Avicin D: A Protein Reactive Plant Isoprenoid Dephosphorylates Stat 3 by Regulating Both Kinase and Phosphatase Activities

Valsala Haridas,Zbigniew Walaszek,Jordan U Gutterman,Fiona Connolly,Margaret Hanausek,Goshi Nishimura,Robert Zoltaszek,Zhi-Xiang Xu,Joseph Alan Bauer

doi:10.1371/journal.pone.0005578

Abstract

Avicins, a class of electrophilic triterpenoids with pro-apoptotic, anti-inflammatory and antioxidant properties, have been shown to induce redox-dependant post-translational modification of cysteine residues to regulate protein function. Based on (a) the cross-talk that occurs between redox and phosphorylation processes, and (b) the role of Stat3 in the process of apoptosis and carcinogenesis, we chose to study the effects of avicins on the processes of phosphorylation/dephosphorylation in Stat3. Avicins dephosphorylate Stat3 in a variety of human tumor cell lines, leading to a decrease in the transcriptional activity of Stat3. The expression of Stat3-regulated proteins such as c-myc, cyclin D1, Bcl2, survivin and VEGF were reduced in response to avicin treatment. Underlying avicin-induced dephosphorylation of Stat3 was dephosphorylation of JAKs, as well as activation of protein phosphatase-1. Downregulation of both Stat3 activity and expression of Stat 3-controlled pro-survival proteins, contributes to the induction of apoptosis in avicin treated tumor cells. Based on the role of Stat3 in inflammation and wounding, and the in vivo inhibition of VEGF by avicins in a mouse skin carcinogenesis model, it is likely that avicin-induced inhibition of Stat3 activity results in the suppression of the pro-inflammatory and pro-oxidant stromal environment of tumors. Activation of PP-1, which also acts as a cellular economizer, combined with the redox regulation by avicins, can aid in redirecting metabolism from growth promoting anabolic to energy sparing pathways.

Highlights

Coincident with the identification of the human genome, as well as increased understanding of gene networks, a quiet renaissance is occurring in the development of natural products as drug candidates [1]
We evaluated the optimal dose of avicin D required for dephosphorylation of both constitutive and IL-6 induced Stat3 in
We studied the effect of avicin D on the localization of both constitutive and IL-6 induced Stat3 in U266 (Fig. 3A) and RPMI-8226 (Fig. 3B) cells

Summary

Introduction

Coincident with the identification of the human genome, as well as increased understanding of gene networks, a quiet renaissance is occurring in the development of natural products as drug candidates [1]. The ability of avicins to interact with, and modify cysteine residues was first demonstrated in a bacterial system with OxyR as a target, wherein we demonstrated that the distal portion of the avicin side chain formed a reversible and covalent thioester bond with the critical cysteine (SH) on the OxyR molecule [10]. This protein modification, termed avicinylation, suggested to us that avicins can induce posttranslational changes in proteins to regulate their function

Methods

Results

Conclusion