REGULATION OF HEME-BIOSYNTHESIS - DISTINCT REGULATORY FEATURES IN ERYTHROID-CELLS

Our previous research has demonstrated that in hemoglobin-synthesizing cells, as compared with nonerythroid cells, a step in iron transport from transferrin localized between the transferrin receptor and ferroc helatase is rate-limiting for the synthesis of heme. In this communica tion we report our more recent studies on the mechanisms involved in t he regulation of the transferrin receptors and ferrochelatase in diffe rentiating erythroid cells. Our studies indicate that transferrin rece ptor gene expression is regulated differently in hemoglobin synthesizi ng as compared with uninduced murine erythroleukemia (MEL) cells: 1) W ith nuclear run-on assays our experiments showed increased transferrin receptor mRNA transcription cells of MEL following induction of eryth roid differentiation with dimethylsulfoxide (DMSO). 2) DMSO treatment of MEL cells does not increase iron-responsive element binding protein (IRE-BP) activity which is, however, increased in uninduced MEL cells by Fe chelators. 3) Following induction of MEL cells there is an incr ease in the stability of transferrin receptor mRNA whose level is only slightly affected by iron excess. Using murine ferrochelatase cDNA as a probe, two ferrochelatase transcripts having lengths of 2.9 kb and 2.2 kb were found in extracts of mouse liver, kidney, brain, muscle an d spleen, the 2.9 kb transcript being more abundant in nonerythroid ti ssues and the 2.2 more predominant in spleen. In MEL cells, the 2.9 fe rrochelatase transcript is also more abundant; however, following indu ction of erythroid differentiation by DMSO there is a preferential inc rease in the 2.2 kb transcript which eventually predominates. With mou se reticulocytes, the purest immature erythroid cell population availa ble, over 90% of the total ferrochelatase mRNA is present as the 2.2 k b transcript. Our further experiments indicate that the 2.2 kb transcr ipt results from the utilization of the upstream polyadenylation signa l and suggest that the preferential utilization of the upstream polyad enylation signal may be an erythroid-specific characteristic of ferroc helatase gene expression. These results provide further evidence for t he idea that iron metabolism and heme synthesis are controlled by dist inct mechanisms in erythroid versus nonerythroid cells.