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.
Uchida
Yoshikazu
Y
Department of Dermatology, School of Medicine, University of California, San Francisco, San Francisco, California, USA. uchiday@derm.ucsf.edu
Houben
Evi
E
Park
Kyungho
K
Douangpanya
Sounthala
S
Lee
Yong-Moon
YM
Wu
Bill X
BX
Hannun
Yusuf A
YA
Radin
Norman S
NS
Elias
Peter M
PM
Holleran
Walter M
WM
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