Beyond phosphorylation cancer cells may alter
Beyond phosphorylation, cancer hesperetin may alter pyrimidine biosynthesis through the activation of the proto-oncogenic transcription factor MYC. MYC is a master regulator of many different pathways and has significant influence on the expression of nucleotide metabolism genes. Previous studies have shown that overexpression of MYC significantly increased expression of nucleotide metabolism enzymes, including DHODH, which was validated as a direct MYC target gene (Liu et al., 2008). Additionally, shRNA knockdown of MYC decreased the expression of nucleotide metabolism genes and lowered the intracellular concentrations of nucleotides (Mannava et al., 2008). These results demonstrate MYC's control over DHODH expression. Surprisingly, inhibition of DHODH by brequinar or teriflunomide down-regulated MYC expression (Dorasamy, Choudhary, Nellore, Subramanya, & Wong, 2017). However, this may be context-dependent as leflunomide (the pro-drug of teriflunomide) demonstrates no effect on MYC expression, possibly due to insufficient conversion of leflunomide to teriflunomide (O'Donnell et al., 2012), but this was not confirmed experimentally. Nonetheless, while these conflicting results complicate the understanding of MYC and DHODH's relationship, nucleotide biosynthesis is among the many pathways MYC activation influences to facilitate cellular proliferation. This is a unique relationship as other transcription factors, such as those in the E2F family, do not appear to increase expression of DHODH (Bester et al., 2011). It is possible that MYC and DHODH expression are linked through glutamine metabolism because MYC increases glutamine flux, which is the first substrate in de novo pyrimidine biosynthesis (Hsieh, Walton, Altman, Stine, & Dang, 2015).
The relationship between glutamine metabolism and de novo pyrimidine biosynthesis is becoming better understood. Low glutamine concentrations have been demonstrated to hinder S-phase progression, presumably by limiting the precursor metabolite for nucleotide production (DeBerardinis & Cheng, 2010; Gaglio, Soldati, Vanoni, Alberghina, & Chiaradonna, 2009). This is in agreement with the recent finding that DHODH inhibitors induce a decrease in the steady-state level of glutamine (Koundinya et al., 2018), and DHODH inhibition is known to cause S-phase arrest. High glutamine concentrations may increase the rate of de novo nucleotide synthesis, however, it does not appear that cancer cells increase glutamine uptake to solely fuel de novo pathways (DeBerardinis & Cheng, 2010; Wise & Thompson, 2010). A significant disparity exists between the rates of glutamine uptake and nucleotide biosynthesis, suggesting that glutamine is utilized by far more pathways than nucleotide biosynthesis alone (DeBerardinis & Cheng, 2010). Nevertheless, glutamine flux may be an indicator of cell sensitivity to DHODH inhibition. Interestingly, cells with a mutant phosphatase and tensin homolog (PTEN) tumor suppressor were observed to possess increased glutamine metabolism and were sensitive to DHODH inhibition (Mathur et al., 2017). While more studies are needed, these results suggest that DHODH inhibitors may be useful in cells with mutant PTEN and possibly increased glutamine metabolism.
DHODH and cell differentiation Beyond directly halting cell growth, DHODH has been implicated as a target to induce cellular differentiation. Previously, DHODH inhibition was shown to induce cell differentiation in neural crest (White et al., 2011) and leukemic cell lines (Sykes et al., 2016). In leukemic cells, DHODH inhibitors, such as brequinar, were observed to decrease the number of self-renewing cells in vivo (Sykes et al., 2016). Recently, a novel class of DHODH inhibitors were found to induce myeloid differentiation in AML cell lines, which led to cytotoxicity of the differentiated cell population (Sainas et al., 2018). Though further studies are needed to fully understand how DHODH inhibitors induce this effect, these findings have significantly increased the interest in DHODH-targeted therapy for cancer. In fact, two newly patented DHODH inhibitors are in clinical trials for the treatment of AML (Table 1); one from Aslan Pharmaceuticals (ASLAN003, Phase II, NCT03451084) and the other from Bayer (BAY2402234, Phase I) (Gradl et al., 2018).