Promoter-binding activity of inflammation-responsive transcription factor SAF is regulated by cyclic AMP signaling pathway

The Molecular Basis for Specificity at the Level of the Protein Kinase a Catalytic Subunit

Assembly of multi enzyme complexes at subcellular localizations by anchoring- and scaffolding proteins represents a pivotal mechanism for achieving spatiotemporal regulation of cellular signaling after hormone receptor targeting [for review, see (1)]. In the 3' 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) signaling pathway it is generally accepted that specificity is secured at several levels. This includes at the first level stimulation of receptors coupled to heterotrimeric G proteins which through stimulation of adenylyl cyclase (AC) forms the second messenger cAMP. Cyclic AMP has several receptors including PKA. PKA is a tetrameric holoenzyme consisting of a regulatory (R) subunit dimer and two catalytic (C) subunits. The R subunit is the receptor for cAMP and compartmentalizes cAMP signals through binding to cell and tissue-specifically expressed A kinase anchoring proteins (AKAPs). The current dogma tells that in the presence of cAMP, PKA dissociates into an R subunit dimer and two C subunits which are free to phosphorylate relevant substrates in the cytosol and nucleus. The release of the C subunit has raised the question how specificity of the cAMP and PKA signaling pathway is maintained when the C subunit no longer is attached to the R subunit-AKAP complex. An increasing body of evidence points toward a regulatory role of the cAMP and PKA signaling pathway by targeting the C subunits to various C subunit binding proteins in the cytosol and nucleus. Moreover, recent identification of isoform specific amino acid sequences, motifs and three dimensional structures have together provided new insight into how PKA at the level of the C subunit may act in a highly isoform-specific fashion. Here we discuss recent understanding of specificity of the cAMP and PKA signaling pathway based on C subunit subcellular targeting as well as evolution of the C subunit structure that may contribute to the dynamic regulation of C subunit activity.

Loss of epigenetic Kruppel-like factor 4 histone deacetylase (KLF-4-HDAC)-mediated transcriptional suppression is crucial in increasing vascular endothelial growth factor (VEGF) expression in breast cancer

Vascular endothelial growth factor (VEGF) is recognized as an important angiogenic factor that promotes angiogenesis in a series of pathological conditions, including cancer, inflammation, and ischemic disorders. We have recently shown that the inflammatory transcription factor SAF-1 is, at least in part, responsible for the marked increase of VEGF levels in breast cancer. Here, we show that SAF-1-mediated induction of VEGF is repressed by KLF-4 transcription factor. KLF-4 is abundantly present in normal breast epithelial cells, but its level is considerably reduced in breast cancer cells and clinical cancer tissues. In the human VEGF promoter, SAF-1- and KLF-4-binding elements are overlapping, whereas SAF-1 induces and KLF-4 suppresses VEGF expression. Ectopic overexpression of KLF-4 and RNAi-mediated inhibition of endogenous KLF-4 supported the role of KLF-4 as a transcriptional repressor of VEGF and an inhibitor of angiogenesis in breast cancer cells. We show that KLF-4 recruits histone deacetylases (HDACs) -2 and -3 at the VEGF promoter. Chronological ChIP assays demonstrated the occupancy of KLF-4, HDAC2, and HDAC3 in the VEGF promoter in normal MCF-10A cells but not in MDA-MB-231 cancer cells. Co-transfection of KLF-4 and HDAC expression plasmids in breast cancer cells results in synergistic repression of VEGF expression and inhibition of angiogenic potential of these carcinoma cells. Together these results identify a new mechanism of VEGF up-regulation in cancer that involves concomitant loss of KLF-4-HDAC-mediated transcriptional repression and active recruitment of SAF-1-mediated transcriptional activation.

Control of VEGF expression in triple-negative breast carcinoma cells by suppression of SAF-1 transcription factor activity

Angiogenesis plays a significant role in cancer by providing increased blood supply to the affected tissues and thus bringing in growth factors, cytokines, and various nutrients for tumor growth. VEGF is the most prominent angiogenic agent that is markedly induced in cancer. Induction of VEGF has been widely studied but as cancer cells are quite adept at acquiring new alternative processes to circumvent surrounding environmental pressures, our understanding of the molecular mechanisms regulating VEGF expression in cancer, especially in triple-negative breast cancer cells, remains incomplete. Here, we present evidence of a novel mode of VEGF induction in triple-negative MDA-MB-231 breast cancer cells that is regulated by serum amyloid A activating factor 1 (SAF-1) transcription factor. Inhibition of SAF-1 by antisense short hairpin RNA profoundly reduces VEGF expression along with reduction in endothelial cell proliferation and migration. By both in vitro and in vivo molecular studies, we show that the effect of SAF-1 is mediated through its direct interaction with the VEGF promoter. In correlation, DNA-binding activity of SAF-1 is found to be significantly higher in MDA-MB-231 breast cancer cells. Examination of several breast cancer samples further revealed that SAF-1 is overexpressed in clinical breast cancer tissues. Taken together, these findings reveal that SAF-1 is a hitherto unrecognized participant in inducing VEGF expression in triple-negative breast cancer cells, an aggressive form of breast cancer that currently lacks effective treatment options. Suppression of SAF-1 activity in these cells can inhibit VEGF expression, providing a possible new method to control angiogenesis.

Docosahexaenoic acid enhances hepatic serum amyloid A expression via protein kinase A-dependent mechanism

Serum amyloid A (SAA) reduces fat deposition in adipocytes and hepatoma cells. Human SAA1 mRNA is increased by docosahexaenoic acid (DHA) treatment in human cells. These studies asked whether DHA decreases fat deposition through SAA1 and explored the mechanisms involved. We demonstrated that DHA increased human SAA1 and C/EBPbeta mRNA expression in human hepatoma cells, SK-HEP-1. Utilizing a promoter deletion assay, we found that a CCAAT/enhancer-binding protein beta (C/EBPbeta)-binding site in the SAA1 promoter region between -242 and -102 bp was critical for DHA-mediated SAA1 expression. Mutation of the putative C/EBPbeta-binding site suppressed the DHA-induced SAA1 promoter activity. The addition of the protein kinase A inhibitor H89 negated the DHA-induced increase in C/EBPbeta protein expression. The up-regulation of SAA1 mRNA and protein by DHA was also inhibited by H89. We also demonstrated that DHA increased protein kinase A (PKA) activities. These data suggest that C/EBPbeta is involved in the DHA-regulated increase in SAA1 expression via PKA-dependent mechanisms. Furthermore, the suppressive effect of DHA on triacylglycerol accumulation was abolished by H89 in SK-HEP-1 cells and adipocytes, indicating that DHA also reduces lipid accumulation via PKA. The observation of increased SAA1 expression coupled with reduced fat accumulation mediated by DHA via PKA suggests that SAA1 is involved in DHA-induced triacylglycerol breakdown. These findings provide new insights into the complicated regulatory network in DHA-mediated lipid metabolism and are useful in developing new approaches to reduce body fat deposition and fatty liver.

Protein kinase A signaling pathway regulates transcriptional activity of SAF-1 by unmasking its DNA-binding domains

Serum amyloid A (SAA) activating factor-1 (SAF-1) is an inducible transcription factor that plays a key role in the regulation of several inflammation-responsive genes including SAA and matrix metalloproteinase-1. Increased synthesis of SAA and matrix metalloproteinase-1 is associated with pathogenesis of several diseases including amyloidosis, arthritis, and atherosclerosis. Previously, we showed in vivo interaction of SAF-1 and protein kinase A (PKA) and presented evidence for induction of SAF-1-regulated genes by a PKA signaling pathway. Here we demonstrate a mechanism by which PKA increases functional activities of SAF-1. Site-directed mutagenesis and phosphorylation analyses revealed two sites in the SAF-1 protein, serine 187 and threonine 386, as the target of PKA. Interestingly, mutation of both PKA phosphorylation sites created a highly active SAF-1 protein with high DNA-binding ability. Furthermore, we found that terminal deletion of SAF-1 protein from either end creates SAF-1 isoforms that are highly transcriptionally active. Partial proteolysis experiments indicated that unphosphorylated and phosphorylated SAF-1 proteins are structurally distinct. Together these results suggest that under native condition, N and C termini of SAF-1 are engaged in an inhibitory intramolecular interaction. PKA-mediated phosphorylation increases transcriptional activity of SAF-1 by unmasking the DNA-binding domain.