Hornsby P J, Hancock J P, Vo T P, Nason L M, Ryan R F, McAllister J M
Proc Natl Acad Sci U S A. 1987 Mar;84(6):1580-4. doi: 10.1073/pnas.84.6.1580.
Senescence in cultured adrenocortical cells involves changes in expression of differentiated functions as well as changes in responses to mitogenic stimulation. Steroid 17 alpha-hydroxylase (steroid 17 alpha-monooxygenase, EC 1.14.99.9) is an adrenal-specific enzyme, the expression of which is dependent on the presence of stimulators of cyclic AMP production, such as cholera toxin. Dot-blot hybridization of RNA from bovine adrenocortical cells that had been incubated with cholera toxin showed a marked decline in 17 alpha-hydroxylase mRNA levels as a function of population doubling level, closely paralleling the decline in induction of 17 alpha-hydroxylase enzyme activity. The lower levels of 17 alpha-hydroxylase induction did not result from a requirement for a longer time period for induction or from a specific defect in response to cholera toxin and were not caused by a general failure of enzyme induction in response to cyclic AMP. The decreased growth rate in older cells results from a general decline in response to several growth factors. However, the decline in 17 alpha-hydroxylase induction did not result from a loss of response of the cells to mitogens, since quiescent cells at a low population doubling level showed stimulation of 17 alpha-hydroxylase mRNA by cholera toxin to levels similar to those in nonquiescent cultures and added mitogens either had no effect on 17 alpha-hydroxylase mRNA levels or decreased them. There was, however, a specific posttranscriptional effect of insulin on 17 alpha-hydroxylase. The loss of 17 alpha-hydroxylase induction is unlikely to result from overgrowth of a minority cell type lacking the ability to induce 17 alpha-hydroxylase, because adrenocortical cell clones that had high levels of 17 alpha-hydroxylase induction gave rise to cells with lower levels of induction on subcloning. Thus, loss of 17 alpha-hydroxylase activity in adrenocortical cellular senescence results from a primary failure of accumulation of 17 alpha-hydroxylase mRNA after incubation with the inducing agent.
培养的肾上腺皮质细胞衰老涉及分化功能表达的变化以及对有丝分裂原刺激反应的变化。类固醇17α-羟化酶（类固醇17α-单加氧酶，EC 1.14.99.9）是一种肾上腺特异性酶，其表达依赖于环磷酸腺苷（cAMP）生成刺激剂的存在，如霍乱毒素。对用霍乱毒素孵育的牛肾上腺皮质细胞的RNA进行斑点印迹杂交显示，17α-羟化酶mRNA水平随群体倍增水平的增加而显著下降，这与17α-羟化酶酶活性诱导的下降密切平行。17α-羟化酶诱导水平较低并非由于诱导需要更长时间或对霍乱毒素反应存在特定缺陷，也不是由对cAMP的酶诱导普遍失败所致。衰老细胞生长速率下降是由于对多种生长因子的反应普遍降低。然而，17α-羟化酶诱导的下降并非由于细胞对有丝分裂原反应丧失，因为处于低群体倍增水平的静止细胞经霍乱毒素刺激后17α-羟化酶mRNA水平与非静止培养物中的相似，而添加有丝分裂原对17α-羟化酶mRNA水平要么无影响，要么使其降低。然而，胰岛素对17α-羟化酶有特定的转录后效应。17α-羟化酶诱导的丧失不太可能是由于缺乏诱导17α-羟化酶能力的少数细胞类型过度生长所致，因为17α-羟化酶诱导水平高的肾上腺皮质细胞克隆在亚克隆时会产生诱导水平较低的细胞。因此，肾上腺皮质细胞衰老过程中17α-羟化酶活性的丧失是由于与诱导剂孵育后17α-羟化酶mRNA积累的原发性失败。
