Pterygia are inflammatory, invasive, and proliferative lesions of the human ocular surface in which the matrix metalloproteinase (MMP) collagenase-1 (MMP-1) is highly expressed. Pterygia development may involve MMP-1 activity against interstitial fibrillar collagen, an abundant extracellular matrix component of the cornea, and its induction by ultraviolet light (UVB). We examined the pathways responsible for enhanced expression of MMP-1 in pterygium epithelial cells after UVB exposure and/or treatment with chemical inhibitors of mitogen-activated protein kinases or epidermal growth factor receptor. The induction of MMP-1 by UVB was comparable to that mediated by heparin-binding epidermal growth factor-like growth factor and epidermal growth factor. The epidermal growth factor receptor inhibitor PD153035 partially blocked the UVB-mediated induction of MMP-1 and totally abrogated its production after stimulation with either heparin-binding epidermal growth factor-like growth factor or epidermal growth factor. UVB exposure enhanced the phosphorylated form of ERK1/2 in a time-dependent manner whereas the ERK1/2 inhibitor PD98059 decreased this induction by at least fivefold. Transcripts for c-jun and c-fos were detected as early as 2 hours after UVB exposure and were suppressed by PD98059. The identification of a specific intracellular signaling pathway responsible for the enhanced production of a key enzyme that denatures intact fibrillar collagen has important implications for understanding the pathophysiology and future therapy for pterygia. Pterygium is a condition of the ocular surface characterized by squamous cell metaplasia and goblet cell hyperplasia. The lesion consists of a wing-shaped mass of fibrovascular conjunctival tissue that invades the normal cornea. Other obvious pathological changes include activation of stromal fibroblasts, a persistent inflammatory component, elastotic degeneration, and destruction of collagenous barriers such as Bowman's layer. Pterygia are particularly prevalent in heavily sun-exposed individuals in whom extensive epidemiological studies link this disease to excessive ultraviolet (UV) radiation. 1-5 Despite this evidence, there is still controversy regarding the actual trigger for the development of pterygia and whether or not there is a genetic predisposition to this disease. 6 Recently, we have identified UVB as an environmental agent likely to be responsible for initiating molecular events that lead to the formation of pterygia. 7,8 From our hypothesis, 9 we postulate that UVB radiation activates cells at or near the limbus. This activation may cause 1) phenotypic alterations in a distinct population of epithelial cells, 2) production of proinflammatory and angiogenic cytokines 7 and growth factors, 8 and 3) increased invasiveness due to enhanced production of matrix metalloproteinases (MMPs) over and above their natural tissue inhibitors (TIMPs). 6,9 -12 To date, we 6,9 -12 and others [13][14][15][16][17] have accumulated data from in vitro culture and in vivo tissue-b...