The Expression And Function Of Riboflavin Transporter2(RFT2) In Esophageal Squamous Cell Carcinoma

Esophageal cancer (EC) is one of the most lethal cancers worldwide with a variable geographic distribution of incidence and histologic subtypes, including esophageal squamous cell carcinoma (ESCC) and esophageal adenocarcinoma (EAC). ESCC is the most common subtype in eastern Asia and southern Africa, and accounts for about90%of all EC cases in China.The etiology of ESCC is still obscure, but it is widely believed that both genetic factors and exposure to environmental risk factors contribute to this malignancy. Over the past few decades, accumulating evidence from epidemiological studies suggests that family history, tobacco and alcohol consumption, nutritional deficiencies, dietary carcinogen exposure and drinking very hot beverages are major risk factors for ESCC.In recent years, genome-wide association studies (GWAS) and genome-wide landscape of genetic and epigenetic studies identified several important susceptibility loci and/or genetic and epigenetic alterations of genes in cancers. Human riboflavin (vitamin B2) transporter2(RFT2, also termed as SLC52A3) encoded by C20orf54was recently identified as a susceptibility locus for esophageal squamous cell carcinoma (ESCC) in a genome-wide association study (GWAS). Although RFT2is known as a trans-membrane transporter that specifically and efficiently transports riboflavin into cells, the role of RFT2in ESCC remains elusive. Riboflavin cannot be synthesized in vivo in mammals, therefore it must be obtained from exogenous sources including dietary and the normal microflora of the large intestine. Riboflavin deficiency has been shown to link with various types of human diseases, especially as a risk factor for ESCC in high incidence areas of China.Here, we examined the expression of RFT2in tissue samples from156ESCC patients. The role of RFT2in ESCC was investigated by RNA interference or ectopic overexpression of RFT2in ESCC cells. Our results showed that RFT2was frequently overexpressed in ESCC tissues and cell lines. Knockdown of RFT2in ESCC cell line KYSE30expressing high levels of RFT2resulted in decreases of intracellular flavin status, mitochondrial membrane potential and cellular ATP levels, and inhibitions of cell proliferation, colony formation and anchorage-independent growth. Knockdown of RFT2increased p21and p27protein levels, decreased their downstream targets cyclin El and Cdk2protein levels and caused pRb hypophosphorylation, leading to cell cycle arrest at G1-G1/S. Knockdown of RFT2also reduced anti-apoptotic proteins Bcl-2, Bcl-xL and survivin levels, caused activation of caspase-3and apoptosis. In contrast, ectopic overexpression of RFT2in ESCC cell line KYSE150expressing low levels of RFT2significantly promoted ESCC cell proliferation under restricted conditions (e.g. anchorage-independent growth) and enhanced cell tumorigenicity in nude mice. In addition, cells overexpressing RFT2displayed anti-apoptotic effects and resistance to cisplatin treatment.Taken together, our results indicate that RFT2is involved in ESCC by maintaining riboflavin homeostasis and cell energy metabolism to sustain cell proliferation and protect against cell death, and it may serve as a potential therapeutic target.