Stimulation of hemoglobin synthesis in chick blastoderms by certain 5beta androstane and 5beta pregnane steroids.

Abstract

Previous studies on the hematopoietic tissue of the chick blastoderm by Levere and Granick' 2 have led to the following tentative interpretations and hypotheses regarding the control of hemoglobin synthesis. The presence of heme appears to be necessary for the globin polypeptide chain to be completed, possibly for its appropriate folding around the heme. The synthesis of heme is limited by a repressor-operator mechanism that controls the synthesis of the limiting enzyme, 6-aminolevulinic acid synthetase. The colorless erythroid cells of the blastoderm appear to contain nonlimiting amounts of stable ribosomes and stable messenger RNA for the synthesis of globin, yet no globin is made until heme is synthesized. In these cells all of the enzymes of the heme biosynthetic chain are at nonlimiting activities except the first enzyme, 8-aminolevulinic acid synthetase. Once the repressor-operator mechanism is derepressed by some as yet unknown derepressor, the messenger RNA for 5-aminolevulinic acid synthetase is made and this enzyme is synthesized. Then follows the synthesis of 5-aminolevulinic acid which is eventually converted to heme. In the presence of heme, globin can be synthesized. In summary, according to this hypothesis, the formation of hemoglobin awaits the appearance of an unknown derepressor that will permit the synthesis of the messenger RNA of 6-aminolevulinic acid synthetase. The studies reported here on the control of hemoglobin synthesis in erythroid cells were prompted by earlier studies by Granick3 on the control of heme synthesis in chick embryo liver cells. Treatment of these cells in culture with certain drugs and chemicals was found to result in an increased de novo synthesis of 6-aminolevulinic acid synthetase and, as a consequence, a marked increase in porphyrin formation. Later, it was found by Granick and Kappas4 that certain 5,B-H steriods which are derived metabolically from several hormonal precursors and intermediates in man could also induce an increase in porphyrin formation in chick embryo liver cells. These results then raised the question of whether the same steroid metabolites, acting on the erythroid precursor cells of the chick blastoderm, could also induce the synthesis of 6-aminolevulinic acid synthetase and thus result in the more rapid formation of hemoglobin. This paper reports that the same steroids which induce enhanced porphyrin synthesis in chick embryo liver cells also induce a more rapid synthesis of hemoglobin in the erythroblasts of the chick blastoderm. It is possible that these, or related, steroids are the physiologic derepressors which have been postulated to cause the increased synthesis of 5-aminolevulinic acid synthetase. Materials and Methods.-The chick blastoderm, de-embryonated and cultured in vitro, was used to study the effect of certain 5,6-H C19 and C21 steroid metabolites on hemoglobin synthesis. As previously described,2 when blastoderms are obtained before the sixth somite stage of embryonic