Human keratinocytes are vanilloid resistant.
BACKGROUND
Use of capsaicin or resiniferatoxin (RTX) as analgesics is an attractive therapeutic option. RTX opens the cation channel inflammatory pain/vanilloid receptor type 1 (TRPV1) permanently and selectively removes nociceptive neurons by Ca(2+)-cytotoxicity. Paradoxically, not only nociceptors, but non-neuronal cells, including keratinocytes express full length TRPV1 mRNA, while patient dogs and experimental animals that underwent topical treatment or anatomically targeted molecular surgery have shown neither obvious behavioral, nor pathological side effects.


METHODS
To address this paradox, we assessed the vanilloid sensitivity of the HaCaT human keratinocyte cell line and primary keratinocytes from skin biopsies.


RESULTS
Although both cell types express TRPV1 mRNA, neither responded to vanilloids with Ca(2+)-cytotoxicity. Only ectopic overproduction of TRPV1 rendered HaCaT cells sensitive to low doses (1-50 nM) of vanilloids. The TRPV1-mediated and non-receptor specific Ca(2+)-cytotoxicity ([RTX]>15 microM) could clearly be distinguished, thus keratinocytes were indeed resistant to vanilloid-induced, TRPV1-mediated Ca(2+)-entry. Having a wider therapeutic window than capsaicin, RTX was effective in subnanomolar range, but even micromolar concentrations could not kill human keratinocytes. Keratinocytes showed orders of magnitudes lower TRPV1 mRNA level than sensory ganglions, the bona fide therapeutic targets in human pain management. In addition to TRPV1, TRPV1b, a dominant negative splice variant was also noted in keratinocytes.


CONCLUSIONS
TRPV1B expression, together with low TRPV1 expression, may explain the vanilloid paradox: even genuinely TRPV1 mRNA positive cells can be spared with therapeutic (up to micromolar) doses of RTX. This additional safety information might be useful for planning future human clinical trials.
Institute of Biochemistry, Biological Research Center of the Hungarian Academy of Sciences, Szeged, Hungary.
László
L
Pecze
Kornélia
K
Szabó
Márta
M
Széll
Katalin
K
Jósvay
Krisztián
K
Kaszás
Erzsébet
E
Kúsz
Tamás
T
Letoha
János
J
Prorok
István
I
Koncz
András
A
Tóth
Lajos
L
Kemény
Csaba
C
Vizler
Zoltán
Z
Oláh
D002118:Calcium / Q000378:metabolism; D002211:Capsaicin / Q000494:pharmacology*; D016923:Cell Death; D002478:Cells, Cultured; D004224:Diterpenes / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance; D006801:Humans; D015603:Keratinocytes / Q000187:drug effects*; D012333:RNA, Messenger / Q000032:analysis; D012867:Skin / Q000166:cytology; D050916:TRPV Cation Channels / Q000032:analysis / Q000235:genetics*
D004224:Diterpenes; D012333:RNA, Messenger; D050916:TRPV Cation Channels; C494482:TRPV1 protein, human; C024353:resiniferatoxin; D002211:Capsaicin; D002118:Calcium