Cancer initiation and progression is driven by a series of changes in DNA that control gene expression, resulting in uncontrolled cellular proliferation. The mutation theory of cancer causation suggests that cancer-associated gene expression arises from random replication errors, exposures to carcinogens (e.g., viruses, radiation, cigarette smoke), or faulty DNA repair processes. Although HPV infection is necessary for cervical cancer (CC) development, progression to cancer occurs in only a small percentage of HPV-infected women, and a number of studies have shown that incident cervicovaginal HPV is self-limited disease generally lasting less than a year in duration (Ho et al., 1998; Woodman et al., 2001; Workowski et al., 2015). From our perspective, human papillomaviruses (HPVs) play a critical role in the etiology of CC, with several lines of epidemiologic and experimental evidence supporting a role for non-viral (co-carcinogens) in controlling the risk for progression to neoplasia among HPV-infected individuals (Bennett et al., 2010; Wei et al., 2014; Goodson et al., 2015). The studies cited here were identified from a systemic review we conducted (Haverkos et al., 2003), reviews conducted by others (Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 2004; International Collaboration of Epidemiological Studies of Cervical Cancer [ICESCC] et al., 2006; International Collaboration of Epidemiological Studies of Cervical Cancer [ICESCC], 2007), and other papers that we have read.

Papillomaviruses are double-stranded DNA viruses that infect skin and mucosa of vertebrates and induce cellular proliferation in a species-specific manner (Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 2007). Over 200 genotypes of HPV, belonging to 49 species, in five genera have been described, and the rate of HPV genotype discovery is rapidly increasing (Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 2007; Bernard et al., 2010; Bzhalava et al., 2015). Among them, HPV-16 and HPV-18 play a major role in the etiology of CC (Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 1995, 2007; Walboomers et al., 1999; Bosch et al., 2002; International Collaboration of Epidemiological Studies of Cervical Cancer [ICESCC] et al., 2006; International Collaboration of Epidemiological Studies of Cervical Cancer [ICESCC], 2007). Over 95% of women with CC are infected with one or more HPV types, most commonly HPV-16 (present in about 50% of all CC) and HPV-18 (present in about 10–15% of cases). Approximately 20 other HPV types (i.e., HPV-45, HPV-31, HPV-59, HPV-58, HPV-33, and HPV-11) have also been associated with CC and thus deemed oncogenic (Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 2007; Guardado-Estrada et al., 2014). Harald zur Hausen was awarded the 2008 Nobel Prize in Medicine for demonstrating a link between HPV infection and development of CC (zur Hausen, 1989).

While HPV is a major factor for developing CC, and given that persistent infection with a high-risk “oncogenic” type of HPV appears to be necessary for the development of invasive CC (Walboomers et al., 1999; Munoz et al., 2003), a number of studies have shown that squamous cell CC may result from an additive or synergistic interaction between oncogenic HPVs and cervical tar exposures, a process generally referred to as co-carcinogenesis (Haverkos, 2004; Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 2004; International Collaboration of Epidemiological Studies of Cervical Cancer [ICESCC] et al., 2006; International Collaboration of Epidemiological Studies of Cervical Cancer [ICESCC], 2007). We highlight coal tar derivatives from cigarette smoking, tar-based vaginal sanitization products, and inhaled smoke from burning biomass (wood, coal, dung) in poorly ventilated kitchens, as potential co-carcinogenic factors that contain bioactive compounds likely to play a determinative role in CC development (Rotkin, 1967; Winkelstein, 1990; Ferrara et al., 2000; Haverkos et al., 2000, 2003; Velema et al., 2002; Steckley et al., 2003; Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 2004; Haverkos, 2005; International Collaboration of Epidemiological Studies of Cervical Cancer [ICESCC] et al., 2006; International Collaboration of Epidemiological Studies of Cervical Cancer [ICESCC], 2007; Bennett et al., 2010). We identify published data from virological and genetic studies linking oncogenic papillomavirus genotypes and chemicals to development of cancer (Rous, 1965; Prokopczyk et al., 2009; Wei et al., 2014).

Ames (1979) suggested that natural and man-made chemicals may contribute to the DNA damage that commits cells to a cancerous state. Carcinogen-induced mutations may also lead to chromosomal rearrangements including aneuploidy, a condition that arises during cell division when the chromosomes do not segregate properly between two daughter cells. An extra or missing chromosome is often seen in CC (McCormack et al., 2013).

Goodson et al. (2015) noted 85 examples of environmental chemicals that disrupted key pathways in carcinogenesis, designated as “hallmarks of cancer.” Those hallmarks include hyperproliferative signaling, insensitivity to growth-factor signals, evasion of apoptosis, sustained angiogenesis, genomic instability, and mutation, promotion of inflammation, and dysregulation of metabolism. Individual chemicals in mixtures, such as tobacco smoke and tar-based vaginal sanitary products, accumulate in cells and tissues and activate important carcinogenic pathways (Goodson et al., 2015).

Tobacco smoke is known to contain about 6,000 compounds, including benzyl pyrenes, polycyclic aromatic compounds, and tobacco specific nitrosamines. For example, benzo[a]pyrene have been identified at higher levels in cervical mucous of smokers than non-smokers (Melikian et al., 1999a). Several investigators have challenged HPV-infected cell lines with benzo[a]pyrene, a known carcinogenic component of cigarette smoke, and demonstrated increased viral titers and DNA damage measured by increased DNA adducts, E6 and E7 oncogene expression, and retinoblastoma protein (pRb) (Sizemore et al., 1995; Melikian et al., 1999b; Alam et al., 2008, 2010; Maher et al., 2011). Investigators in New Mexico exposed cervical cell lines containing episomal HPV genomes to tobacco smoke condensate and observed increased E6 and E7 oncogene expression, suggesting prominent roles for both HPV and tobacco smoke in cancer development (Wei et al., 2014) (Table 1).

Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC] (2007) updated their evaluation of the carcinogenic risks of HPVs. They concluded that HPV-16 is carcinogenic for cancers of the anus, oral cavity, oropharynx, penis, vagina, and vulva (Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 2007; Chaturvedi, 2010). However, whether tar-based factors could be involved in the pathogenesis of other HPV-related cancers remains to be determined. For example, investigators in Maryland conducted a case-control study of 100 patients (86 males, 14 females) with oropharyngeal squamous cell cancers and 200 age- and sex matched patients without cancer seen at the same outpatient otolaryngology clinic. Oropharyngeal cancer was strongly associated with oral HPV type 16 infection (Odds ratio, 14.6), and oral infection with any of 37 HPV types (OR, 12.3). HPV-16 DNA was found in 72 of 100 paraffin-embedded tumor specimens. Additionally, cancer was significantly associated with greater than 28 alcoholic drinks per week (OR, 4.1), more than 20 tobacco pack-years (OR, 2.4), and smoking marijuana monthly for more than 1 year (OR, 2.1). The authors conclude that oropharyngeal cancer is strongly associated with oral HPV infection with or without the established risk factors of alcohol or tobacco use (D’Souza et al., 2007). On the other hand, we propose that oral HPV infection and oral coal tar derivative exposures are component causes of oropharyngeal squamous cell cancers, similar to the co-carcinogenic process inducing CC.

Rous (1910) described the Rous sarcoma virus (RSV) as the causative agent for a sarcoma by inoculating a naïve chicken with a cell-free filtrate from another, infected chicken with sarcoma, and won the Nobel Prize in Medicine for it in 1966. However, some investigators questioned Rous’s attribution of the cancer to RSV alone, and insisted that the cancer was caused by the virus only after inflammation was induced by scraping the area with sand added to the filtrate (Gye, 1925, 1926, 1938; Gye and Mueller, 1929).

In an attempt to replicate Rous’s observations with sarcoma, William E. Gye conducted a series of experiments in chickens, and concluded:

These seminal studies set the stage for new thinking about the etiologic role of viruses and cancer development, but they also resulted in exciting new questions about whether a single oncogenic virus could be sufficient to cause cancer without the influence of other, contributing factors such as viral genetics, environmental irritants, and the immune status of the infected host.

The Rous-Gye debate led to Rous’s later works. Although squamous cell cancers had previously been produced by Kennaway using multiple doses of a polycyclic aromatic hydrocarbon (Waller, 1994; Fujiki, 2014), Rous and colleagues consistently demonstrated the joint action of tar and Shope papillomavirus (or cottontail rabbit PV, CRPV1), now referred to as Sylvilagus floridanus Papillomavirus 1 (SfPV1) (Bernard et al., 2010) in inducing squamous cell carcinomas in rabbits in shorter time periods. They painted rabbits’ ears or abdomens with methylcholanthrene once, before or after a single inoculation with a papillomavirus and noted cancer development within 3–6 months (Rous and Beard, 1934; Rous and Kidd, 1938; Rogers and Rous, 1951). Based on these observations, Rous proposed that cancer resulted from a two-step process of “initiation and promotion” (Rous, 1965).

While the Henle-Koch postulates for causation have been accepted as a basis for establishing that a virus may be causally linked to a specific cancer, Evans (1977, 1982) and Evans and Mueller (1990) recognized several problems that might slight the universal application of such a “standard.” Evans instead proposed that the cause(s) of cancers of any type more likely involve a complex interplay of infectious agents, chemicals, and other factors that modify susceptibility to the oncogenic state.

Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC] (1995) an international body declared “at least” HPV-16 and HPV-18 cause CC. Later, scientists identified 17 other HPV types (26, 31, 33, 35, 39, 45, 51, 52, 53, 56, 58, 59, 66, 67, 68, 73, and 82) as “high-risk” for cancer (Munoz et al., 2003; de Villiers et al., 2004). In 2004, WHO recognized cigarette smoke as another causative agent for cervical cancer (Working Group on the Evaluation of Carcinogenic Risks to Humans [IARC], 2004). We added coal tar derivatives in vaginal douche products and by inhaling smoke from wood and coal burning stoves as probable co-carcinogens (Bennett et al., 2010).

We have formulated our virus-coal tar derivative hypothesis from observational studies. Epidemiologic investigations, especially ecologic studies, have well-known limitations. Case-control and cohort studies provide a higher level of evidence but are subject to confounding and various biases, i.e., recall and selection bias. Although epidemiologic studies may provide additional insights, such questionnaire-based research is not able to isolate individual carcinogens in tobacco and wood-burning smoke. For example, scientists have explored the interaction of benzo[a] pyrene and tobacco smoke condensate on HPV-immortalized cells. We encourage more studies combining “high risk” HPV types and coal tar derivatives, both individual chemicals and various mixtures, in the laboratory and observing effects on viral genetics and cancer induction. One might re-enact the Peyton Rous rabbit model with Shope papillomavirus and replacing methylcholanthrene with chronic administration of coal tar derivatives. Those studies should provide insights into the process of co-carcinogenesis involving viruses and chemicals. One might also find that this concept may apply to other cancers, including oropharyngeal and anal carcinomas.

An overwhelming body of evidence clearly identifies HPV as a major etiologic factor for CC, and we support universal immunization of boys and girls with HPV vaccines (Future II Study Group, 2007; Joura et al., 2015). In addition, Papinicoulaou testing with or without HPV-specific testing remains a life-saving intervention for many women (Schiffman and Solomon, 2013; Centers for Disease Control and Prevention [CDC], 2015). However, the etiology of CC appears to be complex and multi-factorial. Infection with HPV plays a central role in CC development, with host, viral, and environmental factors influencing the risk for progression to neoplasia. A growing number of reports on the pathogenesis of oncogenic HPV genotypes, the relative contributions of co-carcinogens, and the host genetic determinants of cancer development, provide a unifying perspective on etiology and pathogenesis of cancer. In addition, current studies demonstrate a viral genetic basis of pathogenicity in which oncogenic HPV types display unique fitness for persistence. Nonetheless, gaps remain in our understanding of the correlates of CC development, and future research is needed to delineate the genetic factors that influence cancer development. Such efforts should lead to even more effective strategies for prevention and treatment of CC in high risk populations.

HH developed concept and outline of the paper, wrote the first draft, provided infectious diseases expertise and cigarette smoking sections, and is the corresponding author. GH wrote historical section on pharmacologic aspects of vaginal douche products and reviewed all aspects of the paper. MO wrote chemical sections of coal tar and coal tar derivatives sections and reviewed all aspects of the paper.

The views expressed are those of the authors, and do not necessarily reflect the official views of the Uniformed Services University of the Health Sciences, or the Department of Defense (DOD).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.