Targeting C4-Demethylating Genes in the Cholesterol Pathway Sensitizes Cancer Cells to EGF Receptor Inhibitors via Increased EGF Receptor Degradation

Abstract

Persistent signaling by the oncogenic epidermal growth factor receptor (EGFR) is a major source of cancer resistance to EGFR targeting. We established that inactivation of two sterol biosynthesis pathway genes, SC4MOL (sterol C4-methyl oxidase-like) and its partner NSDHL (NADP-dependent steroid dehydrogenase-like), sensitized tumor cells to EGFR inhibitors. Bioinformatics modeling of interactions for the sterol pathway genes in eukaryotes allowed us to hypothesize, and then extensively validate an unexpected role for SC4MOL and NSDHL in controlling the signaling, vesicular trafficking and degradation of EGFR and its dimerization partners, ERBB2 and ERBB3. Metabolic block upstream of SC4MOL with ketoconazole or CYP51A1 siRNA rescued cancer cell viability and EGFR degradation. Inactivation of SC4MOL markedly sensitized A431 xenografts to cetuximab, a therapeutic anti-EGFR antibody. Analysis of Nsdhl-deficient Bpa1H/+ mice confirmed dramatic and selective loss of internalized PDGFR in fibroblasts, and reduced activation of EGFR and its effectors in regions of skin lacking NSDHL.

Endogenous Sterol Metabolites Regulate Growth of EGFR/KRAS-Dependent Tumors via LXR

“…4C). Silencing of CYP51A1, an enzyme acting upstream of NSDHL in the cholesterol synthesis pathway that prevents accumulation of MAS (Sukhanova et al, 2013), reversed the upregulation of ABCA1 in NSDHL-deficient cancer cells (Fig. 4D) supporting the idea that upregulation of MAS metabolites is important in triggering LXR activation.…”

“…We evaluated two additional models. Using a series of SCC61 carcinoma cells in which the expression of NSDHL or SC4MOL was silenced with specific shRNAs, versus vector controls (Sukhanova et al, 2013), we found depletion of these proteins increased levels of cleaved SREBP2 (Radhakrishnan et al, 2008), and expression of ABCA1 (Fig. 4A).…”

“…In an earlier study, we showed that loss of NSDHL or SC4MOL affected EGFR signaling in cancer cells (Sukhanova et al, 2013). We here show that activated LXR showed synergy with EGFR antagonists, thus pointing out the cholesterol pathway as a critical target regulating the growth of human carcinomas.…”

“…Under physiological conditions, these MAS are abundant in male and female gonads and regulate meiosis (Byskov et al, 1995) through a mechanism that involves release of EGFR ligands and activated receptor signaling (Park et al, 2004). In previous work, we had pursued suggestions from an RNAi-based sensitization screen to establish that depletion of SC4MOL and NSDHL, but not enzymes operating upstream or downstream in the sterol synthesis pathway, sensitized cancer cells to the EGFR inhibitors cetuximab and erlotinib (Sukhanova et al, 2013). These EGFR inhibitor-sensitizing effects of SC4MOL and NSDHL were mediated by the MAS sterol metabolites, and were observed both in vitro and in xenograft analysis in vivo (Sukhanova et al, 2013), suggesting further study of SC4MOL, NSDHL, and MAS might be informative in cancers dependent on EGFR pathway signaling.…”

Controlling Cholesterol Synthesis beyond 3-Hydroxy-3-methylglutaryl-CoA Reductase (HMGCR)

“…The EGF receptor (EGFR) may be pivotal in these skin changes, as similarities are observed in mice and humans with EGFR pathway perturbations. Moreover, accumulation of MASs through inactivation of SC4MOL and NSDHL reduces EGFR expression and signaling (10).…”