Factors Involved in Cancer Development

• Most cancers may occur because of environmental factors.  This conclusion takes into account findings that (a) specific cancer incidence rates are not constant (b) cancer rates, even within populations, change and are affected by migration.⁶

• Within the framework of environmental factors one should consider occupation, tobacco use, alcohol use, sun exposure (skin cancers such as melanoma), diet, occupation.  In addition to these factors, underlying risk may occur as a result of hereditary predispositions.  Hereditary effects could influence the sequential processes involved in transformation of normal cell biology to cancer cell biology.⁶

  • Epidemiological analysis has suggested that cancers which present in middle and old age might be reduced by as much as 80% to 90% with overall risk, worldwide,  reduced by 50%.  Along with this suggestion is the realization that attainment of this result might require approaches not considered socially acceptable.  In this epidemiological study considering behavioral and environmental impacts on human cancer risk, proportions of fatal cancers associated with different causes were examined under 17 categories, some noted below.^6,7

• Smoking was determined to be the most important consideration.  Smoking was an identifiable factor in the development of seven cancer types in addition to eight recognized earlier by the International Agency for Research on Cancer (1986).  Smoking was likely responsible for 38% of cancers in men and 6% in women in Germany.  Probably the above noted factors act in a synergistic way with the ultimate risk of developing a particular cancer type modifiable in accord with changing risk factor profiles.⁷

Carcinogens

• Carcinogens:  The definition of carcinogen includes chemical agents, biological agents, and physical factors, such as ionizing radiation, that can cause cancer in animals or humans.

• Examples of cancer induced by radiation include ultraviolet radiation from sunlight causing⁶:

  •  (a) Cutaneous melanoma, which is a melanoma starting in the skin rather than the eye or an internal organ.

  •  (b) Basal cell carcinoma, a skin cancer involving cells in the lower part or base of the outer layer of skin (epidermis).

  •  (c) Squamous cell carcinoma.  This type of skin cancer occurs in the principal cells of the epidermis (squamous cells). 

• Higher intensity, ionizing radiation, can produce⁶:

  •  (a) Sarcomas (Sarcomas refer to malignant tumors associated with connective tissue, e.g. cartilage, or bone.)

  •  (b) Leukemias (Leukemia refers to blood cell cancer.  Blood cell growth and development are abnormal and this abnormality is principally associated with leukocytes, i.e. white blood cells.  Often leukemia can be used in a more general way to reference cancer associated with any cell in blood or bone marrow.  An example would be erythroleukemia.)

  •  (c) Carcinomas. 

• Cancer as a result of ionizing radiation  was described over 400 years ago when it was noted that miners in the Black Forest regions in Eastern Europe exhibited premature death.  The explanation for these deaths was likely radon gas which accumulated to high concentration in relatively unventilated mine tunnels. Probably about 3% of all cancers may be attributable to radiation exposure, most of the time this exposure is from natural sources such as radon and cosmic rays (cosmic rays, which are high-energy radiation originating beyond the Earth’s atmosphere, include protons, alpha particles which consist of positively charged helium nuclei, 2 protons and 2 neutrons,  and other atomic nuclei).⁸  

• Carcinogenic effects from radiation have been assessed mainly in populations exposed to > 20 cGy. (20 rad, 1 rad equals a dose of 0.01 Joules of energy/kilogram; 1 rad = 0.01 Gray).⁸  

  • In order to compare higher exposures to more commonly encountered references, a chest x-ray exposes bone marrow to about 0.026 cGy, whereas a mammogram exposes breast tissue to about 0.3 cGy.  Naturally occurring background radiation contributes about 0.1 to 0.3 cGy per year in whole body radiation. 

  • By contrast, workplace or occupational radiation limits are set at 5 cGy per year. The LD₅₀ for radiation (lethal dose for 50% of the population in one month)  would be about 400 cGy.  In radiation cancer treatment (radiation oncology treatment), targeted exposure would range between 1000 to 6000 cGy.⁸

  • Significant occupational exposure is associated with underground mining, medical technologies, as well as involvement in nuclear fuel/activities.⁸  

    • Although current physicians are probably not at increased risk from radiation used in their practices, early radiologists were likely exposed over time to large levels which were reflected in increased incidence of skin cancer and leukemia. 

    • Many years ago, long before the “digital” age, analog watch dials were painted with radium, allowing illumination in the dark.  Unfortunately, this practice exposed these workers to very high radiation dosages, probably resulting in bone and sinus sarcomas.

    •  Underground miners exposed to radon exhibit at increases in lung cancer incidence.⁸

  • Hazards possibly associated with radon exposure, at low levels, found in the home have been of recent concern since indoor radon may account for over 50% of all general population radiation exposure.  Based on analysis of extrapolated data from high-dose exposure, indoor radon exposure may increase lung cancer deaths per year by an amount in a range between  6000 to 36,000.⁸  

  • Ultraviolet Radiation:   Sunlight emits electromagnetic (photon) radiation at wavelengths encompassing a range of 11 orders of magnitude [10^-4 nm (nanometers) to 10¹² nm).  DNA exhibits an absorption maximum at 260 nm and is probably the primary chromophore for UV radiation (UVR).

    • DNA Absorption Spectrum¹⁹

  • The relationship between radio waves on one hand and gamma rays on the other as well as the intermediate energies in electromagnetic spectrum is noted below.

    • Electromagnetic Energy Spectrum²⁰

  • UVR is subdivided into three regions, UVC (240 to 290 nm), UVB (290 to 320 nm) and UVA (320 to 400 nm).  UVC radiation is absorbed by atmospheric (stratospheric) ozone resulting in limited wavelengths < 300 nm reaching the Earth’s surface.  Most pathological effects of the sun are due to UVB radiation; however, UVR, overall is responsible for a significant proportion of skin cancer in humans.  The most common types of cancer on the face and trunk (men) and face and legs (women) include basal and squamous cell carcinomas.  Increased skin pigmentation tends to decrease skin cancer risk; moreover, certain genetic diseases can increase risk of sunlight-induced skin cancer.  These genetic dysfunctions relate to diminished UVR damage and repair system efficacy.  One such disease exhibiting attenuated DNA repair is xeroderma pigmentosum, associated with increases not only in squamous and basal cell carcinoma frequencies but also increased melanoma incidence.

    •  Following exposure to radiation with wavelengths consistent with the DNA absorbance spectrum, DNA can be altered by photon absorption.  One such alteration involves a conversion of a pyrimidine based to an excited state.  The excited state transitions back to the ground state in one scenario by non-radiative transition or fluorescence (heat or light being liberated).  Another possibility is that the excited pyrimidine state will react with other molecules forming either unstable or stable photoproducts.  One less likely scenario but one which increases the likelihood of photoproduct formation depends on transition of the pyrimidine from an excited singlet state to an excited triplet state which exhibits a longer half-life, thus increasing the likelihood of subsequent photoproduct formation.²¹

    •  Two photoproducts are noted below: one the cyclobutane dimer (below left) derives from the excited triplet state; whereas, the pyrimidine (6-4) pyrimidone is formed by an alternative mechanism.²¹

      • DNA (thymine) Photoproducts following UV radiation²⁰

• Many chemicals have been demonstrated in carcinogenic in humans and are responsible for leukemias, carcinomas, and sarcomas.⁶  In addition to chemicals, viruses are also causative agents for cancer⁶:

  • Hepatitis B and C virus have been implicated in oncogenesis for those patients with liver cancer.  Hepatitis B (HBV) is a circular DNA virus consisting of about 3200 base pairs.  HBV codes for 4 sets of viral products. Hepatitis C virus, by contrast, is a linear, single-strand RNA virus.⁹

  • Chronic hepatitis B, especially in patients infected in early childhood or infancy and patients with the HBeAg envelope protein and/or high levels of HBV DNA, may be associated with increased hepatocellular carcinoma risk.  Patients with chronic hepatitis C are also at increased risk for hepatocellular carcinoma and, in this context, the patients probably have cirrhosis and have had hepatitis C typically for least 30 years.⁹  By way of background, symptoms exhibited by hepatocellular carcinoma (HCC) may include abdominal pain, weight loss, weakness, jaundice, nausea, abdominal fullness and swelling. The most common physical sign is hepatomegaly, occurring in between 50% and 90% of patients.  Other physical signs include abdominal bruits and ascites.¹⁰

  • Human papillomavirus (HPV) subtypes16 and 18 have been associated with anal cancer.  Anal intraepithelial neoplasia may also occur.^6,11

  • The Ebstein Barr virus has been implicated in nasopharyngeal cancer and in certain lymphomas.

    • Epstein-Barr virus (EBV) is typically transmitted by infected saliva and results in oropharynx infection. Pharynx and salivary gland epithelial cells host viral replication. Surface B lymphocytes may also host the infection. Infected B cells promote the spread of infection from a oropharynx initiation site throughout the rest of the body. In addition to nasopharyngeal carcinoma, EBV is a causative agent of Burkett's lymphoma and Hodgkin's disease as well as certain other lymphomas.

      • Serum samples from patients with Burkett's lymphoma or nasopharyngeal carcinoma exhibit high antibodies levels of virus-specific antigens; furthermore, EBV DNA is found in tumor tissue and viral gene expression occurs in tumor samples. With respect to Burkett's lymphoma, a jaw tumor in African children and young adults, EBV DNA and the tumor-expressed EBNA1 antigen are found. By contrast, outside of Africa, only about 1/5 of Burkett's lymphoma contain the EBV viral DNA. Malaria is a recognized cofactor.¹²

  • Human herpesvirus type 8 and Kaposi sarcoma⁶ .

    • Four cancers are associated with AIDS so much so that they are included in the CDC classification. These include Kaposi's sarcoma, non-Hodgkin's lymphoma, primary lymphoma of the brain, and invasive cervical carcinoma. Kaposi's sarcoma lesions can be located anywhere and appear as a purplish, nonblanching and maybe either papular or nodular. About 40% of patients ultimately develop visceral disease for example, gastrointestinal and pulmonary, although the initial manifestations were dermatologic.

      • There are four forms of Kaposi's sarcoma: classic or Mediterranean which is found in elderly men of Mediterranean, Jewish, or Eastern European descent. This form is unlikely to be seen in the US. These lesions are typically limited to extremities, often lower extremities and the lesions would be characterized as a benign and associated with minimal pain. Another form occurs in the post-transplant setting. This form is described as iatrogenic Kaposi's sarcoma and occurs following kidney transplant in some patients who are also receiving cephalosporin. The disorder develops after a number of months of immunosuppressive treatment and the disease may regress if immunosuppressive therapy is discontinued. The third type of Kaposi's sarcoma is considered endemic or "African Kaposi's". This form tends to be more aggressive than those mentioned earlier affecting both the skin and internal organs. Further, endemic Kaposi's sarcoma is associated with significant morbidity and, in African children, there is a variant of endemic Kaposi's that targets lymph nodes and maybe rapidly fatal. The fourth type of Kaposi's sarcoma is AIDS-associated. This form is very aggressive, disseminating to internal organs, often with fatal outcomes.¹³

      • In terms of the differential at least in so far as superficial appearance, Kaposi's lesions might be confused with other lesions (vascular) including angiomas and pyogenic granulomas.

        Pyogenic granuloma (Medline Plus; U.S. NLM and NIH), [http://www.nlm.nih.gov/medlineplus/ency/imagepages/2465.htm]	Pyogenic granuloma (Medline Plus; U.S. NLM and NIH), [http://www.nlm.nih.gov/medlineplus/ency/imagepages/2517.htm]

        

         

       

      • Kaposi's sarcoma was identified early on in the AIDS pandemic as an important marker. Although HIV infection can predispose to Kaposi's sarcoma (KS), KS tumor cells were not found to contain the HIV genome. However, factors released from HIV-infected cells were noted to promote KS tumor cell growth. Epidemiologic studies suggested that a sexually transmitted factor different from HIV was important in KS tumorigenesis. The likelihood of KS development was enhanced by the presence of concurrent sexually transmitted diseases, number of sex partners, and various sexual behaviors. Several factors, taken together research focused on identification of other factors than HIV, i.e. exogenous pathogens in KS tumors. In 1994, utilizing polymerase chain reaction technology (PCR), two small DNA fragments were found present in AIDS-KS specimens yet absent in most non-KS sites. Nucleotide sequencing indicated that these two fragments exhibited homology to two known gamma-(lymphotropic) herpesviruses. This finding suggested that the fragments were derived from a herpes viral genome. The sequence can also be found not only in nearly all AIDS-KS tumor sources but also in KS specimens from HIV-negative patients. The novel herpes viral genome ultimately led to human herpesvirus 8. Most AIDS-related B-cell lymphomas do not appear associated with HHV-8 except for "primary effusion lymphoma" (cavity-based lymphoma).¹⁴

      • HIV infection may predispose to adult T-cell leukemia/lymphoma.⁶   

        • Intermediate-or high-grade B-cell HIV-related lymphoma (HRL) was designated as an AIDS-defining disease in 1985.   AIDS defining illnesses eventually included recurrent bacterial infection, cervical cancer, and tuberculosis to name a few.  Overall incidence declined with the introduction of highly active antiviral therapies (HAART).  The HIV virus is not likely directly involved in malignant T cell transformations; moreover, HIV nucleotide sequences are not always detectable in lymphoma tissue or in reactive B-cell hyperplasia seen in generalized, persistent lymphadenopathy, a condition preceding lymphoma development about a third of the time. In this context PCR (polymerase chain reaction) investigations describe HIV presents only in infiltrating T cells.  What seems to be more important than the HIV infection itself and lymphoma development is HIV-infection-induced immune dysregulation.¹⁵

        • HIV-infected cells synthesize and release multiple cytokines; furthermore, some of these cytokines promote T cell proliferation as well as differentiation.  IL-6 is an example of a cytokine, actually an autocrine growth factor for B-cell cancers, such as multiple myeloma and chronic lymphocytic leukemia.  Other HIV-infection-induced cytokines include IL-1, IL-2, IL-4, IL-7, IL-10, IFN-γ, lymphotoxin, tumor-necrosis factor, and B-cell growth factor.  The HIV virus itself can promote directly polyclonal B-cell activation.¹⁵

        CT Scan, Malignant Lymphoma¹⁶

        "This abdominal CT scan shows tumor masses (malignant lymphoma) in the area behind the peritoneal cavity (retroperitoneal space)"16

  • Metals:

    • Arsenic: (bottle²⁵) Inorganic arsenic is associated with skin cancer development; increased incidence of skin cancer has been noted in individuals drinking arsenic-contaminated water.  Moreover, exposure to inorganic arsenic compounds is likely associated with excess incidence of lung cancer.  This latter exposure appears to be in the workplace setting.  The magnitude of increased risk can be seen, at least in an extreme example, in miners in Rhodesia where the primary objective is gold-mining; however, arsenic is a contaminant.  In that case, the increased likelihood of lung cancer ranged between 6-14 fold. Arsenical pesticides, in which workers may be exposed both in manufacturing in use, also promote increased incidence of lung and skin cancer. Ingested arsenic  also exhibits a dose-response trend with respect to lung cancer risk; furthermore, this risk appeared more prominent among individuals who smoked cigarettes.  Probably lung cancer risk assessment associated with ingested arsenic should consider cigarette smoking as part of the analysis.^22,23

      • The underlying mechanisms which might explain tumorigenic effects of arsenic remain to be fully elucidated.  Arsenic may be effective under some circumstances in treating acute promyelocytic leukemia despite arsenic being a carcinogen.  One proposed mechanism to resolve this apparent paradox was based on a finding that arsenic inhibits transcription of the hTERT gene which encodes for the reverse transcriptase subunit of human telomerase.  A reduction in telomerase enzyme activity results in chromosomal end lesions that could promote either genomic instability and carcinogenesis or cancer cell death.  Further work in this area is still required.²⁴   

    • Cadmium may be associated with increased likelihood of lung cancer and prostate cancer.  The primary subset of the population at risk are those who are exposed to cadmium in the workplace (e.g. battery workers and cadmium smelters).  A notable source of cadmium exposure is cigarette smoking since each pack contains about 1-2 µg cadmium.²²

      • Possible mechanisms: Cadmium itself is only weakly genotoxic; however, cadmium influences cellular activities such as proliferation, apoptosis (cell-programmed cell death) and differentiation.  Indirect mechanisms that might explain cadmium carcinogenesis include:²⁶

        • Influences on gene expression and signal transduction.

        • Interference with cellular antioxidant systems as well as promotion of reactive oxygen species generation.

        • Inhibiting DNA repair and DNA methylation.

        • Alterations of cellular apoptosis.

        • Interference with E-cadherin-mediated cell-cell adhesion.

      • Therefore, gene induction by cadmium results in changes in cell signaling transduction pathways by means of altered protein phosphorylation states and transcription and translation factor activation. The production of reactive oxygen species may cause increased gene expression and apoptosis.  Cadmium-induced alteration of the E-cadherin adhesion system may also promote tumor development.²⁶ 

        • Cell-cell adhesion is typically diminished in human cancers; furthermore, this reduction appears associated with loss of “contact inhibition” of cellular proliferation.  Reduced cell-cell adhesiveness may facilitate metastasis from the primary tumor site.  The rationale is that malignant cells need to detach from the primary site, enter the circulation, eventually lodge in a vascular bed far removed perhaps from the initial site and then ultimately proliferate there.  Easier initial malignant cellular detachment from the primary tumor due to reduced cell-cell adhesion may promote this process.  The principle mediators of calcium-dependent cell-cell adhesion are categorized as members of large group of transmembrane glycoproteins, the cadherins.  One principal subtype is the E-(epithelial) cadherins. Abnormal E-cadherin levels have been demonstrated in invasive carcinomas and carcinoma in situ.²⁷

    • Nickel:  Increased rates of nasal cancer as well as lung cancer have been associated with occupational exposure to nickel during the refining process.  The reduction in cancer risk from nickel exposure was associated with increased environmental oversight in the industry beginning over 70 years ago; however, workers involved in nickel refining in Norway as late as the 1950s were probably at increased cancer risk.  Higher incidence of nasal, laryngeal, and lung cancer observed in that population may have resulted from concomitant exposure to smoking and nickel.²²

      • Underlying molecular mechanisms by which nickel promotes cancer continue to be the subject of investigation.  One possibility is that nickel binds to cell nuclear histones.²⁸

      • Nickel appears to damage, selectively, heterochromatin. [Heterochromatin is a densely-stained chromosomal region.  These regions tend to remain tightly coiled (and darkly staining) throughout the cellular cycle.]  A principal effect of nickel may be to reduce gene expression near heterochromatin by causing a loss of histone H4 and H3 acetylation as well as reduced DNA hypermethylation.  Furthermore, soluble nickel ions, interacting with cell surface receptors, may activate cell signaling promoting cellular gene induction.²⁹  [Histones (5 types) are proteins commonly found in chromosomes.  They bind to DNA and form a "scaffolding" around which DNA winds, thus forming chromatin structure.  Histones, being rich in the amino acids arginine and lysine, are basic proteins and are involved in controlling gene activity.]

        • Figure © 2004 by Griffiths et al³⁰

          "Histones are the major structural proteins of chromosomes. The DNA molecule is wrapped twice around a histone octamer to make a nucleosome. Six nucleosomes are assembled into a solenoid in association with H1 histones. The solenoids are in turn coiled onto a scaffold, which is further coiled to make the chromosomal matrix."-Stephen Carr30.

    • Chromate:  Hexavalent (Cr(VI)) (not trivalent) chromium exposure causes cancer both in humans and in mammalian cells in vitro³¹. Increased lung cancer risk has been noted both in chromate production plants in England (300%) and in Baltimore (200%).²²

      • Chromium VI carcinogenicity probably involves particulate forms of limited solubility.  Cr(VI) oxyanions move across the cell membrane by means of a non-specific anionic transporter.  Cr(VI) is metabolized, reductively, by reactions involving ascorbic acid, glutathione (GSH) and cysteine (Cys).  During this reductive process, “genetic lesions” are formed, including Cr-DNA binary (mono) adducts, Cr-DNA ternary adducts, DNA protein cross-links, bi-functional adducts (DNA interstrand cross-links adducts), single-strand breaks and oxidized bases. Interstrand cross-links could result in apoptosis (cell death) or terminal growth stoppage because of physical barriers to DNA replication and/or transcription.  Ternary DNA adducts might be pre-mutagenic.³²

      • "DNA Damage Resulting from Cr(VI) Reduction by Cys" (Cysteine)³³

        

         

  • Fibers and Dusts²²

    • Asbestos:  The predisposition of asbestosis patients to lung cancer has been described for over 70 years.  Lung cancer induced by asbestos exposure is associated with a long latency period.  Mesothelioma of the lung appears a specific consequence of asbestos exposure.  Subpopulations of the workforce predisposed to asbestos-induced mesothelioma include those working in insulation workers as well as asbestos manufacturing workers and others.²²

      • Background: asbestos references a number of naturally occurring hydrated mineral silicate fibers which are causative of certain pulmonary and pleural diseases.  There are two major groups of asbestos (1) serpentine of which chrysotile is the representative form and (2) amphibole represented by crocidolite, anthophyllite, amosite and tremolite.  Of these, crocidolite is the most oncogenic asbestos type involved in causing malignant mesothelioma (MM).  Malignant mesothelioma targets the serosal lining of the plural, peritoneal and pericardial cavities and the disease causes about 2500 mortalities per year in the United States.  The mechanism by which asbestos causes cancer remains to be elucidated.³⁴  The following represents one pathologic model: ³⁴

        • (1) Crocidolite, pictured above. exposure promotes macrophage accumulation in plural cavities and in the lung. 

        • (2) Macrophages upon interacting with crocidolite releases TNF-α (tumor necrosis factor alpha); concurrently asbestos causes primary human mesothelial cells (HM) to secrete TNF-α (i.e. both paracrine and autocrine affects). 

        • (3) TNF-α activates NF-κB  [nuclear factor-kappa B, a protein complex functioning as a transcription factor with broad genetic regulatory activities].  This protein family, consisting of structurally-similar transcription factors, is involved in regulation of immune responses, inflammatory responses, apoptosis, cellular growth and developmental processes.³⁵  Furthermore, NF-κB appear active in many disease states, not just cancer, including asthma, heart disease, neurodegenerative disorders and others. TNF-α, a proinflammatory cytokine, not only induces NF-κB, but affects oncogenesis because NF-κB is an important regulator of oncogenesis.  Often cell survival is enhanced due to NF-κB activation, since NF-κB activation itself both increases cell proliferation while inhibiting apotosis (programmed cell death).

        • (4) These effects permit human mesothelial cells (HM) with asbestos-caused DNA damage to divide rather than die. Consequently, with sufficient accumulation of DNA damage, malignant mesothelioma could develop.

    • Silica (quartz, picture at left³⁸):  Silica dust exposure in the occupational setting predisposes to enhanced lung cancer risk. Individuals involved in the production of ceramic plumbing would be in such a risk category.³⁶ The principal human exposure silica form is usually quartz, cristobalite and tridymite. Construction, sandblasting, foundries, granite quarrying and processing as well as ceramic industries represent sites for occupational exposure. In terms of dimension, inhalation of crystalline silica suggest particle sizes of about 2-5 µm. From a mechanistic point of view, at least superficially similar to what has been described for asbestos, a pro-inflammatory effect of crystalline silica (quartz) appears involved in lung cancer development, at least in rats. Furthermore, epithelial proliferation is also involved.³⁷  In cell culture, neoplastic transformation by crystalline silica as well as chromosomal damage have been observed. In aqueous media, crystalline silica promotes oxygen free radical production; furthermore, direct binding of crystalline silica to DNA, as assayed by infrared spectrometry, has been noted. These types of studies suggest that silica carcinogenesis might involve DNA binding of silica, thus exposing DNA sites to the damaging effects of oxygen free radicals.³⁹

    • Wood Dust: An increased risk of nasal carcinomas is likely associated with exposure to wood dust in a cell population of workers exposed and furniture manufacturing. Hardwood dusts appear associated with highest risk. Slight increases in risks of other cancers such as lung cancer, Hodgkin's disease and laryngeal cancer may also accompany wood dust occupational exposure.³⁶

      • Although increased risk for human sinonasal cancer (SNC) due to wood dust exposure has been well documented by epidemiological analysis, the underlying mechanism remains to be determined. Increased COX-2 levels have been found in malignant and premalignant tissues and COX-2 may be linked to cancer development. COX-2 expression appears associated with wood dust occupational exposure. This work suggested that this increased COX-2 expression indicated an inflammatory role in sinonasal carcinogenesis.⁴⁰

      • COX-2 is an abbreviation for cycloxygenase-2, an enzyme, which catalyzes prostaglandin production. Specifically, COX-1 and COX-2 convert arachidonic acid to prostaglandin endoperoxide.

        • COX-1 (PGH synthase-1) is continuously (constitutively) expressed; however, COX-2 (PGH synthase-2) is inducible. COX-2 is categorized as an immediate early-response gene product that is up-regulated (increased in expression) by a number of factors including tumor promoters, cytokines, growth factors, and shear stress as might occur in vascular walls. COX-2 activity promotes prostanoid formation associated with inflammation and cancer. Examples of prostanoids include prostaglandins, prostacyclin, and thromboxane.⁴¹

  Arachidonic Acid

   

  • Chemicals:

  • An important group of chemicals that have been implicated in human cancer are polycyclic aromatic hydrocarbon (PAH). Most of these chemicals tend to be produced during combustion and other processes unintentionally; however, some are produced commercially. These examples include naphthalene, acenaphthene, fluorene, anthracene, phenanthracene, fluranthene, and pyrene. Of these industrial products, the most important is probably naphthalene. These agents are typically not chemically synthesized for industrial use but are isolated from coal processing products, mainly from coal-tar. Naphthalene may be occasionally isolated from pyrolysis residue oils, petroleum-derived fractions and olefin fractions.⁴² 
  • Historically, polycyclic aromatic hydrocarbons had been correlated with certain, high incidence cancers within particular occupational subpopulations. For instance, increase risk of scrotal cancer among chimney sweeps is likely associated with exposure to select and tar. Application of coal tar to skin a test animals led to the identification of the chemical constituents responsible for malignancy. These constituents were the polycyclic aromatic hydrocarbons (PAHs). Increased likelihood of lung cancer has also been associated with occupational exposure to PAHs; occupations associated with increased worker risk include roofers and pavers, coke oven workers, some workers involved in steel/iron manufacturing as well as those involved in aluminum production.³⁶  
  • Metabolic, biochemical pathways for PAHs and benzo[a]pyrene have been identified (see figure below). Note the initial step in benzo[a]pyrene metabolism which involves epoxidation of an aromatic double bond by one of the cytochrome P450 monooxygenases (CYP1A1).

    • Polycyclic Aromatic Hydrocarbons Metabolism⁴²

       

  • Epoxide-benzo[a]pyrene intermediates can lead to phenol or glutathione conjugate formation. Another possibility is additional oxidation by epoxide hydrolase to form dihydrodiol-benzo[a]pyrene. An additional, second, oxidation step can lead to a very reactive 7, 8-dihydrodiol-9,10-epoxide benzo[a]pyrene. In human lung and colon tissue benzo[a]pyrene may be metabolized to the likely carcinogen 7, 8-dihydro-7,8-dihydrobenzo[a]pyrene. These results were obtained using tissue culture techniques. Benzo[a]pyrene administered to other test animals orally resulted in benzo[a]pyrene-DNA adduct formation in a number of tissues.³⁶ 

  • Benzo[a]pyrene: a model of PAH Metabolism (IARC, 1983)⁴³ 

  •  

    Structures of Some Important Polynuclear Hydrocarbons⁴³

    

   

   

  • Aromatic Amines: Aromatic amines have been implicated in bladder cancer etiology. Initial observations of increased incidence of bladder cancer in dye industry workers were reported over 100 years ago. 1-naphthylamine, 2-naphthylamine, and benzidine (arylamines) as well as mixed-dyes have been implicated. Aromatic amines are metabolized by a process involving acetylation. The enzyme that catalyzes this reaction is N-acetyltransferase. Genetic polymorphism results in enzyme form variants that exhibit different acetylation rates. Expression of one NAT2 (N-acetyltransferases 2) gene form is associated with relatively slower acetylation rates; moreover, these lower rates may predispose to a higher incidence of aromatic amine-induced bladder cancer.³⁶ 

  1-naphthylamine

  2-naphthylamine

  • Bladder cancer involves transformation of normal urothelial cells to malignant cells with possible subsequent development of metastatic disease. The mechanistic foundation of malignant transformation as a result of exposure to arylamines, such as 1-naphthylamine, or aromatic amines, such as 4-aminobiphenyl, is based on formation of DNA adducts. These modifications in DNA is influenced by activity of certain liver enzymes either involved in phase I (cytochrome P450 system metabolism) or phase II metabolism (N-acetyltransferase 2, i.e. NAT2) and glutathione-S-transferase M1 (GSTM1). One particular cytochrome P450 isozyme especially important in this process appears to be CYP1A2, which is an inducible enzyme catalyzing aromatic amine demethylation with attendant increase in DNA adduct formation. NAT2 catalyzes the formation of acetylated products which tend to be relatively non-reactive; therefore, these acetylation reactions appear to reduce DNA adduct formation.⁴⁵

    Arylamine Metabolism Pathway Associated with Bladder Carcinogenesis^45,46

     

  Urothelial Cells⁴⁷ 

  "Normal urothelial cells in a catheterized urine. Papanicolaou  Stain, 600X magnification."47	"Urothelial carcinoma of low malignant potential.  These cell found in a voided urine are characterized by small, uniform appearing cells with a loss of polarity or cell to cell orientation. The nuclei are round with evenly distributed chromatin. Note the loss of cytoplasm. Papanicolaou stain. 400X magnification."47

   

  Urothelial Cells⁴⁷

  "Urothelial carcinoma of high malignant potential. These cells found in a voided urine specimen demonstrate easily recognized criteria for malignancy. The cells are enlarged with high nuclear to cytoplasmic ratios, irregular nuclear membranes and nucleoli. Papanicolaou stain. 1000X magnification."47	"Urothelial carcinoma in situ. Most of the cells appear singly or in small groups. Papanicolaou stain. 400X magnification."47

   

  • Benzene exposure may result in increased nonlymphocytic leukemia and lymphoma. This risk may be most likely observed in workers who are exposed to solvents associated with rubber film and rubber coating production.
    • The underlying mechanism for benzene-induced leukemogenicity remains to be elucidated but may involve metabolic change, growth factor dysregulations, oxidative stress, DNA damage, cell cycle dysregulation and alterations in apoptotic mechanisms.
    • Benzene induced hematotoxicty has been suggested to be associated with abnormalities in the p53 transcription factor. p53 activity is regulated by control of p53 protein levels. p53 binds to an oncogene product mdm2 and the complex is transported from the nucleus for proteolytic degradation; therefore, nuclear p53 levels can be regulated by the extent of complex formation and translocation. Phosphorylation of p53 as well as mdm2, a response to noxious stimulation, changes the equilibrium between p53, mdm2 and the complex p53-mdm2 in a way that favors free p53. As a result, under some pathological circumstances, increased p53 levels predispose the cell to apoptosis or cell cycle arrest. If a mutagenic event occurs that compromises p53 regulation, one possibility is that reduced p53 efficacy results in reduced apoptosis and reduced cell cycle arrest. Acquired p53 mutation is likely the most common genetic change observed in human cancer.^36,48,49 
  • Aflatoxin is produced by certain fungal strains (Aspergillus flavus and A. parasiticus). Aflatoxin B is a hepatotoxin which under some circumstances can be associated with an increased and high liver cancer rate. Possibly a co-carcinogenic relationship exists between aflatoxin and hepatitis B virus (HBV). Following aflatoxin exposure, at least in amounts in excess of 10-100 µg/day, aflatoxin metabolites as well as aflatoxin-DNA adducts can be assayed in urine. Aflatoxin may exert its hepatocellular carcinogenic activity through p53 dysregulation, as suggested below:

  Hepatocarcinogenic Mechanisms (figure, slightly modified from Farazi PA, DePinho RA)⁵⁰