Heat shock proteins stimulate APOBEC-3-mediated cytidine deamination in the hepatitis B virus.

Apolipoprotein B mRNA-editing enzyme catalytic subunit 3 (APOBEC-3) enzymes are cytidine deaminases that are broadly and constitutively expressed. They are often up-regulated during carcinogenesis and candidate genes for causing the major single-base substitution in cancer-associated DNA mutations. Moreover, APOBEC-3s are involved in host innate immunity against many viruses. However, how APOBEC-3 mutational activity is regulated in normal and pathological conditions remains largely unknown. Heat shock protein levels are often elevated in both carcinogenesis and viral infection and are associated with DNA mutations. Here, using mutational analyses of hepatitis B virus (HBV), we found that Hsp90 stimulates deamination activity of APOBEC-3G (A3G), A3B, and A3C during co-expression in human liver HepG2 cells. Hsp90 directly stimulated A3G deamination activity when the purified proteins were used in in vitro reactions. Hsp40, -60, and -70 also had variable stimulatory effects in the cellular assay, but not in vitro Sequencing analyses further demonstrated that Hsp90 increased both A3G cytosine mutation efficiency on HBV DNA and total HBV mutation frequency. In addition, Hsp90 shifted A3G's cytosine region selection in HBV DNA and increased A3G's 5' nucleoside preference for deoxycytidine (5'-CC). Furthermore, the Hsp90 inhibitor 17-N-allylamino-17-demethoxygeldanamycin dose dependently inhibited A3G and A3B mutational activity on HBV viral DNA. Hsp90 knockdown by siRNA or by Hsp90 active-site mutation also decreased A3G activity. These results indicate that heat shock proteins, in particular Hsp90, stimulate APOBEC-3-mediated DNA deamination activity, suggesting a potential physiological role in carcinogenesis and viral innate immunity.