Down‐regulation of myeloid cell leukemia 1 by epigallocatechin‐3‐gallate sensitizes rheumatoid arthritis synovial fibroblasts to tumor necrosis factor α–induced apoptosis

Apoptosis, or programmed cell death, is a process in which the activation of death receptors or mitochondrial-mediated signaling cascades is initiated in a tightly regulated manner to limit unwanted cell growth (1). In chronic human diseases such as cancer or rheumatoid arthritis (RA), increased expression of antiapoptotic proteins, such as members of the Bcl-2 family, commonly occurs and is associated with disease progression, resistance to therapies, and poor clinical outcome (1,2). Interestingly, among Bcl-2 family members, myeloid cell leukemia 1 (Mcl-1) represents a critical determinant of sensitivity and resistance to apoptosis (2). Cells expressing high levels of Mcl-1 are found to be resistant to a newly developed Bcl-2/Bcl-xL/Bcl-w inhibitor, suggesting the pivotal role of Mcl-1 in suppressing apoptosis (3). This notion was further authenticated by several studies in which knockdown of Mcl-1 dramatically increased drug sensitivity of several tumor cell lines (2). RA is an inflammatory disorder characterized by hyperplasia of the synovial joint lining, which invades and degrades cartilage and underlying bone, ultimately resulting in joint destruction (4). Studies of the mechanisms of RA pathogenesis have established an essential role for RA synovial fibroblasts in arthritis initiation and progression (5). Although the exact molecular mechanisms regulating synovial hyperplasia are presently unclear, accumulating evidence suggests that RA synovial fibroblasts are resistant to both tumor necrosis factor α (TNFα)– and Fas-mediated apoptosis (6). Recent observations at the cellular level have implicated aberrant expression of Mcl-1 in the resistance of RA synovial fibroblasts to TNFα- and Fas-mediated apoptosis (7,8). Furthermore, constitutive activation of survival protein Akt and transcription factor NF-κB in RA synovial fibroblasts suggests the importance of these molecules in RA synovial fibroblast hyperproliferation and invasion into underlying tissue (9,10). Thus, the purposeful induction of apoptosis in RA synovial fibroblasts, either through suppression of these signaling pathways or through inhibition of Mcl-1 expression, is emerging as a therapeutic strategy for halting deleterious tissue growth (6,11). Epigallocatechin-3-gallate (EGCG) is a potent antioxidant, antiinflammatory, and antioncogenic compound derived from green tea (12). We and others have shown that EGCG is capable of inducing apoptosis in a variety of cancers both in vitro and in vivo (13,14). However, the use of EGCG in combination with other therapeutic agents in order to enhance the sensitivity or efficacy of the therapy is gaining attention and could serve as an impetus for such mechanisms (2,12). Recent studies have shown that EGCG sensitizes carcinoma cells to trastuzumab and TRAIL-induced apoptosis (14,15). Recently, we found that EGCG blocked interleukin-1β (IL-1β)–induced chemokine production and matrix metalloproteinase 2 (MMP-2) activation in RA synovial fibroblasts (16). In this study, we evaluated whether EGCG inhibits the uncontrolled growth of RA synovial fibroblasts by inducing apoptosis.