Evaluation of the genotoxic effects of thyroxine using in vivo cytogenetic test on Swiss albino mice

Abstract

Thyroid hormones enhance aerobic metabolism favoring oxidative stress which may lead to covalent damage of various molecules including DNA. Previous investigations revealed that thyroid hormones induce DNA damage on human lymphocytes and sperm in the in vitro Comet assay. However, cytogenetic evaluation of genotoxic effects of thyroxine gave equivocal results: increase of sister chromatid exchanges, and no incerase of micronuclei in cultured human lymphocytes. Therefore, the aim of the present study was to further evaluate the possible genotoxic effects of thyroxine using in vivo cytogenetic test on Swiss albino mice. Three experimental concentrations of thyroxine were used (0.1 mg/kg, 0.5 mg/kg and 2.5 mg/kg). The mice were divided into several groups depending on the duration of the treatment with thyroxine. Thus, we treated mice for 1, 3, 7 and 10 days. Positive (Nmethyl- N'-nitro-N-nitrosoguanidine) and negative controls were also formed for the same time periods. Cytogenetic endpoinds (numerical and structural aberrations, chormosome gaps and breaks) were analysed in bone marrow cells from femures. The results obtained in this investigation showed that thyroxine has not induced chromosome damage or aberrations. This is in agreement with our previous analysis of micronuclei in human peripheral blood lymophocytes treated with thyroxine. On the other hand, we observed a decrease of mitotic index especially in animals treated for a longer period of time with the highest dose of thyroxine. Therefore, it can be concluded that thyroxine does not induce genotoxic effects which could be detected by cytogenetic analysis.

Tireoidni hormoni podstiču aerobni metabolizam favorizujući oksidativni stres koji može da dovede do kovalentnih oštećenja različitih molekula uključujući i DNK. U prethodnim istraživanjima otkriveno je da tireoidni hormoni indukuju oštećenja molekula DNK u humanim limfocitima i spermi u in vitro Komet testu. Međutim, citogenetička evaluacija genotoksičnih efekata tiroksina dala je kontradiktorne rezultate: povećanje razmena sestrinskih hromatida bez porasta učesalosti mikronukleusa u kulturama humanih limfocita. Stoga je cilj istraživanja u ovom radu bio da dodatno ispitamo moguće genotoksične efekte tiroksina koristeć i in vitro citogenetički test na Swiss albino miševima. Upotrebljene su tri eksperimentalne koncentracije tiroksina (0,1 mg/kg, 0,5 mg/kg and 2.5 mg/kg). Miševi su podeljeni u nekoliko grupa zavisno od dužine tretmana tiroksinom: 1, 3, 7 i 10 dana. U istim vremenskim periodima miševi su tretirani pozitivnom (N-metil-N'-nitro-N-nitrozogvanidin) i negativnom kontrolom. Analizirani su citogenetički parametri (numeričke i strukturne aberacije hromozoma, gapovi i prekidi na hromozomima) u ćelijama kostne srži izolovanim iz femura. Rezultati dobijeni u ovom istraživanju ukazuju da tiroksin ne indukuje hromozomske prekide i aberacije, što je u saglasnosti sa našim prethodnim zapažanjima na humanim limfocitima u kulturi. Istovremeno, primetili smo smanjenje mitotskog indeksa, naročito kod životinja tretiranih u dužem vremenskom periodu sa visokim dozama tiroksina. Prema tome, može se zaključiti da tiroksin ne indukuje genotoksične efekte koji mogu da se detektuju citogenetičkim analizama.