Litvin Y, Leiser M, Fleischer N, Erlichman J
Endocrinology. 1986 Aug;119(2):737-45. doi: 10.1210/endo-119-2-737.
The mechanisms by which somatostatin (SRIF) inhibits CRF-induced ACTH secretion from AtT20 cells were characterized by comparing the effects of SRIF on cAMP production, adenylate cyclase activity, and activation of cAMP-dependent protein kinase isoenzymes with its effects on ACTH release. In isolated membranes, CRF (100 nM) stimulated adenylate cyclase activity 4- to 5-fold. SRIF inhibited CRF-stimulated adenylate cyclase in a concentration-dependent manner. However, maximal inhibition was 50%. SRIF did not inhibit basal adenylate cyclase or forskolin-stimulated cyclase in the absence of guanine nucleotides and had only small effects on forskolin-stimulated cyclase when assayed in the presence of guanine nucleotides. CRF (100 nM) induced small rises (2-fold) in intracellular cAMP levels which produced maximal ACTH release. SRIF inhibited basal and CRF-stimulated ACTH release in a concentration-dependent manner, and there was a good correlation between inhibition of ACTH release and inhibition of the activation of cAMP-dependent protein kinases in these cells. Thus, the effect of SRIF on CRF-induced ACTH release appeared to result from its effect on inhibition of adenylate cyclase. In the presence of 3-methylisobutylxanthine (MIX), CRF increased cAMP levels 20-fold and activated a greater proportion of cAMP-dependent protein kinase, but did not stimulate ACTH release more than CRF alone. Under these conditions, SRIF (100 nM) inhibited cAMP accumulation by 90%. ACTH release was also inhibited, but higher concentrations of SRIF were required to block ACTH release compared to cells incubated in the absence of MIX. Sufficient cAMP levels were achieved so that activation of cAMP-dependent protein kinases was only partially blocked. There was still sufficient cAMP to activate cAMP-dependent protein kinase to an extent equal to that seen with CRF without MIX. Similar effects of SRIF on cAMP accumulation and protein kinase activation were seen when cells were stimulated with forskolin. Our results demonstrate that SRIF inhibits ACTH release from AtT20 cells by inhibiting hormone-sensitive adenylate cyclase and thereby prevents the activation of cAMP-dependent protein kinases. However, under conditions where cAMP-dependent protein kinases are still sufficiently active to induce ACTH secretion, high concentrations of SRIF can inhibit ACTH release by a mechanism independent of cAMP-dependent protein kinase.
通过比较生长抑素（SRIF）对环磷酸腺苷（cAMP）生成、腺苷酸环化酶活性以及cAMP依赖性蛋白激酶同工酶激活的影响与其对促肾上腺皮质激素（ACTH）释放的影响，来确定生长抑素抑制促肾上腺皮质激素释放因子（CRF）诱导AtT20细胞分泌ACTH的机制。在分离的细胞膜中，CRF（100 nM）可使腺苷酸环化酶活性提高4至5倍。SRIF以浓度依赖性方式抑制CRF刺激的腺苷酸环化酶。然而，最大抑制率为50%。在不存在鸟嘌呤核苷酸的情况下，SRIF不抑制基础腺苷酸环化酶或福斯高林刺激的环化酶，而在存在鸟嘌呤核苷酸的情况下进行检测时，SRIF对福斯高林刺激的环化酶只有微小影响。CRF（100 nM）可使细胞内cAMP水平小幅升高（2倍），从而导致ACTH释放达到最大值。SRIF以浓度依赖性方式抑制基础和CRF刺激的ACTH释放，并且在这些细胞中，ACTH释放的抑制与cAMP依赖性蛋白激酶激活的抑制之间存在良好的相关性。因此，SRIF对CRF诱导的ACTH释放的作用似乎是由于其对腺苷酸环化酶抑制的作用。在存在3 - 甲基异丁基黄嘌呤（MIX）的情况下，CRF使cAMP水平升高20倍，并激活了更大比例的cAMP依赖性蛋白激酶，但对ACTH释放的刺激并不比单独使用CRF时更强。在这些条件下，SRIF（100 nM）可使cAMP积累抑制90%。ACTH释放也受到抑制，但与在不存在MIX的情况下培养的细胞相比，需要更高浓度的SRIF来阻断ACTH释放。达到了足够的cAMP水平，使得cAMP依赖性蛋白激酶的激活仅被部分阻断。仍然有足够的cAMP来激活cAMP依赖性蛋白激酶，其程度与在不存在MIX的情况下CRF所引起的程度相同。当用福斯高林刺激细胞时，SRIF对cAMP积累和蛋白激酶激活也有类似的作用。我们的结果表明，SRIF通过抑制激素敏感性腺苷酸环化酶来抑制AtT20细胞释放ACTH，从而防止cAMP依赖性蛋白激酶的激活。然而，在cAMP依赖性蛋白激酶仍然有足够活性诱导ACTH分泌的条件下，高浓度的SRIF可通过一种独立于cAMP依赖性蛋白激酶的机制抑制ACTH释放。
