HDAC Class I Inhibition Acetylates a Non-Histone Protein STAT3 by Modulating p300-STAT3-HDAC1 Interaction In Activated B- Cell Like (ABC) Diffuse Large B Cell Lymphoma

Abstract 115 Patients with diffuse large B- cell lymphoma (DLBCL) tumors that have an activated B-cell like (ABC) gene expression profile have a poorer prognosis. Understanding the mechanism(s) used by ABC tumor cells to resist the effects of common chemotherapy agents may lead to alternative approaches for the treatment of these tumors. ABC cell lines have been shown to have high levels of phosphorylated STAT3 (pSTAT3); however, the mechanisms that regulate STAT3 signaling in ABC DLBCL remain unclear. Histone deacetylases (HDACs) are enzymes that can deacetylate both non-histone and histone substrates. In this study we tested the hypothesis that HDACs in the tumor cells target a non-histone protein STAT3 in ABC DLBCL. In studies of HDAC expression in DLBCL tumors, we found over-expression of the type 1 HDACs, specifically HDAC1and HDAC3, in the pSTAT3- positive ABC tumors as compared to germinal centre B like (GCB) tumors. We then performed a co-immunoprecipitation (Co-IP) assay to learn the functional interaction between STAT3 and HDAC1. We found that STAT3 formed complexes with HDAC1 or HDAC3. Further Co-IP studies demonstrated that p300, a histone acetyltransferase (HAT), STAT3, and HDAC1 are all in the same complex. To determine whether p300 acetylates STAT3 in ABC cells, we immuno-precipitated endogenous p300 and blotted with acetylated STAT3 and showed that p300 acetylates STAT3 at lysine 685. We next tested whether HDAC inhibition could affect p300 mediated STAT3 acetylation in ABC cells. Inhibition of HDAC activity through the HDAC inhibitor LBH589 (LBH, Novartis Pharmaceuticals) increased STAT3 acetylation in a dose- dependent manner. Similar results were obtained when we used antiacetyl- lysine antibody. Furthermore HDAC1 over-expression inhibits STAT3 acetylation at lysine 685. This data implies a tight regulation of STAT3 acetylation and deacetylases in vivo in ABC lymphoma. In addition to acetylation, STAT3 can be modified by phosphorylation, thus the effect of HDAC inhibition on pSTAT3 both at serine and tyrosine residues was studied. We observed a dose-dependent decrease in pSTAT3 with some inhibitory effect on total STAT3. LBH was found to mediate STAT3 dephosphorylation by inhibiting the tyrosine phosphorylation of JAK2 and TYK2, the STAT3 upstream activators, in a dose- dependent manner. Since ABC lymphoma has higher levels of HDAC1 or HDAC3 and pSTAT3/STAT3 than GCB, we hypothesized that ABC cells will be more sensitive to HDAC inhibition than GCB. In fact, when ABC and GCB DLBCL cells were treated with LBH we observed that LBH was more cytotoxic to ABC than GCB as evidenced by annexin/PI staining and PARP cleavage. LD90 was 25 nM for ABC cells, however GCB cells required 5 times more LBH to kill 90% cells. STAT3 activation regulates genes involved in cell survival, including Bcl-2, Mcl-1, Bcl-XL, and c-Myc. LBH treatment resulted in down-regulation of Mcl-1 and c-Myc in ABC cells but has no effect in GCB cells; however, Bcl-2 and Bcl-XL levels were not decreased in both the subtype. Having established that HDAC1 physically associated with STAT3 and that LBH treatment elevated STAT3 acetylation in ABC cells, we proceeded to deplete endogenous HDAC1 with siRNA in Ly3 cells and found that HDAC1 knockdown up-regulated STAT3 acetylation indicating that HDAC1 negatively regulates the acetylation in vivo. HDAC1 inhibition also prevented phopshorylation of STAT3 and induces aopotosis in ABC cells. In summary, we have demonstrated that a key consequence of HATs and HDACs expression and activity is modulation of the STAT3 pathway in ABC lymphoma. Inhibition of this pathway with the HDAC inhibitor LBH inhibits constitutive STAT3 signaling and induces Mcl-1 mediated apoptosis. These studies provide the rationale for targeting the poorly responsive ABC-type DLBCL by inhibiting HDAC activity with epigenetic inhibitors such as LBH. We are currently testing LBH589 in relapsed DLBCL in a phase I clinical trial. Disclosures: No relevant conflicts of interest to declare.