Regulated expression of three C/EBP isoforms during adipose conversion of 3T3-L1 cells

In an effort to identify protein factors that play a regulatory role in the differentiation of adipocytes, we have isolated two genes that encode polypeptides related to CCAAT/enhancer-binding protein (C/EBP; hereafter termed C/EBPα). The proteins encoded by these C/EBP-related genes, termed C/EBPβ and C/EBPδ, exhibit similar DNA-binding specificities and affinities compared with C/EBPα. Furthermore, C/EBPβ and C/EBPδ readily form heterodimers with one another as well as with C/EBPα. The transcriptional activating capacity of these two newly identified C/EBP isoforms was demonstrated by transient transfection experiments in which expression vectors encoding C/EBPβ and C/EBPδ were observed to induce transcription from the promoter of the serum albumin gene in cultured hepatoma cells. The mRNAs encoding C/EBPβ and C/EBPδ were detected in a number of tissues, most of which corresponded to sites of expression of C/EBPα. The expression pattern of C/EBPβ and C/EBPδ during adipose conversion of 3T3-L1 cells was examined by Western and Northern blotting assays. In contrast to the expression profile of the gene encoding C/EBPα, whose product is not detectable until the late phase of adipocyte differentiation, the c/ebpβ and c/ebpδ genes were actively expressed very early during adipocyte differentiation. Moreover, transcription of the c/ebpβ and c/ebpδ genes was observed to be induced directly by adipogenic hormones. The accumulation of C/EBPβ and C/EBPδ reached a maximal level during the first 2 days of differentiation and declined sharply before the onset of C/EBPα accumulation. The temporal pattern of expression of these three C/EBP isoforms during adipocyte differentiation may reflect the underpinnings of a regulatory cascade that controls the process of terminal cell differentiation.