Expression of TMBIM6 in Cancers: The Involvement of Sp1 and PKC.

Raghu Patil Junjappa,Seong Yeol Park,Kashi Raj Bhattarai,Kyung-Woon Kim,Han-Jung Chae,Hyun-Kyoung Kim,Hyung-Ryong Kim,Jae-Won Soh

doi:10.3390/cancers11070974

Abstract

Transmembrane Bax Inhibitor Motif-containing 6 (TMBIM6) is upregulated in several cancer types and involved in the metastasis. Specific downregulation of TMBIM6 results in cancer cell death. However, the TMBIM6 gene transcriptional regulation in normal and cancer cells is least studied. Here, we identified the core promoter region (−133/+30 bp) sufficient for promoter activity of TMBIM6 gene. Reporter gene expression with mutations at transcription factor binding sites, EMSA, supershift, and ChIP assays demonstrated that Sp1 is an essential transcription factor for basal promoter activity of TMBIM6. The TMBIM6 mRNA expression was increased with Sp1 levels in a concentration dependent manner. Ablation of Sp1 through siRNA or inhibition with mithramycin-A reduced the TMBIM6 mRNA expression. We also found that the protein kinase-C activation stimulates promoter activity and endogenous TMBIM6 mRNA by 2- to 2.5-fold. Additionally, overexpression of active mutants of PKCι, PKCε, and PKCδ increased TMBIM6 expression by enhancing nuclear translocation of Sp1. Immunohistochemistry analyses confirmed that the expression levels of PKCι, Sp1, and TMBIM6 were correlated with one another in samples from human breast, prostate, and liver cancer patients. Altogether, this study suggests the involvement of Sp1 in basal transcription and PKC in the enhanced expression of TMBIM6 in cancer.

Highlights

Transmembrane Bax Inhibitor Motif-containing 6 (TMBIM6), is known as Bax Inhibitor-1, is an evolutionarily conserved transmembrane protein predominantly localized in the endoplasmic reticulum (ER) membrane [1]
As mentioned in a previous study [21], in rats, two TMBIM6 transcriptional variants have been found: variant-1 is from the distal promoter P2, which is expressed ubiquitously; and variant-2 is from the proximal promoter P1, which is restricted to the testes
We tested the expression pattern of both variants of human TMBIM6 in cells other than testis tissue by RT-PCR using exon-1-specific forward primers and common exon 2 primers, and we found that only variant-1 is expressed in HT1080 and HeLa cells (Supplementary Figure S1A)

Summary

Introduction

Transmembrane Bax Inhibitor Motif-containing 6 (TMBIM6), is known as Bax Inhibitor-1, is an evolutionarily conserved transmembrane protein predominantly localized in the endoplasmic reticulum (ER) membrane [1]. The TMBIM6 gene was first identified while screening the testis cDNA library and was named the testis enhanced gene transcript [2]. While screening cDNA expression libraries for clones that protect yeast from Bax-mediated cell death, TMBIM6 was identified as an antiapoptotic protein that inhibits Bax-mediated cell death [3,4]. The human TMBIM6 gene is located on chromosome 12q13.12 and expressed in two transcriptional variant isoforms [2]. TMBIM6 gene is normally expressed in all tissues but the expression level varies depending on the tissue [5]. The dysregulated expression of TMBIM6 leads to cell transformation, which can Cancers 2019, 11, 974; doi:10.3390/cancers11070974 www.mdpi.com/journal/cancers

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