Disruption of homeostatic signaling

Cell to cell and cell to extra cellular matrix (ECM) contacts within epithelia are crucial in the governance of proliferation . Signaling facilitated by these physical interactions, mediated through membrane spanning proteins, has been observed to convey tumor-suppressive functions. The loss of function of one such protein, E-cadherin, is common in epithelial cancers and its role as an inhibitor of progression to a more aggressive phenotype has been observed in vitro, wherein induced functional loss of this integrin stimulated invasion and migration of prostate cancer cells . The over-abundance of proteolytic enzymes produced during inflammation is known to disrupt epithelial cell to cell and cell to ECM adhesions. Matrix metalloproteinases (MMP), notorious degraders of the ECM, released by stromal cells in response to inflammatory response-derived chemical signaling , have recently been attributed with multiple functions, including the direct cleavage of integrin molecules such as E-cadherin . Such disruptions in cell to cell signaling is thought to lead to genomic instability and subsequent oncogenic transformation by effecting loss of control over cytokinesis-regulating pathways, such as those mediated by TP53. In support of this, Radisky and colleagues reported a series of experiments demonstrating that addition of MMP-3 to murine mammary cells caused a shift toward malignancy, attributed to altered gene expression, and the resulting increase in reactive oxygen species.

Oxidative stress

Oxidative stress is a well-documented cause of genomic instability and has been implicated in the promotion of tumourigenesis. Activated phagocytes (and potentially epithelial cells) at the site of inflammation produce elevated levels of the superoxide anion, a precursor to a variety of reactive oxygen and nitrogen species . Additional oxidative stress through the accumulation of free radicals from the metabolism of saturated fats and paucity of dietary antioxidants characteristic of Western culture may serve to further disrupt the cellular oxidative balance and increase the likelihood of oncogenic initiation. Peroxidases particular to neutrophils and eosinophils have been observed to be responsible for the production of reactive nitrogen species in murine models of acute inflammation , which has additionally been shown to be a precursor of lipid peroxidation . Constant localized exposure to oxidative stress and mediators of the immune response have been implicated in the development of permanent genetic damage and the activation of proto-oncogenes, such as c-fos and c-jun . In a review of a series of human and animal studies, Kawanishi and Hiraku report examples of bacterial, viral, and parasitic infections, as well as noninfectious chronic inflammatory illnesses, that are suspected risk factors for cancer. In these studies, the investigators were able to consistently identify oxidation products of guanine, including those known to comprise mutagenic DNA lesions in inflammatory and epithelial cells at the site of inflammation. The investigators concluded that these oxidative alterations of DNA were probable contributors to the development of carcinomas from chronic or repeated inflammation caused by helicobacter pylori, hepatitis C, inflammatory bowel disease, and oral lichen planus, at their respective sites. Though the reviewed body of research did not include inflammation of the prostate, many of the same pathologic processes are present in models of chronic prostatitis.

In a recent case-controlled study, Zhou and colleagues compared levels of oxidative stress in 70 subjects with chronic bacterial prostatitis to that of an equal number of healthy controls. The investigators observed increased levels of molecular byproducts of oxidizing reactions and lower activities of prominent antioxidant enzymes among cases. This supports a higher exposure to oxidative stress among the former group, which could potentially result in mutagenic DNA lesions and ultimately, carcinogenesis.

Further epidemiologic support for the transforming effect of reactive oxygen and nitrogen species in prostate epithelia is found in case-controlled studies examining the allele frequency of genes encoding enzymes that protect against, and repair, oxidative damage. Manganese superoxide dismutase is an antioxidant enzyme accredited with the detoxification of oxygen-free radicals inside the mitochondria and has been observed to suppress proliferation of prostate cancer cells both in vitro and in vivo . A polymorphism for the encoding gene was discovered that consists of a single valine-alanine substitution, thought to affect the enzyme’s ability to permeate the mitochondrial membrane. Results from a study nested within the α-tocopherol β-carotene Cancer Prevention Study by Woodson and colleagues imply that men homozygous for the alanine allele had an increased risk of prostate cancer. Two independent case-controlled studies reported increased prostate cancer risk with certain polymorphisms of a gene encoding human oxoguanine glycosylase, an enzyme involved in repair of oxidative DNA lesions . An increased risk for men with the Ser362Cys polymorphism was common to both. Evidence that reduced functionality of innate antioxidant mechanisms can increase the likelihood of developing prostate cancer coincides with the idea that sustained, elevated oxidative stress, as encountered in chronic inflammation, is a probable potent carcinogen.

Glutathione-S-transferases (GSTs) are an important family of antioxidant enzymes that, because of their role as detoxifiers of reactive compounds, have been postulated to provide protection against oncogenesis . The enzymes are capable of neutralizing several different forms of potentially damaging reactive products and are basally expressed in normal epithelial cells . However, a recent meta-analysis assessing the association between certain genetic variations of three of the most common GST genes (GSTM1, GSTT1 and GSTP1) and risk of prostate cancer was unable to detect a statistically significant association . The investigators chose studies that compared null and nondeleted genotypes for GSTM1 and GSTT1, and two common polymorphisms for GSTP1. The quantitative synthesis of cases and controls from 11, 10, and 12 studies on GSTM1, GSTT1 and GSTP1, respectively, failed to show significant allelic differences for the GST genes between cases and healthy controls, yet loss of GST functionality is ubiquitous in prostate . The nearly unanimous down-regulation of GSTP1 in high-grade intraepithelial neoplasia (HGPIN) and carcinomas imply a “caretaker” role, described by Kinzler and Vogelstein as a gene whose inactivation facilitates genomic instability, resulting tumor initiation.

This loss of function has been attributed to the somatic silencing of GST genes, corroborated by Lin and colleagues , who observed that 95% of assessed prostatectomy and lymph node dissection samples exhibited hypermethylation of distinct sequences in the promoter region of GSTP1. Parsons and colleagues observed that GSTP1, the most widely studied of the GST enzymes within the human prostate, is highly expressed in normal prostate epithelium, while GSTPA1 is not. The investigators noted that regions of focal prostatic glandular atrophy, potential precursors to HGPIN, display elevated GSTP1 and GSTPA1 activity, while both are down-regulated in HGPIN and adenocarcinomas. The evidence behind such a prominent group of antioxidant enzymes having this caretaker role in prostate tumourigenesis further upholds the hypothesis of inflammation-induced carcinogenesis via disruption of oxidative balance.

Disruption of homeostatic signaling

Cell to cell and cell to extra cellular matrix (ECM) contacts within epithelia are crucial in the governance of proliferation . Signaling facilitated by these physical interactions, mediated through membrane spanning proteins, has been observed to convey tumor-suppressive functions. The loss of function of one such protein, E-cadherin, is common in epithelial cancers and its role as an inhibitor of progression to a more aggressive phenotype has been observed in vitro, wherein induced functional loss of this integrin stimulated invasion and migration of prostate cancer cells . The over-abundance of proteolytic enzymes produced during inflammation is known to disrupt epithelial cell to cell and cell to ECM adhesions. Matrix metalloproteinases (MMP), notorious degraders of the ECM, released by stromal cells in response to inflammatory response-derived chemical signaling , have recently been attributed with multiple functions, including the direct cleavage of integrin molecules such as E-cadherin . Such disruptions in cell to cell signaling is thought to lead to genomic instability and subsequent oncogenic transformation by effecting loss of control over cytokinesis-regulating pathways, such as those mediated by TP53. In support of this, Radisky and colleagues reported a series of experiments demonstrating that addition of MMP-3 to murine mammary cells caused a shift toward malignancy, attributed to altered gene expression, and the resulting increase in reactive oxygen species.

Oxidative stress

Oxidative stress is a well-documented cause of genomic instability and has been implicated in the promotion of tumourigenesis. Activated phagocytes (and potentially epithelial cells) at the site of inflammation produce elevated levels of the superoxide anion, a precursor to a variety of reactive oxygen and nitrogen species . Additional oxidative stress through the accumulation of free radicals from the metabolism of saturated fats and paucity of dietary antioxidants characteristic of Western culture may serve to further disrupt the cellular oxidative balance and increase the likelihood of oncogenic initiation. Peroxidases particular to neutrophils and eosinophils have been observed to be responsible for the production of reactive nitrogen species in murine models of acute inflammation , which has additionally been shown to be a precursor of lipid peroxidation . Constant localized exposure to oxidative stress and mediators of the immune response have been implicated in the development of permanent genetic damage and the activation of proto-oncogenes, such as c-fos and c-jun . In a review of a series of human and animal studies, Kawanishi and Hiraku report examples of bacterial, viral, and parasitic infections, as well as noninfectious chronic inflammatory illnesses, that are suspected risk factors for cancer. In these studies, the investigators were able to consistently identify oxidation products of guanine, including those known to comprise mutagenic DNA lesions in inflammatory and epithelial cells at the site of inflammation. The investigators concluded that these oxidative alterations of DNA were probable contributors to the development of carcinomas from chronic or repeated inflammation caused by helicobacter pylori, hepatitis C, inflammatory bowel disease, and oral lichen planus, at their respective sites. Though the reviewed body of research did not include inflammation of the prostate, many of the same pathologic processes are present in models of chronic prostatitis.

In a recent case-controlled study, Zhou and colleagues compared levels of oxidative stress in 70 subjects with chronic bacterial prostatitis to that of an equal number of healthy controls. The investigators observed increased levels of molecular byproducts of oxidizing reactions and lower activities of prominent antioxidant enzymes among cases. This supports a higher exposure to oxidative stress among the former group, which could potentially result in mutagenic DNA lesions and ultimately, carcinogenesis.

Further epidemiologic support for the transforming effect of reactive oxygen and nitrogen species in prostate epithelia is found in case-controlled studies examining the allele frequency of genes encoding enzymes that protect against, and repair, oxidative damage. Manganese superoxide dismutase is an antioxidant enzyme accredited with the detoxification of oxygen-free radicals inside the mitochondria and has been observed to suppress proliferation of prostate cancer cells both in vitro and in vivo . A polymorphism for the encoding gene was discovered that consists of a single valine-alanine substitution, thought to affect the enzyme’s ability to permeate the mitochondrial membrane. Results from a study nested within the α-tocopherol β-carotene Cancer Prevention Study by Woodson and colleagues imply that men homozygous for the alanine allele had an increased risk of prostate cancer. Two independent case-controlled studies reported increased prostate cancer risk with certain polymorphisms of a gene encoding human oxoguanine glycosylase, an enzyme involved in repair of oxidative DNA lesions . An increased risk for men with the Ser362Cys polymorphism was common to both. Evidence that reduced functionality of innate antioxidant mechanisms can increase the likelihood of developing prostate cancer coincides with the idea that sustained, elevated oxidative stress, as encountered in chronic inflammation, is a probable potent carcinogen.

Glutathione-S-transferases (GSTs) are an important family of antioxidant enzymes that, because of their role as detoxifiers of reactive compounds, have been postulated to provide protection against oncogenesis . The enzymes are capable of neutralizing several different forms of potentially damaging reactive products and are basally expressed in normal epithelial cells . However, a recent meta-analysis assessing the association between certain genetic variations of three of the most common GST genes (GSTM1, GSTT1 and GSTP1) and risk of prostate cancer was unable to detect a statistically significant association . The investigators chose studies that compared null and nondeleted genotypes for GSTM1 and GSTT1, and two common polymorphisms for GSTP1. The quantitative synthesis of cases and controls from 11, 10, and 12 studies on GSTM1, GSTT1 and GSTP1, respectively, failed to show significant allelic differences for the GST genes between cases and healthy controls, yet loss of GST functionality is ubiquitous in prostate . The nearly unanimous down-regulation of GSTP1 in high-grade intraepithelial neoplasia (HGPIN) and carcinomas imply a “caretaker” role, described by Kinzler and Vogelstein as a gene whose inactivation facilitates genomic instability, resulting tumor initiation.

This loss of function has been attributed to the somatic silencing of GST genes, corroborated by Lin and colleagues , who observed that 95% of assessed prostatectomy and lymph node dissection samples exhibited hypermethylation of distinct sequences in the promoter region of GSTP1. Parsons and colleagues observed that GSTP1, the most widely studied of the GST enzymes within the human prostate, is highly expressed in normal prostate epithelium, while GSTPA1 is not. The investigators noted that regions of focal prostatic glandular atrophy, potential precursors to HGPIN, display elevated GSTP1 and GSTPA1 activity, while both are down-regulated in HGPIN and adenocarcinomas. The evidence behind such a prominent group of antioxidant enzymes having this caretaker role in prostate tumourigenesis further upholds the hypothesis of inflammation-induced carcinogenesis via disruption of oxidative balance.

Tumor associated macrophages

Inflammatory factors produced by TAMs directly, through paracrine interaction with other cells or by liberation from the breakdown of stromal tissue, contribute to malignancy by multiple processes, three of the most significant being resistance to apoptosis, induction of proliferation, and angiogenesis . Originating from circulating monocytes, TAMs are a major constituent of the leukocytes that infiltrate the tumor microenvironment . TAM function is modified by exposure to various cytokines and has been observed to confer both tumor-suppressing and promoting activity. Two studies exposing macrophages to interleukin (IL)-12, one directly and the other via injection with a vector containing the IL-12 gene, noted a resulting tumouricidal behavior of TAMs, both in vitro and in vivo, toward chemically induced tumors .

Other research has implicated TAMs as promoters of tumor progression because of the production of proangiogenic growth factors, cytokines, and proteases . Torisu and colleagues , observed that the up-regulation of vascular endothelial growth factor (VEGF), a strong promoter of angiogenesis, corresponded with increased production of proinflammatory cytokines produced by activated macrophages. The investigators noted that degree of macrophage infiltration was correlated with microvessel density and depth of tumor invasion in melanomas, directly implicating TAMs in the promotion of angiogenesis and tumor progression. Positive correlations between TAM infiltration of the tumor environment, VEGF levels, and microvessel density have also been observed in human breast and oral cancers . That TAMs can behave as both tumor suppressors and promoters is consistent with the current concept of a tumor microenvironment that enlists host immune response mechanisms to enhance malignant progression . A key component of this is the reprogramming of TAMs and other immune cells through chronic or recurrent exposure to soluble mediators of inflammation.

Soluble inflammatory mediators

Chemokines, originally identified as directors of leukocyte migration , provide another example of how inflammation-related host systems might promote tumor progression. These soluble mediators have been attributed with a number of homeostatic roles, including mediation of the infiltration and activation of leukocytes during the inflammatory response . Altered chemokine receptor expression has been observed in neoplastic tissue, particularly near tumor and host tissue borders . Evidence for inflammation-associated chemokines as potentiators of malignancy, specific to prostate cancer, is found in recent studies reporting elevated expression of them and their receptors in human prostate cancer cell lines. Increased expression of members of the CXC family of chemokines, implicated in the promotion of angiogenesis, was found to be correlated with invasiveness of prostate cancer cell lines . This was associated with the cytoplasmic relocalization of corresponding CXC receptors from the cell surface. Murphy and colleagues reported a correlation between increased expression of another member of the CXC chemokine family, IL-8, its receptors, and high-grade disease in human biopsies. Recently, Vaday and colleagues discovered the expression of CCL5 and its receptor, CCR5, in human prostate cancer cells, a chemokine known to stimulate cell migration in breast cancer and one that has been correlated with the progression of other malignant diseases. In proof of a similar aggressiveness-enhancing effect in prostate cancer, the investigators reported that incubation with exogenous CCL5 stimulated proliferation and increased invasiveness of prostate cancer cells. Cytokines, other soluble mediators of inflammation, have been implicated in the signaling of malignant progression. A key proinflammatory cytokine, tumor necrosis factor-alpha (TNF-α), has demonstrated tumor-promoting behavior at multiple cancer sites .

TNF-α is regulated by the proteolytic activity of stromal metalloproteinases that liberate it from membranes of somatic cells and is produced by TAMs directly . Selective blockade of TNF-α and TNF-α receptor expression has resulted in resistance to chemically induced epidermal and hepatic cancers, indicating an important role of TNF-α in tumourigenesis at these, and perhaps other sites. Pikarsky and colleagues used Mdr2 knockout mice, artificially provoking inflammation-derived carcinogenesis, to elucidate mediators involved in this process. The researchers discovered that suppression of nuclear transcription factor (NF)-κB through inhibition of TNF-α resulted in apoptosis of transformed hepatocytes and the arrest of further progression to carcinogenesis. In addition, it was observed that TNF-α at the site of inflammation was not hepatocyte-derived, and concentrations of the cytokine were five-fold in Mdr2 knockout mice relative to wild types, implicating inflammatory or other parenchymal cells as the source. Recently, Huerta-Yepez and colleagues presented evidence of TNF-α having a role in conveying resistance to Fas-induced apoptosis through a similar pathway involving NF-κB in human prostate cancer cells.

NF-κB, known as a master inflammatory transcriptional regulator, has been shown to be activated by reactive oxygen species and various other carcinogens, in addition to pro-inflammatory chemokines and cytokines such as TNF-α . It is a potentially multi-faceted instigator of progression to more aggressive phenotypes because of its regulation of gene products that have been observed to be involved in mediating resistance to apoptosis, insensitivity to growth inhibition, tissue invasion, immortalization, and angiogenesis . NF-κB is constitutively active in many tumor cell lines, including transformed androgen-independent PC-3 and DU-145 prostate cells , though not in normal epithelial tissue. Kukreja and colleagues report findings that implicate NF-κB activity in the increased expression of a particular chemokine receptor, CXCR4, that was observed to confer heightened invasiveness in the former of these cell lineages. Recently, Yemelyanov and colleagues , were able to induce apoptosis and inhibit proliferation and invasive activity in human androgen-independent prostate carcinoma cell lines using a novel NF-κB inhibitor. The investigators noted that inactivation of NF-κB coincided with reduced production of VEGF from both cell lines, supporting the transcription factor’s positive regulation of angiogenesis. Because of its low-specificity for activation and multifarious transcriptional regulation of genes that serve to promote survival and proliferation, NF-κB has been regarded as an overall mediator of the stress response . The idea of inflammation as a predominant stressor of the tumor microenvironment portrays the NF-κB pathway as a key enabler of aggressive tumor characteristics.