Cell Apoptosis And Apoptosis Related Genes In The Tumor Promotion Process In The Rat Hepatoma

Study of apoptosis and apoptosis-related gene expressinin rat liver tumor promotionAbstractCarcinogenesis involves inutiple stages that have distinct biological features. The process is operationally divided into stages termed initiation, promotion and progression. Carcinogenesis is a complex multistage process in which normal cellular growth processes and genes become altered. Chemical carcinogens may act by modulating or inducing mutation in genes that control normal growth, with subsequent clonal growth of the resulting precancerous or cancerous cells.Many carcinogens classified as non-genotoxic carcinogens do not interact directly with DNA and are generally negative in vitro and in vivo tests of genotoxicity, they acted as tumor promoter.The aim of this study was to duplicate a rat liver initiaing-model by DEN-initiating, hepatectomy-stimulating and 2-AAF-selectively supressing, and followed by establised the rat liver long-term promoting model, to study the effects of tumor promoter PB on apoptosis of rat liver precancerous foci and expression levels of apoptosis-related genes such as bcl-2, bax, p53, Fas, FasL,TGF-13 land TI3R I, TI3RIJ.The main results are as the followings:1. We found that preneoplastic cell foci in rat liver showed high rates of apoptosis, which partially antagonized the effect of cell replication in these lesions. Tumor promoters such as PB inhibited apoptosis, thereby accelerating growth of preneoplastic cell foci and cancer development.On the other hand, tumor promoter withdrawal resulted in increase of apoptosis in prenioplastic foci, suggesting the possibility of regression of tumor pormotion in the liver.These findings show that tumor promoters act as survival factors forpreneoplastic cells. SuPpressing of apoptosis plays an important role in tumorpromotion since diSrUPtion of dPoPtosis processes would result in the survivaland oatgrowth of damaged of initiated cells.2.The results showed that: Bcl-2 protein level increased during high-dose(500mg/kg) and middle edose (l00mg/kg) PB treatment detected byimmunohistOchemical assay. The results detected by ISH and RT-PCR indicatedthat of bc1-2 mRNA level was similar to protein level. Bax protein decreasedduring high-dose and middle-dose PB administration detected byimmunohistochemical assap which was similar to bax mRNA level measuredby ISH and RT-PCR. It is well known that the tumour suppressor gene p53 isresponse to DNA-damage, its protein product blocks cells in the G1-phase of thecell cycle. This gives cells additional time to repair their DNA-damage. Ourresults showed that the m[ltant tyPe p53 (mtp53 ) was not observed in normal ratlivef, but observed in DEN-initiating grouPs. mtp53 protein level increasedwith high-dose and middle-dose PB treatment. On the other hand,wtP53mRNA level increased with carciongen DEN treatment compared tocontrol grouP, wherease wtP53 mRNA level was declined along with tumorpromoter PB treatmellt.The results suggested that p53 may play an importamrole in Promoting Stage of experimental hepatocarcinogenesis. Tumor promoterPB may induce mtP53 expression and inhibit exprssion of wtp53, which resultsin inhibiting aPoptosis and cell cycle arrest, thereby may favor clonalexpansion of preneoplastic hepatocytes in rats after administration of tumorpromoter PB. Meanwhi1e, inhibition of aPoptosis was related touPregulation of anti-aPoptosis gene bcl-2 and downregulation of pro-aPoptosisgene bax. It was suggested that this aPoptotic pathway may be impaired inpreneoplastic cells. It seems that bcl-2 protCin mediates the suppression ofapoPtosis in Preneoplastic cells durng treatment with tumor promoter PB.lVDown-regulation of bcl-2 expression after withdrawal of PB suPports this idea,which suggests increased exPression of bcl-2 involve in suppression ofaPoptosis.3. It was showed that Fas eXPression level in all initiatd groups (high-dose,middle-dose, low-dose and initiating grouP) was hi...