Cellular uptake of cobalamin (Cbl) is mediated by the transcobalamin receptor (TCblR) that binds and internalizes transcobalamin (TC) saturated with Cbl. These receptors are expressed in actively proliferating cells and are down-regulated in quiescent cells. The 5' region of TCblR gene was analyzed for promoter activity to determine transcriptional regulation of TCblR expression. The region -668 to -455 appears to regulate TCblR expression. We have identified transcription factors MZF-1 (myeloid zinc finger 1)/RREB-1 (Ras-responsive element binding protein 1), C/EBP (CCAAT/enhancer binding protein)/HNF-3beta (hepatocyte nuclear factor 3) and AP-1(activator protein 1) as proteins likely to be involved in this regulation with the former region primarily involved in up regulation and the latter two regions involved in suppression of TCblR expression. These transcription factors are involved in cell proliferation and differentiation. Thus the cell cycle associated expression of TCblR appears to be tightly regulated in synchrony with the proliferative phase of the cell cycle.
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