Products of Vitamin D3 or 7-Dehydrocholesterol Metabolism by Cytochrome P450scc Show Anti-Leukemia Effects, Having Low or Absent Calcemic Activity

BackgroundCytochrome P450scc metabolizes vitamin D3 to 20-hydroxyvitamin D3 (20(OH)D3) and 20,23(OH)2D3, as well as 1-hydroxyvitamin D3 to 1α,20-dihydroxyvitamin D3 (1,20(OH)2D3). It also cleaves the side chain of 7-dehydrocholesterol producing 7-dehydropregnenolone (7DHP), which can be transformed to 20(OH)7DHP. UVB induces transformation of the steroidal 5,7-dienes to pregnacalciferol (pD) and a lumisterol-like compounds (pL).
Methods and FindingsTo define the biological significance of these P450scc-initiated pathways, we tested the effects of their 5,7-diene precursors and secosteroidal products on leukemia cell differentiation and proliferation in comparison to 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3). These secosteroids inhibited proliferation and induced erythroid differentiation of K562 human chronic myeloid and MEL mouse leukemia cells with 20(OH)D3 and 20,23(OH)2D3 being either equipotent or slightly less potent than 1,25(OH)2D3, while 1,20(OH)2D3, pD and pL compounds were slightly or moderately less potent. The compounds also inhibited proliferation and induced monocytic differentiation of HL-60 promyelocytic and U937 promonocytic human leukemia cells. Among them 1,25(OH)2D3 was the most potent, 20(OH)D3, 20,23(OH)2D3 and 1,20(OH)2D3 were less active, and pD and pL compounds were the least potent. Since it had been previously proven that secosteroids without the side chain (pD) have no effect on systemic calcium levels we performed additional testing in rats and found that 20(OH)D3 had no calcemic activity at concentration as high as 1 µg/kg, whereas, 1,20(OH)2D3 was slightly to moderately calcemic and 1,25(OH)2D3 had strong calcemic activity.
ConclusionsWe identified novel secosteroids that are excellent candidates for anti-leukemia therapy with 20(OH)D3 deserving special attention because of its relatively high potency and lack of calcemic activity.

背景：细胞色素P450scc（Cytochrome P450scc）可将维生素D3代谢为20-羟基维生素D3（20(OH)D3）与20,23-二羟基维生素D3（20,23(OH)2D3），亦可将1-羟基维生素D3转化为1α,20-二羟基维生素D3（1,20(OH)2D3）。该酶还可裂解7-脱氢胆固醇的侧链，生成7-脱氢孕烯醇酮（7DHP），后者可进一步转化为20(OH)7DHP。紫外线B（UVB）可诱导甾体类5,7-二烯烃转化为孕烷钙化醇（pregnacalciferol，pD）及类光甾醇化合物（pL）。
方法与结果：为明确上述由P450scc启动的代谢通路的生物学意义，我们以1α,25-二羟基维生素D3（1,25(OH)2D3）为对照，检测了其5,7-二烯烃前体与开链甾醇（secosteroid）类产物对白血病细胞分化与增殖的影响。结果显示，此类开链甾醇可抑制K562人慢性髓系白血病细胞及MEL小鼠白血病细胞的增殖，并诱导其红系分化：其中20(OH)D3与20,23(OH)2D3的活性与1,25(OH)2D3相当或略弱，而1,20(OH)2D3、pD及pL化合物的活性则稍弱或中等程度偏弱。此类化合物还可抑制HL-60早幼粒细胞白血病细胞与U937幼单核细胞白血病细胞的增殖，并诱导其单核细胞分化：其中1,25(OH)2D3活性最强，20(OH)D3、20,23(OH)2D3及1,20(OH)2D3活性次之，pD与pL化合物活性最弱。既往研究已证实，不含侧链的开链甾醇（pD）不会对机体血钙水平产生影响；我们在大鼠中开展补充实验后发现，当给药剂量高达1微克/千克时，20(OH)D3无血钙活性，而1,20(OH)2D3仅表现出轻度至中度的血钙活性，1,25(OH)2D3则具有强烈的血钙活性。
结论：本研究鉴定出一类极具抗白血病治疗潜力的新型开链甾醇，其中20(OH)D3凭借其较高的活性与无血钙毒性的特性，值得重点关注。