The influence of pentoxifylline on damage responses in tumour cells

Theron, Catherina S (2000-04)

Thesis (PhD)--Stellenbosch University, 2000.

Thesis

ENGLISH ABSTRACT: Pentoxifylline enhances the toxicity of radiation and has emerged as an effective modulator of the radiation response of tumour cells. The molecular mechanisms involved in the enhancement of radiotoxicity by pentoxifylline have not yet been elucidated. Cell cycle blocks, DNA repair and programmed cell death (apoptosis) are all pert of the cellular response to DNA damage and as such must be considered as targets of the drug. In this study, the influence of pentoxifylline on radiosensitisation, G2 block abrogation, DNA repair inhibition and the induction of apoptosis have been investigated in 8e11 and MeWo melanoma and 4197 and 4451 squamous cell carcinoma (SCC) cell lines. The influence of pentoxifylline on radiation-induced apoptosis in Jurkat J5 T-lymphocytic leukemia cells has also been assessed. Hela cervical carcinoma cells were used to investigate the molecular events involved in the abrogation of the G2 block by pentoxifylline. It is shown that pentoxifylline preferentially sensitises the TP53 mutant MeWo and 4451 cell lines and enhances radiotoxicity by factors of up to 14.5. In the MeWo melanoma, but not in the 4451 SCC cell line, radiosensitisation is accompanied by inhibition of DNA repair. No significant enhancement of radiation-induced apoptosis was observed in MeWo melanoma and 4451 SCC cells. However, Jurkat J5 cells showed an increase in apoptosis after irradiation in the presence of the drug. In irradiated Hela cervical carcinoma cells, pentoxifylline affects the expression of the two components of the mitosis promoting factor (MPF), namely cyclin 81 and p34cdC2, and rapidly restores cyclin 81/p34cdC2 ratios to control levels. Analysis of cyclin 81 expression in whole cells and isolated nuclei furthermore reveals an influence of the drug on the subcellular translocation of the MPF. It is concluded that G2 block abrogation is not the only mechanism involved in the radiosensitisation of tumour cells by pentoxifylline, but that DNA repair inhibition plays a role in certain cell types. Although pentoxifylline induces apoptosis in Jurkat J5 thymocytes, radiation-induced apoptosis plays no role in the radiosensitisation of the two TP53 mutant melanoma and sec cell lines. Abrogation of the G2 block by pentoxifylline, which sensitises tumour cells to a second irradiation or chemotherapeutic challenge, involves a modulation of the levels of cyclin 81 and p34cdC2, and the subcellular location of the MPF. These results are of utmost importance for the clinical potential of pentoxifylline as a dose modifier in cancer therapy.

AFRIKAANSE OPSOMMING: Pentoxifylline verhoog die toksisiteit van bestraling en het dus na vore getree as 'n effektiewe modulator van die sellulêre stralingsrespons in kankerselle. Die molekulêre meganismes betrokke by die verhoging van stralingstoksisiteit deur pentoxifylline is egter nog nie duidelik nie. Blokkering van die selsiklus, die herstel van ONS skade en geprogrammeerde seldood (apoptose) vorm almal deel van die sellulêre respons ná bestraling en word as sulks beskou as potensiële teikens van die middel. In hierdie studie is die invloed van pentoxifylline op stralings-sensitiwiteit, G2 blok verwydering, die vertraging van ONS herstel en die indusering van apoptose ondersoek in die Be11 en MeWo melanoom en 4197 en 4451 plaveisel-sel karsinoom sellyne. Die invloed van pentoxifylline op stralings-geïnduseerde apoptose in Jurkat J5 T-limfosiete is ook bestudeer. Hela servikale karsinoom selle is gebruik om die molekulêre gebeurtenisse rondom die verwydering van die G2 blok deur pentoxifylline te ondersoek. Dit word aangetoon dat pentoxifylline by voorkeur die radiosensitiwiteit van die TP53 mutante MeWo en 4451 sellyne verhoog, en stralingstoksisiteits verhogingsfaktore van tot 14.5 genereer. Hierdie effek gaan gepaard met die vertraging van ONS herstel in die MeWo melanoom, maar nie in die 4451 plaveisel-sel karsinoom sellyn nie. Die meting van apoptose toon geen noemenswaardige verhoging van stralings-geïnduseerde apoptose in MeWo melanoom óf in 4451 plaveisel-sel karsinoom selle nie. Jurkat J5 T-limfosiete toon egter wel 'n verhoging in apoptose wanneer bestraling in die teenwoordigheid van pentoxifylline gedoen word. In Hela servikale karsinoom selle affekteer pentoxifylline die uitdrukking van die twee komponente van die mitose promoverings faktor (MPF), naamlik siklien B1 en p34CdC2 , en restoreer die siklien 81/p34cdC2 verhoudings vinnig na normale vlakke. Ontleding van die siklien 81 uitdrukking in heel selle en in geïsoleerde selkerne toon verder dat die middelook die sub-sellulêre ligging van die MPF affekteer. Die gevolgtrekking word gemaak dat G2 blok verwydering nie die enigste meganisme is waardeur pentoxifylline radiosensitiwiteit verhoog nie, maar dat die vertraging van ONS herstel in sommige seltipes 'n rol speel. Alhoewel pentoxifylline apoptose verhoog in T-limfosiete, speel dit nie 'n rol in die verhoogde radiotoksisiteit wat waargeneem is in die TP53 mutante melanoom en plaveisel-sel karsinoom sellyne nie. Verwydering van die G2 blok deur pentoxifylline, wat selle meer sensitief kan maak vir 'n tweede stralings- of chemoterapie aanslag, behels die modulasie van siklien 81 en p34cdc2 vlakke en die sub-sellulêre ligging van die MPF. Hierdie resultate is van uiterste belang vir die kliniese aanwending van pentoxifylline as 'n dosis-modifiseerder in kankerterapie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/51987
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