Targeted therapies and the consequent adoption of “personalized” oncology have achieved notable successes in some cancers; however, significant problems remain with this approach. Many targeted therapies are highly toxic, costs are extremely high, and most patients experience relapse after a few disease-free months. Relapses arise from genetic heterogeneity in tumors, which harbor therapy-resistant immortalized cells that have adopted alternate and compensatory pathways (i.e., pathways that are not reliant upon the same mechanisms as those which have been targeted). To address these limitations, an international task force of 180 scientists was assembled to explore the concept of a low-toxicity “broad-spectrum” therapeutic approach that could simultaneously target many key pathways and mechanisms. Using cancer hallmark phenotypes and the tumor microenvironment to account for the various aspects of relevant cancer biology, interdisciplinary teams reviewed each hallmark area and nominated a wide range of high-priority targets (74 in total) that could be modified to improve patient outcomes. For these targets, corresponding low-toxicity therapeutic approaches were then suggested; many of which were phytochemicals. Proposed actions on each target and all of the approaches were further reviewed for known effects on other hallmark areas and the tumor microenvironment. Potential contrary or procarcinogenic effects were found for 3.9% of the relationships between targets and hallmarks, and mixed evidence of complementary and contrary relationships was found for 7.1%. Approximately 67% of the relationships revealed potentially complementary effects, and the remainder had no known relationship. Among the approaches, 1.1% had contrary, 2.8% had mixed and 62.1% had complementary relationships. These results suggest that a broad-spectrum approach should be feasible from a safety standpoint. This novel approach has potential to help us address disease relapse, which is a substantial and longstanding problem, so a proposed agenda for future research is offered.

Pure and mixed isoprenoid end products of plant mevalonate metabolism trigger actions that suppress 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity. These actions modulate HMG CoA reductase mRNA translation and the proteolytic degradation of HMG CoA reductase. Such post-transcriptional events, we propose, are activated directly by acyclic isoprenoids and indirectly by cyclic isoprenoids. Isoprenoids, acting secondarily to the dominant transcriptional effector of sterologenesis, modestly lower cholesterol levels, if and only if, sterologenesis is not repressed by a saturating imput of dietary cholesterol. An anomaly associated with tumor growth-a sterol feedback-resistant HMG CoA reductase activityensures a pool of sterologenic pathway intermediates. Such intermediates provide lipophilic anchors essential for membrane attachment and biological activity of growth hormone receptors, nuclear lamins A and B, and oncogenic ras. Tumor HMG CoA reductase retains high sensitivity to the isoprenoid-mediated secondary regulation. Repression of mevalonate synthesis by plant-derived isoprenoids reduces ras and lamin B processing, arrests cells in G1, and initiates cellular apoptosis. This unique tumor cell-specific sensitivity allows isoprenoids to be used for tumor therapy, an application emulating that of the statins, but one free of adverse effects. When evaluated at levels provided by a typical diet, isoprenoids individually have no impact on cholesterol synthesis and tumor growth. Nonetheless, isoprenoid-mediated activities are additive, and, sometimes synergistic. Therefore, the combined actions of the estimated 23,000 isoprenoid constituents of plant materials, acting in concert with other chemopreventive phytochemicals, may explain the lowered cancer risk associated with a diet rich in plant products. In contrast, that lowering of cancer risk does not correspond to supplemental intake of other dietary factors associated with fruits, vegetables, and cereal grains, namely fiber, p-carotene, vitamin C, and vitamin E, and only weakly to supplemental folate.

Elevated 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase expression supports synthesis of prenyl pyrophosphate intermediates required for tumor growth. In this study, the copy number of HMG-CoA reductase mRNA was determined in solid tumor and leukemic cell lines using competitive reverse transcriptase-polymerase chain reaction. Reductase mRNA was increased about eight-fold in Caco2 human colon adenocarcinoma cells compared with that in CCD18 normal colon cells. We also found a 50-fold enhancement of reductase mRNA in stimulated human lymphocytes compared with unstimulated cells. In CEM human leukemia cells, reductase mRNA was increased 8.6 times compared with that in stimulated lymphocytes. Greater low density lipoprotein receptor mRNA was also observed in tumor cells compared with normal counterparts. We hypothesized that elevated reductase mRNA was due to attenuation of sterol-mediated control of tumor reductase promoter activity. We first compared the methylation status of CpG dinucleotides in the promoters of reductase and p16 tumor suppressor genes from solid tumor, leukemic, and normal cells. As reported for other tumor cells the p16 promoter region was hypermethylated in Caco2 and CEM cells but was hypomethylated in corresponding normal cells. However, reductase promoter sequences in both normal and tumor cells were hypomethylated, demonstrating that methylation is not involved in sterol-independent reductase regulation. We addressed altered transcription factor binding to the tumor cell reductase promoter by transiently transfecting Caco2 and CCD18 with a plasmid vector containing a hamster HMG-CoA reductase promoter fused to the luciferase gene. We found that increased reductase mRNA was partially due to an approximately three-fold higher reductase promoter activity in Caco2 than in CCD18, measured by luciferase reporter assays. Thus, differential binding of transcription factor or factors on the tumor cell reductase promoter attenuates normal sterol-mediated regulation of reductase activity.

Pure and mixed isoprenoid end products of plant mevalonate metabolism trigger actions that suppress 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity. These actions modulate HMG CoA reductase mRNA translation and the proteolytic degradation of HMG CoA reductase. Such post-transcriptional events, we propose, are activated directly by acyclic isoprenoids and indirectly by cyclic isoprenoids. Isoprenoids, acting secondarily to the dominant transcriptional effector of sterologenesis, modestly lower cholesterol levels, if and only if, sterologenesis is not repressed by a saturating imput of dietary cholesterol. An anomaly associated with tumor growth-a sterol feedback-resistant HMG CoA reductase activity-ensures a pool of sterologenic pathway intermediates. Such intermediates provide lipophilic anchors essential for membrane attachment and biological activity of growth hormone receptors, nuclear lamins A and B, and oncogenic ras. Tumor HMG CoA reductase retains high sensitivity to the isoprenoid-mediated secondary regulation. Repression of mevalonate synthesis by plant-derived isoprenoids reduces ras and lamin B processing, arrests cells in G1, and initiates cellular apoptosis. This unique tumor cell-specific sensitivity allows isoprenoids to be used for tumor therapy, an application emulating that of the statins, but one free of adverse effects. When evaluated at levels provided by a typical diet, isoprenoids individually have no impact on cholesterol synthesis and tumor growth. Nonetheless, isoprenoid-mediated activities are additive, and, sometimes synergistic. Therefore, the combined actions of the estimated 23,000 isoprenoid constituents of plant materials, acting in concert with other chemopreventive phytochemicals, may explain the lowered cancer risk associated with a diet rich in plant products. In contrast, that lowering of cancer risk does not correspond to supplemental intake of other dietary factors associated with fruits, vegetables, and cereal grains, namely fiber, beta-carotene, vitamin C, and vitamin E, and only weakly to supplemental folate.

One of the hallmarks of malignant cell populations is the ability to undergo continuous proliferation. This property allows clonal lineages to acquire sequential aberrations that can fuel increasingly autonomous growth, invasiveness, and therapeutic resistance. Innate cellular mechanisms have evolved to regulate replicative potential as a hedge against malignant progression. When activated in the absence of normal terminal differentiation cues, these mechanisms can result in a state of persistent cytostasis. This state, termed “senescence,” can be triggered by intrinsic cellular processes such as telomere dysfunction and oncogene expression, and by exogenous factors such as DNA damaging agents or oxidative environments. Despite differences in upstream signaling, senescence often involves convergent interdependent activation of tumor suppressors p53 and p16/pRB, but can be induced, albeit with reduced sensitivity, when these suppressors are compromised. Doses of conventional genotoxic drugs required to achieve cancer cell senescence are often much lower than doses required to achieve outright cell death. Additional therapies, such as those targeting cyclin dependent kinases or components of the PI3K signaling pathway, may induce senescence specifically in cancer cells by circumventing defects in tumor suppressor pathways or exploiting cancer cells’ heightened requirements for telomerase. Such treatments sufficient to induce cancer cell senescence could provide increased patient survival with fewer and less severe side effects than conventional cytotoxic regimens. This positive aspect is countered by important caveats regarding senescence reversibility, genomic instability, and paracrine effects that may increase heterogeneity and adaptive resistance of surviving cancer cells. Nevertheless, agents that effectively disrupt replicative immortality will likely be valuable components of new combinatorial approaches to cancer therapy.