Cornified Envelope Formation by Anthralin, Simple Analogues, and Related Anthracenones

Cornified Envelope Formation by Anthralin, Simple Analogues, andRelated Anthracenones✩

Klaus Müllera)* and Hans Reindlb)

a) Westfälische Wilhelms-Universität Münster, Institut für Pharmazeutische Chemie, Hittorfstraße 58–62, D-48149 Münster, Germany

b) Institut für Pharmazie, Pharmazeutische Chemie I, Universität Regensburg, D-93040 Regensburg, Germany

Key Words: Anthracenones; anthralin; cornified envelope; dithranol; HaCaT cell; keratinocyte differentiation

Summary

Introduction

Psoriasis is a chronic relapsing skin disease affecting 2% ofthe people of European ancestry [1]. The underlying patho-genesis involves abnormal differentiation of keratinocytes,hyperproliferation of keratinocytes, and the infiltration ofinflammatory components into the skin [2]. The anthracenonederivative anthralin (dithranol, 1) has been the gold standardin the treatment of psoriasis for many decades. Even thougha number of therapeutic agents have been added to topicalantipsoriatic therapy, anthralin is the most remarkably con-sistent and time-honored antipsoriatic drug [3] and still com-pares favorably in efficacy with newer treatment methodssuch as vitamin A and D derivatives [4]. Common side-effects

of anthralin are severe inflammation and staining of thehealthy, perilesional skin. The specific molecular mode ofaction of the drug is poorly understood, but there is substantialdata suggesting the involvement of oxygen radicals in thetherapeutic as well as toxic action [5]. Oxygen radicals gen-erated from anthralin have been shown to be responsible forDNA damage [6,7], induction of inflammatory cytokines [8],and activation of transcription factors [9].

In vitro and in vivo studies on the interference of anthralinwith several enzymes or other targets associated with two ofthe three predominant biological processes of psoriasis [10],cutaneous inflammation [11,12], and epidermal proliferation[13–16], have been extensively documented. However, notmuch is known about the action of the drug with respect to athird important aspect of the disease, disturbance of celldifferentiation. Epidermal differentiation is responsible forthe maintenance of epidermal homeostasis. The multilayeredstratified squamous epithelium undergoes the process of ter-minal differentiation to result in corneocytes. One of the moststriking aspects of keratinocyte differentiation is the forma-tion of insoluble cornified envelopes [17], which are com-posed predominantly of an array of cross-linked proteins [18].The cornified envelope, the specialized external envelope ofthe terminally differentiated corneocyte, confers rigidity andmechanical resistance. The balance between keratinocyteproduction in the basal layer and corneocyte shedding at theskin surface maintains the equilibrium of the normal epider-mis, but the small number of dividing keratinocytes can berapidly expanding in hyperproliferative diseases such as pso-riasis [10].

In previous studies we have focussed on strategies to mini-mize the potential of skin inflammation by anthralin and theunderlying concepts of controlling and manipulating oxygen-radical generation which are essential for the rational designof novel anthracenones [19]. Thus, various studies haveshown that by altering the structure of anthralin the in vitroprofile can be markedly altered from an only moderate inhibi-tor of leukotriene biosynthesis to a more potent inhibitor,which may reflect improved activity against the inflamma-tory component of psoriasis. Furthermore, the potent antipro-liferative activity of anthralin was retained, and the structuralchange was also accompanied by diminished oxygen-radicalgeneration from the novel compounds, which resulted insignificantly reduced damaging properties to cell membranesas compared to parent anthralin [20,21]. As a further indicationthat such molecules exhibit potential antipsoriatic activity,we were interested to see if the compounds are capable ofinducing epidermal differentiation. Therefore, we examined

Arch. Pharm. Pharm. Med. Chem. © WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0365-6233/01/0303/0086 $17.50 +.50/0

———Abbreviations: DMEM; Dulbecco’s modified Eagle’s medium; DMSO,dimethylsulfoxide; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetra-zoliumbromide; PBS, phosphate-buffered saline; SDS, sodium dodecyl sul-fate; β-ME, β-mercaptoethanol; TCA, trichloroacetic acid.

The ability of the antipsoriatic anthralin to induce HaCaT kerati-nocyte differentiation was investigated and correlated with itspotency to inhibit proliferation of keratinocytes. To determine thestructural requirements for this effect, anthralin and seventeensimple analogues or related anthracenones were examined for theirability to induce the formation of cornified envelope as a markerof terminal differentiation. Covalently cross-linked protein wasmeasured as a key feature of this process. Induction of keratinocytedifferentiation was significant at a concentration of 0.5 µM an-thralin after 48 h exposure. The presence of the 1,8-dihydroxygroups is a critical determinant of cross-linking activity, sinceremoving or exchanging these groups prevented the induction ofkeratinocyte differentiation. Furthermore, at least one hydrogenatom at the 10-position of anthralin is required. Moreover, an-thralin, anthralin dimer, and anthralin triacetate exhibited antipro-liferative and antirespiratory activity at concentrations required toinduce keratinocyte differentiation, suggesting a causality betweenthese effects. In addition, cornified envelope formation was ob-served for a number of related anthracenones at concentrations aslow as 1–5 µM. In general, compounds containing benzoyl substit-uents, independent of the position in the anthralin nucleus, weremore potent than those having benzyl substituents. Only marginaldifferences in cross-linking potency were observed within a num-ber of phenylpropionyl substituted analogues, suggesting that theability to induce keratinocyte differentiation is independent of thenature of substituents at the side chain.

86 Müller and Reindl

the ability of anthralin to induce cross-linking of cell proteinsas a marker of terminal differentiation of keratinocytes, andwe also determined the structural requirements for this effect.In the present paper, we report the results of these studies andthe potential implications for other compounds of the an-thracenone class of antipsoriatic agents.

Chemistry

Anthralin analogues and related anthracenones were avail-able from previous work, and these are referenced in theExperimental Part.

Results and Discussion

Keratinocyte Line and Assay Conditions

The use of epidermal cell lines in studies of keratinocytedifferentiation has been largely limited because, in most casesestablished cells give rise to irregular proliferation and lossof terminal differentiation [22]. This limitation can be partiallyovercome by the use of primary culture. However, limitedgrowth potential restricts these studies. Nevertheless, theimmortalized human keratinocyte line HaCaT [23], althoughcytogenetically abnormal, shares many features of differen-tiation with normal keratinocytes [22], and the highly pre-served epidermal characteristics make this cell line anexcellent candidate for studying external modulators of epi-dermal differentiation [24].

The terminal differentiation of human epidermal keratino-cytes is a complex morphological and biochemical shift froma mitotically active cell to an insoluble cornified envelope,which is perhaps the most characteristic feature of squamousdifferentiation [18]. The envelope consists of covalentlycross-linked proteins that are held together by intermolecular

ε-(γ-glutamyl) peptide bonds [25]. Formation of the cross-linked protein is catalyzed by the action of a keratinocyte-spe-cific transglutaminase, which is a calcium-dependentenzyme [26]. Increased extracellular calcium, resulting inelevated intracellular calcium, is perhaps the best knownsignal for the induction of keratinocyte differentiation [27].This was confirmed by the action of calcium ionophoreA23187 on HaCaT cell differentiation. Figure 1 shows thatthis substance at 1–10 µM concentrations extensively in-duced the formation of cross-linked protein with a maximaleffect at 5 µM calcium ionophore. In the following cross-link-ing experiments, HaCaT cells were incubated in the presenceof the test compounds at the same calcium concentration(1.8 mM) that was used in the proliferation and mitochondrialfunction assay.

Cornified Envelope Formation, Antiproliferative and An-tirespiratory Activity of Anthralin and Simple Analogues

HaCaT keratinocytes were grown for six days. At this pointthe various concentrations of test compounds were added, andincubation was continued for 48 h, as preliminary studies onthe time course for the effects of anthralin on cornifiedenvelope formation showed that the amount of cross-linkedprotein markedly increased within this period of time (datanot shown).

The ability of anthralin to induce the formation of cross-linked protein as a measure of terminal differentiation in thekeratinocyte cell line HaCaT was concentration-dependent(Figure 2). Induction of keratinocyte differentiation was sig-nificant at a concentration of 0.5 µM anthralin after 48 hexposure, maximum stimulation was observed at 10 µM andstagnated at higher anthralin concentrations.

Figure 3 presents the structures of some simple analoguesof anthralin which were also screened for cornified envelope

Figure 1. Cornified envelope formation in the presence of calcium ionophoreA23187. HaCaT cells were grown for 144 h before addition of the indicatedconcentrations of calcium ionophore. Incubation was continued for 48 h at37 °C. Amounts of cornified envelopes were measured as described inMaterials and Methods and are expressed as µg cross-linked protein per mgtotal amount of protein/mL. Results are means ± SEM of three independentexperiments. *Values are significantly different with respect to vehiclecontrol (P < 0.05; Student’s t-test).

Figure 2. Concentration-dependent cornified envelope formation by an-thralin. HaCaT cells were grown for 144 h before addition of the indicatedconcentrations of anthralin. Incubation was continued for 48 h at 37 °C.Amounts of cornified envelopes were measured as described in Materialsand Methods and are expressed as µg cross-linked protein per mg totalamount of protein/mL. Results are means ± SEM of three independentexperiments. *Values are significantly different with respect to vehiclecontrol (P < 0.05; Student’s t-test).

Cornified Envelope Formation 87

Arch. Pharm. Pharm. Med. Chem. 334, 86–92 (2001)

formation at concentrations ranging up to 10 µM. This seriesof compounds examined specific features necessary for thecross-linking effects. Questions of particular interest centeredon what role the 1,8-dihydroxy substitution pattern and thecritical 10-position of anthralin played. The anthralin meta-bolites danthron (2) and anthralin dimer (3), which are boththerapeutically inactive [28], as well as 2,2′-dihydroxybenzo-phenone (7) maintained the hydroxy substitution pattern ofanthralin, with the methylene moiety in position 10 of themolecule being oxidized, substituted, or omitted, respec-tively. The unsubstituted 9(10H)-anthracenone (5) and 1,8-dichloro-9(10H)-anthracenone (6) maintained the 9-ketofunction of anthralin and replaced the 1,8-dihydroxy groupswith hydrogen or chloro atoms, while 6 also maintained the

1,8-arrangement of the substituents. Compound 4, whichexplored the effect of esterification of the oxygen functions,changed the anthracenone to the tautomeric anthracenolform.

As shown in Table 1, analogues 2, 5, 6, and 7 did not exhibitappreciable activity within the concentration range. By con-trast, anthralin triacetate (4) was significantly active at 5 µMconcentration, while the dimer 3 induced extensive cross-linking even at the lowest concentration.

To determine whether the ability of the compounds toinduce cornified envelope formation correlated with theirantiproliferative activity, proliferation of the keratinocytes inthe presence of anthralin and simple analogues was deter-mined directly by counting the dispersed cells under a phase-contrast microscope after 48 h of treatment. Anthralin and itsdimer 3 inhibited proliferation of HaCaT keratinocytes withIC50 values of 0.6 µM (Table 1), and also the triacetate 4 wasa highly potent inhibitor of cell growth. The other analogueswere not active at concentrations up to 5 µM. In another setof experiments, the effects of the compounds on mitochon-drial function was independently assessed under the condi-tions of the cornified envelope assay. These studies wereperformed to measure the potential cytotoxicity of the com-pounds using a colorimetric assay based on the reductivecleavage of the tetrazolium salt MTT into a blue formazanproduct, which is dependent on the mitochondrial dehydro-genase activity in living cells [29].

In general, there was a good agreement between the resultsof enumerating the keratinocytes and those obtained from thecolorimetric measurement for evaluating the antirespiratoryactivity. With the exception of the dimer 3, compounds withpotent growth-inhibitory activity such as 1 and 4 also mark-edly impaired mitochondrial function in HaCaT keratino-

OHOH O

1

OH O OH

OHOOH

3

OH O OH

O2

OH O OH

7

O

5

ClCl O

6

O O O

O

H3C

O

CH3

O CH3

4

1 8

10

Figure 3. Chemical structures of anthralin and simple analogues: anthralin(1), danthron (2), anthralin dimer (3), anthralin triacetate (4), 9(10H)-an-thracenone (5), 1,8-dichloro-9(10H)-anthracenone (6), 2,2′-dihydroxyben-zophenone (7).

Table 1. Induction of keratinocyte differentiation, antiproliferative and antirespiratory activity of anthralin and simple analogues.——————————————————————————————————————————————————————

Cornified envelope assayb AAc MFd

µg cross-linked protein/mg protein IC50 IC50Compounda 1 µM 5 µM 10 µM (µM) (µM)

——————————————————————————————————————————————————————

anthralin (1) 3.7 ± 1.8e 4.9 ± 0.8e 5.8 ± 1.4e 0.6 5

danthron (2) 0.6 ± 1.8 –0.1 ± 0.7 0.0 ± 1.0 >5 (5%)

anthralin dimer (3) 4.0 ± 0.6e 5.4 ± 1.4e 5.5 ± 2.0e 0.6 (33%)

anthralin triacetate (4) 2.5 ± 0.6 3.8 ± 1.2e 3.3 ± 1.8e 0.5 8

9(10H)-anthracenone (5) 0.5 ± 1.2 0.3 ± 0.6 0.8 ± 1.8 >5 (4%)

1,8-(Cl)2-anthracenone (6) 0.8 ± 1.2 0.4 ± 0.4 0.2 ± 1.0 >5 (14%)

2,2′-(OH)2-benzophenone (7) –1.0 ± 0.3 –0.8 ± 0.1 0.9 ± 0.6 >5 (21%)

——————————————————————————————————————————————————————aStructures are given in Fig. 3.b Differences of the amounts of cross-linked protein as a measure of HaCaT keratinocyte differentiation at indicated concentrations oftest compounds and vehicle control. 144-h post-confluent cells were incubated for 48 h at 37 °C. Amounts of cornified envelopes were

measured as described in Materials and Methods. Results are the means ± SEM of three independent experiments. cAntiproliferative activity against HaCaT keratinocytes. Inhibition of cell growth was significantly different with respect to that of the control (N = 3, P < 0.01). Cells were grown for 24 h at 37 °C and then incubated with the test compounds for an additional 48 h. Cells were enumerated by phase-contrast microscopy.dInhibition of mitochondrial function (antirespiratory activity) in HaCaT keratinocytes (N = 3, P < 0.01); values in parenthesis are

% inhibition at 10 µM test compound. 144-h post-confluent cells were incubated with the test compounds for 48 h at 37 °C. Mitochondrial activity was determined by addition of MTT, and formazan production was determined after 2 h.eValues are significantly different with respect to vehicle control (P < 0.05; Student’s t-test).



cytes. This is in line with earlier reports that the inhibition ofcell proliferation by anthralin, at least in keratinocytes, is theconsequence of impaired mitochondrial function rather thanof DNA damage [13]. Moreover, anthralin, its dimer 3, andthe triacetate 4 exhibited antiproliferative and antirespiratoryactivity at concentrations required to induce keratinocytedifferentiation, suggesting a causality between these effects.While growth arrest may be a necessary consequence ofkeratinocyte differentiation [30], growth arrest does not nec-essarily result in the onset of differentiation [31,32].

Furthermore, the data of Table 1 indicate that the presenceof the 1,8-dihydroxy groups is a critical determinant of cross-linking activity, since removing or exchanging these groups,as in 5 and 6, prevented the induction of keratinocyte differ-entiation. In addition, at least one hydrogen atom at the10-position of anthralin is required. Where the 1,8-dihydroxygroups were intact but the methylene moiety was replacedwith a carbonyl group or the middle ring was opened, as in 2or 7, respectively, the ability of cornified envelope formationwas lost.

Although the structure of anthralin triacetate (4) is notconsistent with the presence of free hydroxy groups, itsactivity is likely dependent on hydrolytic or enzymatic re-moval of the ester groups to give the parent 1. In support of

this, 4 has been reported to be a prodrug, which is transformedto the parent 1 [28].

Surprisingly, the dimer 3 not only inhibited the proliferationof keratinocytes in vitro but also induced their differentiation.Even though this dimer has no apparent clinical efficacy, ithas been reported to be almost as effective as anthralin forinhibition of keratinocyte proliferation in other studies [13,33].The apparent inconsistencies between the in vivo and in vitroactivities of 3 may be explained by postulating 3 as a meta-bolite that is too reactive to survive penetration of the skin.In order to reach its critical biological target, it must begenerated from anthralin beneath the protective barrier of thestratum corneum in, or in close proximity to, dividing kerat-inocytes [33].

Substituents at Positions 2,3, or 10 of Anthralin

Next we examined the effect of substituents at variouspositions of the anthralin nucleus on cornified envelope for-mation. The structures of these substituted anthracenones arelisted in Figure 4, and Table 2 presents the relevant biologicalproperties.

In general, benzyl substituents were less active or inactive,independent of the position at the anthralin nucleus. Thefailure of benzyl derivatives 8 and 12 to induce appreciablecornified envelope formation correlated with their inabilityto inhibit keratinocyte growth at concentrations up to 5 µM.The observation that 10-benzylanthralin (12) did not exhibitappreciable activity within the concentration range indicatesthat structural features necessary for cross-linking do notreside necessarily in the anthralin chromophore, and meresubstitution of one hydrogen atom, as in dimer 3, does notnecessarily result in active analogues. By contrast, 10 wassignificantly active at 5 µM and was also a potent cell growthinhibitor. Furthermore, exchanging the methylene linker of12 with a sulfur atom (13) significantly increased the amountof cross-linked protein, but only at 10 µM concentration.However, exchanging the methylene linker with a carbonylgroup resulted in compounds (9, 11, 14) that were all able toinduce cross-linking. Among the benzoyl analogues, com-pound 11 was the most potent inhibitor of keratinocyte

OHOH O OH O OH

Z

Z

OHOH O

Z

8: Z = CH29: Z = CO

10: Z = CH211: Z = CO

12: Z = CH213: Z = S14: Z = CO

Figure 4. Chemical structures of benzyl, benzoyl, and thiophenyl substitutedanthralin derivatives.

Table 2. Induction of keratinocyte differentiation, antiproliferative and antirespiratory activity of 2-, 3-, or 10-substituted anthracenones.

—————————————————————————————————————————————————————



—————————————————————————————————————————————————————

2-Bn-anthralin (8) 1.0 ± 0.8 1.9 ± 0.5 1.3 ± 0.1 >5 8

2-Bz-anthralin (9) 2.1 ± 0.7 2.9 ± 0.1e 3.1 ± 0.3e 1.6 4

3-Bn-anthralin (10) 1.8 ± 1.4 4.2 ± 0.2e 3.6 ± 0.2e 0.5 3

3-Bz-anthralin (11) 2.9 ± 0.1e 3.9 ± 0.4e 4.5 ± 0.4e 0.3 1

10-Bn-anthralin (12) –1.0 ± 0.3 –0.8 ± 0.1 0.9 ± 0.6 >5 (43%)

10-S-Ph-anthralin (13) –1.0 ± 0.7 1.9 ± 0.6 2.9 ± 0.3e 3.7 4

10-Bz-anthralin (14) 2.7 ± 1.5 4.2 ± 1.2e 5.1 ± 1.0e ND 9

—————————————————————————————————————————————————————aStructures are given in Fig. 4.b–e See footnotes of Table 1. ND = not determined.



growth and caused also the strongest impairment of mito-chondrial function.

10-Phenylpropionyl-substituted Analogues of Anthralin

Compounds 15–18 (Figure 5) were used to explore theinfluence of ortho-dihydroxy groups in the terminal phenylring of a propionyl linker at position 10 of anthralin. In aprevious study, potent cross-linking of cellular protein wasobserved for a number of protein-tyrosine kinase inhibitorsrelated to erbstatin, which required ortho- or para-dihydroxy-lation of the aromatic ring [34]. A mechanism was suggestedwhich involves initial oxidation of the compounds to reactivequinone intermediates that subsequently cross-linked proteinnucleophiles by a non-physiological, chemical reaction.Since anthralin has been reported to reduce tyrosine-specificprotein phosphorylation in HaCaT keratinocytes [35], it wasof interest to examine if these structural features were alsoimportant for the anthralin analogues.

Catechol 15 contains two hydroxy groups arranged inortho-orientation. Removing one hydroxy group (16) or pro-tection of the ortho-dihydroxy groups by acylation (17),which would inhibit oxidation to a quinone, was also studied.

Finally, a derivative in which the ortho-hydroxy groups wereincorporated into a fused ring (18) was also being examinedto assess whether the free hydroxy groups were not restoredfrom the acetate 17 through intracellular cleavage by ester-ases. As shown in Table 3, the ability of 15 to induce proteincross-linking could not reside in the ortho-dihydroxy ar-rangement, since analogues 16–18 were also able to inducesignificant cross-linking even at the lowest concentration of1 µM. Further, there was no considerable difference in cross-linking potency between 15 and analogues 16–18. Therefore,a mere chemical effect of 15 was unlikely, as compounds15–18 were thought to act by a common physiologicalmechanism. Also in contrast to the study with erbstatin ana-logues, which required comparatively high concentrations of50–100 µM to induce cross-linking [34], which were abovethose required to inhibit proliferation, compounds 15–18inhibited keratinocyte growth and induced their differentia-tion at comparable concentrations.

In conclusion, the process of differentiation examined,cornified envelope formation, is unique to the keratinocyteand gives the terminally differentiated keratinocyte its name,corneocyte. Since differentiation is an irreversible process,corneocytes are then lost by shedding at the skin surface.Accordingly, agents that induce this process can be used inthe treatment of hyperproliferative conditions. Anthralin andits analogues were established not only as inhibitors of kera-tinocyte proliferation, but also as potent regulators of kera-tinocyte differentiation. The structural requirements for theability to induce cornified envelope formation are the pres-ence of 1,8-dihydroxy groups and at least one hydrogen atposition 10 of anthralin.

Experimental Part

Anthralin [36], anthralin dimer [36] and anthralin triacetate [37] were pre-pared as described, and the required anthracenone analogues were availablefrom previous work: 6 [38], 8–11 [39], 12 [40], 13 [20], 14 [41], 15–17 [42].

1,8-Dihydroxy-10-[3-(3,4-methylenedioxyphenyl)-1-oxopropyl]-9(10H)-anthracenone (18)

A solution of 1,8-dihydroxy-10-[3-(3,4-methylenedioxyphenyl)-1-oxo-propenyl]-9(10H)-anthracenone [42] (0.25 g, 0.62 mmol) in absolute THF(30 mL) was hydrogenated over 10% Pd/C at room temperature for 24 h. Thereaction mixture was filtered, and the filtrate was evaporated. The residuewas purified by column chromatography (SiO2, CH2Cl2) and recrystallizedfrom benzene/petroleum ether (6/4) to afford 18 as a yellow powder; 40%

Table 3. Induction of keratinocyte differentiation, antiproliferative and antirespiratory activity of 1,8-dihydroxy-10-phenylpropionyl-9(10H)-anthracenones.

——————————————————————————————————————————————————————



——————————————————————————————————————————————————————

3,4-(OH)2 analogue (15) 2.8 ± 0.3e 4.9 ± 0.2e 5.5 ± 0.1e 0.5 5

4-OH analogue (16) 2.9 ± 0.2e 5.0 ± 0.3e 5.8 ± 0.3e 2.7 3

3,4-(OAc)2 analogue (17) 2.8 ± 0.8e 5.2 ± 0.7e 5.1 ± 0.2e 1.4 ND

3,4-OCH2O analogue (18) 2.3 ± 0.6e 5.2 ± 0.4e 5.7 ± 0.8e 0.9 ND

——————————————————————————————————————————————————————aStructures are given in Fig. 5.b–e See footnotes of Table 1. ND = not determined.

OH O OH

15

O

OH

OH O OH

O

OHOH

16

OH O OH

17

O

OH O OH

O

OO

18

CH3

O

OO

H3C

O

Figure 5. Chemical structures of 1,8-dihydroxy-10-phenylpropionyl-9(10H)-anthracenones.



yield; mp 134–137 °C; FTIR 1711 (COOH), 1628 cm–1 (CO...HO); 1H NMR(DMSO-d6) δ 11.92 (s, 2H, OH), 7.61–6.47 (m, 9H, Ar), 5.94 (s, 2H,O-CH2-O), 5.64 (s, 1H, 10-CH), 2.92 (t, J = 7.3 Hz, 2H, COCH2), 2.54 (t, J= 7.3 Hz, 2H, PhCH2). Anal. (C24H18O6) C, H.

All other compounds were of the highest grades available from Sigma(Deidenhofen, Germany), Aldrich (Steinheim, Germany) or from Merck(Darmstadt, Germany).

Keratinocyte Culture and Growth Inhibition

HaCaT cells [23] were cultured as described [39]. In brief, the cells weregrown in DMEM (No. 041-11965A, Gibco) supplemented with 10% fetalcalf serum, penicillin (100 U/mL), streptomycin (100 µg/mL) in a humidi-fied incubator containing 8% CO2 at 37 °C. Cells (2.5 × 104/1.1 mL suspen-sion per well) were seeded on 24-well multidishes and grown in DMEM.After 24 h growth the medium was replaced, and the test compounds (0.1–5 µM) were added from stock solutions. These were prepared in DMSO andthen diluted with DMEM, the final concentration of DMSO was 0.2% in theculture medium. Controls were performed with DMSO or medium alone.Forty-eight hours after addition of the test compounds to the culture, themedium was removed and each well was rinsed with PBS (100 µL). The cellswere then incubated with sterile 0.5% trypsin, 0.2% EDTA in PBS for 20 minat 37 °C. The detached cells from each well were suspended in DMEM anddispersed into single cells by gentle pipetting through an Eppendorf pipette,and cell growth was determined directly by counting the keratinocytes. Thecells were enumerated in Neubauer counting chambers by phase contrastmicroscopy. Inhibition was calculated by the comparison of the mean valuesof the test compound (N = 3) with the control (N = 6–8) activity: (1 – testcompound/control) × 100. Inhibition was statistically significant comparedto that of the control (Student’s t-test; P < 0.01). IC50 values were derived byinterpolation of a log inhibitor concentration versus response plot using fouror more concentrations of the growth inhibitor, spanning the 50% inhibitionpoint.

Cornified Envelope Assay

The assay measures insoluble cross-linked protein envelopes based onprevious reports [18,34]. The medium of 144-h post-confluent cultures wasreplaced, and the test compounds were added from stock solutions (1, 5,10 µM). These were prepared in DMSO and then diluted with DMEM; thefinal concentration of DMSO was 0.2% in the culture medium. Controls wereperformed with DMSO or medium alone. Forty-eight hours at 37 °C afteraddition of the test compounds to the culture, the medium was removed andeach well was rinsed with PBS (100 µL). The cells were then incubated withsterile 0.5% trypsin, 0.2% EDTA in PBS for 20 min at 37 °C. The detachedcells from each well were treated with 10% SDS and 2% β-ME in water(100 µL) with vigorous agitation. A sample (250 mL) of the suspension wasapplied to a presoaked, regenerated cellulose sheet (RC 60, Schleicher andSchuell, Dassel, Germany) over a dot blot apparatus (Bio-DotTM, Bio-Rad,Munich, Germany) using a presoaked, protein-free cellulose sheet (GB 002,Schleicher and Schuell) as a backing. The suspension was drained by gentlesuction from below, and wells were washed with SDS/β-ME (3 × 400 µL).The RC 60 sheet was dried and then removed from the apparatus, submergedin ice-cold 7.5% TCA solution (500 mL), heated to 80 °C for 30 min, washedwith ether/EtOH (1/1) for 10 min and then with ether (250 mL) for 10 min.The RC 60 sheet was dried and stained with a solution of 1% Coomassie blueG-250 in 7% acetic acid (500 mL) at 50 °C for 15 min. Then it was washedwith 7% acetic acid at 50 °C for 5 min. This was repeated until the back-ground was white (usually twice). The sheet was then transferred to a scanner(Hewlett-Packard Scan Jet 4c), and image analysis was performed withOptimas 6.1 (Media Cybernetics, Göttingen, Germany) on a Pentium Vectracomputer. Intensity of Coomassie blue stain is proportional to the amount ofenvelope protein. Indicated values are the differences of the amount ofcross-linked protein in the presence of test compounds and vehicle control.Amounts of cornified envelopes are expressed as µg cross-linked protein permg of the total amount of protein/mL. Protein in envelope preparations wasdetermined by the method of Bradford [43].

Inhibition of Mitochondrial Function

HaCaT keratinocytes were grown for 144 h and incubated with the testcompounds or controls as described for the cornified envelope assay. Thecell suspension (100 µL) was incubated in 96-well plates for an additional2 h with 0.5% MTT (30 µL), and then the culture plates were centrifuged at600 g. Formazan was dissolved in DMSO (100 µL), and the absorbance wasmeasured on a titertek twinreader (Flow) at 540 nm. IC50 values were derivedas described for growth inhibition.

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Received: October 11, 2000 [FP523]



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