All cancer registries constantly show striking differences in cancer incidence by age and among tissues. For example, lung cancer is diagnosed hundreds of times more often at age 70 than at age 20, and this cancer in nonsmokers occurs thousands of times more frequently than heart cancer in smokers. An analysis of these differences using basic concepts in cell biology indicates that cancer is the end-result of the accumulation of cell divisions in stem cells. In other words, the main determinant of carcinogenesis is the number of cell divisions that the DNA of a stem cell has accumulated in any type of cell from the zygote. Cell division, process by which a cell copies and separates its cellular components to finally split into two cells, is necessary to produce the large number of cells required for living. However, cell division can lead to a variety of cancer-promoting errors, such as mutations occurring during DNA replication, chromosome aberrations arising during mitosis, errors in the distribution of cell-fate determinants between the daughter cells, and failures to restore physical interactions with other tissue components. Some of these errors are spontaneous, others are promoted by endogenous DNA damage occurring during quiescence, and others are influenced by pathological and environmental factors. The cell divisions required for carcinogenesis are primarily caused by multiple local and systemic physiological signals rather than by errors in the DNA of the cells. As carcinogenesis progresses, the accumulation of DNA errors promotes cell division and eventually triggers cell division under permissive extracellular environments. The accumulation of cell divisions in stem cells drives not only the accumulation of the DNA alterations required for carcinogenesis, but also the formation and growth of the abnormal cell populations that characterize the disease. This model of carcinogenesis provides a new framework for understanding the disease and has important implications for cancer prevention and therapy.

Cancer cells cannot proliferate and survive unless they obtain sufficient levels of the 20 proteinogenic amino acids (AAs). Unlike normal cells, cancer cells have genetic and metabolic alterations that may limit their capacity to obtain adequate levels of the 20 AAs under challenging metabolic environments. However, since normal diets provide all AAs at relatively constant levels and ratios, these potentially lethal genetic and metabolic defects are eventually harmless to cancer cells. If we temporarily replace the normal diet of cancer patients with artificial diets in which the levels of specific AAs are manipulated, cancer cells may be unable to proliferate and survive. This article reviews in vivo studies that have evaluated the antitumor activity of diets restricted or supplemented with the 20 proteinogenic AAs, individually and in combinations. It also reviews our recent studies that show that manipulating the levels of several AAs simultaneously can lead to marked survival improvements in mice with metastatic cancers.

Almost 6% of cancers worldwide are attributable to alcohol consumption. Approximately half of them occur in tissues highly exposed to ethanol, such as the oral cavity, pharynx, upper larynx and esophagus. However, since ethanol is not mutagenic and the carcinogenic metabolite of ethanol (acetaldehyde) is mainly produced in the liver, it is unclear why alcohol consumption preferentially causes a local carcinogenic effect. We recently hypothesized that the cytotoxic activity of ethanol could explain the high risk of these cancers in alcohol users. Here we report that short-term exposures (2-3 seconds) to ethanol concentrations between 10% and 15% start to cause a marked cytotoxic effect on human epithelial keratinocytes in a concentration-dependent manner. After discussing new evidence that cancer is the end-result of the accumulation of cell divisions in stem cells, we explain why regular alcohol consumption imposes a high risk of cancer on these tissues. Briefly, the cytotoxicity of ethanol reduces the lifespan of the cells lining these tissues. The stem cells located in deeper layers need to divide more often than usual to renew the damaged epithelia. The accumulation of cell divisions in stem cells leads to the accumulation of cancer-promoting errors (e.g., mutations arising during DNA replication) that increase their risk of malignant transformation. Cell division also exposes the DNA of the stem cells to the genotoxic activity of acetaldehyde and tobacco carcinogens. We propose that choosing alcoholic beverages containing non-cytotoxic concentrations of ethanol, or diluting ethanol to non-cytotoxic concentrations, is a simple way to reduce the risk of cancer of the oral cavity, pharynx, larynx and esophagus in alcohol users. This preventive strategy may also abolish the known synergistic effect of alcohol drinking and tobacco smoking on the risk of these cancers.

Extremely low-frequency electromagnetic fields (ELF-EMFs) are non-ionizing radiations typically emitted by power lines, electrical wiring and electrical appliances. Epidemiological studies have repeatedly shown a positive association between ELF-EMFs and childhood leukemia. Exposures greater than 0.3-0.4 µT increase the risk by approximately 1.5-2 fold, and estimates indicate that up to 2% of childhood leukemia cases in Europe may be attributable to ELF-EMFs. However, it is considered unlikely that ELF-EMFs can cause cancer, because carcinogenesis requires the accumulation of DNA alterations and ELF-EMFs do not have enough energy to damage the DNA. Lack of biological plausibility is a barrier to accept the evidence of carcinogenicity in human studies and to take measures to protect pregnant women and children from ELF-EMFs. Recent evidence indicates that non-mutagenic agents can cause DNA alterations and increase the risk of cancer by promoting the accumulation of cell divisions in stem cells. Cell division generates DNA alterations (e.g., mutations arising during DNA replication), which occur even in the absence of DNA-damaging agents. Importantly, ELF-EMFs can trigger the division of stem cells; this effect is under development in the field of regenerative medicine. A possible mechanism by which ELF-EMFs induce the malignant transformation of hematopoietic stem cells is discussed.

Epidemiological data indicate that 5.8% of cancer deaths world-wide are attributable to alcohol consumption. The risk of cancer is higher in tissues in closest contact on ingestion of alcohol, such as the oral cavity, pharynx and esophagus. The risk of these cancers is increased even in people who have only one alcoholic drink per day. However, since ethanol is not mutagenic and the carcinogenic metabolite of ethanol (acetaldehyde) is mostly produced in the liver, it is not clear why alcohol use preferentially exerts a local carcinogenic effect. It is well known that ethanol causes cell death at the concentrations present in alcoholic beverages; however, this effect has been overlooked probably because dead cells cannot give rise to cancer. Here I discuss that the cytotoxic effect of ethanol on the cells lining the oral cavity, pharynx and esophagus activates the division of the stem cells located in deeper layers of the mucosa to replace the dead cells. Every time stem cells divide, they become exposed to unavoidable errors associated with cell division (e.g., mutations arising during DNA replication and chromosomal alterations occurring during mitosis) and also become highly vulnerable to the genotoxic activity of endogenous and exogenous DNA-damaging agents (e.g., reactive oxygen species, acetaldehyde and tobacco carcinogens). Alcohol consumption probably increases the risk of developing cancer of the oral cavity, pharynx and esophagus by promoting the accumulation of cell divisions in the stem cells that maintain these tissues in homeostasis. Because the cytotoxic activity of ethanol is concentration-dependent, the risk of these cancers will not only increase with increasing amounts of ethanol, but also with increasing concentrations; an ounce of whisky is probably more carcinogenic when taken undiluted than when taken mixed with non-alcoholic beverages. The local cytotoxic effect of ethanol can also explain the known synergistic effect of alcohol and tobacco use on the risk of these cancers. Understanding the mechanisms of carcinogenicity of alcohol is important to reinforce the epidemiological evidence and to raise public awareness of the strong link between alcohol consumption and cancer.

Tobacco use, most people would say. Smoking tobacco increases the risk of developing many types of cancer and is responsible for approximately one-third of all cancer deaths. The association between tobacco use and lung cancer is well known; lung cancer occurs about 20 times more often in heavy smokers than in nonsmokers [1]. However, many lung cancers are diagnosed in never smokers [2], and most smokers do not develop lung cancer [3,4].

The sulfur-containing amino acids methionine (Met), cysteine (Cys) and taurine (Tau) are common dietary constituents with important cellular roles. Met restriction is already known to exert in vivo anticancer activity. However, since Met is a precursor of Cys, and Cys produces Tau, the role of Cys and Tau in the anticancer activity of Met-restricted diets is poorly understood. In this work, we screened the in vivo anticancer activity of several Met-deficient artificial diets supplemented with Cys, Tau or both. Diet B1 (6% casein, 2.5% leucine, 0.2% Cys and 1% lipids) and diet B2B (6% casein, 5% glutamine, 2.5% leucine, 0.2% Tau and 1% lipids) showed the highest activity and were selected for further studies. Both diets induced a marked anticancer activity in two animal models of metastatic colon cancer, which were established by injecting CT26.WT murine colon cancer cells in the tail vein or peritoneum of immunocompetent BALB/cAnNRj mice. Diets B1 and B2B also increased the survival of mice with disseminated ovarian cancer (intraperitoneal ID8 Tp53 -/- cells in C57BL/6JRj mice) and renal cell carcinoma (intraperitoneal Renca cells in BALB/cAnNRj mice). The high activity of diet B1 in mice with metastatic colon cancer may be useful in colon cancer therapy.

Patients with metastatic triple negative breast cancer (TNBC) need new therapies to improve the low survival rates achieved with standard treatments. In this work, we show that the survival of mice with metastatic TNBC can be markedly increased by replacing their normal diet with artificial diets in which the levels of amino acids (AAs) and lipids are strongly manipulated. After observing selective anticancer activity in vitro, we prepared five artificial diets and evaluated their anticancer activity in a challenging model of metastatic TNBC. The model was established by injecting 4T1 murine TNBC cells into the tail vein of immunocompetent BALB/cAnNRj mice. First-line drugs doxorubicin and capecitabine were used as positive controls. AA manipulation led to modest improvements in mice survival when the levels or lipids were normal. Reducing lipid levels to 1% markedly improved the activity of several diets with different AA content. Mice fed the artificial diets as monotherapy lived longer than mice treated with doxorubicin and capecitabine. An artificial diet without 10 non-essential AAs, with reduced levels of essential AAs, and with 1% lipids improved the survival not only of mice with TNBC but also of mice with other types of metastatic cancers.

Targeted therapies with antiangiogenic drugs (e.g., sunitinib) and immune checkpoint inhibitors (e.g., anti-PD-1 antibodies) are the standard of care for patients with metastatic renal cell carcinoma. Although these treatments improve patient survival, they are rarely curative. We previously hypothesized that advanced cancers might be treated without drugs by using artificial diets in which the levels of specific amino acids (AAs) are manipulated. In this work, after showing that AA manipulation induces selective anticancer activity in renal cell carcinoma cells in vitro, we evaluated the anticancer activity of 17 artificial diets in a challenging animal model of renal cell carcinoma. The model was stablished by injecting murine renal cell carcinoma (Renca) cells into the peritoneum of immunocompetent BALB/cAnNRj mice. Mice survival was markedly improved when their normal diet was replaced with our artificial diets. Mice fed a diet lacking six AAs (diet T2) lived longer than mice treated with sunitinib or anti-PD-1 immunotherapy; several animals lived very long or were cured. Controlling the levels of several AAs (e.g., cysteine, methionine and leucine) and lipids was important for the anticancer activity of the diets. Additional studies are needed to further evaluate the therapeutic potential of this simple and inexpensive anticancer strategy.

In our continuous search for selective anticancer treatments, we have screened 65 extracts from 45 plants collected in several areas of Western Andalusia (Spain) for cytotoxic activity against lung cancer cells and lung normal cells. Active extracts were also tested against 11 cell lines from other tissues. An extract from the leaves of Tetraclinis articulata (Vahl) Mast. (Cupressaceae) showed a marked cytotoxicity (IC₅₀ = 0.37 ± 0.03 µg/mL) and selectivity (selectivity index = 378.3) against the lung cancer cells; cisplatin, 5-fluorouracil and an extract from the leaves of Taxus baccata L. (Taxaceae) were less cytotoxic and selective.

Although several plant-derived drug groups (vinca alkaloids, taxanes, podophyllotoxin derivatives and camptothecins) continue to be widely used in cancer therapy, the anticancer potential of the Plant Kingdom remains largely unexplored. In this work, we have carried out a random screening for selective anticancer activity of 57 extracts from 45 plants collected in Grazalema Natural Park, an area in the South of Spain of high plant diversity and endemism. Using lung cancer cells (A549) and lung non-malignant cells (MRC-5), we found that several extracts were more cytotoxic and selective against the cancer cell line than the standard anticancer agent cisplatin. Five active extracts were further tested in cancer and normal cell lines from other tissues, including three skin cell lines with increasing degree of malignancy. An extract from the leaves of Daphne laureola L. (Thymelaeaceae) showed a striking potency and selectivity on lung cancer cells and leukemia cells; the IC₅₀ values against these cancer cells were approximately 10000-fold lower than against the normal cells. Daphnane-type diterpene orthoesters may be responsible for this highly selective anticancer activity.

We recently screened a series of new aziridines β-D-galactopyranoside derivatives for selective anticancer activity and identified 2-methyl-2,3-[N-(4-methylbenzenesulfonyl)imino]propyl 2,3-di-O-benzyl-4,6-O-(S)-benzylidene-β-D-galactopyranoside (AzGalp) as the most promising compound. In this article, we explore possible mechanisms involved in the cytotoxicity of this aziridine and evaluate its selective anticancer activity using cancer cells and normal cells from a variety of tissues. Our data show that AzGalp induces DNA damage (detected with the comet assay). Cells deficient in the DNA repair pathway nucleotide excision repair (NER) were hypersensitive to the cytotoxicity of this compound. These results suggest that AzGalp induces bulky DNA adducts, and that cancer cells lacking a functional NER pathway may be particularly vulnerable to the anticancer effects of this aziridine. Several experiments revealed that neither the generation of oxidative stress nor the inhibition of glycolysis played a significant role in the cytotoxicity of AzGalp. The combinations of AzGalp with either oxaliplatin or 5-fluorouracil slightly improved the ability of both anticancer drugs to selectively kill cancer cells. AzGalp also displayed selective cytotoxicity against a panel of malignant cells versus normal cells; the highest selectivity was observed for two acute promyelocytic leukemia cell lines. Additional preclinical studies are necessary to evaluate the anticancer potential of AzGalp.