*3.3. Vascular Smooth Muscle Cells*

The UPS in VSMCs can also regulate BP. Peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear regulator superfamily of transcription factors, which is an important regulator of lipid and glucose metabolism. PPARγ is expressed in numerous tissues, including VSMCs. Importantly, studies have shown that mutations (P467L or V290M) in the ligand-binding domain of PPARγ cause not only insulin resistance but also early-onset hypertension [86,87], indicating its role in BP regulation. Moreover, dominant negative mice model of PPARγ (S-P467L) in VSMCs developed arterial sti ffness and vascular dysfunction, accompanied by hypertension [88,89]. These results indicate that PPARγ in VSMCs may play an essential role in regulating BP.

Recent studies have suggested that the e ffect of PPARγ in VSMCs is mediated by its downstream effector molecule, Rho-related BTB domain-containing protein 1 (RhoBTB1) [90]. RhoBTB1, a new subfamily of Rho GTPases [91], is expressed in various tissues [92]. Several genome-wide association studies have demonstrated that RhoBTB1 loci are associated with BP [93,94]. RhoBTB1 interacts with the N-terminal of CUL3 through its first BTB domain [95]. Recently, Mukohda et al. demonstrated that RhoBTB1 protects against hypertension and arterial sti ffness by restoring the activity of phosphodiesterase 5 (PDE5) [89]. They generated tamoxifen-inducible and VSMC-specific RhoBTB1 transgenic mice (S-RhoBTB1) and found that Rho-BTB1 expression was reduced in S-P467L mice, whereas S-P467L/S-RhoBTB1 mice exhibited the restoration of RhoBTB1 expression and improvement of vasocontraction in VSMCs, which was accompanied by the reduced PDE5 activity, leading to the attenuation of hypertension. In addition, tadalafil, a PDE5 inhibitor, reduced BP in the S-P467L/S-RhoBTB1 mice. It is interesting to note that RhoBTB1 promoted PDE5 ubiquitylation in the presence of CUL3, which was blunted upon treatment with an inhibitor (MLN4924) of neddylation, a modification that is required for CUL3 activation. The authors concluded that RhoBTB1 is involved in the PPARγ-mediated regulation of BP by regulating PDE5 activity through CUL3-dependent ubiquitylation [89]. Accumulating data indicate that phosphodiesterase 3 (PDE3), another member of the phosphodiesterase family, also critically regulates BP. Recent studies have demonstrated that six missense mutations of PDE3A in six unrelated families with Mendelian hypertension exhibit severe salt-independent but age-dependent hypertension [96]. In vitro analyses of mesenchymal stem cell-derived VSMCs demonstrated that the mutations increased the PKA-mediated PDE3A phosphorylation and resulted in gain of function, with increased cAMP-hydrolytic activity [96]. Whether PDE3 is regulated through UPS needs to be determined.

CUL3 also mediates the ubiquitylation and degradation of RhoA by interacting with a BTB domain-containing adaptor, BACURD [97], which regulates vascular contraction. It has been demonstrated that hypertension-causing mutations in CUL3 impair RhoA ubiquitylation [98] and that selective expression of mutant CUL3 in VSMCs results in augmented RhoA signaling and vascular dysfunction, leading to elevation of BP [99,100].
