br Results br Discussion Many

Results
Discussion Many tumor d-biotin rely on EAAs to provide fundamental building blocks for macromolecular biosynthesis because they cannot synthesize EAAs de novo (Hattori et al., 2017; Sheen et al., 2011). Therefore, they must use highly effective transporters to maximize EAA uptake from the extracellular milieu to satisfy rapid cellular metabolism. However, the underlying mechanisms of how EAA metabolism is regulated and how this cellular process affects tumor cell growth remain to be clarified. Using Mycn-amplified neuroblastomas and C-Myc-overexpressing Burkitt’s lymphomas as model systems, we show that oncogenic MYC and SLC7A5/SLC43A1 transporters create a positive interconnected autoregulatory circuit that is essential for sustaining mutual high expression in tumor cells. MYC activates SLC7A5 and SLC43A1 expression and promotes effective EAA delivery into tumor cells. Efficient EAA incorporation, in turn, promotes MYC synthesis, constituting a feedforward activation loop that reinforces MYC-regulated oncogenic programs (Figure 7I). It should be noted that MYC also activates gene networks involved in ribosomal and mitochondrial biogenesis (Li et al., 2005; Morrish and Hockenbery, 2014; van Riggelen et al., 2010), glucose and glutamine metabolism (Gao et al., 2009; Qing et al., 2010, 2012; Ren et al., 2015; Wise et al., 2008) and lipid and nucleotide biosynthesis (Carroll et al., 2015; Ye et al., 2014). Thus, SLC7A5/SCL43A1-mediated EAA import maximizes the entire MYC oncogenic program, conferring advantages to acquiring nutrients and utilizing nutrients during malignant transformation. Two opposing models have been proposed to describe MYC function in shaping cellular transcriptomes. One posits that MYC amplifies transcription at all active loci (Lin et al., 2012; Nie et al., 2012), whereas the other claims that MYC differentially controls discrete sets of genes through binding to specific E box elements (Kress et al., 2015; Sabò et al., 2014; Walz et al., 2013, 2014). Here we systematically analyzed the transcription of sixteen active transporter genes from the Slc6, Slc7, Slc38, and Slc43 families in multiple tumor cells upon MYC activation or inactivation. Interestingly, among the transporters analyzed, we show that MYC only activates Slc7a5 and Slc43a1 transcription through direct binding to the specific E box sequences, suggesting that differential transcriptional activation plays a major role in upregulation of SLC7A5 and SLC43A1 expression. Nevertheless, we cannot rule out the possibility that MYC may contribute to Slc7a5 and Slc43a1 upregulation through transcriptional amplification. In addition to MYC, recent studies also showed that hypoxia inducible factor 2α, the Hippo pathway effectors, the estrogen receptor, and ATF4, respectively, upregulate SLC7A5 expression in clear cell renal carcinoma cells, hepatocellular carcinoma cells, breast cancer cells, and nutrient-deprived prostate cancer cells (Elorza et al., 2012; Furuya et al., 2012; Hansen et al., 2015; Wang et al., 2013), whereas the androgen receptor signaling pathway enhanced SLC43A1 expression in prostate cancer (Wang et al., 2011), leading to elevated leucine uptake and mTORC1 activation. Most likely, all of these factors act in concert to maximize SLC7A5 and SLC43A1 expression in tumor cells. Although glucose is the most highly consumed nutrient by tumor cells, amino acids account for the majority of cell mass during cell growth and proliferation because most of the glucose-derived carbon is excreted as lactate (Hosios et al., 2016). In addition to glutamine, EAAs are also avidly utilized by tumor cells for central carbon metabolism and cell mass duplication (Hosios et al., 2016). Here we reveal a MYC-SLC7A5/SLC43A1 signaling circuit that underlies EAA metabolism, MYC deregulation, mTORC1 activation, and tumor progression. We show that SLC7A5/SLC43A1-mediated EAA uptake stimulates MYC protein synthesis and downstream target gene transcription, leading to reprogramming of the entire metabolic processes, including glycolysis, glutaminolysis, and lipogenesis. A recent study showed that glutamine-derived nucleotides activate C-MYC protein synthesis through enhanced mRNA translation (Dejure et al., 2017). In principle, EAAs and glutamine would cooperate in MYC deregulation and malignant transformation.