Journal Article

Effect of sulforaphane on metallothionein expression and induction of apoptosis in human hepatoma HepG2 cells

Chi-Tai Yeh and Gow-Chin Yen

in Carcinogenesis

Volume 26, issue 12, pages 2138-2148
Published in print December 2005 | ISSN: 0143-3334
Published online July 2005 | e-ISSN: 1460-2180 | DOI: http://dx.doi.org/10.1093/carcin/bgi185
Effect of sulforaphane on metallothionein expression and induction of apoptosis in human hepatoma HepG2 cells

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The molecular mechanism of sulforaphane on the induction of metallothionein (MT) genes in HepG2 cells and the antiproliferative effects of sulforaphane were investigated in this study. Treatment of the cells with sulforaphane at non-toxicity concentration (0–20 µM) resulted in coordinate increases in the induction of MT-I and MT-II mRNA, followed by corresponding increases in MT protein expression. Western blot analysis revealed the increased level of the transcription factor, Nrf2 in a time-dependent manner from sulforaphane-treated cells. Furthermore, sulforaphane activated the extracellular signal-regulated protein kinase (ERK), p38 and c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) pathways. SB203580, a specific inhibitor of p38 and PD98059, a specific inhibitor of ERK, abolished sulforaphane-induced MT protein expression, whereas SP600125, a specific inhibitor of JNK, had no significant effect. At relatively high concentration (30–100 µM), sulforaphane is a cell growth modulator, as it induced apoptotic cell death characterized by internucleosomal DNA fragmentation and caused a rapid induction of caspase 3 activity, according to the appearance of the caspase 3 fragments and stimulated proteolytic cleavage of poly (ADP-ribose) polymerase in a time-dependent manner. Moreover, sulforaphane-induced apoptotic cell death was accompanied by upregulation of Bax and downregulation of Bcl-2 and Bcl-Xl protein. Sulforaphane-induced DNA fragmentation was blocked by the N-acetyl-l-cysteine and catalase, suggesting that the death signaling was triggered by oxidative stress. Taken together these results strongly suggest that at low concentrations of sulforaphane, activation of MAPKs, such as ERK and p38 pathway, lead to Nrf2-mediated MT gene expression. Whereas at a higher concentration, sulforaphane is an effective apoptosis inducer in HepG2 cells through regulation of Bcl-2 family molecular and activation of ICE/Ced-3 protease (caspase 3) cascade. The results from this study may provide more evidence for its chemopreventive function.

Keywords: ARE, antioxidant response element; CAT, catalase; GRE, glucocorticotid-response elements; GSTA1, glutathione S-transferase A1; ERK, extracellular signal-regulated protein kinase; IC50 value, concentration causing 50% cell death; JNK, c-Jun N-terminal kinase; Keap1, kelch-like ECH-associating protein 1; MT, metallothionein; MAPK, mitogen-activated protein kinase; MRE, metal response element; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide; NAC, N-acetyl-l-cysteine; NQO1, NAD(P)H: quinone oxidoreductase; Nrf2, nuclear factor E2-related factor 2; PEITC, phenethyl isothiocyanate; PARP, poly (ADP-ribose) polymerase; RT–PCR, reverse transcription–polymerase chain reaction; SFN, sulforaphane; TBST, Tris-buffered saline with Tween-20

Journal Article.  8888 words.  Illustrated.

Subjects: Clinical Cytogenetics and Molecular Genetics

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