Solubility shift and SUMOylaltion of promyelocytic leukemia (PML) protein in response to arsenic(III) and fate of the SUMOylated PML

Abstract

Promyelocytic leukemia (PML), which is a tumor suppressor protein that nevertheless plays an important role in the maintenance of leukemia initiating cells, is known to be biochemically modified by As(3+). We recently developed a simple method to evaluate the modification of PML by As(3+) resulting in a change in solubility and the covalent binding of small ubiquitin-like modifier (SUMO). Here we semi-quantitatively investigated the SUMOylation of PML using HEK293 cells which were stably transfected with PML-VI (HEK-PML). Western blot analyses indicated that PML became insoluble in cold RadioImmunoPrecipitation Assay (RIPA) lysis buffer and was SUMOylated by both SUMO2/3 and SUMO1 by As(3+). Surprisingly SUMO1 monomers were completely utilized for the SUMOylation of PML. Antimony (Sb(3+)) but not bismuth (Bi(3+)), Cu(2+), or Cd(2+) biochemically modified PML similarly. SUMOylated PML decreased after removal of As(3+) from the culture medium. However, unSUMOylated PML was still recovered in the RIPA-insoluble fraction, suggesting that SUMOylation is not requisite for changing the RIPA-soluble PML into the RIPA-insoluble form. Immunofluorescence staining of As(3+)-exposed cells indicated that SUMO2/3 was co-localized with PML in the nuclear bodies. However, some PML protein was present in peri-nuclear regions without SUMO2/3. Functional Really Interesting New Gene (RING)-deleted mutant PML neither formed PML nuclear bodies nor was biochemically modified by As(3+). Conjugation with intracellular glutathione may explain the accessibility of As(3+) and Sb(3+) to PML in the nuclear region evading chelation and entrapping by cytoplasmic proteins such as metallothioneins.

Keywords: Arsenic; Immunofluorescence; Metalloid; Promyelocytic leukemia; SUMO.