Brasaemle D L, Barber T, Kimmel A R, Londos C
Laboratory of Cellular and Developmental Biology, NIDDK, National Institutes of Health, Bethesda, Maryland 20892-2715, USA.
J Biol Chem. 1997 Apr 4;272(14):9378-87. doi: 10.1074/jbc.272.14.9378.
The perilipins are a family of polyphosphorylated proteins found exclusively surrounding neutral lipid storage droplets in adipocytes and steroidogenic cells. In steroidogenic cells, the cholesterol ester-rich lipid storage droplets are encoated with perilipins A and C. This study describes the dependence of perilipin levels on neutral lipid storage in cultured Y-1 adrenal cortical cells. The addition of fatty acids and cholesterol to the culture medium of Y-1 adrenal cortical cells greatly increased the storage of cholesterol esters and triacylglycerols concomitant with the formation of many new lipid storage droplets. The addition of fatty acids to the culture medium also produced a transient 6-fold increase in levels of perilipin A, but not C, mRNA, while much larger and stable increases in both perilipin A and C proteins were observed. The increases in perilipin protein levels were dependent upon the metabolism of fatty acids to triacylglycerol or cholesterol esters, since the incubation of cells with bromopalmitate, a poorly metabolized fatty acid, failed to yield large increases in lipid content or perilipin levels. Constitutive expression of epitope-tagged perilipins in transfected Y-1 adrenal cortical cells was regulated by lipid similarly to expression of the endogenous perilipins despite an absence of untranslated perilipin mRNA sequences in the expression constructs. Epitope-tagged perilipin A mRNAs were efficiently loaded with polyribosomes whether or not fatty acids were added to the culture medium; therefore, the increase in perilipin levels in the presence of fatty acids is likely due to factors other than increased translational efficiency. We suggest that the large increase in cellular perilipin levels upon lipid loading of cells is the result of post-translational stabilization of newly synthesized perilipins by stored neutral lipids.
围脂滴蛋白是一类多磷酸化蛋白家族，仅存在于脂肪细胞和类固醇生成细胞中中性脂质储存小滴的周围。在类固醇生成细胞中，富含胆固醇酯的脂质储存小滴被围脂滴蛋白A和C包裹。本研究描述了在培养的Y-1肾上腺皮质细胞中围脂滴蛋白水平对中性脂质储存的依赖性。向Y-1肾上腺皮质细胞的培养基中添加脂肪酸和胆固醇，可大大增加胆固醇酯和三酰甘油的储存量，同时形成许多新的脂质储存小滴。向培养基中添加脂肪酸还可使围脂滴蛋白A的mRNA水平瞬时增加6倍，但围脂滴蛋白C的mRNA水平无此变化，而围脂滴蛋白A和C的蛋白质水平均出现更大且稳定的增加。围脂滴蛋白水平的增加依赖于脂肪酸代谢生成三酰甘油或胆固醇酯，因为用难以代谢的脂肪酸溴棕榈酸孵育细胞，未能使脂质含量或围脂滴蛋白水平大幅增加。尽管表达构建体中不存在未翻译的围脂滴蛋白mRNA序列，但在转染的Y-1肾上腺皮质细胞中，表位标记的围脂滴蛋白的组成型表达与内源性围脂滴蛋白的表达一样受到脂质的调节。无论是否向培养基中添加脂肪酸，表位标记的围脂滴蛋白A的mRNA都能有效地与多核糖体结合；因此，在脂肪酸存在下围脂滴蛋白水平的增加可能是由于翻译效率提高以外的因素。我们认为，细胞脂质负载后细胞围脂滴蛋白水平的大幅增加是储存中性脂质对新合成的围脂滴蛋白进行翻译后稳定作用的结果。
