Seshadri T, Uzman J A, Oshima J, Campisi J
Department of Cell and Molecular Biology, Lawrence Berkeley Laboratory, University of California, Berkeley 94720.
J Biol Chem. 1993 Sep 5;268(25):18474-80.
Normal eukaryotic cells divide only a limited number of times before proliferation ceases due to cellular senescence. We previously reported that a constitutively expressed, non-cell cycle-regulated transcript of unknown identity declines severalfold when human fibroblasts become senescent. We show here, from the sequence of cDNA and genomic clones, that this transcript encodes L7, a structural protein of the large ribosomal subunit. The human L7 protein shares > 90% amino acid identity with the mouse and rat L7 proteins but is shorter than either rodent protein due to fewer basic repetitive motifs at the amino terminus. The position of the first intron is conserved between the mouse and human genes. The L7 mRNA was abundant, stable (t1/2 > 10 h), and polyadenylated in presenescent and senescent human fibroblasts; however, steady state mRNA levels were 5-10-fold lower in senescent cells, whether derived from fetal lung or neonatal foreskin. Quiescent and senescent cells synthesized protein at similar rates, yet only senescent cultures showed a decline in L7 mRNA. The mRNAs encoding five other ribosomal proteins (L5, P1, S3, S6, and S10) behaved similarly. The results suggest that the senescence-associated decline in L7 and other ribosomal protein mRNAs is unrelated to growth state or protein synthetic rate per se and support the view that senescence and quiescence are dissimilar states.
正常真核细胞在因细胞衰老而停止增殖之前仅能分裂有限次数。我们之前报道过，一种持续表达、身份不明且不受细胞周期调控的转录本，在人类成纤维细胞衰老时会下降数倍。我们在此通过cDNA和基因组克隆序列表明，该转录本编码L7，即大核糖体亚基的一种结构蛋白。人类L7蛋白与小鼠和大鼠的L7蛋白氨基酸同源性超过90%，但由于氨基末端的碱性重复基序较少，其长度比两种啮齿动物蛋白都短。小鼠和人类基因中第一个内含子的位置是保守的。L7 mRNA在衰老前和衰老的人类成纤维细胞中含量丰富、稳定（半衰期>10小时）且有polyA尾；然而，无论来源于胎儿肺还是新生儿包皮，衰老细胞中的稳态mRNA水平都低5 - 10倍。静止和衰老细胞以相似的速率合成蛋白质，但只有衰老培养物中L7 mRNA出现下降。编码其他五种核糖体蛋白（L5、P1、S3、S6和S10）的mRNA表现类似。结果表明，L7和其他核糖体蛋白mRNA与衰老相关的下降与生长状态或蛋白质合成速率本身无关，并支持衰老和静止是不同状态的观点。
