High-Mobility Group Boxes Mediate Cell Proliferation and Radiosensitivity via Retinoblastoma-Interaction-Dependent and -Independent Mechanisms

Abstract

Our previous studies have shown that high-mobility group box 1 (HMGB1) could physically associate with the retinoblastoma (RB) protein via an LXCXE (leucine-X-cysteine-X-glutamic; X=any amino acid) motif. An identical LXCXE motif is present in the HMGB1–3 protein sequences, whereas a near-consensus LXCXD (leucine-X-cysteine-X-asparagine; X=any amino acid) motif is found in the HMGB4 protein. In this study, we have demonstrated that like HMGB1, HMGB2–3 also associated with the RB in vitro and in vivo, as evidenced by glutathione-s-transferase capture and immunoprecipitation–Western blot assays. A point mutation of the LXCXE or LXCXD motif led to disruption of RB:HMGB1–4 interactions. Enforced expression of HMGB1–3 or HMGB4 by adenoviral-vector-mediated gene transfer resulted in significant inhibition of breast cancer cell proliferation through an LXCXE- or LXCXD-dependent mechanism and an increased radiosensitivity through an LXCXE- or LXCXD-independent mechanism. These results suggest an important role of the LXCXE/D motif in RB:HMGB1–4 association and modulation of cancer cell growth, but not radiosensitivity.