Induction of the Connexin 32 Gene by Epigallocatechin-3-Gallate Potentiates Vinblastine-Induced Cytotoxicity in Human Renal Carcinoma Cells

Abstract

Background/Aim: Enforced expression of the connexin (Cx) 32 gene, a member of the gap junction gene family and a tumor suppressor gene in human renal cell carcinoma (RCC), enhanced vinblastine (VBL)-induced cytotoxicity in RCC cells due to suppression of multidrug resistance 1 (MDR1) expression. Furthermore, in RCC the Cx32 gene is silenced by hypermethylation of CpG islands in a promoter region of the Cx gene. In this study, we investigated if the green tea polyphenol epigallocatechin-3-gallate (EGCG) could enhance susceptibility of RCC cells (Caki-1, a human metastatic RCC cell) to VBL. Methods: The effects of EGCG on Caki-1 cells were estimated by WST-1 (cell viability), real-time RT-PCR (mRNA level) and immunoblotting (protein level). We estimated the methylation status in the promoter region of the Cx32 gene in RCC cells by methylation-specific PCR. Each protein function was inhibited by small interfering RNA (siRNA) and specific inhibitors. Results: The EGCG treatment elicited significant upregulation of Cx32 in Caki-1 cells, and the induction of the Cx led to the suppression of MDR1 mRNA expression through inactivation of Src and subsequent activation of c-Jun NH2-terminal kinase (JNK). Chemical sensitivity to VBL in Caki-1 cells was increased by EGCG pretreatment, and this effect was abrogated by siRNA-mediated knockdown of Cx32. Conclusion: This study suggests that the restoration of Cx32 by EGCG pretreatment improves chemical tolerance on VBL in Caki-1 cells via the inactivation of Src and the activation of JNK.

<p>Connexin 32 downregulation is critical for chemoresistance in oxaliplatin-resistant HCC cells associated with EMT</p>

“…The chemosensitization effect of Cx on various cytotoxic drugs (eg, OXA) has been confirmed by several studies 20,22,39. Although its exact mechanism of action remains not fully elucidated, research has focused on the following aspects: 1) The role of GJ-dependent communication functions.…”

“…This gene was originally considered a tumor-suppressor gene13,14 and has been demonstrated to be implicated in multiple hepatocellular processes such as carcinogenesis, cell proliferation, apoptosis, invasion, and metastasis 15–18. More importantly, Cx32 can affect sensitivity to many chemotherapeutic drugs, including platinum drugs 19,20…”

Potential Therapeutic Targets of Epigallocatechin Gallate (EGCG), the Most Abundant Catechin in Green Tea, and Its Role in the Therapy of Various Types of Cancer

“…Epigallocatechin gallate treatment provoked important upregulation of Cx32. Chemical sensitivity to vinblastine in cells was increased by EGCG pre-treatment, and this effect was withdrawn by siRNA-mediated knockdown of connexin 32 [ 97 ].…”

Connexins and Gap Junctions in Cancer of the Urinary Tract

“…The group led by Yano published several studies on the role of connexin 32 in renal cell carcinoma and claimed that connexin 32 regulates “proliferation, invasion, and metastasis” in renal cell carcinoma [28,29]. A more recent report demonstrated that induction of the connexin 32 gene in human renal carcinoma cells enhanced the vinblastine-induced cytotoxicity in human renal carcinoma cells [30].…”