Transcription factor MafB may play an important role in secondary hyperparathyroidism.

The transcription factor MafB is essential for development of the parathyroid glands, the expression of which persists after morphogenesis and in adult parathyroid glands. However, the function of MafB in adult parathyroid tissue is unclear. To investigate this, we induced chronic kidney disease (CKD) in wild-type and MafB heterozygote (MafB+/-) mice by feeding them an adenine-supplemented diet, leading to secondary hyperparathyroidism. The elevated serum creatinine and blood urea nitrogen levels in heterozygous and wild-type mice fed the adenine-supplemented diet were similar. Interestingly, secondary hyperparathyroidism, characterized by serum parathyroid hormone elevation and enlargement of parathyroid glands, was suppressed in MafB+/- mice fed the adenine-supplemented diet compared to similarly fed wild-type littermates. Quantitative RT-PCR and immunohistochemical analyses showed that the increased expression of parathyroid hormone and cyclin D2 in mice with CKD was suppressed in the parathyroid glands of heterozygous CKD mice. A reporter assay indicated that MafB directly regulated parathyroid hormone and cyclin D2 expression. To exclude an effect of a developmental anomaly in MafB+/- mice, we analyzed MafB tamoxifen-induced global knockout mice. Hypocalcemia-stimulated parathyroid hormone secretion was significantly impaired in MafB knockout mice. RNA-sequencing analysis indicated PTH, Gata3 and Gcm2 depletion in the parathyroid glands of MafB knockout mice. Thus, MafB appears to play an important role in secondary hyperparathyroidism by regulation of parathyroid hormone and cyclin D2 expression. Hence, MafB may represent a new therapeutic target in secondary hyperparathyroidism.

Retinoic acid and the transcription factor MafB act together and differentially to regulate aggrecan and matrix metalloproteinase gene expression in neonatal chondrocytes.

Vitamin A (VA) and its active form, retinoic acid (RA), are regulators of skeletal development and chondrogenesis. MafB, a transcription factor, has been identified as an important mediator in monocyte and osteoclast differentiation. However, the presence and function of MafB in chondrocytes is not clear. In this study, MafB gene expression was regulated by both the VA status of the mother (VA-marginal, adequate, and supplemented diets) and by direct oral supplementation of the neonates with VARA (VA mixed with 10% RA). Expression was highest in neonates of VA-supplemented versus VA-marginal dams (P < 0.05), and in VARA-treated versus placebo-treated neonates across all diet groups (P < 0.05). To examine cellular changes, primary chondrocytes derived from neonatal rat ribs were cultured in the presence of RA for up to 48 h. MafB mRNA exhibited a time- and dose-dependent increase in response to RA, while the induction of MafB mRNA was attenuated by BMS-493, a pan-RAR inverse agonist, implicating RAR signaling in the regulation of MafB. The genetic knockdown of MafB in chondrocytes using siRNA (MafB(SI) chondrocytes) abrogated the RA-induced increase in MafB expression. MafB(SI) chondrocytes expressed higher levels of aggrecan mRNA. Additionally, the increased matrix metalloproteinase (MMP)3 and MMP13 gene expression due to RA was attenuated in MafB(SI) chondrocytes, while total extracellular matrix staining was increased. These results support a role for MafB as a regulator of chondrocyte gene expression and matrix formation via control of aggrecan, MMP3 and MMP13 expression, and indicate an important role for RA in the regulation of MafB.