Photomodulation of the Melanocyte Cell Cycle by Indoleamines

Melanocytes are photoresponsive cells which respond to varying doses of UV exposure in the G2 phase of the cell cycle by prominent dendricity. This photoresponse is related to indoleamine light sensitivity. The present study highlights the role of indoleamines in the photomodulation of the melanocyte cell cycle. The study was conducted on 40 whole-skin organ cultures taken from the marginal zone of vitiligo. Twenty organ cultures were subjected to G2-phase arrest, while 20 were incubated in tryptamine. The organ cultures were incubated in the dark, exposed to a pulse of 120 s UV at 2 h of incubation and harvested 3 and 6 h after UV exposure. It has been reported that the photosensitive enzymes N-acetyl transferase (NAT) and hydroxyindole-o-methyl transferase (HIOMT) are activated during the G2 phase. The conversion of serotonin to melatonin is inhibited by UV exposure as seen at 3 h. This activity recovers on continued dark incubation 6 h after UV exposure. On incubation with tryptamine, UV exposure results in utilisation of tryptamine as seen by prominent indoleamine positivity. Three hours after UV exposure, there is 75% dendricity indicating G2 phase traverse. There is a corresponding high serotonin positivity with a low melatonin positivity. This is reversed following 6 h of dark incubation with high melatonin positivity indicating reactivation of NAT and HIOMT. This is accompanied by a doubling of the melanocyte number due to mitotic traverse and an arrest in G1 phase with low utilisation of tryptamine. Thus tryptamine is utilised by melanocytes on UV exposure to be synchronised and traversed into G2 phase activating the photosensitive enzymes NAT and HIOMT. When followed by a dark phase, melatonin accumulates to traverse the melanocytes through M-phase of the cell cycle with doubling of the cell number. Thus the uptake and metabolisation of indoleamine precursors photomodulate the melanocyte cell cycle on UV exposure.