Persistent γ-H2AX Formation and Expression of Stem Cell Markers in N-Butyl-N-(4-Hydroxybutyl)Nitrosamine-Induced Bladder Carcinogenesis in Rats

Abstract

We investigated γ-H2AX formation, a biomarker of DNA damage, and expression of stem cell markers (SCMs), including cytokeratin 14, aldehyde dehydrogenase 1A1 (ALDH1A1), and CD44, in the development of rat bladder tumors induced by short-term administration of N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN). Histopathological examination showed that diffuse simple hyperplasia of the bladder urothelium induced by BBN recovered to the normal-appearing urothelium after withdrawal, whereas focal proliferative lesions were newly developed and subsequently progressed to benign papilloma and carcinoma. Immunohistochemical analysis revealed that BBN-induced γ-H2AX formation and ALDH1A1 and CD44 expression persisted at higher levels in the normal-appearing urothelium than those in the control group for long periods after withdrawal. Since persistent chronic inflammation was observed even after withdrawal, targeted gene expression analysis of inflammation-related factors revealed 101 genes, including Stat3 and Myc, that showed persistent high expression. Pathway analysis suggested that Stat3 and/or Myc activation may be associated with SCM expression. We focused on hepatocyte growth factor (Hgf), one of the genes predicted in relation to Stat3/Myc, and confirmed that HGF-positive cells increased by BBN persisted in the normal-appearing urothelium after withdrawal and colocalized with γ-H2AX and SCMs. These results suggested that the long-term persistence of γ-H2AX formation and SCM expression, which occurred during the early stages of bladder tumorigenesis, is not a transient response to exposure and might contribute to bladder tumorigenesis. Although further studies are needed, BBN-induced rat bladder tumors may originate from focal hyperplasia arising from SCM-positive cells via activation of the STAT3/MYC pathway after DNA damage involving γ-H2AX formation.

Keywords: N-butyl-N-(4-hydroxybutyl)nitrosamine; carcinogenicity; stem cell marker; urinary bladder; γ-H2AX.