Abstract：
Methionine dependence, the inability to grow in culture when methionine in the medium is replaced by its metabolic precursor homocysteine, occurs in many tumor cell lines. In most affected lines, the cause of methionine dependence is not known. An exception is the melanoma-derived cell line MeWo-LC1, in which hypermethylation of the MMACHC gene is associated with decreased MMACHC expression. Decreased expression results in decreased provision of the methylcobalamin cofactor required for activity of methionine synthase and thus decreased conversion of homocysteine to methionine. Analysis of data in the Cancer Cell Line Encyclopedia Archive demonstrated that MMACHC hypermethylation and decreased MMACHC expression occurred more frequently in melanoma cell lines when compared to other tumor cell lines. We further investigated methionine dependence and aspects of MMACHC function in a panel of six melanoma lines, including both melanoma lines with known methionine dependence status (MeWo, which is methionine independent, and A375, which is methionine dependent). We found that the previously unclassified melanoma lines HMCB, Colo829 and SH-4 were methionine dependent, while SK-Mel-28 was methionine independent. However, despite varying levels of MMACHC methylation and expression, none of the tested lines had decreased methylcobalamin and adenosylcobalamin synthesis as seen in MeWo-LC1, and the functions of both cobalamin-dependent enzymes methionine synthase and methylmalonyl-CoA mutase were intact. Thus, while melanoma lines were characterized by relatively high levels of MMACHC methylation and low expression, the defect in metabolism observed in MeWo-LC1 was unique, and decreased MMACHC expression was not a cause of methionine dependence in the other melanoma lines.
摘要：
蛋氨酸依赖，当培养基中的蛋氨酸被其代谢前体高半胱氨酸取代时，无法在培养物中生长，发生在许多肿瘤细胞系中。在大多数受影响的线路中，蛋氨酸依赖的原因尚不清楚。一个例外是源自黑素瘤的细胞系MeWo-LC1,其中MMACHC基因的超甲基化与降低的MMACHC表达相关。表达降低导致甲硫氨酸合酶活性所需的甲基钴胺素辅因子的提供减少，从而降低了高半胱氨酸向甲硫氨酸的转化。癌细胞系百科全书档案中的数据分析表明，与其他肿瘤细胞系相比，MMACHC超甲基化和MMACHC表达降低在黑色素瘤细胞系中更频繁地发生。我们进一步研究了一组六个黑色素瘤细胞系中MMACHC功能的甲硫氨酸依赖性和方面，包括两个已知蛋氨酸依赖状态的黑色素瘤细胞系(MeWo，它是蛋氨酸独立的，和A375，它是蛋氨酸依赖性的）。我们发现以前未分类的黑色素瘤系HMCB，Colo829和SH-4是蛋氨酸依赖性的，而SK-Mel-28是不依赖蛋氨酸的。然而,尽管MMACHC甲基化和表达水平不同，如MeWo-LC1所示，测试品系均未降低甲基钴胺素和腺苷钴胺素的合成，并且钴胺素依赖性酶蛋氨酸合酶和甲基丙二酰辅酶A变位酶的功能都完整。因此，虽然黑色素瘤细胞系的特征是MMACHC甲基化水平相对较高，表达较低，在MeWo-LC1中观察到的代谢缺陷是独特的，在其他黑色素瘤细胞系中，MMACHC表达降低不是蛋氨酸依赖的原因。
