A beta 2RARE-LacZ transgene identifies retinoic acid-mediated transcriptional activation in distinct cutaneous sites

The P450 system and mTORC1 signalling in acne

In this issue, Hellmann-Regen et al. suggested that anti-acne effects of erythromycin and tetracyclines may be related to their inhibitory effect of cytochrome P450-mediated degradation of all-trans-retinoic acid (ATRA). We have recently proposed that all anti-acne agents function by attenuation of increased mTORC1 signalling. This commentary links the P450 system to mTORC1 regulation in acne. Drug-mediated induction of P450 activity or P450 mutants with increased catabolic activity may reduce cellular ATRA levels and FoxO1 expression, thus reducing FoxO-mediated mTORC1 inhibition. In contrast, agents blocking ATRA degradation such as erythromycin and tetracyclines may improve acne by increasing FoxO1 expression with consecutive inhibition of mTORC1 signalling.

Do tetracyclines and erythromycin exert anti-acne effects by inhibition of P450-mediated degradation of retinoic acid?

For decades, retinoic acid (RA) is known as the most potent therapeutic option in the therapy of acne and altered homeostasis of endogenous retinoids has been discussed in the context of acne pathogenesis. Besides retinoids, antibiotics such as tetracyclines or erythromycin are well established in acne pharmacotherapy. Accumulating evidence points towards common molecular pathways being targeted by both RA and anti-acne antibiotics; however, a precise 'common denominator' connecting these chemically diverse anti-acne agents has not yet been identified. Interestingly, tetracyclines are associated with the occurrence of pseudotumor cerebri, a rare neurological side effect otherwise associated with retinoid intoxication or RA exposure. This association at the clinical level suggests an interaction between tetracyclines and endogenous RA signalling. As erythromycin does not cross the blood brain barrier, CNS side effects are not to be expected, yet not precluding a possible local interaction of erythromycin with endogenous RA metabolism in the skin. We hypothesize tetracyclines and erythromycin to locally inhibit endogenous RA metabolism in the skin and thus mimic therapeutic action of RA. This readily testable hypothesis suggests inhibition of endogenous RA metabolism and amplification of endogenous RA signalling as a mechanism underlying the biochemical actions of antibiotics in acne therapy. Elucidation of such interactions may ultimately enhance our understanding of acne therapy and pathogenesis and may yield a sound, scientific basis for hypothesis-driven development of novel therapeutic compounds.

Nephric duct insertion requires EphA4/EphA7 signaling from the pericloacal mesenchyme

The vesico-ureteric junction (VUJ) forms through a complex developmental program that connects the primordium of the upper urinary tract [the nephric duct (ND)] with that of the lower urinary tract (the cloaca). The signals that orchestrate the various tissue interactions in this program are poorly understood. Here, we show that two members of the EphA subfamily of receptor tyrosine kinases, EphA4 and EphA7, are specifically expressed in the mesenchyme surrounding the caudal ND and the cloaca, and that Epha4−/−;Epha7+/− and Epha4−/−;Epha7−/− (DKO) mice display distal ureter malformations including ureterocele, blind and ectopically ending ureters with associated hydroureter, megaureter and hydronephrosis. We trace these defects to a late or absent fusion of the ND with the cloaca. In DKO embryos, the ND extends normally and approaches the cloaca but the tip subsequently looses its integrity. Expression of Gata3 and Lhx1 and their downstream target Ret is severely reduced in the caudal ND. Conditional deletion of ephrin B2 from the ND largely phenocopies these changes, suggesting that EphA4/EphA7 from the pericloacal mesenchyme signal via ephrin B2 to mediate ND insertion. Disturbed activity of this signaling module may entail defects of the VUJ, which are frequent in the spectrum of congenital anomalies of the kidney and the urinary tract (CAKUT) in human newborns.

Endogenous retinoids in the hair follicle and sebaceous gland

Vitamin A and its derivatives (retinoids) are critically important in the development and maintenance of multiple epithelial tissues, including skin, hair, and sebaceous glands, as shown by the detrimental effects of either vitamin A deficiency or toxicity. Thus, precise levels of retinoic acid (RA, active metabolite) are needed. These precise levels of RA are achieved by regulating several steps in the conversion of dietary vitamin A (retinol) to RA and RA catabolism. This review discusses the localization of RA synthesis to specific sites within the hair follicle and sebaceous gland, including their stem cells, during both homeostasis and disease states. It also discusses what is known about the specific roles of RA within the hair follicle and sebaceous gland. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Involvement of thyroid hormone and its alpha receptor in avian neurulation

We have analyzed the expression pattern of c-erb A alpha and c-erb A beta which encode the thyroid hormone receptors (T3R alpha and T3R beta) during early chicken embryogenesis. Only c-erb A alpha expression was detected by RT-PCR and whole-mount in situ hybridization. c-erb A alpha transcripts were found to be already present at low level in embryos before egg incubation. During neurulation a marked increase was observed in neurectoderm. A reporter cell line was then constructed and used to demonstrate the release of significant amount of thyroid hormone (T3) from egg yolk by area opaca cells before gastrulation. During gastrulation T3 was found to be enriched in the primitive streak and Hensen's node. Introduction of excess T3 frequently resulted in abnormal development of anterior structures, mainly neural tube defects and anencephalia. These observations suggest that T3R alpha, like the closely related retinoic acid receptors, fulfills functions which are important for embryonic development well before the onset of thyroid gland function.

Direct repeats bind the EcR/USP receptor and mediate ecdysteroid responses in Drosophila melanogaster

The steroid hormone 20-hydroxyecdysone plays a key role in the induction and modulation of morphogenetic events throughout Drosophila development. Previous studies have shown that a heterodimeric nuclear receptor composed of the EcR and USP proteins mediates the action of the hormone at the transcriptional through binding to palindromic ecdysteroid mediates the action of the hormone at the transcriptional level through binding to palindromic ecdysteroid response elements (EcREs) such as those present in the promoter of the hsp27 gene or the fat body-specific enhancer of the Fbp1 gene. We show that in addition to palindromic EcREs, the EcR/USP heterodimer can bind in vitro with various affinities to direct repetitions of the motif AGGTCA separated by 1 to 5 nucleotides (DR1 to DR5), which are known to be target sites for vertebrate nuclear receptors. At variance with the receptors, EcR/USP was also found to bind to a DR0 direct repeat with no intervening nucleotide. In cell transformation assays, direct repeats DR0 to DR5 alone can render the minimum viral tk or Drosophila Fbp1 promoter responsive to 20-hydroxyecdysone, as does the palindromic hsp27 EcRE. In a transgenic assay, however, neither the palindromic hsp27 element nor direct repeat DR3 alone can make the Fbp1 minimal promoter responsive to premetamorphic ecdysteroid peaks. In contrast, DR0 and DR3 elements, when substituted for the natural palindromic EcRE in the context of the Fbp1 enhancer, can drive a strong fat body-specific ecdysteroid response in transgenic animals. These results demonstrate that directly repeated EcR/USP binding sites are as effective as palindromic EcREs in vivo. They also provide evidence that additional flanking regulatory sequences are crucially required to potentiate the hormonal response mediated by both types of elements and specify its spatial and temporal pattern.