Effects of short-term methylmercury exposure on metallothionein mRNA levels in the testis and epididymis of the rat.

Abstract

Methylmercury (MeHg) is a widespread environmental contaminant that causes reproductive dysfunction in men. Metallothioneins (MTs) are low-molecular-weight proteins that can bind heavy metals and protect the cell from metal toxicity. MT levels are increased by exposure to metals and physiological stressors. Although MTs have been identified in the testis and epididymis, little is known about their distribution and regulation in the epididymis or the effects of MeHg on MT levels in male reproductive tissues. The objective of this study was to determine whether MT I, II, and III mRNA are present in the epididymis, if their relative levels differ between epididymal segments, and if MeHg alters cellular mRNA levels for MT I, II, and III in the testis and epididymal segments of the rat. Northern blot analysis was done on total cellular RNA isolated from each of the four epididymal segments (initial segment [IS], caput [CT], corpus [CS], and cauda [CA] epididymidis) using a cDNA probe for MT I and MT II. MT I transcripts were present in all epididymal segments. The lowest mRNA levels were observed in the IS; these levels were 4-fold less than in the CT and CS and 5.5-fold less than in the CA. MT II mRNA levels were similar in the IS and CT but were eightfold higher in the CS and CA. A cDNA probe for MT III was generated by reverse transcription-polymerase chain reaction using testicular RNA. MT III mRNA was detected only in the IS and CT and not in the CS and CA. To assess whether exposure to MeHg alters MT mRNA levels, rats were exposed for 14 days to one of five MeHg doses (0, 25, 50, 100, and 200 [microg/kg/day] via a subdermal osmotic pump. No changes were observed in either body weight or in the weights of the testis, epididymis, seminal vesicles, or ventral prostate between MeHg-treated and control rats. Serum testosterone levels were significantly decreased only at the highest MeHg dose. In the testis, MeHg treatment resulted in 2.5- to 7-fold increases in MT I mRNA levels. There were no changes in either MT II or MT III mRNA levels. In the initial segment of the epididymis, MT I mRNA levels were significantly increased only at the 50 microg/kg/ day dose, whereas there were no significant differences in MT II mRNA levels. In the caput epididymis, MT I mRNA levels were significantly lower at the 50 and 100 microg/kg/day dose. MT II mRNA levels were also lower, with the exception of the 50 microg/kg/day dose. Although MT III mRNA levels were lower at the two lower doses, levels were not different from controls in the two highest doses tested. In the corpus epididymidis, MeHg did not alter MT I mRNA levels, and MT II was higher only in the 50 microg/kg/day group. In the cauda epididymidis, MT I mRNA levels were decreased in a dose-dependent manner by up to 63%. MT II levels were unaltered. Together these data indicate that exposure of adult rats to MeHg can modulate MT mRNA levels in both the testis and epididymal segments. Furthermore, changes in MT mRNA levels following exposure to MeHg differ between epididymal segments, suggesting either differences in MeHg accumulation or differences in MT modulation.