Induction of tyrosinase gene transcription in human iris organ cultures exposed to latanoprost

Objective: Topical administration of latanoprost sometimes induces gradual iris darkening. The present study was undertaken to determine if latanopros t can increase transcription of the gene for tyrosinase, an important enzym e in the biosynthesis of melanin. Results from brown, hazel, and blue iride s were compared. Methods: Iris tissue was isolated from 30 pairs of postmortem human donor e yes, and 2 iris segments from each eye were incubated in tissue culture med ium supplemented with 200nM latanoprost acid or vehicle for 7 days. Tyrosin ase messenger RNA (mRNA) was determined using real-time polymerase chain re action analysis (TaqMan quantitative polymerase chain reaction). Results fo r tyrosinase mRNA were normalized according to glyceraldehyde-3-phosphate d ehydrogenase (GAPDH) mRNA in each sample. Results: Tyrosinase mRNA expression was similar in blue and hazel irides, a nd ranged from 0.7-fold to 12.6-foId greater than GAPDH expression. In cont rast, control brown iris culture tyrosinase expression ranged from 6.4-fold to 265-fold greater than GAPDH expression. Induction of tyrosinase mRNA by latanoprost was below threshold in all the blue iris cultures (n=8 pairs), present in 1 of 9 hazel iris cultures, and present in 5 of 13 brown iris c ultures. Mean induction in the responding hazel iris cultures was 1.40-fold . Mean induction among the responding brown iris cultures was 2.97-fold. Conclusions: These observations support the view that iris darkening associ ated with latanoprost treatment reflects induction of tyrosinase expression . Moreover, they suggest that the probability that latanoprost will increas e tyrosinase expression is directly related to the magnitude of tyrosinase expression before treatments are initiated. Clinical Relevance: The variabi lity of iris darkening with latanoprost may reflect natural variation in th e basal transcription of tyrosinase.