Acne is a multifactorial disease of the pilosebaceous unit resulting from increased sebum production, altered keratinization, inflammation, and bacterial hypercolonization.With respect to hyperseborrhea as one of the contributing factors, it is of interest to study pathways that regulate sebaceous gland differentiation.Interestingly, mouse strains deficient in the enzyme stearoyl-CoA desaturase 1 (SCD1) exhibit severe hypoplasia of sebaceous glands (Zheng et al., 1999; Sundberg et al., 2000; Miyazaki et al., 2001; Georgel et al., 2005).SCDs are microsomal enzymes that catalyze the biosynthesis of monounsaturated fatty acids from saturated acyl-CoAs (Ntambi and Miyazaki, 2003); specifically they introduce cis-double bonds in the Δ9 position, the preferred substrates being palmitoyl-CoA and stearoyl-CoA.Here, we asked whether pharmacol. inhibition of SCD activity would have an impact on sebaceous glands.From a series of novel SCD1 inhibitors (Zhang et al., 2013), compound XEN103, (6-[4-(5-fluoro-2-trifluoromethylbenzoyl)-piperazin-1-yl]pyridazine-3-carboxylic acid (2-cyclopropylethyl)amide (Figure 1a), was chosen for further characterization.XEN103 inhibited SCD1 activity in mouse liver microsomes with IC50=14 nM.The compound featured 700-fold selectivity for mouse SCD1 activity relative to the closely related Δ5- and Δ6-desaturases (FADS1 and FADS2), indicating low potential for blockade of essential fatty acid and sapienic acid biosynthesis, in which these enzymes are involved.In human HepG2 cells, XEN103 blocked SCD1 activity with IC50=12 nM.Furthermore, XEN103 treatment of human SZ95 sebocytes led to reduction of monounsaturated acyl chains in phosphatidylcholine with IC50=11 nM (Figure 1b); these cells express SCD1 (Harrison et al., 2007) but not SCD5 (our unpublished data), the only other known human Δ9-desaturase.Expression of SCD1 in SZ95 cells is increased by treatment with testosterone and linoleic acid; this increase was shown previously to be blocked by XEN103 using a semiquant. method (Zouboulis et al., 2011).Quant. reverse transcriptase-PCR showed half-maximal inhibition between 10 and 100 nM XEN103 (Figure 1c).Thus, XEN103 not only acts as an enzyme inhibitor but also blocks androgen-induced transcription of the gene.One may speculate that a product of SCD1 required for SCD1 transcription is involved in a pos. feedback loop.We assessed the effect of XEN103 on mouse skin after topical application.Mice were treated for 8 days on the shaved back with solutions containing 0.1% or 1% XEN103 in propylene glycol/ethanol 7:3, to assess local tolerance and activity.There were no signs of skin irritation at the test sites during the treatment period visible by macroscopic inspection or by histol. examinationAs assessed by histomorphometry, application of 1% XEN103 reduced the numbers of sebaceous glands by∼65% (P<0.001) and the size of the remaining glands by∼50% (P<0.01) (Figure 2a).In a total of three independent studies, reduction of sebaceous gland numbers and size varied in the range of 50-75%.Representative photographs of vehicle- and XEN103-treated skin samples stained with hematoxylin-eosin or with oil red O are shown in Figure 2b and c, resp.While numerous normally sized and lipid-loaded sebaceous glands were seen at vehicle-treated sites, only very small atrophic sebaceous glands with hardly any associated lipid staining were visible at XEN103-treated sites.Thus, the SCD1 inhibitor XEN103 clearly induces an atrophogenic effect on the sebaceous glands in mouse skin, which should consequently result in reduced sebum lipids.When XEN103 was applied in solution at a concentration of 0.1%, XEN103 induced little atrophogenic effect (Figure 2a); in a sep. study we determined the minimal effective concentration to 0.5% (data not shown).While a trend toward a decrease in sebaceous gland size was already observed after 2 days, the effect increased with time and was statistically significant after 6 days for both size and numbers (Figure 2d).When treatment was stopped after 7 days, sebaceous gland numbers and size reverted to normal values within 2 wk (data not shown).At skin areas distant from the XEN103-treated sites, there was no significant reduction in the number of sebaceous glands (Figure 2a).This indicates that efficacy is restricted to the topically treated area, excluding activity at distant sites due to percutaneous absorption.This observation is in line with the observed low plasma exposure with XEN103, while at least 1000-fold higher concentrations of XEN103 were found in the skin (Supplementary Table S1).To determine whether topical application of XEN103 indeed blocked SCD activity in the skin, we determined the ratio of palmitoleic and palmitic acid as a measure of Δ9-desaturase activity.The desaturation of fatty acids in the triglycerides was reduced by about 50% when mice were treated with 1% XEN103 (Figure 2e).In the 0.1% XEN103-treated group, reduction was by 20%.This result correlates well with the sebaceous glands' atrophy end point and the skin exposure to XEN103.To summarize, XEN103 is a potent inhibitor of SCD activity as shown in microsomal preparations, cultured cells, and in rodents in vivo.The compound is active on its target enzyme across species and, importantly, it also reduces both the activity and the androgen-induced expression of SCD1 in human sebocytes.Using this tool compound, we show that pharmacol. inhibition of SCD in mouse skin rapidly induces pronounced sebaceous gland atrophy after topical application in a dose- and time-dependent manner.The effect is due to the local action of the compound on SCD activity in the skin and occurs in the presence of only minimal plasma exposure, thus excluding any potentially undesirable effects of systemic SCD inhibition.In mice, there are three Δ9-desaturases apart from SCD1, namely SCD2-4, which potentially could be inhibited by XEN103; however, Scd2-deficient mice have normal sebaceous glands (Miyazaki et al., 2005), the Scd3 gene cannot compensate Scd1 deficiency in asebia mice (Zheng et al., 2001), and Scd4 expression is restricted to the heart (Miyazaki et al., 2003).Thus, we consider that sebaceous gland atrophy in mouse skin is most likely due to inhibition of SCD1.In human skin, SCD1 is present as the only Δ9-desaturase.It will be interesting to learn from a clin. study whether an SCD1 inhibitor such as XEN103 also induces human sebaceous gland atrophy when applied in a topical formulation allowing for adequate exposure at the target.
