Objective--Degradation of IB is an essential step in nuclear factor (NF)-B activation. However, the determinants regulating this process have not been defined in vascular smooth muscle cells (VSMCs). The hypothesis was that the E3-ligase, -transducin repeat-containing protein 1 (-TrCP1), was a rate-determining mediator that regulates the ubiquitin-mediated degradation of IB.

Methods and Results--Upregulation of -TrCP1 accelerated the rate of IB degradation, leading to increased NF-B activity. In contrast, VSMCs harboring a dominant-negative -TrCP1 transgene lacking the F-box domain exhibited a reduction in serum-stimulated NF-kB activity but no alteration in response to tumor necrosis factor (TNF). These findings suggest that -TrCP1 increases the rate of NF-B activation but is not rate-limiting in response to TNF in VSMCs. Endogenous -TrCP1 expression was regulated through the conserved Wnt cascade. Upregulation of Wnt1 resulted in -catenin-mediated activation of Tcf-4, leading to increased -TrCP1 expression and NF-B activity. Furthermore, VSMCs harboring a Tcf-4 mutant lacking a -catenin binding domain exhibited a significant reduction in -TrCP1 expression along with abolishment of NF-B activity.

Conclusions--We provide the first evidence of crosstalk between the Wnt cascade and NF-B signaling in VSMCs. This crosstalk is mediated through the E3-ligase, -TrCP1.