MicroRNA 33 Regulates the Population of Peripheral Inflammatory Ly6C^high Monocytes through Dual Pathways

Area of Science:

• Immunology
• Molecular Biology
• Cardiovascular Research

Background:

• MicroRNA 33 (miR-33) targets ATP-binding cassette transporter A1 (ABCA1), influencing HDL-cholesterol and atherosclerosis.
• Previous studies indicated miR-33 deficiency increases peripheral Ly6C^high monocytes in ApoE-deficient mice.
• The precise role of miR-33 in monocyte population dynamics, particularly in wild-type settings, remains incompletely understood.

Purpose of the Study:

• To elucidate the effect of miR-33 deficiency on monocyte populations in a wild-type mouse model.
• To investigate the mechanisms by which miR-33 influences monocyte distribution and hematopoietic stem cell function.
• To explore the dual pathways through which miR-33 deficiency impacts inflammatory monocytes.

Main Methods:

• Analysis of monocyte populations (Ly6C^high) in peripheral blood and bone marrow of miR-33 knockout mice.
• Assessment of myeloid progenitor expansion and apoptosis in Lin^- Sca1⁺ c-Kit⁺ (LSK) cells.
• Bone marrow transplantation and competitive repopulation assays to distinguish hematopoietic and non-hematopoietic effects.
• Investigation of high-mobility group AT-hook 2 (HMGA2) expression and its role in apoptosis.
• Evaluation of high-density lipoprotein cholesterol (HDL-C) levels and monocyte populations following apolipoprotein A1 and ABCA1 transduction.

Main Results:

• miR-33 deficiency led to decreased peripheral Ly6C^high monocytes and increased bone marrow populations.
• Expansion of myeloid progenitors and reduced apoptosis were observed in LSK cells from miR-33 deficient mice.
• Hematopoietic deficiency of miR-33 increased peripheral Ly6C^high monocytes, while non-hematopoietic deficiency reduced them.
• miR-33 deficiency upregulated HMGA2 in LSK cells, contributing to reduced apoptosis.
• Increased HDL-C levels correlated with reduced peripheral Ly6C^high monocytes.

Conclusions:

• miR-33 deficiency influences inflammatory monocyte distribution via two distinct pathways.
• One pathway involves enhanced HMGA2 expression in hematopoietic stem cells, increasing Ly6C^high monocytes.
• The other pathway involves elevated HDL-C, which decreases peripheral Ly6C^high monocytes.