Krüppel-like Factor 4 Inhibits Proliferation by Platelet-derived Growth Factor Receptor β-mediated, Not by Retinoic Acid Receptor α-mediated, Phosphatidylinositol 3-Kinase and ERK Signaling in Vascular Smooth Muscle Cells

Proliferation inhibition of vascular smooth muscle cells (VSMCs) is governed by the activity of a transcription factor network. Kruppel-like factor 4 (Klf4), retinoic acid receptor (RARα), and platelet-derived growth factor receptor (PDGFR) are expressed in VSMCs and are components of such a network. However, the relationship among them in the regulation of VSMC proliferation remains unknown. Here, we investigated the mechanisms whereby Klf4 mediates the growth inhibitory effects in VSMCs through RARα and PDGFRβ. We demonstrated that Klf4 directly binds to the 5′ regulatory region of RARα, down-regulates RARα expression, and specifically inhibits RARα-mediated phosphatidylinositol 3-kinase (PI3K) and ERK signaling in cultured VSMCs induced by the synthetic retinoid Am80. Of particular interest, Klf4 inhibits RARα and PDGFRβ expression while blocking PI3K and ERK signaling induced by Am80 and PDGF-BB, respectively. The anti-proliferative effects of Klf4 on neointimal formation depend largely on PDGFR-mediated PI3K signaling without involvement of the RARα-activated signaling pathways. These findings provide a novel mechanism for signal suppression and growth inhibitory effects of Klf4 in VSMCs. Moreover, the results of this study suggest that Klf4 is one of the key mediators of retinoid actions in VSMCs.