Kruppel-like factors

The Krüppel-like family of transcription factors (Klfs), so named for their homology to the Drosophila Krüppel protein, have been extensively studied for their roles in cell proliferation, differentiation and survival, especially in the context of cancer. All KLF family members are characterised by their three Cys2 His2 zinc fingers located at the C-terminus (Fig. 1A), separated by a highly conserved H/C link, (Fig. 1B and C; Fig. 2). DNA binding studies demonstrated that the KLFs have similar affinities for different GC-rich sites, , or sites with CACCC homology,  and can compete with each other for the occupation of such sites. KLFs also share a high degree of homology between the specificity protein (Sp) family of zinc-finger transcription factors and bind similar, if not the same sites, in a large number of genes.

Krüppel-like factors 4 & 5
Klf4 (Fig. 2A), known also as gut-enriched Krüppel-like factor (GKLF) acts as a transcriptional activator or a transcriptional repressor  depending on the promoter context and/or cooperation with other transcription factors. For example, Klf4 transactivates the iNOS promoter in cooperation with p65 (RelA), and the p21Cip1/Waf1 promoter in cooperation with p53, but it directly suppresses the p53 promoter and inhibits ornithine decarboxylase promoter activity by competing with specificity protein-1 (Sp-1). Klf4 also interacts with the p300/CBP transcription co-activators. Klf5, also known as intestinal enriched Krüppel-like factor (IKLF) or basic transcription element binding protein 2 (Bteb2) has been assigned purely transcriptional activation activity (Fig. 1A) but, similar to Klf4, binds p300 which acetylates the first zinc finger conferring a trans-activating function. Importantly for Klf4 & Klf5, the amino acids that are predicted by the Klevit model to interact with DNA are identical (Fig. 1B and Fig. 2) and the two compete for the same CACCC element found in a wide variety of promoters. Klf4 & Klf5 can act antagonistically during cellular proliferation, differentiation,  and promoter activation,  either via direct competition or via alterations in their own gene expression. The expression of Klf4 in terminally-differentiated, post-mitotic intestinal epithelial cells as opposed to proliferating crypt cells which contain high levels of Klf5 is one example of such opposing effects. Klf4 inhibits proliferation through activation of p21Cip1/Waf1, and direct suppression of cyclin D1 and cyclin B1  gene expression. Both Klf4 & Klf5 proteins act on the Klf4 promoter where Klf4 increases expression and Klf5 decreases expression of Klf4 mRNA.

Krüppel-like factors 4 & 5 in the vascular system
Klf4 is upregulated in vascular injury. It dramatically represses SRF/myocardin-induced activation of gene expression, and directly inhibits myocardin gene expression in vascular smooth muscle cells (VSMCs), therefore inhibiting the transition to a proliferative phenotype. Furthermore, Klf4 has been identified as an anti-proliferative shear stress-responsive gene,49 and forced over-expression of Klf4 in VSMCs induces growth arrest. ,  Klf4 may therefore be an important protective factor in disease states affected by shear stress, such as thrombosis, restenosis and atherosclerosis. Klf4 also mediates the vascular response to nitric oxide (NO) by activating the promoters of inducible nitric oxide synthase (iNOS) in endothelial cells and cGMP-dependent protein kinase 1α/protein kinase G 1α (PKG 1α) in VSMCs. PKG 1α is activated by NO and mediates VSMC relaxation. This trans-activating effect of Klf4 on the PKG 1α promoter is inhibited by RhoA-induced actin polymerisation, possibly via G-actin regulation of a Klf4 co-activator or co-repressor. RhoA signalling pathways and RhoA activation are implicated in hypertension and increased vascular resistance  which to some extent can be explained by this interaction with Klf4 and its effects on the response to NO. Klf5 has no effect on the PKG 1α promoter though the protein expression and nuclear localisation of Klf5 was similar to that of Klf4.

Krüppel-like factors 4 & 5 in the myocardium
Little is known of the Klfs in the myocardium. Klf5 activates the promoter of the hypertrophic agonist platelet derived growth factor (PDGF-A) in cardiac fibroblasts a factor previously identified as being upregulated by ET-1,  and Klf5+/- transgenic mice heterozygotes (described earlier) exhibited less cardiac fibrosis and hypertrophy when stimulated with angiotensin II compared with controls. Klf5 is itself regulated by the immediate early gene egr-1 in VSMCs, which, if similarly regulated in the cardiomyocyte, places Klf5 potentially in a position to co-ordinate the acute response to external stress and tissue remodelling in the myocardium.