多環芳香烴感受器(Aryl hydrocarbon receptor, AhR)是一個被ligand活化的轉錄因子，其ligands主要是多環芳香烴類環境污染物，譬如Benzo[a]pyrene (B[a]P)或Dioxin，AhR可調控很多與藥物代謝有關的基因。當細胞(特別是皮膚細胞)的生長環境中含有B[a]P或Dioxin時，AhR會被活化進而將cytochrome P450 (CYP1A1)轉錄，活化的CYP1A1則可將毒物代謝活化。從老鼠實驗知以化學藥物處理，誘發皮膚腫瘤後，AhR的功能會降低，因而造成CYP1A1的基因誘導降低。由此可知，AhR所控制的基因表現與腫瘤形成有密切關係。最近，在我們研究室以B[a]P處理Skin basal cell carcinoma (BCC)與非癌化HaCaT細胞，結果發現CYP1A1的基因誘導性在BCC細胞會降低，從RT-PCR知兩細胞株AhR的mRNA均無差異，但從Western blot知AhR蛋白在BCC細胞中明顯較低。本計劃深入探討在BCC癌細胞AhR蛋白量及功能降低的原因。我們發現(1)AhR蛋白表現程度與皮膚角質細胞分化程度有關，分化程度越高，AhR表現越多；(2)AhR蛋白結構不穩定，以超音波破碎細胞時，AhR蛋白量減少；(3)Genistein及Herbimycin A可增加CYP1A1被誘導之程度，因此BCC細胞株中可能存在某種Kinase蛋白過度活化，而影響B[a]P誘導CYP1A1基因表現之過程。(4)B[a]P處理24小時後BCC細胞質中AhR蛋白量減少，但細胞核中AhR蛋白不受影響，以上這些因素都可能造成BCC細胞對B[a]P沒有反應。
Aryl hydrocarbon receptor (AhR) is a ligand-activated cytosolic transcription factor which regulates a number of genes encoding drug-metabolizing enzymes, such as cytochrome P4501Al (CYP1A1). Polycyclic aromatic hydrocarbons, such as benzo[a]pyrene (B[a]P) and dioxin (TCDD), are AhR agnoists. When keratinocytes, the major cell type in the epidermis of the skin, are exposed to TCDD, AhR is activated and increases CYP1A1 gene transcription. AhR agonists increase metabolic activation of various procarcinogens. It is recently reported that AhR function and CYP1A1 inducibility are downregulated in chemically induced skin tumors in mice. CYP1A1 mRNA level in the normal tissue adjacent to papillomas is increased after treatment with TCDD but no increase in the tumors. We have two human keratinocyte cell lines: a nontumorigenic HaCaT cells and a cell line derived from skin basal cell carcinoma (BCC). Our preliminary data show that CYP1A1 gene expression is induced by AhR agonist, B[a]P, in HaCaT cells, but not in BCC cells. Consistent with CYP1A1 inducibility, AhR protein level is much lower in BCC than in HaCaT cells. In this project we further investigate why BCC cells were non-responsive to B[a]P. We found that (1) BCC cells were less differentiated than HaCaT cells; (2) AhR protein was unstable and sensitive to ultrasonication; (3) Treatment with genistein and herbimycin A increased CYP1A1 inducibility in BCC cells. Therefore, certain protein kinases may overactivated in BCC cells, which interfere CYP1A1 inducibility; (4) Treatment with B[a]P decreased cytosolic AhR levels. Thus ligand (B[a]P) binding may enhance AhR protein degradation. All these factors may contribute to the non-responsiveness to B[a]P in BCC cells.
