Measurement of mRNAs for TGF beta and extracellular matrix proteins in corneas of rats after PRK

PURPOSE. To assess the role of the transforming growth factor (TGF)beta sys tem in formation of corneal haze after excimer laser photorefractive kerate ctomy (PRK), levels of mRNAs for three TGF beta isoforms (TGF beta1, TGF be ta2, and TGF beta3), the TGF beta type II receptor (T beta RII), and extrac ellular matrix (ECM) genes including fibronectin (FN), collagen I, collagen III, and collagen IV were measured in rat corneas. METHODS. Corneas were graded for corneal haze at 0, 1.5, 7, 21, 42, and 91 days after PRK. Total RNA was isolated from pooled corneas, and the levels of mRNAs were measured using competition-based quantitative reverse transcr iption-polymerase chain reaction (RT-PCR). RESULTS. Severe corneal haze developed by day 42 and persisted to day 91. L evels of TGF beta1 mRNA were high in rat corneas before PRK and remained re latively constant. In contrast, levels of TGF beta2 and TGF beta3 mRNAs wer e very low in normal corneas, increased 300-fold and 25-fold, respectively, on day 21, and remained elevated on day 91. Levels of mRNA for T beta RII increased, with a peak elevation of 50-fold on day 42 after PRK. Levels of mRNAs for ECM proteins also increased. Fibronectin mRNA was nondetectable i n normal corneas but rapidly increased to 675 copies/cell on day 7 and rema ined elevated to day 91. Collagen III mRNA levels peaked on day 21 with a 7 00-fold increase compared with a very low level of expression in normal cor nea, and then decreased on day 91. Expression of collagen I mRNA lagged exp ression of collagen III mRNA and peaked at day 42 after PRK with a 1200-fol d increase over normal cornea. In contrast, mRNA for collagen alpha (1)IV, a major component in basement membranes, remained relatively stable through day 21 and then increased slightly on days 42 and 91. CONCLUSIONS. The synchronized increase in mRNA synthesis for both the TGF b eta system and key ECM genes supports the hypothesis that TGF beta is a key growth factor promoting stromal haze formation in corneas after PRK and su ggests that limiting TGF beta system may reduce corneal scarring after exci mer laser ablation.