A variety of studies indicate that the TGF-α plays an important role in the modulation of differentiation and proliferation cells. Moreover, altered cellular responsiveness to growth factors is one of the factors involved in carcinogenesis. Epidemiological studies had shown that humans develop tumors in urinary bladder after chronic drinking water iAs exposure (Chiou et al., 1995
; Hopenhayn-Rich et al., 1996
; Steinmaus et al., 2006
The goal of this study was to characterize the urinary pattern of iAs metabolites and their relationship with concentration of TGF-α in BUC of people from an arsenic-endemic area. A positive linear regression between urinary TAs and TGF-α concentration in BUC was observed (). Additionally, we found a statistically significant difference between the low and high As exposure groups in which we observed higher concentrations of TGF-α in BUC from the high exposure group than in the low exposure group (). The association between TGF-α levels and urinary As was also observed by Do et al. (2001)
, in a population of an iAs endemic area of Bangladesh; although in this study the TGF-α levels in urine were evaluated.
Moreover, the present study shows that individuals with skin lesions, which have long been known to be the hallmark signs of chronic As exposure, present higher concentration of TGF-α in the urothelial cells (). Recent evidence suggests that the development of skin lesions from arsenic exposure may be mediated by increases in the expression of various growth factors, including TGF-α (Hsu et al., 2006
). This may suggest that individuals with skin lesions are more sensitive to present alterations in urothelial cells. This association resembled those found in skin samples obtained from residents of Taiwan, in where Germolec et al. (1998)
observed that TGF-α mRNA transcripts were highly expressed in hyperpigmentation and hyperkeratosis areas in arsenic-exposed individuals. Elevated levels of TGF-α may play an essential role in mitogenic stimulation during tumor promotion by diverse mechanisms.
It is difficult to explain why in our study we observed a better correlation of TGF-α with pentavalent metabolites rather than trivalent arsenic species (Tables and ), which had been suggested to be the more toxic species in the arsenic metabolism (Styblo et al., 2000
). However, it is important to consider that trivalent methylated arsenic metabolites are mainly accumulated in the urothelial cells (Drobna et al., 2005
), so this can increase their toxicity whereas pentavalent forms are readily excreted. Further studies including the analysis of As species in BUC may provide more information for risk analysis of diseases associated with iAs exposure.
This study has several methodological advantages, including individual exposure assessment, and inclusion of all arsenic metabolites in human urine in order to determine whether a particular urinary metabolite is associated with increased TGF-α levels in BUC from individuals chronically exposed to iAs. The significant associations among arsenic exposure with TGF-α levels in BUC support, but do not specifically define causality. Importantly in this study, we found multivariate associations among all the urinary As species (trivalent and pentavalent species) with TGF-α levels in BUC after adjusting by age, in which we observed a strong correlation only in the models of the As skin lesions group, but not in the non-As skin lesions group (). These results suggest that the induction of TGF-α concentrations in BUC by As species is only present in people who are susceptible to present the toxic effects after chronic As exposure. Therefore, the presence of TGF-α in exfoliated epithelial cells could be used as an early marker of human susceptibility before the development of the classical arseniasis associated with chronic iAs exposure. Furthermore, this hypothesis is consistent with the finding that the TWE to iAs is an indicator of the effects of chronic exposure to iAs that is closely linked to the hyper-hypopigmentation and keratosis but not associated with the increased TGF-α concentrations in BUC.
In the multivariate analyses we observed the relation between age and TGF-α concentrations in BUC. Because of the small number of subjects, this association was not significant. Moreover, smoking is one of the principal confounding factors in the development of bladder cancer (Di Menza et al., 1992
; Murata et al., 1996
). Because our study population included only a small number of smokers, we were unable to estimate the association between TGF-α concentrations and smoking habits. An additional limitation of this study was that we cannot evaluate the difference in response of TGF-α and urinary arsenic metabolites between genders, because this study only includes women. However, in the literature it has been reported that women had more risk of bladder cancer than men following arsenic exposure (Chen et al., 1992
Another important confounding factor that had been reported to increase the TGF-α expression is the presence of urinary infection (Tungekar and Linehan, 1998
). In our studies this was evaluated as a categorical variable measuring the presence of leukocytes in urine by a semiquantitative technique. As a result, we adjusted for this factor in multivariate analyses that examined the association between the urinary arsenical concentrations and TGF-α. The presence of inflammatory urologic diseases and urothelial carcinoma has also been associated with increased TGF-α levels in the urine (Chow et al., 1998
), emphasizing need for this adjustment.
In previous studies, we observed the patterns of methylated metabolites in people exposed to iAs, and noted that individuals with skin lesions have higher urinary concentrations of MAsIII
, despite the fact that DMAsIII
was the major metabolite present in the urine (49%) (Valenzuela et al., 2005
). It has been documented that the trivalent methylated species are the most toxic metabolites in the arsenic pathway in several target tissues (Mass et al., 2001
; Styblo et al., 2000
). In this work we observed that people exposed to iAs have increased concentrations of TGF-α in BUC, and that these significantly higher levels of TGF-α showed a positive and linear association with urinary TAs. After adjusting by age, alcohol consumption, and urinary infection we found an association between TGF-α in BUC and urinary levels of MAsIII
, which have higher uptakes and retention in urothelial cells.
Interestingly, statistically significant differences were found between the group with and without skin lesions, with a stronger association in the presence of melanosis. One problem, common in the interpretation of data from cross-sectional studies, is that the exposure is measured at the same time as the effects, which may not be the etiologically relevant period. This may be problematic for TGF-α concentrations in urothelial cells, because it largely reflects recent exposures. This is not the case for arsenic-induced skin lesions, which are a good estimate of the chronic arsenic exposure over decades. Our data suggest that TGF-α in BUC of individuals exposed to iAs can be used as an early human biomarker of the toxicity of this metalloid in bladder epithelium, and may be useful to detect susceptible individuals prior to the development of bladder cancer.