Human Keratinocytes Are Vanilloid Resistant

Abstract

BackgroundUse 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 Ca2+-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.MethodsTo address this paradox, we assessed the vanilloid sensitivity of the HaCaT human keratinocyte cell line and primary keratinocytes from skin biopsies.ResultsAlthough both cell types express TRPV1 mRNA, neither responded to vanilloids with Ca2+-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 Ca2+-cytotoxity ([RTX]>15 µM) could clearly be distinguished, thus keratinocytes were indeed resistant to vanilloid-induced, TRPV1-mediated Ca2+-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.ConclusionTRPV1B 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.

Epidermal keratinocyte polarity and motility require Ca2+ influx through TRPV1

“…We found that inhibition of TRPV1, but not TRPV4 or TRPM7, impaired keratinocyte polarity and migration. TRPV1 expression has been defined mostly in sensory ganglia (dorsal root, trigeminal and nodose) (Caterina et al, 2000), although it has also been detected in epithelial cells of the bladder lumen (Birder et al, 2002), epidermal keratinocytes (Denda et al, 2001;Pecze et al, 2008), as well as other non-neuronal tissue (Southall et al, 2003;Li et al, 2007). We detected robust levels of trpv1 in trigeminal sensory neurons in 24-hour embryos, which is consistent with its well-documented role in neuronal sensory function.…”

“…Whereas TRPV1 agonists do not appear to be cytotoxic to human keratinocytes over brief periods (Pecze et al, 2008), reduced cellular growth and increased apoptosis occur in epidermal cells of hair follicles during organ culture after a 5-day exposure to 10 mM capsaicin (Bodó et al, 2005). In zebrafish keratinocytes, TRPV1 agonists induced increases in [Ca 2+ ] i that are intermittent and not chronic, and that do not result in cytotoxic effects for up to 2 hours of exposure.…”

Cocarcinogenic Effect of Capsaicin Involves Activation of EGFR Signaling but Not TRPV1

“…TRPV1 activation is involved in inflammatory responses in human keratinocytes through the release of inflammatory mediators, such as prostaglandin E 2 and interleukin-8 (28), and UV irradiation-increased MMP-1 is mediated by TRPV1 in keratinocytes (29). In contrast, keratinocytes were reportedly resistant to vanilloid-induced TRPV1 activation and subsequent calcium influx (30). In our previous studies, we confirmed that TRPV1 expression was depleted in skin cancer tissues, and lack of TRPV1 corresponded with increased incidence of mouse skin cancer development mediated by high levels of EGFR (15).…”

Retinoids activate the irritant receptor TRPV1 and produce sensory hypersensitivity

“…Thus, we do not expect that retinoids directly activate epidermal keratinocytes through TRPV1 since keratinocytes are vanilloid resistant (65). On the other hand, retinoid signaling can also regulate TRPV1 function indirectly.…”