The effects of increased LIPA, a coronary heart disease risk gene, on macrophage and neutrophil function

Genome-wide association studies (GWAS) have revealed a robust association between genetic variation at the LIPA (lysosomal acid lipase) locus and risks of coronary heart diseases (CHD). Fine mapping, expression quantitative trait analysis and colocalization analysis strongly support LIPA as the candidate gene, but the causal cell types and the biological mechanisms remain unknown. Our preliminary data and published work show that the risk alleles of the CHD variants are associated with higher LIPA expression in monocytes/macrophage and neutrophils, but not lymphocytes, suggesting that myeloid-specific increased LIPA function may confer the risk of CHD by affecting myeloid cells. To elucidate how increased LIPA expression and activity regulate macrophage and neutrophil function, I use human THP-1 macrophage (THP-1-Mfs) with lentiviral-mediated LIPA overexpression (OE) and transgenic mice with LysMCre+/-¿mediated myeloid-specific Lipa overexpression (LipaTg). My preliminary data suggest that: (1) LIPA OE increased macrophage proliferation in both human and murine macrophages; (2) LipaTg mice showed increased number of circulating neutrophils, but not monocytes and lymphocytes, which is consistent with previous human blood cell GWAS. LIPA hydrolyzes neutral lipids delivered to the lysosome through uptake of lipoproteins. I therefore hypothesize that LIPA OE regulates macrophage and neutrophil function by affecting cellular lipid composition and downstream signaling. Aim 1 will determine how LIPA OE increases macrophage proliferation in vitro and in vivo. My lipidomics and RNA-seq data suggest increased intracellular phosphatidylinositol, and upregulated pathways related to phosphatidylinositol metabolism and PI3K/Akt signaling in LIPA OE THP-1-Mfs. I will test whether LIPA OE will: (1) increase levels of phosphatidylinositol-derived signaling molecules and activation of downstream pathways in macrophage in vitro; (2) increase macrophage proliferation in LipaTg mice in vivo. Aim 2 will determine how LIPA OE increases blood neutrophils in vivo. I will test whether LipaTg mice will show: (1) increased neutrophil clearance and G-CSF production by BM macrophages; (2) increased circulating G-CSF level; (3) increased BM and blood granulocyte progenitors. This project will reveal novel mechanisms of LIPA as a candidate gene for CHD and human blood trait GWAS and will pave the way to ultimately address the fundamental role of LIPA genetic variation in atherosclerosis. (AHA Program: Postdoctoral Fellowship)