Valrubicin Activates PKCα in Keratinocytes: A Conceivable Mode of Action in Treating Hyper-Proliferative Skin Diseases

October 2013 | Volume 12 | Issue 10 | Original Article | 1156 | Copyright © 2013

Ina Groenkjaer Laugesen MD,a Eva Hauge,a Stine Maria Andersen MD,a Karin Stenderup PhD,a
Elisabeth de Darkó MD,b Tomas Norman Dam MD PhD,c and Cecilia Rosada PhDa

aDepartment of Dermatology, Aarhus University Hospital, Aarhus, Denmark
bValderm ApS, Lyngby, Denmark
cDepartment of Dermatology, Roskilde Hospital, Roskilde, Denmark

Abstract

BACKGROUND: Valrubicin is a semisynthetic anthracycline developed as an anti-cancer drug able to ameliorate psoriasis and non-melanoma skin cancer (NMSC) by topical application in animal models. Valrubicin decreases cell proliferation, and induces apoptosis; however its mode of action is still unknown. Valrubicin localizes in the cytoplasm and its valerate moiety resembles diacylglycerol, an activator of protein kinase C (PKC) α, which belongs to the PKC family of cytoplasmic serine/threonine protein kinases. PKCα is observed in the suprabasal layers of normal skin and is associated to keratinocyte growth arrest and differentiation processes. In hyper-proliferative skin diseases the presence of PKCα is altered.
OBJECTIVE: The aim of the present study was to investigate valrubicin’s mode of action in keratinocytes by studying its possible effect on PKCα activation.
METHODS: PKCα's characteristic to translocate from the cytoplasm to the cellular membrane when activated was assessed by measuring the amount of PKCα in the soluble and membrane-bound protein fractions isolated from valrubicin stimulated keratinocytes and by visualizing PKCα in stimulated cells over time. Downstream signaling was investigated by measuring the amount of phosphorylated Myristoylated Alanine-rich C-kinase substrate (MARCKS) and extracellular signal-regulated kinases (ERK) 1/2 of valrubicin-stimulated keratinocytes. To investigate whether there was a direct interaction between valrubicin and PKCα, an activity assay employing purified PKCα protein was used.
RESULTS: Valrubicin activates PKCα in vitro as shown by PKCα's translocation and phosphorylation of downstream signaling molecules.
CONCLUSION: Valrubicin stimulates PKCα activity and downstream signaling which may contribute to its beneficial effect in psoriasis and NMSC.

J Drugs Dermatol. 2013;12(10):1156-1162.

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INTRODUCTION

Valrubicin (N-trifluoroacetyladriamycin-14-valerate) is a semisynthetic anthracycline originally developed from doxorubicin as an anti-cancer drug and currently approved as Valstar (Endo Pharmaceuticals, Lexington, MA) for the treatment of bladder cancer.1,2 Valrubicin is a drug with a favorable safety profile shown as limited induction of resistant cancer cells, lack of toxicity in healthy tissue, and decreased incidence of severe heart insufficiency.1,3-6 The absence of skin toxicity and the lipophilic characteristic of the molecule permit the use of valrubicin as topical treatment with focus on hyper-proliferative skin diseases. Recently, we showed that valrubicin has a beneficial effect in treating psoriasis and non-melanoma skin cancer (NMSC) by topical application in animal models. The effect was suggested to be partly due to an inhibition of proliferation and an induction of apoptosis in the keratinocytes.1,4,5,7-9

Nevertheless, valrubicin cellular activity is still unknown. Elucidation of valrubicin mode of action aims at optimizing the use of valrubicin as treatment, and unravels new and useful steps in valrubicin signaling pathway that may have potential as targets for future treatments of hyper-proliferative skin diseases. Hyper-proliferation and abnormal differentiation are part of the pathogenesis of several common skin disorders such as psoriasis, NMSC, actinic keratosis, and acne. Consequently a compound which targets this aspect of the pathogenesis of the diseases may offer a future novel treatment.

Valrubicin localizes in the cytoplasm,8,10,11 implicating a mode of action distinct from the former anthracyclines that explicate their effects in the nucleus. The valerate moiety of valrubicin structurally resembles diacyglycerol (DAG), a natural ligand of protein kinase C (PKC), thus binding of valrubicin to PKC is suggested. Moreover, valrubicin has been demonstrated to inhibit TPA induced PKC activity12 and AD198, an analog to valrubicin also possessing a valerate chain, has been described to activate PKCα.13

PKC is a family of cytoplasmic serine/threonine protein kinases that take part in a variety of cellular processes by playing a crucial role in the initial events of signal transduction.14,15 The PKC family consists of nine isoenzymes that, in a concerted manner, regulate cell function upon differential activation by phosphatidylserine (PS) alone or in association with Ca2+ and/or DAG.16 Five isoforms

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