Vella Veronica, Puppin Cinzia, Damante Giuseppe, Vigneri Riccardo, Sanfilippo Mariangela, Vigneri Paolo, Tell Gianluca, Frasca Francesco
Department of Internal Medicine, Endocrinology Unit, University of Catania, Catania, Italy.
Int J Cancer. 2009 Jun 1;124(11):2539-48. doi: 10.1002/ijc.24221.
DeltaNp73 is a N-terminally truncated p53 family member with a dominant negative function, which is upregulated in cancer. PTEN is a lipid phosphatase, which is involved in the attenuation of tyrosine kinase signaling. PTEN expression is increased by p53, and its function is blunted in several malignancies. Because in most of the thyroid carcinomas, DeltaNp73alpha is upregulated, whereas PTEN expression down regulated, we investigated whether DeltaNp73alpha may influence PTEN expression in this cell model. We found that DeltaNp73alpha overexpression in thyroid cancer cells reduces PTEN expression, whereas DeltaNp73alpha down-regulation by siRNA increases PTEN expression. Real-time PCR indicated that overexpression of DeltaNp73alpha is able to reduce PTEN mRNA levels. Moreover, chromatin immunoprecipitation (ChIP) and luciferase assays indicated that DeltaNp73alpha binds to -1031-779 region of the PTEN promoter, which is a different site than that for p53, thereby inhibiting promoter activity. Interestingly, also the transcriptionally active p73 isoforms (TAp73alpha and TAp73beta) bound to this DNA sequence and, at variance with DeltaNp73alpha, stimulated PTEN promoter activity to an extent similar to that of p53. In accordance with its effect on PTEN protein levels, DeltaNp73alpha increased phospho-Akt protein content and, as a consequence, Mdm2-mediated p53 degradation. This effect of DeltaNp73alpha resulted in increased thyroid cancer cell proliferation and reduced apoptosis and was reverted by the PI3-kinase inhibitor LY294002, indicating the role of Akt pathway in this effect. Taken together, these results indicate a novel p73 regulated mechanism for PTEN expression in thyroid cancer cells, and that, also through this mechanism, DeltaNp73alpha exerts its protumorigenic effect.
DeltaNp73是一种N端截短的p53家族成员，具有显性负性功能，在癌症中上调。PTEN是一种脂质磷酸酶，参与酪氨酸激酶信号传导的减弱。PTEN的表达由p53上调，其功能在几种恶性肿瘤中减弱。由于在大多数甲状腺癌中，DeltaNp73α上调，而PTEN表达下调，我们研究了DeltaNp73α是否可能在该细胞模型中影响PTEN表达。我们发现甲状腺癌细胞中DeltaNp73α的过表达降低了PTEN表达，而通过小干扰RNA（siRNA）下调DeltaNp73α则增加了PTEN表达。实时聚合酶链反应（PCR）表明，DeltaNp73α的过表达能够降低PTEN信使核糖核酸（mRNA）水平。此外，染色质免疫沉淀（ChIP）和荧光素酶测定表明，DeltaNp73α与PTEN启动子的-1031 - 779区域结合，这是一个与p53不同的位点，从而抑制启动子活性。有趣的是，转录活性的p73亚型（TAp73α和TAp73β）也与该DNA序列结合，与DeltaNp73α不同的是，它们刺激PTEN启动子活性的程度与p53相似。与它对PTEN蛋白水平的影响一致，DeltaNp73α增加了磷酸化Akt蛋白含量，结果导致Mdm2介导的p53降解。DeltaNp73α的这种作用导致甲状腺癌细胞增殖增加和凋亡减少，并被磷脂酰肌醇-3激酶（PI3-激酶）抑制剂LY294002逆转，表明Akt途径在这种作用中的作用。综上所述，这些结果表明甲状腺癌细胞中PTEN表达存在一种新的p73调节机制，并且DeltaNp73α也通过这种机制发挥其促肿瘤作用。
