Borghetti A F, Piedimonte G, Tramacere M, Severini A, Ghiringhelli P, Guidotti G G
J Cell Physiol. 1980 Oct;105(1):39-49. doi: 10.1002/jcp.1041050107.
The transport of selected neutral and cationic amino acids has been studied in Balb/c 3T3, SV3T3, and SV3T3 revertant cell lines. After properly timed preincubations to control the size of internal amino acid pools, the activity of systems A, ASC, L, and Ly+ has been discriminated by measurements of amino acid uptake (initial entry rate) in the presence and absence of sodium and of transport-specific model substrates. L-Proline, 2-aminoisobutyric acid, and glycine were primarily taken up by system A; L-alanine and L-serine by system ASC; L-phenylalanine by system L; and L-lysine by system Ly+ in SV3T3 cells. L-Proline and L-serine were also preferential substrates of systems A and ASC, respectively, in 3T3 and SV3T3 revertant cells. Transport activity of the Na+-dependent systems A and ASC decreased markedly with the increase of cell density, whereas the activity of the Na+-independent systems L and Ly+ remained substantially unchanged. The density-dependent change in activity of system A occurred through a mechanism affecting transport maximum (Vmax) rather than substrate concentration for half-maximal velocity (Km). Transport activity of systems A and ASC was several-fold higher in transformed SV3T3 cells than in 3T3 parental cells at all the culture densities that could be compared. In SV3T3 revertant cells, transport activity by these systems remained substantially similar to that observed in transformed SV3T3 cells. The results presented here add cell density as a regulatory factor of the activity of systems A and ASC, and show that this control mechanism of amino acid transport is maintained in SV40 virus-transformed 3T3 cells that have lost density-dependent inhibition of growth, as well as in SV3T3 revertant cells that have resumed it.
已在Balb/c 3T3、SV3T3和SV3T3回复细胞系中研究了特定中性和阳离子氨基酸的转运。在进行适当时间的预孵育以控制细胞内氨基酸池的大小后，通过在有钠和无钠情况下以及存在转运特异性模型底物时测量氨基酸摄取（初始进入速率），区分了系统A、ASC、L和Ly+的活性。在SV3T3细胞中，L-脯氨酸、2-氨基异丁酸和甘氨酸主要通过系统A摄取；L-丙氨酸和L-丝氨酸通过系统ASC摄取；L-苯丙氨酸通过系统L摄取；L-赖氨酸通过系统Ly+摄取。在3T3和SV3T3回复细胞中，L-脯氨酸和L-丝氨酸也分别是系统A和ASC的优先底物。依赖钠的系统A和ASC的转运活性随着细胞密度的增加而显著降低，而不依赖钠的系统L和Ly+的活性基本保持不变。系统A活性的密度依赖性变化是通过影响转运最大值（Vmax）而非半最大速度的底物浓度（Km）的机制发生的。在所有可比较的培养密度下，转化的SV3T3细胞中系统A和ASC的转运活性比3T3亲本细胞高几倍。在SV3T3回复细胞中，这些系统的转运活性与在转化的SV3T3细胞中观察到的基本相似。此处给出的结果增加了细胞密度作为系统A和ASC活性的调节因子，并表明这种氨基酸转运的控制机制在已失去密度依赖性生长抑制的SV40病毒转化的3T3细胞以及已恢复该抑制的SV3T3回复细胞中均得以维持。
