Retrograde transport of protein toxins under conditions of COPI dysfunction.

Retrograde transport dependent on coat protein I (COPI) was impaired using two different approaches and the effects on the retrograde transport of protein toxins were investigated. One approach was to study ldlF cells that express a temperature-sensitive defect in the epsilon-COP subunit of COPI. The second approach was to treat cells with 1,3-cyclohexanebis(methylamine) (CBM), a drug that interferes with the binding of COPI to Golgi membranes. With both approaches, cells remained sensitive to a variety of protein toxins regardless of whether the toxins contained a KDEL motif. Moreover, cholera toxin, which contains a KDEL sequence, was observed by immunofluorescence microscopy to enter the endoplasmic reticulum of Vero cells in the presence of CBM. These data support published evidence indicating the presence in cells of a COPI- and KDEL receptor-independent pathway of retrograde transport from the Golgi complex to the endoplasmic reticulum. In addition, the results suggest that certain toxins containing a KDEL motif may use either the COPI-dependent or COPI-independent pathway of retrograde transport.

Hepatic peroxisomal fatty acid beta-oxidation is regulated by liver X receptor alpha.

Peroxisomes are the exclusive site for the beta-oxidation of very-long-chain fatty acids of more than 20 carbons in length (VLCFAs). Although the bulk of dietary long-chain fatty acids are oxidized in the mitochondria, VLCFAs cannot be catabolized in mitochondria and must be shortened first by peroxisomal beta-oxidation. The regulation of peroxisomal, mitochondrial, and microsomal fatty acid oxidation systems in liver is mediated principally by peroxisome proliferator-activated receptor alpha (PPARalpha). In this study we provide evidence that the liver X receptor (LXR) regulates the expression of the genetic program for peroxisomal beta-oxidation in liver. The genes encoding the three enzymes of the classic peroxisomal beta-oxidation cycle, acyl-coenzyme A (acyl-CoA) oxidase, enoyl-CoA hydratase/L-3-hydroxyacyl-CoA dehydrogenase, and 3-ketoacyl-CoA thiolase, are activated by the LXR ligand, T0901317. Accordingly, administration of T0901317 in mice promoted a dose-dependent and greater than 2-fold increase in the rate of peroxisomal beta-oxidation in the liver. The LXR effect is independent of PPARalpha, because T0901317-induced peroxisomal beta-oxidation in the liver of PPARalpha-null mice. Interestingly, T0901317-induced peroxisomal beta-oxidation is dependent on the LXRalpha isoform, but not the LXRbeta isoform. We propose that induction of peroxisomal beta-oxidation by LXR agonists may serve as a counterregulatory mechanism for responding to the hypertriglyceridemia and liver steatosis that is promoted by potent LXR agonists in vivo; however, additional studies are warranted.

Sonic hedgehog is essential for first pharyngeal arch development.

The secreted protein sonic hedgehog (Shh) is essential for normal development of many organs. Targeted disruption of Shh in mouse leads to near complete absence of craniofacial skeletal elements at birth, and mutation of SHH in human causes holoprosencephaly (HPE), frequently associated with defects of derivatives of pharyngeal arches. To investigate the role of Shh signaling in early pharyngeal arch development, we analyzed Shh mutant embryos using molecular markers and found that the first pharyngeal arch (PA1) was specifically hypoplastic and fused in the midline, and remaining arches were well formed at embryonic day (E) 9.5. Molecular analyses using specific markers suggested that the growth of the maxillary arch and proximal mandibular arch was severely defective in Shh-null PA1, whereas the distal mandibular arch was less affected. TUNEL assay revealed an increase in the number of apoptotic signals in PA1 of Shh mutant embryos. Ectodermal expression of fibroblast growth factor (Fgf)-8, a cell survival factor for pharyngeal arch mesenchyme, was down-regulated in the PA1 of Shh mutants. Consistent with this observation, downstream transcriptional targets of Fgf8 signaling in neural crest-derived mesenchyme, including Barx1, goosecoid, and Dlx2, were also down-regulated in Shh-null PA1. These results demonstrate that epithelial-mesenchymal signaling and transcriptional events coordinated by Shh, partly via Fgf8, is essential for cell survival and tissue outgrowth of the developing PA1.

Farnesoid X receptor agonist reduces serum asymmetric dimethylarginine levels through hepatic dimethylarginine dimethylaminohydrolase-1 gene regulation.

The farnesoid X receptor (FXR, NR1H4) is a bile acid-responsive nuclear receptor that plays critical roles in the transcriptional regulation genes involved in cholesterol, bile acid, triglyceride, and carbohydrate metabolism. By microarray analysis of hepatic genes from female Zucker diabetic fatty (ZDF) rats treated with the FXR agonist GW4064, we have identified dimethylarginine dimethylaminohydrolase-1 (DDAH1) as an FXR target gene. DDAH1 is a key catabolic enzyme of asymmetric dimethylarginine (ADMA), a major endogenous nitric-oxide synthase inhibitor. Sequence analysis of the DDAH1 gene reveals the presence of an FXR response element (FXRE) located 90 kb downstream of the transcription initiation site and within the first intron. Functional analysis of the putative FXRE demonstrated GW4064 dose-dependent transcriptional activation from the element, and we have demonstrated that the FXRE sequence binds the FXR-RXR heterodimer. In vivo administration of GW4064 to female ZDF rats promoted a dose-dependent and >6-fold increase in hepatic DDAH1 gene expression. The level of serum ADMA was reduced concomitantly. These findings provide a mechanism by which FXR may increase endothelium-derived nitric oxide levels through modulation of serum ADMA levels via direct regulation of hepatic DDAH1 gene expression. Thus, beneficial clinical outcomes of FXR agonist therapy may include prevention of atherosclerosis and improvement of the metabolic syndrome.

Tbx1 regulates fibroblast growth factors in the anterior heart field through a reinforcing autoregulatory loop involving forkhead transcription factors.

Birth defects, which occur in one out of 20 live births, often affect multiple organs that have common developmental origins. Human and mouse studies indicate that haploinsufficiency of the transcription factor TBX1 disrupts pharyngeal arch development, resulting in the cardiac and craniofacial features associated with microdeletion of 22q11 (del22q11), the most frequent human deletion syndrome. Here, we have generated an allelic series of Tbx1 deficiency that reveals a lower critical threshold for Tbx1 activity in the cardiac outflow tract compared with other pharyngeal arch derivatives, including the palatal bones. Mice hypomorphic for Tbx1 failed to activate expression of the forkhead transcription factor Foxa2 in the pharyngeal mesoderm, which contains cardiac outflow precursors derived from the anterior heart field. We identified a Fox-binding site upstream of Tbx1 that interacted with Foxa2 and was necessary for pharyngeal mesoderm expression of Tbx1, revealing an autoregulatory loop that may explain the increased cardiac sensitivity to Tbx1 dose. Downstream of Tbx1, we found a fibroblast growth factor 8 (Fgf8) enhancer that was dependent on Tbx1 in vivo for regulating expression in the cardiac outflow tract, but not in pharyngeal arches. Consistent with its role in regulating cardiac outflow tract cells Tbx1 gain of function resulted in expansion of the cardiac outflow tract segment derived from the anterior heart field as marked by Fgf10. These findings reveal a Tbx1-dependent transcriptional and signaling network in the cardiac outflow tract that renders mouse cardiovascular development more susceptible than craniofacial development to a reduction in Tbx1 dose, similar to humans with del22q11.

Tbx1 is regulated by tissue-specific forkhead proteins through a common Sonic hedgehog-responsive enhancer.

Haploinsufficiency of Tbx1 is likely a major determinant of cardiac and craniofacial birth defects associated with DiGeorge syndrome. Although mice deficient in Tbx1 exhibit pharyngeal and aortic arch defects, the developmental program and mechanisms through which Tbx1 functions are relatively unknown. We identified a single cis-element upstream of Tbx1 that recognized winged helix/forkhead box (Fox)-containing transcription factors and was essential for regulation of Tbx1 transcription in the pharyngeal endoderm and head mesenchyme. The Tbx1 regulatory region was responsive to signaling by Sonic hedgehog (Shh) in vivo. We show that Shh is necessary for aortic arch development, similar to Tbx1, and is also required for expression of Foxa2 and Foxc2 in the pharyngeal endoderm and head mesenchyme, respectively. Foxa2, Foxc1, or Foxc2 could bind and activate transcription through the critical cis-element upstream of Tbx1, and Foxc proteins were required, within their expression domains, for Tbx1 transcription in vivo. We propose that Tbx1 is a direct transcriptional target of Fox proteins and that Fox proteins may serve an intermediary role in Shh regulation of Tbx1.