Coordinate regulation of Col11a2 and Col27a1 by the transcription factor Lc‐Maf

Abstract

The process of endochondral ossification requires precisely-coordinated expression of extracellular matrix proteins. In this study, enhancer/reporter assays showed that the transcription factor Lc-Maf is capable of both activating and inhibiting enhancer elements from two cartilage-specific collagen genes. We demonstrated that Lc-Maf binds to a previously-reported recognition element within two Col11a2 enhancer elements to inhibit transcriptional activity. However, a COL27A1 enhancer region was strongly activated by Lc-Maf. Site-directed mutagenesis identified the binding region within the COL27A1 enhancer, but no known consensus binding sequence was present. This suggests the possibility that Lc-Maf heterodimerizes with a different binding partner than has previously been recognized and activates transcription through the recognition of a novel DNA binding element within the COL27A1 enhancer element. Interestingly, immunohistochemistry and in situ hybridization data demonstrated that type XXVII collagen and the Col27a1 transcript have only a small region of overlap in prehypertrophic chondrocytes. As Lc-Maf was also expressed in this region, we propose that Lc-Maf activates the expression of a short-lived translatable Col27a1 mRNA in the prehypertrophic chondrocytes, and that the Col27a1 transcript identified throughout the resting and proliferating zone is not translated. This hypothesis is supported by a previous report of a Col27a1 transcript that is retained in the nucleus. Our work suggests that Lc-Maf plays a critical role during the cellular transition from proliferating to hypertrophic chondrocytes by coordinately downregulating Col11a2 and upregulating Col27a1 collagen gene expression.