Björnsson O G, Sparks J D, Sparks C E, Gibbons G F
Metabolic Research Laboratory, Radcliffe Infirmary, Oxford, UK.
Eur J Clin Invest. 1994 Feb;24(2):137-48. doi: 10.1111/j.1365-2362.1994.tb00979.x.
When hepatocytes were cultured for 24 h in the presence of forskolin (10(-4) mol l-1) or isobutylmethylxanthine (IBMX, 10(-3) mol l-1), the intracellular cAMP concentration peaked (320-380 pmol mg-1 protein) after 10-20 min of culture. This increase was accompanied by a decrease in the secretion of triacylglycerol, cholesterol and apoprotein B associated with VLDL. After 4 h cAMP levels had returned almost to basal values but the inhibition of VLDL secretion persisted. There was a small intracellular accumulation of triacylglycerol but not of apoprotein B. Addition of forskolin and IBMX together led to a further increase in intracellular cAMP and a further suppression of VLDL output. Similar effects on the secretion of VLDL were also observed after addition of Bt2cAMP. Exposure of cell cultures to glucagon (10(-7) mol l-1) for only 10 min raised cellular cAMP levels to > 200 pmol mg-1 protein, and suppressed VLDL secretion during the next 24 h to < 40% of control. All of the substances tested inhibited de novo synthesis of fatty acids but had little or no effect on cholesterol synthesis and did not inhibit oleate esterification to triacylglycerol. The cAMP-dependent protein kinase antagonist Rp-cAMPS prevented suppression of VLDL triacylglycerol secretion induced by glucagon (10(-7) mol l-1) and abolished glucagon-induced ketogenesis. Rp-cAMPS also inhibited Bt2cAMP (7.5 x 10(-6) mol l-1)-induced suppression of VLDL secretion and enhancement of ketogenesis. It is concluded that rat hepatic VLDL metabolism can be regulated by cAMP and cAMP-dependent protein kinases, and that the initial transient rise in cellular cAMP levels induced by glucagon is sufficient to maintain a long-term inhibitory effect on assembly and secretion of VLDL.
当肝细胞在福斯可林（10⁻⁴ mol l⁻¹）或异丁基甲基黄嘌呤（IBMX，10⁻³ mol l⁻¹）存在的情况下培养24小时时，细胞内cAMP浓度在培养10 - 20分钟后达到峰值（320 - 380 pmol mg⁻¹蛋白质）。这种增加伴随着与极低密度脂蛋白（VLDL）相关的三酰甘油、胆固醇和载脂蛋白B分泌的减少。4小时后，cAMP水平几乎恢复到基础值，但对VLDL分泌的抑制仍然存在。有少量三酰甘油在细胞内积累，但载脂蛋白B没有。同时添加福斯可林和IBMX会导致细胞内cAMP进一步增加，并进一步抑制VLDL输出。添加Bt₂cAMP后也观察到对VLDL分泌有类似影响。细胞培养物仅暴露于胰高血糖素（10⁻⁷ mol l⁻¹）10分钟，细胞内cAMP水平就升高至> 200 pmol mg⁻¹蛋白质，并在接下来的24小时内将VLDL分泌抑制至对照的< 40%。所有测试物质均抑制脂肪酸的从头合成，但对胆固醇合成几乎没有影响，也不抑制油酸酯化生成三酰甘油。cAMP依赖性蛋白激酶拮抗剂Rp - cAMPS可阻止胰高血糖素（10⁻⁷ mol l⁻¹）诱导的VLDL三酰甘油分泌抑制，并消除胰高血糖素诱导的生酮作用。Rp - cAMPS还抑制Bt₂cAMP（7.5×10⁻⁶ mol l⁻¹）诱导的VLDL分泌抑制和生酮作用增强。结论是大鼠肝脏VLDL代谢可受cAMP和cAMP依赖性蛋白激酶调节，并且胰高血糖素诱导的细胞内cAMP水平的初始短暂升高足以对VLDL的组装和分泌维持长期抑制作用。
