Krüppel-like Factor 2 (KLF2), also known aslung Krüppel-like Factor (LKLF), is aprotein that in humans is encoded by theKLF2gene onchromosome 19.[5][6] It is in theKrüppel-like factor family ofzinc finger transcription factors, and it has been implicated in a variety of biochemical processes in the human body, includinglung development, embryonicerythropoiesis,epithelial integrity,T-cell viability, andadipogenesis.[7]
Erythroid Krüppel-like Factor (EKLF or KLF1) was the first Krüppel-like Factor discovered. It is vital for embryonic erythropoiesis in promoting the switch from fetalhemoglobin (Hemoglobin F) to adult hemoglobin (Hemoglobin A)gene expression by binding to highly conserved CACCC domains.[8]EKLFablation in mouse embryos produces a lethalanemicphenotype, causing death by embryonic day 14, and naturalmutations lead toβ+ thalassemia in humans.[9] However, expression ofembryonic hemoglobin and fetal hemoglobin genes is normal inEKLF-deficient mice, and since all genes on thehuman β-globin locus exhibit the CACCC elements, researchers began searching for other Krüppel-like factors.[10]
KLF2, initially called lung Krüppel-like Factor due to its high expression in the adult mouse lung, was first isolated in 1995 by using thezinc finger domain of EKLF as ahybridization probe.[11] Bytransactivationassay in mousefibroblasts, KLF2 was also noticed to bind to theβ-globingene promoter containing the CACCC sequence shown to be the binding site for EKLF, confirming KLF2 as a member of the Krüppel-like Factor family.[11] Since then, many other KLF proteins have been discovered.
The main feature of the KLF family is the presence of three highly conservedCysteine2/Histidine2 zinc fingers of either 21 or 23amino acid residues in length, located at theC-terminus of the protein. These amino acid sequences eachchelate a singlezinc ion,coordinated between the two cysteine and two histidine residues. These zinc fingers are joined by a conserved seven-amino acid sequence;TGEKP(Y/F)X. The zinc fingers enable all KLF proteins to bind to CACCCgene promoters, so although they may complete varied functions (due to lack ofhomology away from the zinc fingers), they all recognize similarbinding domains.[7]
KLF2 also exhibits these structural features. ThemRNA transcript is approximately 1.5kilobases in length, and the 37.7kDa protein contains 354 amino acids.[11] KLF2 also shares some homology with EKLF at theN-terminus with aproline-rich region presumed to function as thetransactivation domain.[11]
KLF2 was first discovered, and is highly expressed in, the adult mouselung, but it is also expressed temporally duringembryogenesis inerythroid cells,endothelium,lymphoid cells, thespleen, andwhite adipose tissue.[7][11] It is expressed as early as embryonic day 9.5 in the endothelium.
KLF2 has a particularly interesting expression profile in erythroid cells. It is minimally expressed in the primitive and fetal definitive erythroid cells, but is highly expressed in adult definitive erythroid cells, particularly in theproerythroblast and thepolychromatic andorthochromatic normoblasts.[12]
Homologous recombination ofembryonic stem cells was used to generateKLF2-deficient mouse embryos. Bothvasculogenesis andangiogenesis were normal in the embryos, but they died by embryonic day 14.5 from severehemorrhaging. Thevasculature displayed defective morphology, with thintunica media andaneurysmal dilation that led to rupturing. Aortic vascular smooth muscle cells failed to organize into a normal tunica media, andpericytes were low in number. TheseKLF2-deficient mice thus demonstrated the important role ofKLF2 in blood vessel stabilization during embryogenesis.[13]
Due to embryonic lethality inKLF2-deficient embryos, it is difficult to examine the role ofKLF2 in normalpost-natalphysiology, such as inlung development and function.[14]
Lung buds removed fromKLF2-deficient mouse embryos and cultured from normaltracheobronchial trees. In order to circumvent embryonic lethality usually observed inKLF2-deficient embryos,KLF2homozygous null mouse embryonic stem cells were constructed and used to producechimeric animals. TheseKLF2-deficient embryonic stem cells contribute significantly to development of skeletal muscle, spleen, heart, liver, kidney, stomach, brain, uterus, testis, and skin, but not to the development of the lung. These embryos had lungs arrested in thelate canalicular stage of lung development, with undilatedacinar tubules. In contrast,wild type embryos are born in thesaccular stage of lung development with expanded alveoli. This suggests that KLF2 is an importanttranscription factor required in late gestation for lung development.[7]
KLF2 is now believed to play an important role in embryonic erythropoiesis, specifically in regulating embryonic andfetal β-like globin gene expression. In amurineKLF2-deficient embryo, expression of β-like globin genes normally expressed in primitive erythroid cells was significantly decreased, althoughadult β-globin gene expression was unaffected.[15]
The role of KLF2 in human β-like globin gene expression was further elucidated bytransfection of a murineKLF2-deficient embryo with the human β-globin locus. It was found that KLF2 was important forε-globin (found in embryonic hemoglobin) andγ-globin (found infetal hemoglobin) gene expression. However, as before, KLF2 plays no role in adult β-globin gene expression; this isregulated by EKLF.[15]
However, KLF2 and EKLF have been found to interact in embryonic erythropoiesis.Deletion of bothKLF2 andEKLF in mouse embryos results in fatal anemia earlier than in either single deletion at embryonic day 10.5. This indicates that KLF2 and EKLF interact inembryonic and fetal β-like globin gene expression.[16] It has been shown usingconditional knockout mice that both KLF2 and EKLF bind directly to β-like globinpromoters.[17] There is also evidence to suggest that KLF2 and EKLFsynergistically bind to theMycpromoter, atranscription factor that is associated with gene expression ofα-globin and β-globin in embryonicproerythroblasts.[18]
KLF2 expression is induced byfluid laminar flowshear stress, as is caused by blood flow in normal endothelium.[19][20]
This activatesmechanosensitive channels, which in turn activates two pathways; theMEK5/ERK5 pathway, which activatesMEF2, atranscription factor that upregulatesKLF2 gene expression; andPI3K inhibition, which increases the stability ofKLF2 mRNA. Binding of cytokines such asTNFα andIL-1β to theirreceptors activatestranscription factorp65, which also inducesKLF2 expression. KLF2 then has four key functions in endothelium:
Thus KLF2 has an important role in regulating normal endothelium physiology. It is hypothesized thatmyeloid-specific KLF2 plays a protective role inatherosclerosis.[22] Gene expression changes in endothelial cells induced by KLF2 have been demonstrated to be atheroprotective.[20]
KLF2 has an important function inT-lymphocytedifferentiation. T-cells are activated and more prone toapoptosis without KLF2, suggesting that KLF2 regulates T-cellquiescence and survival.[7]KLF2-deficientthymocytes also do not express several receptors required for thymusemigration and differentiation into mature T-cells, such assphingosine-1 phosphate receptor 1.[23]
KLF2 is anegative regulator ofadipocyte differentiation. KLF2 is expressed inpreadipocytes, but not mature adipocytes, and it potently inhibitsPPAR-γ (peroxisome proliferator-activated receptor-γ) expression by inhibitingpromoter activity. This prevents differentiation of preadipocytes into adipocytes, and thus prevents adipogenesis.[24]
This article incorporates text from theUnited States National Library of Medicine, which is in thepublic domain.
