Interplay of transcriptional and post-transcriptional regulation in the epidermis

Defective permeability barrier is a feature of many skin diseases and causes mortality in premature infants. The epidermal stratification process, crucial for barrier formation, involves basal proliferating keratinocytes that undergo a series of differentiation steps giving rise in succession to the spinous, granular and terminally differentiated cornified layer. This process is regulated by a complex and elegant interplay of several transcription factors. We identified Get1/Grhl3, an evolutionarily conserved transcription factor, to be important for terminal differentiation and barrier formation of the epidermis. Grhl3 deleted mice exhibit barrier function defects associated with impaired differentiation of epidermis with differential expression of genes involved in lipid metabolism and cell-cell adhesion. However, we identified relatively few genes with conserved Grhl3 binding sites in proximal promoters of differentially expressed genes in Grhl3 -/- mice. The lack of conserved Grhl3 binding sites in differentially expressed genes led us to the hypothesis that Grhl3 might directly regulate specific miRNA genes. We identified 12 miRNAs that were differentially regulated in the Grhl3-/- mice using miRNA profiling. We focused on miRNA(miR)-21, which is upregulated in the Grhl3 -/- mice and expressed in the post-mitotic suprabasal layer of the epidermis. Its spatial and temporal expression in mouse skin correlates with epidermal differentiation suggesting a role in inhibiting proliferation and a potential role in suppressing the basal cell program. We found that Grhl3 binds to miR-21 promoter region and repress its activity suggesting they may be involved in a regulatory loop to maintain homeostasis in the epidermis. miR-21 overexpression in keratinocytes impedes cell cycle and results in reduced number of cells in S-phase. Expression profiling on keratinocytes either overexpressing or deficient in miR-21, identified several cell cycle associated genes. However, the potential targets were mildly altered in these keratinocytes. Interestingly, when keratinocytes are Ras-modified, we observe a more dramatic downregulation of cell cycle genes like CDC25A, CDK6, E2F3 and as well as of other known targets of miR-21 such as PDCD4, NFIB, and MSH2. These findings indicate a role for miR-21 in promoting tumorigenesis in pathological skin conditions.