Hoffman Robert M, Stern Peter H, Coalson Dennis W, Douglas Wallace C, Erbe Richard W
AntiCancer, Inc., San Diego, CA, USA.
Department of Surgery, University of California, San Diego, CA, USA.
Methods Mol Biol. 2019;1866:13-26. doi: 10.1007/978-1-4939-8796-2_2.
Many different types of cancer cells have been shown to be methionine (MET) dependent. Cancer cells, unlike normal cells, grow poorly or not at all when MET is restricted. Cancer cells have an elevated requirement for exogenous MET for growth, despite high levels of endogenous synthesis. This requirement reflects increased utilization of MET by cancer cells, analogous to increased utilization glucose by cancer cells (Warburg effect). To answer the critical question of whether MET-dependent cancer cells synthesize normal amounts of MET, we determined the levels of MET, S-adenosylmethionine (AdoMET), and S-adenosylhomocysteine (AdoHCY) that were synthesized by MET-dependent cancer cells under conditions of MET restriction. We demonstrated that MET-dependent cells synthesize a normal amount of endogenously synthesized MET but are still deficient in AdoMET. In contrast, exogenously supplied MET results in normal AdoMET levels. The ratio of AdoMET to AdoHCY is low in MET-dependent cells growing in MET-restricted medium but is normal when MET is supplied. Under conditions of MET restriction, the low AdoMET/AdoHCY ratio probably limits proliferation of MET-dependent cancer cells. The amount of free MET is also low in MET-dependent cancer cells under MET restriction. The elevated MET requirement for cancer cells may be due to enhanced overall rates of transmethylation compared to normal human cells. Thus, MET-dependent cancer cells have low levels of free MET, low levels of AdoMET, and elevated levels of AdoHCY under conditions of MET restriction probably due to overuse of MET for transmethylation reactions ("Hoffman effect"), thereby blocking cellular proliferation.
许多不同类型的癌细胞已被证明对甲硫氨酸（MET）有依赖性。与正常细胞不同，当MET受到限制时，癌细胞生长不良或根本不生长。尽管内源性合成水平很高，但癌细胞对外源性MET的生长需求却有所增加。这种需求反映了癌细胞对MET的利用率提高，类似于癌细胞对葡萄糖的利用率增加（瓦伯格效应）。为了回答MET依赖性癌细胞是否合成正常量的MET这个关键问题，我们测定了在MET限制条件下MET依赖性癌细胞合成的MET、S-腺苷甲硫氨酸（AdoMET）和S-腺苷同型半胱氨酸（AdoHCY）的水平。我们证明，MET依赖性细胞合成的内源性MET量正常，但AdoMET仍然缺乏。相比之下，外源性提供的MET会使AdoMET水平正常。在MET限制培养基中生长的MET依赖性细胞中，AdoMET与AdoHCY的比值较低，但在提供MET时则正常。在MET限制条件下，低AdoMET/AdoHCY比值可能会限制MET依赖性癌细胞的增殖。在MET限制下，MET依赖性癌细胞中的游离MET量也很低。癌细胞对MET的需求增加可能是由于与正常人类细胞相比，转甲基化的总体速率提高。因此，在MET限制条件下，MET依赖性癌细胞的游离MET水平低、AdoMET水平低、AdoHCY水平升高，这可能是由于过度使用MET进行转甲基化反应（“霍夫曼效应”），从而阻碍了细胞增殖。
