Ectopic expression of the transcription factor MafB in basal keratinocytes induces hyperproliferation and perturbs epidermal homeostasis.

Mammalian epidermis is composed of four morphologically and functionally distinct layers of keratinocytes. The innermost basal layer consists of proliferating self-renewing keratinocytes, which also undergo asymmetric cell division to differentiate into postmitotic suprabasal cells throughout life. Control of the balance between growth and differentiation of basal cells is important for epidermal homeostasis to prevent skin disorders including malignancies; however, the underlying mechanism remains to be elucidated. Recently, MafB was identified as one of the transcription factors that regulate epidermal keratinocyte differentiation. MafB is expressed in postmitotic differentiating keratinocytes, and epidermal differentiation is partially impaired in MafB-deficient mice. To further establish the roles of MafB in the epidermis in vivo, we generated mice transgenic for MafB under the control of the basal cell-specific keratin (Krt) 14 promoter. In the epidermis of transgenic mice at embryonic day 18.5, the number of proliferating Krt14-positive basal-like cells was increased, and the granular and cornified layers were thickened. Furthermore, these MafB transgenic mice developed papillomas spontaneously with age. Therefore, MafB promotes differentiation in postmitotic keratinocytes and simultaneously has potential to promote growth when ectopically expressed in undifferentiated basal keratinocytes.

Phosphorylation of MafA enhances interaction with Beta2/NeuroD1.

AIMS: MafA is a critical regulator of insulin expression and mature ß-cell function. MafA binds to the insulin promoter through its carboxyl-terminal basic domain-leucine zipper (bZip) region and activates transcription synergistically with the ß-cell-enriched transactivators Beta2 (NeuroD1) and Pdx1. MafA protein is highly phosphorylated in ß-cells, and phosphorylation at multiple sites within its amino-terminal region is critical for its DNA-binding and transactivating abilities, as well as for regulation of its degradation. Here, we investigated whether phosphorylation of MafA affects its interaction with Beta2. METHODS: By mutational analysis, we identified interaction domains of MafA and Beta2. Using in situ proximity ligation assay (PLA), we explored mechanism of phosphorylation-dependent binding of MafA with Beta2. We also searched for a pathophysiological condition that would induce lower levels of MafA phosphorylation. RESULTS: Mutational analysis revealed that the phosphorylation sites within the amino-terminal region of MafA were not necessary for interaction with Beta2. In situ PLA suggested that phosphorylation induces conformational or configurational changes in MafA, thereby regulating the interaction with Beta2. We also found that long-term culture of the MIN6 insulinoma cell line under high-glucose conditions resulted in a decrease in ß-cell-specific transcripts including insulin, along with a decrease in MafA phosphorylation and DNA binding. CONCLUSION: Phosphorylation of MafA plays a critical role in ß-cell function by regulating multiple functionalities, including binding to DNA, interaction with Beta2, and transactivation.

Gata3 cooperates with Gcm2 and MafB to activate parathyroid hormone gene expression by interacting with SP1.

Haploinsufficiency of the Gata3 gene, which encodes a zinc-finger transcription factor, is associated with the disorder hypoparathyroidism, deafness, and renal dysplasia (HDR) syndrome in humans. However, the roles of Gata3 in transcriptional regulation in the parathyroid glands are not well-understood. In this study, we show that Gata3 activates transcription of parathyroid hormone (PTH), which is secreted from parathyroid glands and is critical for regulating serum calcium and phosphate homeostasis. Gata3 interacted with Gcm2 and MafB, two known transcriptional regulators of parathyroid development, and synergistically stimulated the PTH promoter. An SP1-binding element (GC box) located within the PTH-promoter proximal region was critical for activating transcription by Gata3. In addition, the ubiquitous transcription factor SP1 also interacted with Gata3 as well as MafB and Gcm2, and HDR syndrome-associated Gata3 mutants were defective in activating the PTH promoter. These results suggest that Gata3 is a critical regulator of PTH gene expression.