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. 2008;3(10):e3419.
doi: 10.1371/journal.pone.0003419. Epub 2008 Oct 14.

Human keratinocytes are vanilloid resistant

László Pecze  1 , Kornélia Szabó, Márta Széll, Katalin Jósvay, Krisztián Kaszás, Erzsébet Kúsz, Tamás Letoha, János Prorok, István Koncz, András Tóth, Lajos Kemény, Csaba Vizler, Zoltán Oláh
Affiliations

Human keratinocytes are vanilloid resistant

László Pecze et al. PLoS One. 2008.

Abstract

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.

Conclusion: 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.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Keratinocytes were not killed by low dose vanilloids.
Primary human keratinocytes (NHEK), proliferating and differentiating HaCaT keratinocytes, TRPV1-expressing HaCaT keratinocytes, HEK293 human embryonal cells, NIH3T3 mouse fibroblasts and sf9 insect cells were grown for 24 hr on 96-well plates in the presence of indicated concentrations of RTX (A) and CAP (B, C). Cell survival was evaluated by the MTS bioassay. Only HaCaT cells stably transformed with TRPV1 transgene were sensitive to cytotoxicity triggered by low dose vanilloids, while normal keratinocytes were only killed by extremely high vanilloid doses, likewise toxic to the negative control insect cells, HEK293 or NIH3T3 cells. Data from a representative experiment repeated three times with similar results is shown. Values are shown as means±SD. (*: p
Figure 2
Figure 2. Transformed cells expressing low levels of TRPV1 were resistant to TRPV1-mediated capsaicin toxicity.
NIH3T3 mouse fibroblasts expressing variable levels of TRPV1-YFP fusion protein were incubated in the absence of capsaicin (A) or with different capsaicin concentrations, 0.1 µM (B), 1 µM (C), 10 µM (D) for 30′, then propidium iodide was added and the flow-cytofluorometric analysis was performed. The M1 and M2 regions contain the intact (propidium iodide negative) TRPV1-YFP high and low positive cells, respectively. Only TRPV1 high positive cells were sensitive to dose dependent, TRPV1 mediated killing by capsaicin (E).
Figure 3
Figure 3. Keratinocytes express low levels of the full length TRPV1 transcript together with transcript of the TRPV1b dominant negative splice form.
Detection of TRPV1 transcript in differentiating human cultured keratinocytes (NHEK) (A) and HaCaT cells (B) by real-time RT-PCR. TRPV1 threshold cycle numbers normalized to the internal control 18S rRNA (delta CT) are plotted. The amount of mRNAs is inversely proportional to the plotted threshold cycle numbers. Data are presented as means of three independent experiments. (*: p
Figure 4
Figure 4. The TRPV1 protein level in the keratinocytes was under detection limit in Western blotting assay.
Proteins were extracted from human trigeminal ganglion (TG), primary human keratinocytes (NEHK) and the HaCaT cell lines. With a commercially available polyclonal antibody only the TG expressed 94 kDa TRPV1 can be detected (A). The equal loading of protein extracts was validated by β-actin (∼50 kDa), a common, ubiquitously expressed cytoskeletal protein (B).
Figure 5
Figure 5. Neither endo-, nor exo-vanilloids caused 45Ca2+-influx in keratinocytes.
Primary human keratinocytes (NHEK), primary culture of DRG neurons, HaCaT keratinocytes and TRPV1 transformed HaCat keratinocytes were seeded in 96-well plates, then incubated for 10-min with different TRPV1 agonists (capsaicin, CAP; resiniferatoxin, RTX; N-arachidonoyl-dopamine, NADA; Anandamide, ANA) and combinations of agonists and antagonists (CAP or RTX+capsazepine, capZ) in the presence of 45Ca2+. Cell-bound radioactivity was measured with liquid scintillation. The bars represent mean scintillation counts per minute (c.p.m.)+SD of eight parallel samples from a representative experiment repeated three times with similar results.
Figure 6
Figure 6. Fluorometric Ca2+ imaging showed no capsaicin-induced Ca-signal in keratinocytes.
Primary human keratinocytes and HaCaT cells were loaded with Fluo4-AM Ca2+-indicator then imaged in an appropriately equipped fluorescence microscope setup. The bars represent the increase in fluorescence upon capsaicin (2 µM) treatment as a % of the fluorescence increase after ionomycin treatment (50 µM). While TRPV1 transformed cells exposed to CAP showed readily detectable fluorescence changes, neither untransformed HaCaT cells, nor primary keratinocytes responded to this algesic vanilloid substance. Values are shown as means±SD for 6 measurement. The significance of differences compared with TRPV1ε/HaCaT was determined with the paired t-test, *P<0.05.
Figure 7
Figure 7. Cobalt histochemistry did not show functional TRPV1 in keratinocytes.
A: TRPV1ε/HaCaT and rat DRG culture (insert) treated with 50 µM RTX in the presence of cobalt cations shows dark CoS precipitate. B: TRPV1ε/HaCaT without RTX C–D: HaCaT and NHEK treated with 50 µM RTX did not produce the characteristic precipitate formed in the presence of intracellular cobalt cations. Bar represents 0.2 mm.
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