The c-Myc Oncogene Maintains Corneal Epithelial Architecture at Homeostasis, Modulates p63 Expression, and Enhances Proliferation During Tissue Repair.

ABSTRACT

Purpose: The transcription factor c-Myc (Myc) plays central regulatory roles in both self-renewal and differentiation of progenitors of multiple cell lineages. Here, we address its function in corneal epithelium (CE) maintenance and repair. Methods: Myc ablation in the limbal-corneal epithelium was achieved by crossing a floxed Myc mouse allele (Mycfl/fl) with a mouse line expressing the Cre recombinase gene under the keratin (Krt) 14 promoter. CE stratification and protein localization were assessed by histology of paraffin and plastic sections and by immunohistochemistry of frozen sections, respectively. Protein levels and gene expression were determined by western blot and real-time quantitative PCR, respectively. CE wound closure was tracked by fluorescein staining. Results: At birth, mutant mice appeared indistinguishable from control littermates; however, their rates of postnatal weight gain were 67% lower than those of controls. After weaning, mutants also exhibited spontaneous skin ulcerations, predominantly in the tail and lower lip, and died 45 to 60 days after birth. The mutant CE displayed an increase in stratal thickness, increased levels of Krt12 in superficial cells, and decreased exfoliation rates. Accordingly, the absence of Myc perturbed protein and mRNA levels of genes modulating differentiation and proliferation processes, including ΔNp63ß, Ets1, and two Notch target genes, Hey1 and Maml1. Furthermore, Myc promoted CE wound closure and wound-induced hyperproliferation. Conclusions: Myc regulates the balance among CE stratification, differentiation, and surface exfoliation and promotes the transition to the hyperproliferative state during wound healing. Its effect on this balance may be exerted through the control of multiple regulators of cell fate, including isoforms of tumor protein p63.