Cytoplasmic sequestration of the tumor suppressor p53 by a heat shock protein 70 family member, mortalin, in human colorectal adenocarcinoma cell lines

Abstract

While it is known that cytoplasmic retention of p53 occurs in many solid tumors, the mechanisms responsible for this retention have not been positively identified. Since heatshock proteins like mortalin have been associated with p53 inactivation in other tumors, the current study sought to characterize this potential interaction in never before examined colorectal adenocarcinoma cell lines. Six cell lines, one with 3 different fractions, were examined to determine expression of p53 and mortalin and characterize their cellular localization. Most of these cell lines displayed punctate p53 and mortalin localization in the cell cytoplasm with the exception of HCT-8 and HCT116 379.2 cells, where p53 was not detected. Nuclear p53 was only observed in HCT-116 40-16, LS123, and HT-29 cell lines. Mortalin was only localized in the cytoplasm in all cell lines. Co-immunoprecipitation and immunohistochemistry revealed that p53 and mortalin were bound and co-localized in the cytoplasmic fraction of four cell lines, HCT-116 (40-16 and 386; parental and heterozygous fractions respectively of the same cell line), HT-29, LS123 and LoVo, implying that p53 nuclear function is limited in those cell lines by being restricted to the cytoplasm. Mortalin gene expression levels were higher than gene expression levels of p53 in all cell lines. Cell lines with cytoplasmic sequestration of p53, however, also displayed elevated p53 gene expression levels compared to cell lines without p53 sequestration. Our data reveal the characteristic cytoplasmic sequestration of p53 by the heat shock protein mortalin in human colorectal adenocarcinoma cell lines, as is the case for other cancers, such as glioblastomas and hepatocellular carcinomas.