What is p53?
After the identification of the p53 protein and the subsequent cloning of p53 genes from several species, early observations suggested that p53 may function as an ontogeny, because over expression of p53 appeared to cause monogenic transformation of cells. In the late 1980s, however, several critical discoveries defined the normal function of p53 to be anti-monogenic. Wild-type p53 genes, when introduced into cells, were found to be growth suppressive. The screening of DNA from colon cancer patients revealed that p53 mutations occur with unusually high frequency in tumor tissue, an observation that was extended to most of the other major forms of human cancer. Indeed, members of Li-Freemen cancer-prone families were shown to carry germ-line p53 mutations. The importance of these observations was underscored by the finding that mice that are homozygous null for p53, although developmentally competent, are highly predisposed to tumors.
The functional character of the p53 protein was determined by experiments showing that p53 contains a strong transcriptional activation domain within its amino terminus and that it is a tetramer, sequence-specific DNA-biding protein with a defined cognate binding site containing two copies of the 10-mer (5'-RRRCA/TT/AGYYY-3'). Although the p53 protein acts as a transcriptional activator of genes containing p53-binding sites, it is also capable of strongly inhibiting transcription from many genes lacking p53-binding sites. Several monogenic DNA viruses express viral gene products that associate with and inhibit the trans-activation function of p53, notably SV40 large T antigen, the adenovirus E1B 55-kD protein, and the E6 protein of monogenic forms of human papillomavirus (HPV E6). In cells, p53 can associate with a 90-kD protein, identified as the product of the mdm-2 ontogeny, which is amplified in some types of tumors. When bound to mdm-2, p53 can no longer function as an activator of transcription.
P53 plays multiple roles in cells. Expression of high levels of wild-type (but not mutant) p53 has two outcomes: cell cycle arrest or apoptosis. The observation that DNA-damaging agents induce levels of p53 in cells led to the definition of p53 as a checkpoint factor, akin, perhaps, to the product of the fad9 gene in yeast. While dispensable for viability, in response to geotaxis stress, p53 acts as an "emergency brake" inducing either arrest or apoptosis, protecting the genome from accumulating excess mutations. Consistent with this notion, cells lacking p53 were shown to be genetically unstable and thus more prone to tumors.
p53是存在人體細胞內的一種抗癌白質，它有抑制細胞生長及維持遺傳物質完整性的功能。事實上，半數以上的癌癥細胞內都有p53的突變，可見其在細胞生長控制上扮演了重要的角色。在正常狀況下，p53的半衰期約只有30分鐘，相當不穩定;然而當細胞經紫外線，離子化射線(如X光，伽傌照射)，或當細胞缺氧、缺養時，p53被活化，同時它的穩定性提高，造成細胞內的p53大量增加，除了上述刺激外，化學治療上常用的藥物也有同效。這種p53的活化與增加常導致兩種可能的結果：一是細胞長停止在G1或G2期;另一是細胞采自殺行為(apoptosis)而死亡。細胞由此得以修補損壞(前者)，或過度受損的細胞得以從人體除去(后者)。這種依賴p53的"自衛措施"在一些細胞中常因p53的突變而失去功能，使得這些有"缺陷"的細胞能繼續不受控制的生長分裂，導致突變的累積和癌癥的生長。
雖然環境因子影響p53活性及穩定性的事實已知已久，其間的分子機轉仍不清楚。蛋白質的磷酸化(phosphorylation)一向被認為在訊息傳遞上扮演重要的角色。事實上，經由我們及其他實驗室的研究發現，p53在經過紫外線，伽傌射線照射后，其N端的數個胺基酸(第15，20，33，37)有磷酸化的現象。這種磷酸化發生極為快速，幾乎是在照射后數分鐘內即已產生，而持續多久則視胺基酸位置、刺激型態，及細胞種類而異。至于這些磷酸化與p53的反應之關聯性則仍有待證明。最近我們發現有兩個在細胞分裂(Cell cycle)的檢查點(checkpoint)上扮演著重要調控功能的磷酸化酵素(kinas) hCHK1，CHK2可以有效的磷酸化p53。有趣的是，磷酸化的胺基酸中包括了那些可以被紫外線、伽傌線引起的位置，即第15，20及37胺基酸。我們正著手研究可能的CHKs的上游分子及p53在CHKs磷酸化后功能之變化。此外， 不同的環境因子與p53聯系的方式可能各異，有些可能透過磷酸化以外的方式進行。 我們希望能先定出p53序列中與環境因子互動有關的區域(domain)，再由此找出與調節p53穩定性有關的機制及分子。
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