)-B, this finding suggests that rates of constitutive NF-B activation, and thus IB degradation, are elevated in activated stromal cells. Both freshly isolated and activated stromal cells degraded IB in response to IL-1; yet, only activated stromal cells maintained autocrine IL-1 expression after 24 hours. UPP induction was coincident with a more than 90% loss of tissue transketolase (TKT) and aldehyde dehydrogenase (ALDH) class 1. TKT was stabilized during the repair phenotype transition by proteasome inhibition and was degraded (>30%/h) by the UPP in cell-free assays.

CONCLUSIONS. Coordinate induction of the UPP during stromal cell activation alters levels of IB and TKT, two UPP substrates that are implicated in the loss of tissue stasis and corneal clarity after injury.

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