Current paradigms suggest that two macrophage subsets, termed M1 and M2, are involved in inflammation and host defense. While the distinct functions of M1 and M2 macrophages have been intensively studied -the former are considered proinflammatory and the latter antiinflammatory -the determinants of their speciation are incompletely understood. Here we report our studies that identify Krüppel-like factor 4 (KLF4) as a critical regulator of macrophage polarization. Macrophage KLF4 expression was robustly induced in M2 macrophages and strongly reduced in M1 macrophages, observations that were recapitulated in human inflammatory paradigms in vivo. Mechanistically, KLF4 was found to cooperate with Stat6 to induce an M2 genetic program and inhibit M1 targets via sequestration of coactivators required for NF-κB activation. KLF4-deficient macrophages demonstrated increased proinflammatory gene expression, enhanced bactericidal activity, and altered metabolism. Furthermore, mice bearing myeloid-specific deletion of KLF4 exhibited delayed wound healing and were predisposed to developing diet-induced obesity, glucose intolerance, and insulin resistance. Collectively, these data identify KLF4 as what we believe to be a novel regulator of macrophage polarization.

Background Although metabolic reprogramming is critical in the pathogenesis of heart failure, studies to date have focused principally on fatty acid and glucose metabolism. Contribution of amino acid metabolic regulation in the disease remains understudied. Methods and Results Transcriptomic and metabolomic analyses were performed in mouse failing heart induced by pressure-overload. Suppression of branched-chain amino acids (BCAAs) catabolic gene expression along with concomitant tissue accumulation of branched-chain α-keto acids (BCKAs) was identified as a significant signature of metabolic reprogramming in mouse failing hearts, and validated to be shared in human cardiomyopathy hearts. Molecular and genetic evidence identified the transcription factor KLF15 as a key upstream regulator of the BCAA catabolic regulation in the heart. Studies using a genetic mouse model revealed that BCAA catabolic defect promoted heart failure associated with induced oxidative stress and metabolic disturbance in response to mechanical overload. Mechanistically, elevated BCKA directly suppressed respiration and induced superoxide production in isolated mitochondria. Finally, pharmacological enhancement of branched-chain α-keto acid dehydrogenase activity significantly blunted cardiac dysfunction following pressure-overload. Conclusions BCAA catabolic defect is a metabolic hallmark of failing heart resulted from KLF15 mediated transcriptional reprogramming. BCAA catabolic defect imposes a previously unappreciated significant contribution to heart failure.

Monocyte differentiation involves the participation of lineage-restricted transcription factors, although the mechanisms by which this process occurs are incompletely defined. Within the hematopoietic system, members of the Kruppellike family of factors (KLFs) play essential roles in erythrocyte and T lymphocyte development. Here we show that KLF4/GKLF is expressed in a monocyte-restricted and stage-specific pattern during myelopoiesis and functions to promote monocyte differentiation. Overexpression of KLF4 in HL-60 cells confers the characteristics of mature monocytes. Conversely, KLF4 knockdown blocked phorbol ester-induced monocyte differentiation. Forced expression of KLF4 in primary common myeloid progenitors (CMPs) or hematopoietic stem cells (HSCs) induced exclusive monocyte differentiation in clonogenic assays, whereas KLF4 deficiency inhibited monocyte but increased granulocyte differentiation. Mechanistic studies demonstrate that KLF4 is a target gene of PU.1. Consistently, KLF4 can rescue PU.1À/À fetal liver cells along the monocytic lineage and can activate the monocytic-specific CD14 promoter. Thus, KLF4 is a critical regulator in the transcriptional network controlling monocyte differentiation.

Abstract-Macrophages are key regulators of many organ systems, including innate and adaptive immunity, systemic metabolism, hematopoiesis, vasculogenesis, malignancy, and reproduction. The pleiotropic roles of macrophages are mirrored by similarly diverse cellular phenotypes. A simplified schema classifies macrophages as M1, classically activated macrophages, or M2, alternatively activated macrophages. These cells are characterized by their expression of cell surface markers, secreted cytokines and chemokines, and transcription and epigenetic pathways. Transcriptional regulation is central to the differential speciation of macrophages, and several major pathways have been described as essential for subset differentiation. In this review, we discuss the transcriptional regulation of macrophages.