We have shown that spontaneously hypertensive rat (SHR)-derived vascular smooth muscle cells (VSMCs) change to the synthetic phenotype and show increased expression of complement 3 (C3) and that C3 plays a role in the change to the synthetic phenotype. To determine the mechanisms underlying the effects of C3 on this phenotypic change, we examined the effects of C3a on transcription factors involved in VSMC phenotype and found that C3a increased the expression of Krüppel-like zinc-finger transcription factor 5 (KLF5) mRNA. C3a increased KLF5 promoter activity in a concentration-dependent manner. Deletion analysis of the promoter region of the KLF5 gene revealed that the region between nucleotides-991 and -699 contains the transcriptional regulatory element stimulated by C3a. C3a induced extracellular signal-regulated kinase (ERK) phosphorylation, and C3a-increased KLF5 promoter activity was completely inhibited by the MEK inhibitor U0126. These findings suggest that C3 increases KLF5 promoter activity and gene expression via ERK signaling.