Targeting the Mevalonate Pathway in Cancer

Abstract

The mevalonate synthesis inhibitors, statins, are mainstay therapeutics for cholesterol management and cardiovascular health. Thirty years of research have uncovered supportive roles for the mevalonate pathway in numerous cellular processes that support oncogenesis, most recently macropinocytosis. Central to the diverse mechanisms of statin sensitivity is an acquired dependence on one mevalonate pathway output, protein geranylgeranylation. New chemical prenylation probes and the discovery of a novel geranylgeranyl transferase hold promise to deepen our understanding of statin mechanisms of action. Further, insights into statin selection and the counterproductive role of dietary geranylgeraniol highlight how we should assess statins in the clinic. Lastly, rational combination strategies preview how statins will enter the oncology toolbox.

Keywords: cholesterol; combinations; mevalonate; prenylation; statins.

Figure 1.. Inhibition of the mevalonate pathway by statins affects several outputs and lipid-regulating transcription factors.

The synthesis of mevalonate begins with a two-step condensation of three acetyl-CoA molecules to produce 3-hydroxy-3-methylglutaryl-Coenzyme A (HMG-CoA). HMG-CoA is the substrate of the committed step enzyme, HMG-CoA reductase (HMGCR), that generates mevalonate. Mevalonate is further processed by two phosphorylation steps and decarboxylation to synthesize isopentenyl pyrophosphate (IPP), which can be isomerized to dimethylallyl pyrophosphate (DMAPP). These two molecules are building blocks for larger isoprenoid pyrophosphates. The synthesis of the 15-carbon isoprenoid farnesyl pyrophosphate (FPP) is a major branch point in the mevalonate pathway. FPP can be shuttled to the production of cholesterol through the synthesis and cyclization of the 30-carbon isoprenoid, squalene. FPP is also used in the synthesis of dolichol phosphate, an oligosaccharide carrier for asparagine glycosylation and mannose-donor for GPI-anchor production. The direct addition of FPP to biomolecules can occur during the formation of ubiquinone (Coenzyme Q) and Heme A of cytochrome c oxidase. Finally, FPP is converted to geranylgeranyl pyrophosphate (GGPP) by geranylgeranyl diphosphate synthase 1 (GGPS1). Both FPP and GGPP can be post-translationally added to proteins, especially RHO, RAS and RAB family proteins, in a process known as prenylation by farnesyltransferase (FTase) and geranylgeranyltransferase (GGTase) respectively. GGPP also can be used for the synthesis of ubiquinone, the conversion of Vitamin K1 to Vitamin K2, and the generation of a newly discovered glutathione conjugate, s-geranylgeranyl-l-glutathione (GGG). SREBP maturation (mSREBP) is important for supporting mevalonate pathway gene expression.

Figure 2.. Oncogenic signaling pathways play a dual role in generating tumor hallmarks that increase demand for mevalonate pathway intermediates while concurrently upregulating mevalonate pathway meet the demand.

The mevalonate pathway supports signaling by cholesterol rich microdomains. Cilia act as a rheostat for WNT signaling. When cilia loss occurs, ß-catenin (ß-cat) interacts with SREBP2 to enforce expression of the mevalonate pathway to support tumorigenesis. Adhesion signaling from the surrounding extracellular matrix (ECM) finely controls YAP/TAZ localization to promote survival in epithelial cells while reduced adhesion signaling, as encountered by mesenchymal cells, strongly promotes SREBP processing and epithelial-to-mesenchymal transition (EMT). PI3K-AKT signaling, a major mediator of Warburg metabolism, can activate mTORC1-mediated SREBP processing as well as ACLY-mediated conversion of citrate to acetyl-CoA for mevalonate synthesis. The MAPK pathway may also contribute to SREBP maturation as well as promote several tumor hallmarks. Statins affect tumor hallmarks by blocking synthesis of necessary intermediates, while farnesyltransferase inhibitors (FTI) and geranylgeranyltransferase inhibitors (GGTI) block the use of specific intermediates (prenylation substrates) to meet the demand generated by tumor hallmarks, like macropinocytosis.

Figure 3.. Novel insights uncover diverse mechanisms driving mevalonate pathway dependence.

Cholesterol may support early premalignant cell signaling, while strong evidence bolsters a role for protein prenylation in macropinocytosis. The cancer stem cell dependence on mevalonate pathway intermediates is mechanistically tied to a feedforward loop that promotes MYC expression and cancer stem cell maintenance. This feedforward loop can be disrupted by statin treatment. Lastly, the mesenchymal state has demonstrated a unique sensitivity to statins in various cancer models.

Figure 4.. Tumor suppressors act as a checkpoint for mevalonate pathway flux by suppressing SREBP activity.

Tumor suppressors are in blue boxes with thick lines. Primary cilia, supported by VHL, Tg737, and Kif3a, are lost after transformation resulting in loss of their tumor suppressive function as a WNT rheostat and leading to enforced SREBP transcriptional activity. Wild type p53 suppresses the mevalonate by transcription of Abca1, a retrograde transporter of cholesterol that can suppress SREBP maturation. Certain p53 mutations may increase mSREBP activity through direct interaction. PTEN dampens PI3K-AKT-mTORC1 signaling, and thus mTORC1-mediated SREBP maturation; however, co-deletion of PTEN and the tumor suppressor PML is required in prostate cancer to promote the lipogenic SREBP program.

Figure 5.. Prenyltransferase biology is key to identifying targets of statin anti-cancer effects.

Prenyltransferases mediate the prenylation of proteins through recognition of CAAX or dicysteine motifs. There is some overlap in the composition of individual complexes: FNTA pairs with different catalytic subunits (FNTB, PGGT1B) to form FTase and GGTase I; the catalytic subunit RABGGTB pairs with different subunits (RABGGTA/PTAR3, PTAR1) to form GGTase 2 (also known as RabGGTase) and GGTase 3. Chaperone proteins REP and SKP1 are respectively required for geranylgeranylation by GGTase 2 and newly identified GGTase 3. FTase inhibitors (FTI) have clinical potential in tumors driven by H-RAS, which is exclusively farnesylated. Currently available GGTase inhibitors (GGTI) competitively inhibit GGTase I by acting as a CAAX peptidomimetic though their activity towards GGTase 3 is not well characterized. Clickable prenylation probes provide tools to assess the substrates of each prenyltransferase using either targeted or global approaches using fluorescent tags like tetramethylrhodamine (TAMRA) azide (N3-TAMRA) or biotin-labeling with biotin azide (N3-Biotin).

Figure 6.. Rational combinations enhancing the effects of statins at the mitochondria.

The mevalonate pathway supports mitochondrial function by providing ubiquinone and heme A that support the electron transport chain and geranylgeranyl pyrophosphate (GGPP) for GTPases that promote survival. Cancers that have lost p53 function (p53 LOF) depend on the mevalonate pathway to support pyrimidine synthesis through the ubiquinone-reducing, dihydroorotate (DHO) dehydrogenase (DHODH) enzyme. Statins induce the production of reactive oxygen species. A MEK-NRF2-mediated compensatory increase of the cystine/glutamate antiporter, xCT, provides resistance to oxidation by supporting glutathione synthesis, which can be prevented by combining statins with the clinical MEK inhibitor, selumetinib. Furthering oxidative phosphorylation (Ox Phos) dysfunction with Ox Phos inhibitors synergize with statins to induce cell death. GPX4 and FSP1 depend on mevalonate pathway outputs to prevent lipid peroxidation and sensitization to ferroptosis. Inhibition of protein geranylgeranylation in blood cancers (via statins or GGTIs) results in the increase of p53-upregulated modulator of apoptosis (PUMA) in a p53 independent manner. Statin-induced PUMA sensitizes to the clinical BCL-2 inhibitor, venetoclax (Veneto), by disrupting anti-apoptotic interactions of BCL-2 as well as MCL-1 Release of the BH3 activator, BIM, induces mitochondrial outer membrane permeability (MOMP) resulting in cytochrome c (Cyto C) release and blood cancer apoptosis.