Chronic inflammation is a risk factor for several cancer types . Asthmatics and individuals with COPD are at an elevated lifetime risk for developing lung cancer . The importance of inflammation in augmenting pulmonary carcinogenesis is further supported by a wide range of pharmaceutical compounds that inhibit neoplastic development  as well as evidence from transgenic mouse models [52, 53]. Because tumor promotion involves changes in gene expression, most likely epigenetic in nature, and is the only reversible stage of carcinogenesis, studying promoters may identify additional pathways to target for preventive strategies against human lung cancer.
In the current investigation, we provide evidence that V2O5 functions as an in vivo tumor promoter among differentially susceptible inbred strains of mice. Using a two-stage model of carcinogenesis, a significant increase in tumor multiplicity was observed in both A/J (10.3 ± 0.9 tumors/mouse) and BALB (2.2 ± 0.36) mice exposed to the carcinogen MCA followed by 5 weekly aspirations of V2O5. The effect of V2O5 was limited to tumor promotion, as no significant increase in tumor numbers were observed in animals exposed to V2O5 alone. Susceptibility to promotion paralleled relative strain sensitivity to V2O5-induced inflammation: A/J mice were most sensitive and BALB were intermediate. B6 mice were found to be most resistant to V2O5-induced inflammation, however were used as a control since they are not initiated by the low dose of MCA administered in this study .
Differences between the two susceptible strains of mice (A/J and BALB) are not unusual based on past genome mapping studies demonstrating distinct genes responsible for tumorigenesis in these specific strains [54, 55]. While both strains are susceptible to lung tumor development, differences in sensitivity between these two strains has been linked to quantitative trait loci containing both tumor suppressor genes as well as inflammatory mediators, such as myeloperoxidase (Mpo), colony stimulating factor (Csf)3, CC chemokine receptor (Ccr10), and Ccl2 (Mcp-1) [54, 55]. Although MCA was used as an initiating agent in this study, additional control experiments further demonstrated that carcinogen treatment alone did not influence inflammatory indices between strains. Because significant strain responses were observed only in response to V2O5, our findings suggest that that genetic (host) factors contributing to V2O5-induced pulmonary inflammation are also strongly associated to lung tumor promotion.
Vanadium is thought to mediate pulmonary inflammation through generation of multiple reactive oxygen species (O2-, H2O2, and ·OH) in target cells [56–58]. Production of ROS is associated with phosphorylation of EGF-R and activation of MAPK signaling [57, 59–63] as well as the transcription factors NFκB [59, 63], AP-1 [59, 64], and STAT-1 . Furthermore, vanadium is known to be a phosphatase inhibitor  and likely prolongs phosphorylation and signaling along ROS-sensitive pathways. These events, in turn can influence the synthesis and release of pro-inflammatory cytokines and chemokines mediating acute lung injury [29, 65, 67, 68]. Pretreatment of human bronchial epithelial cells with metal chelators and/or free radical scavengers reduces vanadium-generated ROS, MAPK activation, as well as release of chemokines, further supporting a role for oxidative stress in vanadium-induced inflammation .
In our study, differential strain induction of chemokines and upstream signaling molecules in response to V2O5 correlated to the extent and duration of inflammatory cells recovered in pulmonary tissue. MIP-2 and KC are principle neutrophil chemoattractants in rodent models, homologous to IL-8 in humans , whereas MCP-1 induces monocyte and lymphocyte chemotaxis and migration . We observed moderate, although significant induction of MIP-2 in all strains at 6 hr following vanadium exposure, which likely involved initial PMN influx. However, strain differences in the peak PMN response were more closely associated with pulmonary levels of KC. MCP-1 was highly induced in A/J and BALB mice and expression coincided with the influx of both monocytes and lymphocytes into pulmonary tissue. The transcription factors NFκB and c-Fos as well as the MAPK pERK1/2 were also found to be differentially regulated in the sensitive (A/J) and resistant (B6) mice and corresponded to both altered chemokine induction and BALF cellularity.
The microenvironment is becoming increasingly recognized as actively contributing to the tumorigenic process. Evidence suggests that PMNs and macrophages appear to be involved in tumor development through multiple mechanisms, including more direct, such as induction of DNA damage and regulation of cell cycle , as well as indirect mechanisms, such as promotion of angiogenesis by cytokines and chemokines and suppression of adaptive immune responses [71, 72]. Local production of cytokines and chemokines may also stimulate expansion of initiated cells by influencing cell proliferation and apoptotic pathways . Several signaling molecules altered by V2O5 in this study have been implicated in lung cancer development. For example, IL-8 has been reported to serve as an autocrine growth factor in lung cancer cell lines [73, 74] and both IL-8 and MCP-1 are elevated in bronchiolar epithelium from patients with COPD [75, 76] and non-small cell lung cancer (NSCLC) . In mouse models, neutralization of CXCR2, the principle receptor for KC and MIP-2 reduces PMN infiltration  as well as tumor growth and angiogenesis, suggesting a role in tumor progression [53, 79, 80]. Constitutive activation of pERK1/2 [81, 82] and the transcription factors NFκB  and c-Fos  have well known effects on cell cycle regulation. Additional evidence for ERK1/2 in pulmonary tumorigenesis was recently demonstrated in transgenic mice overexpressing mutant B-raf and K-ras. Pharmacological inhibition of pERK1/2 resulted in tumor regression by inhibiting cell proliferation and restoring apoptosis . Constitutive activation of ERK1/2 was also observed in V2O5-induced mouse carcinomas from the NTP study containing both K-ras mutations and loss of heterozygosity , which supports findings in this model and suggests involvement of ERK1/2 as one pathway driving tumor promotion by V2O5.