Vascular smooth muscle cells (VSMCs) exhibit remarkable phenotypic plasticity, whereby differentiated contractile VSMCs switch to a synthetic state under pathological conditions. Synthetic VSMCs are manifested by their high susceptibility to inflammatory activation and loss of contractility, contributing largely to various vascular disorders such as post-angioplasty restenosis and aneurysm. Targeting early activation of VSMC inflammation may represent an attractive strategy to block vascular pathologies. However, molecular mechanisms involving key regulator(s) that drive the proinflammatory VSMC phenotype are incompletely understood. One possibility that has yet to be explored is the pervasive class of long noncoding RNAs (lncRNAs). Through an unbiased RNA-seq study, we discovered a novel human-specific lncRNA, KILN, which is enriched in proinflammatory VSMCs and diseased human vessels (aneurysm). RNA-seq revealed that loss of KILN in cultured VSMCs suppresses expression of multiple inflammatory genes. Notably, KILN is highly responsive to inflammatory insults in humanized Bacterial Artificial Chromosome (BAC) transgenic mice carrying either KILN (BAC-KILN) or both KILN and its neighboring gene IL8 (BAC-KILN/IL8). Both transgenic lines display an exacerbated inflammatory response triggered by vascular injury. These results support a critical role of KILN in promoting VSMC inflammation and vascular disease. Mechanistically, KILN interacts with MKL1, a potent transcriptional cofactor with a recognized role in transactivating VSMC contractile genes. Intriguingly, Mkl1 null mice are protected from aneurysm formation and depletion of MKL1 in cultured VSMCs impairs the proinflammatory gene program. These results suggest a potential link between MKL1 and KILN in VSMC inflammation and vascular disease. Indeed, loss of KILN decreases MKL1 protein levels and MKL1/p65 physical interaction, the latter being critical for transactivation of proinflammatory genes. These exciting preliminary findings support a central hypothesis that KILN interacts with MKL1 to stabilize MKL1 protein, which potentiates MKL1/p65-activated VSMC inflammation and vascular pathologies. We propose two specific aims to probe this hypothesis. Aim 1 will elucidate the regulation and function of KILN during pathological vascular remodeling using BAC transgenic mice, and evaluate KILN expression in vessels and plasma exosomes from aneurysm patients. Aim 2 will elucidate the molecular mechanism of KILN-mediated vascular inflammation and disease involving KILN disruption of MKL1 ubiquitination to potentiate MKL1/p65 transactivation of the proinflammatory gene program. Successful completion of the proposed studies will define a new molecular switch comprising a novel VSMC-enriched lncRNA (KILN) that associates with MKL1 for VSMC inflammation and pathological vascular remodeling. These studies will advance our knowledge of lncRNA and MKL1 vascular pathophysiology, potentially providing novel insights into therapeutic strategies for various inflammatory vascular diseases.