Arlow F L, Walczak S M, Luk G D, Majumdar A P
Veterans Administration Medical Center, Allen Park, Michigan 48101.
Cancer Res. 1989 Nov 1;49(21):5884-8.
Two in vivo and one in vitro studies were performed to evaluate the chemoprotective role of calcium during the early period of azoxymethane (AOM) induction. In the first set of experiments, groups of male Fischer 344 rats were s.c. injected with either AOM (20 mg/kg) or water (controls) and sacrificed immediately (0 time), and 1, 3, 5, and 7 days postinjection. In the second set of experiments, animals were injected with the same dose of AOM and subsequently pair-fed with rat chow containing either calcium carbonate or diet devoid of added calcium. The amount of calcium consumed was calculated to be 250 mg/kg b.w. In both experiments, colonic mucosa was assayed for ornithine decarboxylase (ODC). In addition, tyrosine kinase (Tyr-k) activity as well as tyrosine specific phosphorylation of membrane proteins were determined. Results revealed that maximal stimulation by AOM of ODC and Tyr-k activity occurred 5 days postinjection. This stimulation was significantly suppressed by calcium. AOM also produced an increase in the rate of tyrosine specific phosphorylation of two distinct colonic mucosal membrane proteins with Mr of 57,000 and 59,000. Again, dietary calcium suppressed the stimulation. In the third set of experiments, organ culture was utilized. Methylazoxymethanol, the active metabolite of AOM, was used instead of AOM in this part of the study. Four hour exposure of mucosal explants to methylazoxymethanol (1 microgram/ml) resulted in a significant (20-30%) increase in ODC and Tyr-k activity when compared to controls. Addition of either CaCl2 (2 mumol/ml) or difluoromethylornithine (2 nmol/ml) the irreversible inhibitor of ODC, significantly suppressed the methylazoxymethanol-induced activity of both ODC and Tyr-k. We conclude that calcium may have a chemoprotective role and tyrosine kinases may have a regulatory role in the early stages of AOM induction of colon cancer.
进行了两项体内研究和一项体外研究，以评估钙在早期偶氮甲烷（AOM）诱导过程中的化学保护作用。在第一组实验中，将雄性Fischer 344大鼠分组，皮下注射AOM（20mg/kg）或水（对照组），并在注射后立即（0小时）、1、3、5和7天处死。在第二组实验中，给动物注射相同剂量的AOM，随后成对喂饲含碳酸钙的大鼠饲料或不含添加钙的饲料。计算得出钙的摄入量为250mg/kg体重。在两项实验中，均对结肠黏膜进行鸟氨酸脱羧酶（ODC）检测。此外，还测定了酪氨酸激酶（Tyr-k）活性以及膜蛋白的酪氨酸特异性磷酸化。结果显示，AOM对ODC和Tyr-k活性的最大刺激作用出现在注射后5天。这种刺激作用被钙显著抑制。AOM还使两种分子量分别为57,000和59,000的不同结肠黏膜膜蛋白的酪氨酸特异性磷酸化速率增加。同样，膳食钙抑制了这种刺激作用。在第三组实验中，采用了器官培养。在本研究的这一部分中，使用AOM的活性代谢产物甲基偶氮甲醇代替AOM。与对照组相比，黏膜外植体暴露于甲基偶氮甲醇（1μg/ml）4小时导致ODC和Tyr-k活性显著增加（20%-30%）。添加氯化钙（2μmol/ml）或ODC的不可逆抑制剂二氟甲基鸟氨酸（2nmol/ml）可显著抑制甲基偶氮甲醇诱导的ODC和Tyr-k活性。我们得出结论，钙可能具有化学保护作用，酪氨酸激酶可能在AOM诱导结肠癌的早期阶段发挥调节作用。
