Ectopic expression of the TERE1 (UBIAD1) protein inhibits growth of renal clear cell carcinoma cells: altered metabolic phenotype associated with reactive oxygen species, nitric oxide and SXR target genes involved in cholesterol and lipid metabolism.

Current studies of the TERE1 (UBIAD1) protein emphasize its multifactorial influence on the cell, in part due to its broad sub-cellular distribution to mitochondria, endoplasmic reticulum and golgi. However, the profound effects of TERE1 relate to its prenyltransferase activity for synthesis of the bioactive quinones menaquinone and COQ10. Menaquinone (aka, vitamin K-2) serves multiple roles: as a carrier in mitochondrial electron transport, as a ligand for SXR nuclear hormone receptor activation, as a redox modulator, and as an alkylator of cellular targets. We initially described the TERE1 (UBIAD1) protein as a tumor suppressor based upon reduced expression in urological cancer specimens and the inhibition of growth of tumor cell lines/xenografts upon ectopic expression. To extend this potential tumor suppressor role for the TERE1 protein to renal cell carcinoma (RCC), we applied TERE1 immunohistochemistry to a TMA panel of 28 RCC lesions and determined that in 57% of RCC lesions, TERE1 expression was reduced (36%) or absent (21%). Ectopic TERE1 expression caused an 80% decrease in growth of Caki-1 and Caki-2 cell lines, a significantly decreased colony formation, and increased caspase 3/7 activity in a panel of RCC cell lines. Furthermore, TERE1 expression increased mitochondrial oxygen consumption and hydrogen production, oxidative stress and NO production. Based on the elevated cholesterol and altered metabolic phenotype of RCC, we also examined the effects of TERE1 and the interacting protein TBL2 on cellular cholesterol. Ectopic TERE1 or TBL2 expression in Caki-1, Caki-2 and HEK 293 cells reduced cholesterol by up to 40%. RT-PCR analysis determined that TERE1 activated several SXR targets known to regulate lipid metabolism, consistent with predictions based on its role in menaquinone synthesis. Loss of TERE1 may contribute to the altered lipid metabolic phenotype associated with progression in RCC via an uncoupling of ROS/RNS and SXR signaling from apoptosis by elevation of cholesterol.

The TERE1 protein interacts with mitochondrial TBL2: regulation of trans-membrane potential, ROS/RNS and SXR target genes.

We originally discovered TERE1 as a potential tumor suppressor protein based upon reduced expression in bladder and prostate cancer specimens and growth inhibition of tumor cell lines/xenografts upon ectopic expression. Analysis of TERE1 (aka UBIAD1) has shown it is a prenyltransferase enzyme in the natural bio-synthetic pathways for both vitamin K-2 and COQ10 production and exhibits multiple subcellular localizations including mitochondria, endoplasmic reticulum, and golgi. Vitamin K-2 is involved in mitochondrial electron transport, SXR nuclear hormone receptor signaling and redox cycling: together these functions may form the basis for tumor suppressor function. To gain further insight into mechanisms of growth suppression and enzymatic regulation of TERE1 we isolated TERE1 associated proteins and identified the WD40 repeat, mitochondrial protein TBL2. We examined whether disease specific mutations in TERE1 affected interactions with TBL2 and the role of each protein in altering mitochondrial function, ROS/RNS production and SXR target gene regulation. Biochemical binding assays demonstrated a direct, high affinity interaction between TERE1 and TBL2 proteins; TERE1 was localized to both mitochondrial and non-mitochondrial membranes whereas TBL2 was predominantly mitochondrial; multiple independent single amino acid substitutions in TERE1 which cause a human hereditary corneal disease reduced binding to TBL2 strongly suggesting the relevance of this interaction. Ectopic TERE1 expression elevated mitochondrial trans-membrane potential, oxidative stress, NO production, and activated SXR targets. A TERE1-TBL2 complex likely functions in oxidative/nitrosative stress, lipid metabolism, and SXR signaling pathways in its role as a tumor suppressor.