KRAS: feeding pancreatic cancer proliferation

Abstract

Oncogenic KRAS mutation is the signature genetic event in the progression and growth of pancreatic ductal adenocarcinoma (PDAC), an almost universally fatal disease. Although it has been appreciated for some time that nearly 95% of PDAC harbor mutationally activated KRAS, to date no effective treatments that target this mutant protein have reached the clinic. A number of studies have shown that oncogenic KRAS plays a central role in controlling tumor metabolism by orchestrating multiple metabolic changes including stimulation of glucose uptake, differential channeling of glucose intermediates, reprogrammed glutamine metabolism, increased autophagy, and macropinocytosis. We review these recent findings and address how they may be applied to develop new PDAC treatments.

Keywords: autophagy; glutaminolysis; glycolysis; macropinocytosis; metabolism.

Figure 1

Human Ras proteins are small GTPases. (a) The human RAS genes encode 188 or 189 amino acid proteins that share strong (82–90% overall) amino acid identity; percentages indicate identity with H-Ras. KRAS encodes two related proteins (K-Ras4A or K-Ras4B; 90% identity) due to alternative exon four utilization, with KRAS4B the predominant transcript in pancreatic tissue. Residues 1–164 comprise the G domain that binds and hydrolyses GTP (93–99% sequence identity). The remaining 24/25 C-terminal residues (shown in inset) comprise the membrane targeting sequence (16–40% identity), where the C-terminal four residues comprise the CAAX motif (shaded in pink; C = cysteine, A = aliphatic; X = terminal amino acid) that signals for farnesyltransferase-catalyzed covalent addition of a C15 farnesyl group to the cysteine residue. The 20/21 amino acids upstream of the CAAX motif comprise the hypervariable (HV) region (shaded in blue) where the Ras proteins exhibit the greatest sequence divergence. Within the HV domain are additional membrane targeting sequence elements that include cysteines that are covalently modified by addition of a palmitate fatty acid (green, underlined ‘C’) or polybasic sequences that promote association with the membrane (blue K). K-Ras4B contains a serine (S181) that is reversibly phosphorylated (yellow circle), regulating localization between the plasma and endomembranes. (b) Mutant K-Ras is persistently GTP-bound and active. Wild type K-Ras cycles between an active GTP-bound and an inactive GDP-bound state. In normal quiescent cells, K-Ras is predominantly GDP-bound. Upon growth factor stimulation, RasGEF activation promotes transient formation of K-Ras-GTP, promoting its binding to downstream effectors (E; e.g., Raf, PI3K). The cycle is terminated by the action of RasGAPs, returning K-Ras to the inactive GDP-bound state. Single amino acid substitutions at G12, G13 or Q61 impair the intrinsic GTP binding and hydrolytic activity of K-Ras, and additionally render the protein insensitive to GAP stimulation, favoring accumulation of persistently GTP-bound and active K-Ras in PDAC. Arrow line thickness corresponds to the level of signaling, i.e. in normal cells Ras is predominantly in the GDP-bound state due to relatively higher GAP activity and thus signals less to downstream effectors.

Figure 1

Human Ras proteins are small GTPases. (a) The human RAS genes encode 188 or 189 amino acid proteins that share strong (82–90% overall) amino acid identity; percentages indicate identity with H-Ras. KRAS encodes two related proteins (K-Ras4A or K-Ras4B; 90% identity) due to alternative exon four utilization, with KRAS4B the predominant transcript in pancreatic tissue. Residues 1–164 comprise the G domain that binds and hydrolyses GTP (93–99% sequence identity). The remaining 24/25 C-terminal residues (shown in inset) comprise the membrane targeting sequence (16–40% identity), where the C-terminal four residues comprise the CAAX motif (shaded in pink; C = cysteine, A = aliphatic; X = terminal amino acid) that signals for farnesyltransferase-catalyzed covalent addition of a C15 farnesyl group to the cysteine residue. The 20/21 amino acids upstream of the CAAX motif comprise the hypervariable (HV) region (shaded in blue) where the Ras proteins exhibit the greatest sequence divergence. Within the HV domain are additional membrane targeting sequence elements that include cysteines that are covalently modified by addition of a palmitate fatty acid (green, underlined ‘C’) or polybasic sequences that promote association with the membrane (blue K). K-Ras4B contains a serine (S181) that is reversibly phosphorylated (yellow circle), regulating localization between the plasma and endomembranes. (b) Mutant K-Ras is persistently GTP-bound and active. Wild type K-Ras cycles between an active GTP-bound and an inactive GDP-bound state. In normal quiescent cells, K-Ras is predominantly GDP-bound. Upon growth factor stimulation, RasGEF activation promotes transient formation of K-Ras-GTP, promoting its binding to downstream effectors (E; e.g., Raf, PI3K). The cycle is terminated by the action of RasGAPs, returning K-Ras to the inactive GDP-bound state. Single amino acid substitutions at G12, G13 or Q61 impair the intrinsic GTP binding and hydrolytic activity of K-Ras, and additionally render the protein insensitive to GAP stimulation, favoring accumulation of persistently GTP-bound and active K-Ras in PDAC. Arrow line thickness corresponds to the level of signaling, i.e. in normal cells Ras is predominantly in the GDP-bound state due to relatively higher GAP activity and thus signals less to downstream effectors.

Figure 2

KRAS mutations in PDAC. (a) Missense mutations result in single amino acid substitutions primarily at G12 (98%), and at lower frequencies at G13 (21%) or Q61 (28%). (b) At G12, eight different amino acid substitutions have been identified, with G12D the predominant mutation (51%). (c) At G13, four different mutations have been described, with the majority G13D. (d) Mutation at Q61 also results in constitutive activation, and among the three mutations described, Q61H occurs most frequently. There is increasing evidence that the different activating mutations may not have the same biochemical and biological consequences. Data were compiled from COSMIC (http://cancer.sanger.ac.uk/cancergenome/projects/cosmic/).

Figure 3

Histologic and genetic progression of pancreatic ductal adenocarcinoma. Pancreatic intraepithelial neoplasias (PanINs), small microscopic abnormal duct structures, are believed to be precursors to invasive pancreatic cancer. While the true precursor cell for PDAC development remains to be fully elucidated, mouse model studies where targeted KRAS activation in acinar cells causes PanIN lesion development suggest that this is the cell of origin for PDAC. The four predominant gene mutations appear to occur in a temporal fashion with PanIN progression. KRAS activating mutations can be found in normal pancreas and PanIN-1. CDKN2A inactivating mutations appear to occur early, in PanINs with low- to intermediate-grade dysplasia, whereas TP53 and SMAD4 inactivating mutations appear to be late events, and occur in PanINs with high-grade dysplasia and in invasive cancer. CDKN2A encodes p14/Arf and p16/Ink4a, an inhibitor of CDK4/6 and G1 cell cycle progression. TP53 encodes the p53 tumor suppressor, a regulator of the cell cycle that is responsible for maintaining cellular and genetic stability. SMAD4 encodes Smad4, a downstream component of the TGF-β signaling network. Oncogenes are indicated with green arrows and tumor suppressors with red arrows.

Figure 4

Anti-Ras drug discovery. Past and ongoing efforts to develop anti-K-Ras inhibitors have included inhibitors of membrane association and downstream effector signaling. Inhibitors of FTases advanced to Phase III clinical evaluation but did not show significant anti-tumor activity for pancreatic cancer. Currently, there are 22 inhibitors of Raf, MEK and/or ERK under clinical evaluation (clinicaltrials.gov). Similarly, 43 inhibitors of the class I PI3K lipid kinases and their downstream targets, the AKT and mTOR protein kinases, are currently under clinical evaluation. Only recently have small molecules that directly bind to K-Ras and cause perturbations in function been described and characterized in cell culture models. Unbiased genetic functional RNAi screens have identified genes (synthetic lethal interactors) whose functions are essential for the growth of KRAS mutant but not wild type cells. The dashed two-headed arrow indicates a functional relationship between KRAS and the interactor that is typically not mediated through a clear signaling mechanism that connects the two components.

Figure 5

Glucose and glutamine metabolism is altered by oncogenic K-Ras. Oncogenic K-Ras directs glucose metabolism into biosynthetic pathways in PDAC by upregulating many key enzymes in glycolysis. Oncogenic K-Ras induces nonoxidative pentose phosphate pathway (PPP) flux to fuel increased nucleic acid biosynthesis and activates the hexosamine biosynthesis and glycosylation pathways. PDAC cells also utilize a non-canonical pathway to process glutamine, through which it is used to maintain redox and support growth. Enzymes that show increased activity under the control of oncogenic K-Ras are shown in red, and metabolites that are increased are boxed in orange. Glut1, glucose transporter 1; Hk 1/2, hexokinase 1/2; Pfk1, phosphofructokinase 1; Eno1, enolase 1; Pkm, pyruvate kinase; Ldha, lactate dehydrogenase A; Gfpt1, glucosamine-fructose-6-phosphate aminotransferase-1; GlcN, glucosamine; GlcNAc, N-acetylglucosamine; Rpe, ribulose-5-phosphate-3-epimerase; Rpia, ribulose-5-phosphate isomerase; GLUD1, glutamate dehydrogenase 1; GOT, aspartate transaminase; GLS, glutaminase; ME1, malic enzyme.

Figure 6

KRAS-driven transcriptional reprogramming contributes to the utilization of autophagy and macropinocytosis to meet the metabolic needs of PDAC cells. Macropinocytosis is utilized to transport extracellular protein into the cell. Following degradation, the internalized protein yields amino acids such as glutamine, which can enter the mitochondria to fuel central carbon metabolism. Autophagy recycles cellular components to basic building blocks such as nucleosides, sugars, and amino acids that can be exploited to fuel nucleic acid biosynthesis and glutaminolysis. Through mitophagy, Ras-driven tumor cells can remove damaged mitochondria, which would otherwise increase ROS stress.