Hydrolytic pathway protects against ceramide-induced apoptosis in keratinocytes exposed to UVB.
Although ceramides (Cers) are key constituents of the epidermal permeability barrier, they also function as apoptogenic signals for UVB irradiation-induced apoptosis in epidermal keratinocytes. As epidermis is continuously exposed to UV irradiation, we hypothesized that Cer hydrolysis protects keratinocytes from UVB-induced apoptosis by attenuating Cer levels. Both low-dose UVB (L-UVB) (< 35 mJ cm(-2)) and high-dose UVB (H-UVB) (> or = 45 mJ cm(-2)) irradiation inhibited DNA synthesis in cultured human keratinocytes, but apoptosis occurred only after H-UVB. Whereas Cer production increased after both L- and H-UVB, it normalized only in L-UVB-exposed keratinocytes, but remained elevated after H-UVB. Both acidic ceramidase (aCDase) and neutral ceramidase (nCDase) activities declined after L- and H-UVB, but returned to normal only in L-UVB cells, with decreased CDase activities or mRNA or protein levels being sustained in H-UVB cells. Inhibition of CDase using either a CDase inhibitor, N-oleoylethanolamine, or small interfering RNA (siRNA) (either to a- and/or n-CDase(s)) sensitized keratinocytes to L-UVB-induced apoptosis in parallel with further Cer accumulation. Blockade of sphingosine kinase 1 (SPHK1) (but not SPHK2) by siRNA also increased apoptosis in L-UVB keratinocytes, revealing that conversion of sphingosine to sphingosine-1-phosphate (S1P) further protects keratinocytes from UVB-induced cell death. Thus, Cer → sphingosine → S1Pmetabolic conversion protects against UVB-induced, Cer-mediated apoptosis in keratinocytes, but excessive UVB overwhelms this mechanism, thereby leading to keratinocyte apoptosis.
Department of Dermatology, School of Medicine, University of California, San Francisco, San Francisco, California, USA. uchiday@derm.ucsf.edu
Yoshikazu
Y
Uchida
Evi
E
Houben
Kyungho
K
Park
Sounthala
S
Douangpanya
Yong-Moon
YM
Lee
Bill X
BX
Wu
Yusuf A
YA
Hannun
Norman S
NS
Radin
Peter M
PM
Elias
Walter M
WM
Holleran
D055573:Acid Ceramidase / Q000235:genetics / Q000378:metabolism*; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D002478:Cells, Cultured; D002518:Ceramides / Q000633:toxicity; D019610:Cytoprotection / Q000187:drug effects; D015971:Gene Expression Regulation, Enzymologic / Q000528:radiation effects; D006801:Humans; D006868:Hydrolysis; D015603:Keratinocytes / Q000166:cytology / Q000187:drug effects / Q000528:radiation effects; D055576:Neutral Ceramidase / Q000235:genetics / Q000378:metabolism*; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D034741:RNA, Small Interfering; D015398:Signal Transduction / Q000187:drug effects / Q000528:radiation effects; D013110:Sphingosine / Q000378:metabolism; D014466:Ultraviolet Rays / Q000009:adverse effects*; D000099157:Sphingosine Kinase
D055573:Acid Ceramidase; D002518:Ceramides; D055576:Neutral Ceramidase; D017853:Phosphotransferases (Alcohol Group Acceptor); D012333:RNA, Messenger; D034741:RNA, Small Interfering; D013110:Sphingosine; D000099157:Sphingosine Kinase