Differential regulation of membrane-associated mucins in the human ocular surface epithelium

Membrane-associated mucins present in the apical cells of the ocular surface epithelium (MUC1, -4, and -16) are believed to contribute to the maintenance of a hydrated and wet-surfaced epithelial phenotype. Serum and retinoic acid (RA) have been used to treat drying ocular surface diseases. The goal of this study was to determine whether serum or RA regulates the production of membrane-associated mucins in human conjunctival epithelial cells.

A telomerase-immortalized human conjunctival epithelial cell line (HCjE) was used. Cells were cultured in serum-free medium to confluence and then cultured with either 10% calf serum or with 100 nM RA for 0 to 72 hours. Conventional RT-PCR was used to determine the expression of retinoic acid receptors (RARs) and quantitative real-time PCR was used to investigate the mRNA expression of MUC1, -4, and -16. Protein levels were assayed by immunoblot analysis, using the antibodies HMFG-2, 1G8, or OC125, which are specific to MUC1, -4 and -16, respectively. To determine whether RA-associated MUC4 mRNA induction is a direct or indirect effect, HCjE cells were treated with RA and the protein synthesis inhibitor cycloheximide (1.0 microg/mL) for 12 hours.

MUC1 and -16, but not -4, mRNAs were detectable in HCjE cells grown in serum-free medium. Real-time PCR revealed that MUC4 mRNA was significantly induced by serum 3 hours after its addition, and that MUC1 and MUC16 mRNA levels were significantly upregulated at 72 hours. Western blot analysis demonstrated that the MUC1, -4, and -16 proteins increased over time after addition of serum. Conventional RT-PCR analysis demonstrated that RAR-alpha and -gamma mRNA were expressed in native human conjunctival tissue as well as in the HCjE cells. Treatment with RA upregulated the expression of both MUC4 and -16 mRNA and protein, but MUC1 was unaffected. Because the protein synthesis inhibitor cycloheximide did not prevent the RA-associated induction of MUC4 mRNA, the action of RA on the MUC4 promoter may be direct.

The membrane-associated mucins of the ocular surface epithelia, MUC1, -4, and -16, are differentially regulated by serum and RA in the telomerase-immortalized human conjunctival epithelial cell line. Serum derived from vessels in the conjunctiva may play an important role in mucin regulation in the ocular surface epithelia. These data also support the clinical efficacy of autologous serum and RA application in patients with ocular surface diseases. Furthermore, the data suggest that MUC4 and -16 are particularly important hydrophilic molecules involved in maintenance of a healthy ocular surface.