Lactate Dehydrogenase B Is Critical for Hyperactive mTOR-Mediated Tumorigenesis

Xiaojun Zha,Fang Wang,Ying Wang,Yanling Jing,Hongbing Zhang,Shaozong He,Xueyan Wu

doi:10.1158/0008-5472.can-10-1668

Abstract

Mammalian target of rapamycin (mTOR) is a major downstream effector of the receptor tyrosine kinase (RTK)-phosphoinositide 3-kinase (PI3K)-v-akt murine thymoma viral oncogene homologue 1 (AKT) signaling pathway. Although this signaling network is frequently altered in cancer, the underlying mechanisms that cause tumorigenesis as a result of activated mTOR remain largely unknown. We report here that expression of lactate dehydrogenase B (LDHB), a critical enzymatic activator of glycolysis, was upregulated in an mTOR-dependent manner in TSC1(-/-), TSC2(-/-), PTEN(-/-), or activated AKT1-expressing mouse embryonic fibroblasts (MEF). LDHB gene expression was transactivated by signal transducer and activator of transcription 3 (STAT3), a key tumorigenic driver in many cancers, acting as a downstream mTOR effector in both mouse MEFs and human cancer cells. LDHB attenuation blunted the tumorigenic potential of oncogenic TSC2-null cells in nude mice. We concluded that LDHB is a downstream target of mTOR that is critical for oncogenic mTOR-mediated tumorigenesis. Our findings offer proof of concept for targeting LDHB as a therapeutic strategy in cancers driven by aberrant activation of the RTK-PI3K-AKT-mTOR signaling cascade.

Highlights

Because of gain-of-function mutations of proto-oncogene epidermal growth factor receptor, phosphoinositide 3-kinase (PI3K), or v-akt murine thymoma viral oncogene homologue 1 (AKT), and by loss-of-function mutations of tumor suppressor phosphatase and tensin homologue (PTEN), LKB1, TSC1, or TSC2, the receptor tyrosine kinase (RTK)–PI3K-AKT–mammalian target of rapamycin (RTK-PI3K-AKT-mTOR) signaling cascade is one of the most frequently activated signaling pathways in human cancers [1,2,3,4,5]. mTOR, a serine/threonine kinase, integrates various inputs from upstream pathways and plays a central role in regulating cell growth and proliferation [2]
Because TSC1 and TSC2 protein complex is the major suppressor of mTOR signaling, mTOR hyperactivation is responsible for multiorgan tumors in tuberous sclerosis complex (TSC) disease due to inactive mutations of either TSC1 or TSC2 [12]
Among the differentially expressed genes, the mRNA abundance of lactate dehydrogenase B (LDHB) but not LDHA was significantly increased in TSC2À/À mouse embryonic fibroblasts (MEF) (Supplementary Table 1) and this finding was confirmed by quantitative real-time PCR (qRT-PCR) analysis (Fig. 1A, left and middle)

Summary

Introduction

Because of gain-of-function mutations of proto-oncogene epidermal growth factor receptor, phosphoinositide 3-kinase (PI3K), or v-akt murine thymoma viral oncogene homologue 1 (AKT), and by loss-of-function mutations of tumor suppressor phosphatase and tensin homologue (PTEN), LKB1, TSC1, or TSC2, the receptor tyrosine kinase (RTK)–PI3K-AKT–mammalian target of rapamycin (mTOR) (RTK-PI3K-AKT-mTOR) signaling cascade is one of the most frequently activated signaling pathways in human cancers [1,2,3,4,5]. mTOR, a serine/threonine kinase, integrates various inputs from upstream pathways and plays a central role in regulating cell growth and proliferation [2]. MTOR, a serine/threonine kinase, integrates various inputs from upstream pathways and plays a central role in regulating cell growth and proliferation [2]. The precise mechanisms downstream of the mTOR signaling leading to cancer development remain elusive. Pyruvate, is often activated in many types of cancer [7]. It is a tetrameric enzyme composed of combinations of 2 subunits (LDHA and LDHB). LDHA is elevated and activated in many cancers, and plays a crucial role in tumor initiation, maintenance, and progression [8], whereas the significance of LDHB in tumor development remains more elusive and the regulation of LDHB expression is less characterized. Because LDHB is critical for hyperactive mTORmediated tumorigenesis, LDHB may be a drugable target for the treatment of diseases associated with aberrant mTOR signaling

Materials and Methods

Results and Discussion

STATV3C STAT3VC STAT3C Primer 3 Primer 2 Primer 1