Ishizuka Aya, Hashimto Yuki, Naka Ryosuke, Kinoshita Mitsuhiro, Kakehi Kazuaki, Seino Junichi, Funakoshi Yoko, Suzuki Tadashi, Kameyama Akihiko, Narimatsu Hisashi
School of Pharmacy, Kinki University, Kowakae 3-4-1, Higashi-Osaka, Osaka 577-8502, Japan.
Biochem J. 2008 Jul 15;413(2):227-37. doi: 10.1042/BJ20071562.
During the N-glycosylation reaction, it has been shown that 'free' N-glycans are generated either from lipid-linked oligosaccharides or from misfolded glycoproteins. In both cases, occurrence of high mannose-type free glycans is well-documented, and the molecular mechanism for their catabolism in the cytosol has been studied. On the other hand, little, if anything, is known with regard to the accumulation of more processed, complex-type free oligosaccharides in the cytosol of mammalian cells. During the course of comprehensive analysis of N-glycans in cancer cell membrane fractions [Naka et al. (2006) J. Proteome Res. 5, 88-97], we found that a significant amount of unusual, complex-type free N-glycans were accumulated in the stomach cancer-derived cell lines, MKN7 and MKN45. The most abundant and characteristic glycan found in these cells was determined to be NeuAcalpha2-6Galbeta1-4GlcNAcbeta1-2Manalpha1-3Manbeta1-4GlcNAc. Biochemical analyses indicated that those glycans found were cytosolic glycans derived from lysosomes due to low integrity of the lysosomal membrane. Since the accumulation of these free N-glycans was specific to only two cell lines among the various cancer cell lines examined, these cytosolic N-glycans may serve as a specific biomarker for diagnosis of specific tumours. A cytosolic sialidase, Neu2, was shown to be involved in the degradation of these sialoglycans, indicating that the cytosol of mammalian cells might be equipped for metabolism of complex-type glycans.
在N-糖基化反应过程中，研究表明“游离”N-聚糖可由脂质连接的寡糖或错误折叠的糖蛋白产生。在这两种情况下，高甘露糖型游离聚糖的出现都有充分的文献记载，并且其在细胞质中分解代谢的分子机制也已得到研究。另一方面，关于在哺乳动物细胞细胞质中积累更多加工后的复合型游离寡糖的情况，即便有了解，也知之甚少。在对癌细胞膜组分中的N-聚糖进行全面分析的过程中 [中田等人（2006年）《蛋白质组研究杂志》5卷，88 - 97页]，我们发现胃癌来源的细胞系MKN7和MKN45中积累了大量异常的复合型游离N-聚糖。在这些细胞中发现的最丰富且具特征性的聚糖被确定为NeuAcalpha2 - 6Galbeta1 - 4GlcNAcbeta1 - 2Manalpha1 - 3Manbeta1 - 4GlcNAc。生化分析表明，所发现的这些聚糖是由于溶酶体膜完整性低而源自溶酶体的细胞质聚糖。由于这些游离N-聚糖的积累仅在所检测的各种癌细胞系中的两个细胞系中具有特异性，所以这些细胞质N-聚糖可能作为特定肿瘤诊断的特异性生物标志物。一种细胞质唾液酸酶Neu2被证明参与了这些唾液酸聚糖的降解，这表明哺乳动物细胞的细胞质可能具备代谢复合型聚糖的能力。
