Rediscovering contact inhibition in the embryo.

The cellular reaction called contact inhibition of locomotion was initially characterised by Michael Abercrombie more than 60 years ago. In his most general definition, it is defined as the stopping of the continued locomotion of a cell in the direction which has produced a collision with another cell. This deceptively simple response has been widely studied in vitro in a number of cell types over the years, yet it is still often misunderstood by the scientific community. Abercrombie spent much of his life studying the failure of the response shown by cancer cell types and how this might lead to malignant invasion of normal tissue. However, since Abercrombie's time, a role for this response in living organisms has been left to the realm of speculation. Here, we discuss the history of contact inhibition research, clarify some of the misconceptions about the response and reclaim misused terminology. We will also highlight our recent work, which for the first time elucidates a functional role for contact inhibition in vivo during embryogenesis.

Induction of the ubiquitin-proteasome pathway during the keratocyte transition to the repair fibroblast phenotype.

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 (125)I-labeled H2A and with progressive (>6-fold) increases in the UPP substrate, inhibitor of kappaBalpha (IkappaBalpha). Because IkappaBalpha expression is induced by nuclear factor (NF)-kappaB, this finding suggests that rates of constitutive NF-kappaB activation, and thus IkappaBalpha degradation, are elevated in activated stromal cells. Both freshly isolated and activated stromal cells degraded IkappaBalpha in response to IL-1alpha; yet, only activated stromal cells maintained autocrine IL-1alpha 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 IkappaBalpha and TKT, two UPP substrates that are implicated in the loss of tissue stasis and corneal clarity after injury.