The Pro-Apoptotic Protein Par-4 Facilitates Vascular Contractility by Cytoskeletal Targeting of Zipk

Par-4 (prostate apoptosis response 4) is a pro-apoptotic protein and tumor suppressor that was originally identified as a gene product upregulated during apoptosis in prostate cancer cells. Here, we show, for the first time, that Par-4 is expressed and co-localizes with the actin filament bundles in vascular smooth muscle. Furthermore, we demonstrate that targeting of ZIPK to the actin filaments, as observed upon PGF-2 alpha stimulation, is inhibited by the presence of a cell permeant Par-4 decoy peptide. The same decoy peptide also significantly inhibits PGF-2 alpha induced contractions of smooth muscle tissue. Moreover, knockdown of Par-4 using antisense morpholino nucleotides results in significantly reduced contractility, and myosin light chain and myosin phosphatase target subunit phosphorylation. These results indicate that Par-4 facilitates contraction by targeting ZIPK to the vicinity of its substrates, myosin light chain and MYPT, which are located on the actin filaments. These results identify Par-4 as a novel regulator of myosin light chain phosphorylation in differentiated, contractile vascular smooth muscle.