Role of posttranscriptional processes in the regulation of glutathione S-transferase P1 gene expression in human breast cancer cells

Increased glutathione S-transferase P1-1 expression by mRNA stabilization in hemin-induced differentiation of K562 cells

GSTP1-1 gene expression mechanisms were investigated in hemin-induced erythroid differentiation of K562 cells. Hemoglobin production during differentiation was followed by a significant increase in GSTP1-1 mRNA (1.7-fold, P < 0.01) and protein (1.2-fold, P < 0.01) after 4 days of induction. This increase in mRNA production was not due to transcriptional up-regulation by GATA-1 previously shown to regulate GSTP1-1 during erythroid and megakaryocytic differentiation. Moreover, a drastic decrease in differentiation-specific GATA-1 mRNA expression was correlated to a reduction in GATA-1 promoter binding activity. Neither AP-1 nor NF-kappaB transcription factor binding activities could provide an explanation to the GSTP1-1 mRNA overexpression in hemin-treated cells. GSTP1-1 mRNA stability analysis using actinomycin D as an inhibitor of mRNA neosynthesis showed that mRNA half-life was doubled in hemin-induced erythroid differentiation of K562 cells. These results allow us to add stabilization of GSTP1-1 mRNA as a novel regulatory mechanism during hemin-mediated differentiation of K562 cells.

High Level of Glutathione-S-Transferase π Expression in Mantle Cell Lymphomas

PURPOSE

Prognosis of mantle cell lymphoma (MCL) remains poor. Patients who achieve a response to first line therapy usually relapse, and the probability of cure remains low. Glutathione-S-transferase pi (GST-pi) overexpression has been associated with alkylating agents and anthracycline resistance. GST-pi gene is located in 11q13 and is coamplified along with CCND1 gene in some human solid tumors.

EXPERIMENTAL DESIGN

We performed immunohistochemical analysis of GST-pi expression in 24 consecutive MCLs, 12 follicular lymphomas (FLs), and 69 diffuse large B-cell lymphomas (DLBCLs). Cases were classified in three groups: high GST-pi expression (> 50% of cells were stained), moderate (5 to 50% cells were stained), or absent (< 5% cells were stained). GST-pi and CCND1 mRNA levels were also assessed by real-time reverse transcription-PCR analysis.

RESULTS

All MCLs exhibit high GST-pi protein expression, compared with 29% of the DLBCLs and none of the FLs. MCLs expressed high levels of GST-pi and CCND1 mRNAs compared with DLBCLs and FLs. There was a strong relation between GST-pi and CCND1 mRNAs transcript levels in MCLs but not in DLBCLs. In conclusion, protein and mRNA GST-pi expression is high in MCL compared with FL and DLBCL.

CONCLUSIONS

Overexpression of CCND1 in MCL is associated with a transcriptional up-regulation of the GST-pi gene. Our results suggest that the glutathione system could play a role in drug resistance in MCL.

Methylation-mediated regulation of the glutathione S-transferase P1 gene in human breast cancer cells

Understanding the mechanisms that regulate the human pi class GST (GSTP1) gene expression in breast cancer cells is of particular importance to the study of breast cancer biology. In cultured human breast cancer cell lines, GSTP1 is exclusively expressed in estrogen receptor-negative (ER-) cells but is undetectable in receptor-positive (ER+) cells. Previously, we examined transiently transfected GSTP1 promoter activities, in vitro GSTP1 promoter-DNA interactions, and GSTP1 mRNA stability. These studies indicated that transiently transfected GSTP1 promoter elements and GSTP1 mRNA stability could only partially explain cell line-specific expression of endogenous GSTP1. In the present study, we examined whether the methylation status of the GSTP1 CpG island plays an important role in GSTP1 regulation. Southern blot analysis revealed that the GSTP1 CpG island is hypermethlyated in ER+, GSTP1 non-expressing cell lines but is undermethylated in ER-, GSTP1 expressing cell lines. Moreover, partial demethylation of the GSTP1 CpG island by treatment with 5-aza-2'-deoxycytidine resulted in de novo gene expression in ER+ cell lines, as detected by RT-PCR, Northern blot and Western blot analyses. Our data strongly indicate that methylation status of the promoter contributes significantly to the levels of GSTP1 expressed in ER- and ER+ breast cancer cell lines.

Contribution of proximal promoter elements to the regulation of basal and differential glutathione S-transferase P1 gene expression in human breast cancer cells

Glutathione S-transferase P1 (GST P1-1) is normally expressed exclusively in estrogen receptor negative (ER-) but not receptor positive (ER+) cultured breast cancer cells. We examined the role of proximal promoter elements in GST P1 gene expression in MCF7 (ER+, GST P1-) and HS578T (ER-, GST P1+) breast cancer cells. Transient transfection of GST P1 promoter-CAT reporter genes confirmed that the GST P1 TRE (-69 to -60) and the adjacent distal GC box (-56 to -51) are required for basal promoter activity in both cell lines. Other studies identified differences in the GST P1 promoter activity and DNA-protein interactions between the two cell lines. Electrophoretic mobility shift assay revealed a protein-TRE interaction that is unique to nuclear proteins derived from GST P1 expressing HS578T cells. Furthermore, a putative silencer region contained within sequences -130 to -70 selectively reduced GST P1 promoter-CAT reporter gene expression in MCF7 but not HS578T cells. While this cell-line specific silencer contributed to the level of GST P1 promoter activity observed in the two cell lines, analysis of cells stably transfected with a novel genomic GST P1 minigene vector established that the silencer is insufficient to completely repress GST P1 transcription in ER+, MCF7 cells that do not normally express endogenous GST P1.