While the mechanism of DHODH inhibition
While the mechanism of DHODH inhibition-induced differentiation is not fully understood, this phenomenon may be caused by pyrimidine depletion. Pyrazofurin, an inhibitor of OMP decarboxylase, was able to induce differentiation and suggests pyrimidine depletion as a potential mechanism (Sykes et al., 2016). Additionally, differentiation caused by DHODH inhibition was rescued in the presence of uridine (Sainas et al., 2018). A possible link between pyrimidine depletion and cellular differentiation may be transcriptional elongation. The PHD-finger Phf5a is known to modulate transcriptional elongation of genes involving cell differentiation (Strikoudis et al., 2016). Pyrimidine depletion has been observed to inhibit transcriptional elongation of tumorigenic genes (Tan et al., 2016), and in fact, direct inhibition of DHODH produced a similar effect in melanoma MK-8931 (White et al., 2011). However, the connection between DHODH and cellular differentiation is still not well understood. Despite this, considerable interest exists for DHODH-targeted therapy as a potential option to induce cellular differentiation.
Relevance of DHODH in cancer DHODH\'s relevance in cancer was recognized nearly six decades ago when Smith et al. noted elevated DHODH activity in leukemic cells (Smith, Baker, & Sullivan, 1960; Smith Jr. & Baker, 1959). Following that observation, several investigations focused on DHODH that culminated in the discovery and clinical evaluation of brequinar, a potent human DHODH inhibitor (Dexter et al., 1985; Schwartsmann et al., 1988). However, brequinar failed to produce objective responses in clinical trials (discussed in further detail below), leading to questions about the viability of DHODH inhibition as a therapeutic approach. Recent reports have revisited the link between DHODH inhibition and antiproliferative effects on cells. However, no studies have thoroughly evaluated the clinical relevance of DHODH in cancer. To address this gap in understanding, we analyzed data from several different in vitro and in vivo profiling projects to provide an unbiased summary of the potential significance of DHODH in cancer. Cell growth is suppressed when clinically relevant anticancer targets are knocked down. The Achilles Project (Aguirre et al., 2016; Cowley et al., 2014; Tsherniak et al., 2017) evaluates large-scale cellular responses in the presence of shRNA across a catalogue of cancer cell lines and has been utilized to gauge the importance of DHODH in cancer cells. Changes in cell viability were averaged for each cell line and then ranked by essentiality; a negative shRNA score indicates that a cell line responded poorly (i.e., showed growth inhibition) to treatment with shRNA. Achilles\' project results (Fig. 4A) depict a resounding negative response of cancer cell lines to shRNA knockdown of DHODH. The results suggest that DHODH inhibition correlates with decreased cell growth in most cancer cell lines. Cell lines that were most sensitive to DHODH knockdown were derived from cancers of the small and large intestines (highlighted in yellow, Fig. 4A), which had a total of 17 out of 21 cell lines with lower than average shRNA scores (Fig. 4B). Consistent with this finding, significantly higher DHODH expression was observed in the Sabetes-Bellver colorectal dataset for adenoma tumor tissues versus normal samples (Fig. 4C) (Baldwin et al., 2005; Rhodes et al., 2004; Rhodes et al., 2007; Sabates-Bellver et al., 2007). Cells overexpressing DHODH may have higher sensitivity to its inhibition. Using data from the Cancer Cell Line Encyclopedia (CCLE), we evaluated mRNA levels of DHODH in various cancer cell lines (Fig. 5) (Barretina et al., 2012). The mRNA expression levels of DHODH in 1019 cancer cell lines were obtained and converted to z-scores. The top four highest and lowest expressing cell lines are listed in Fig. 5. Frequent overexpression of DHODH mRNA occurs in lung (38% of top 50) and haematopoietic/lymphoid (24% of top 50) tissues. This data correlates well with the Achilles database (Fig. 4). Both lung and haematopoietic/lymphoid are cell lines that frequently possess higher than average cell growth inhibition when exposed to DHODH shRNA. Collectively, this expression data suggests that both lung and haematopoietic/lymphoid cell lines may be more sensitive to DHODH inhibitors than others.