The Role of β-Transducin Repeat-Containing Protein (β-TrCP) in the Regulation of NF-κB in Vascular Smooth Muscle Cells

Objective-Degradation of IκB is an essential step in nuclear factor (NF)-κB activation. However, the determinants regulating this process have not been defined in vascular smooth muscle cells (VSMCs). We hypothesized that the E3-ligase, β-transducin repeat-containing protein 1 (β-TrCP1), was a rate-determining mediator that regulates the ubiquitin-mediated degradation of IκBα (in VSMC). Methods and Results-Upregulation of β-TrCP1 accelerated the rate of IκBα degradation, leading to increased NF-κB activity. In contrast, VSMCs harboring a dominant-negative β-TrCP1 transgene lacking the F-box domain exhibited a reduction in serum-stimulated NF-κB activity but no alteration in response to tumor necrosis factor (TNF). These findings suggest that β-TrCP1 increases the rate of NF-κB activation but is not rate-limiting in response to TNF in VSMCs. Endogenous β-TrCP1 expression was regulated through the conserved Wnt cascade. Upregulation of Wnt1 resulted in β-catenin-mediated activation of Tcf-4, leading to increased β-TrCP1 expression and NF-κB activity. Furthermore, VSMCs harboring a Tcf-4 mutant lacking a β-catenin binding domain exhibited a significant reduction in β-TrCP1 expression along with abolishment of NF-κB activity. Conclusions-We provide the first evidence of crosstalk between the Wnt cascade and NF-κB signaling in VSMCs. This crosstalk is mediated through the E3-ligase, β-TrCP1.