Wang Lei, Duke Lindsay, Zhang Peter S, Arlinghaus Ralph B, Symmans W Fraser, Sahin Aysegul, Mendez Richard, Dai Jia Le
Department of Molecular Pathology, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
Cancer Res. 2003 Aug 1;63(15):4724-30.
Spleen tyrosine kinase (Syk) is a candidate tumor (metastasis) suppressor that is highly expressed in mammary epithelial cells. Loss of Syk expression through promoter hypermethylation is associated with increased invasiveness in a subset of breast cancer. Here, we show that in addition to full-length Syk [Syk(L)], an alternatively spliced variant, Syk(S), is frequently expressed in breast cancer cells. Syk(S) is identical to Syk(L), except that it lacks 23 amino acid residues (deletion) within the interdomain B (IDB) of Syk. We also show that the aberrant expression of Syk(S) occurs frequently in primary breast tumors but never in matched normal mammary tissues, suggesting a contribution of Syk(S) to mammary tumor progression. Expression of Syk(L) suppressed breast cancer cell invasiveness. In contrast, Syk(S) expression did not affect the cell invasion potential. This differential phenotypic response is accompanied by their different subcellular localization. Immunocytochemical studies and nuclear and cytoplasmic fractionation experiments indicated that Syk(L) could enter the nucleus, whereas Syk(S) was located exclusively in the cytoplasm. Five basic residues in deletion were found to be critical in determining Syk(L) nuclear transport and invasion suppression activity; mutations completely excluded Syk(L) from the nucleus and blocked Syk(L)-inducible invasion suppression. Moreover, IDB acted as an autonomous nuclear localization signal to facilitate nuclear transport of a heterologous protein. Thus, the IDB of Syk(L) contains a nuclear localization signal that is responsible for Syk(L) nuclear translocation. The correlation of the nuclear localization and invasion suppression function of Syk(L) indicated that nuclear Syk possesses biological activities associated with tumor suppression in mammary epithelial cells.
脾酪氨酸激酶（Syk）是一种候选肿瘤（转移）抑制因子，在乳腺上皮细胞中高表达。通过启动子高甲基化导致的Syk表达缺失与一部分乳腺癌侵袭性增加有关。在此，我们发现，除了全长Syk [Syk(L)]外，一种选择性剪接变体Syk(S)也经常在乳腺癌细胞中表达。Syk(S)与Syk(L)相同，只是它在Syk的结构域间B（IDB）内缺少23个氨基酸残基（缺失）。我们还发现，Syk(S)的异常表达在原发性乳腺肿瘤中频繁出现，但在配对的正常乳腺组织中从未出现，这表明Syk(S)对乳腺肿瘤进展有作用。Syk(L)的表达抑制了乳腺癌细胞的侵袭性。相反，Syk(S)的表达不影响细胞侵袭潜能。这种不同的表型反应伴随着它们不同的亚细胞定位。免疫细胞化学研究以及细胞核和细胞质分级分离实验表明，Syk(L)可以进入细胞核，而Syk(S)仅位于细胞质中。发现缺失区域的五个碱性残基对于确定Syk(L)的核转运和侵袭抑制活性至关重要；突变完全阻止Syk(L)进入细胞核并阻断Syk(L)诱导的侵袭抑制。此外，IDB作为一个自主的核定位信号促进异源蛋白的核转运。因此，Syk(L)的IDB包含一个负责Syk(L)核转运的核定位信号。Syk(L)的核定位与侵袭抑制功能的相关性表明，核Syk具有与乳腺上皮细胞肿瘤抑制相关的生物学活性。
