FHL3 Regulates Vascular Smooth Muscle Cell Phenotypic Switching Through the MRTFB-SRF Signaling Axis

Genome-wide association studies have uncovered many coronary artery disease (CAD) loci, but mechanisms linking risk variants to vascular biology remain unclear. We screen candidate causal CAD genes for regulators of the vascular smooth muscle cell (VSMC) contractile phenotype using a pooled CRISPR-Cas9 knockout library in primary human VSMCs with MYH11 protein levels as the readout. FHL3 (four-and-a-half LIM domains protein 3) emerges as the top repressor of the contractile state. Colocalization analyses in a VSMC biobank and vascular tissues associate a CAD risk allele with reduced FHL3 expression. Biochemical and imaging studies show FHL3 is associated and co-localizes with the transcriptional co-activator MRTFB (myocardin-related transcription factor-B). Functionally, FHL3 overexpression attenuates MRTFB-serum response factor (SRF)-driven induction of contractile markers (MYH11, transgelin), whereas FHL3 knockdown increases their expression in an MRTFB-dependent manner. FHL3 knockdown or MRTFB overexpression reduces VSMC proliferation and migration, and FHL3 overexpression mitigates MRTFB-driven effects on these behaviors. These data identify FHL3 as a regulator of VSMC plasticity that inhibits the contractile phenotype by interfering with MRTFB-SRF signaling, link a CAD-associated genetic signal to the VSMC phenotype, and nominate the FHL3-MRTFB interaction as a potential therapeutic target.