The Cytochrome P450 and GST System and Carcinogenesis

being฀twice฀as฀inducible฀as฀the฀m1฀mutant฀ [81] .฀Smokers฀with฀the฀m2฀mutant฀have฀more฀ DNA฀adducts฀in฀their฀leucocytes฀than฀smokers฀without฀this฀mutant฀ [82] .฀These฀adducts฀are฀ also฀increased฀in฀the฀cord฀venous฀blood฀of฀neonates฀with฀CYP1A1-MspI฀polymorphism฀ [82] .฀ B[a]PDE฀ concentration฀ and฀ PAH-DNA฀ adducts฀ correlate฀ positively฀ with฀ enzyme฀ activity฀ in฀ the฀ parenchymal฀ lung฀ tissue฀ of฀ smokers฀ [36] .฀ Homozygous฀ CYP1A1-MspI฀ alleles฀are฀observed฀more฀rarely฀in฀Caucasians฀than฀in฀Japanese฀ [83] . Associations฀with฀CYP1A1฀activity฀have฀been฀demonstrated฀in฀more฀than฀20฀studies฀in฀ different฀ethnic฀populations฀see฀Table฀ 5.3 .฀According฀to฀studies฀from฀Japan,฀the฀lung฀can- cer฀risk฀was฀demonstrated฀with฀both฀m1฀and฀m2฀mutants฀ [85] .฀The฀CYP1A1฀phenotype฀was฀ important฀in฀terms฀of฀the฀development฀of฀SCCs฀only฀in฀moderate฀smokers฀ [88] .฀The฀results฀ were฀not฀corroborated฀in฀Norwegian,฀Finnish,฀Swedish฀and฀US฀populations,฀a฀inding฀that฀ was฀possibly฀attributable฀to฀the฀smaller฀occurrence฀of฀the฀m1฀allele฀in฀Caucasians.฀Studies฀ conducted฀in฀a฀mixed฀US฀population฀and฀in฀French฀patients฀showed฀no฀evidence฀of฀a฀pre- ferred฀allele฀m1฀or฀m2฀for฀increased฀cancer฀prevalence.฀In฀one฀study,฀individuals฀with฀the฀ m3฀mutation฀m1m1,฀m2฀had฀an฀increased฀AC฀risk฀with฀an฀OR฀of฀8.4฀ [97] ,฀whereas฀other฀ cancer฀types฀did฀not฀occur฀with฀increased฀frequency฀Table฀ 5.3 ฀ [30,฀97–100] .฀Where฀the฀ m1฀and฀m2฀variants฀of฀CYP1A1฀occurred฀in฀combination฀with฀the฀GSTM1฀00฀genotype,฀ there฀was฀an฀increased฀incidence฀of฀SCC฀in฀Japanese฀subjects฀ [63,฀85,฀101] . CYP1A2:฀ This฀ enzyme฀ activates฀ numerous฀ procarcinogens฀ from฀ tobacco,฀ preferen- tially฀ aromatic฀ and฀ heterocyclic฀ amines,฀ and฀ metabolises฀ nicotine.฀ The฀ sequence฀ of฀ CYP1A2฀is฀72฀identical฀with฀that฀of฀CYP1A1.฀However,฀it฀is฀formed฀principally฀in฀the฀ liver฀and฀to฀a฀very฀much฀smaller฀extent฀in฀the฀lungs฀ [102] .฀The฀enzyme฀is฀of฀marginal฀ importance฀for฀an฀increased฀risk฀of฀lung฀cancer. CYP2D6:฀This฀enzyme฀metabolises฀debrisoquine฀and฀poor฀metabolisers฀PMs฀have฀a฀ lesser฀ risk฀ for฀ the฀ development฀ of฀ lung฀ cancer฀ than฀ extensive฀ metabolisers฀ EMs฀ [71] .฀ However,฀the฀enzyme฀also฀activates฀NNK฀and฀metabolises฀nicotine฀ [103] .฀Associations฀evi- dently฀exist฀between฀nicotine฀dependence฀and฀metaboliser฀status฀ [104] .฀An฀increased฀lung฀ cancer฀risk฀has฀been฀demonstrated฀in฀EMs฀ [63,฀93,฀105–107] ,฀whereas฀two฀meta-analyses฀ have฀rejected฀such฀an฀association฀on฀the฀basis฀of฀the฀data฀reviewed฀ [22,฀107,฀108] . CYP2E1:฀CYP2E1,฀which฀is฀inducible฀by฀ethanol฀and฀numerous฀other฀agents,฀is฀capa- ble฀ of฀ metabolising฀ NNK,฀ NNN฀ and฀ other฀ soluble฀ nitrosamines฀ from฀ tobacco฀ smoke.฀ Structural฀ enzyme฀ variants฀ are฀ less฀ important฀ for฀ an฀ increased฀ lung฀ cancer฀ risk฀ in฀ Caucasians,฀with฀only฀2฀out฀of฀11฀studies฀showing฀such฀a฀inding฀ [63,฀109,฀110] .฀RsaI฀and฀ DraI฀polymorphisms฀of฀CYP2E1฀have฀not฀been฀shown฀to฀correlate฀with฀the฀development฀ of฀SCC;฀where฀these฀enzyme฀variants฀were฀present,฀there฀was฀a฀tenfold฀reduction฀in฀the฀ Polymorphism Point฀mutation Systematic฀nomenclature฀ for฀the฀mutation Wild฀type฀allele,฀m1 None wt MspI฀allele,฀3′,฀non-coding฀region,฀m2 6,235฀T→C m1 Ile→Val,฀exon฀7,฀codon฀462 4,889฀A→G m2 Afro-American-speciic฀allele,฀intron฀7 5,639฀T→C m3 Thr→Asn,฀exon฀7,฀codon฀461 4,887฀C→A m4 Table 5.2 ฀฀฀Overview฀of฀CYP1A1฀subforms฀ [79] SCC฀squamous฀cell฀carcinoma;฀AC฀adenocarcinoma;฀SCLC฀small฀cell฀lung฀cancer;฀LCLC฀large฀cell฀ lung฀cancer;฀LC฀lung฀cancer,฀wt฀wild฀type;฀PM฀poor฀metaboliser;฀EM฀extensive฀metaboliser;฀HEM฀ heterozygous฀extensive฀metaboliser,฀m1฀to฀m4:฀see฀Table฀ 5.1 ,฀C฀minor฀Allele;฀D฀general฀allele,฀ DraI,฀RsaI:฀see฀text Gene,฀ mutant Cancer฀ types Cases controls Genotype,฀frequency฀ Signiicance฀odds฀ ratio;฀95฀CI References 1A1฀m1 SCC,฀AC,฀ SCLC,฀ LCLC 68104 m1m1:฀23.510.6;฀ wtwt:฀35.349.0 LC:฀3.1;฀SCC:฀4.6 [84] 1A1฀m2 SCC,฀AC 212358 m2m2:฀12.34.7;฀ wtwt:฀56.7665.1 LC:฀2.97฀1.59–5.57;฀ SCC:฀3.34฀ 1.49–7.52;฀AC:฀ 2.54฀1.48–4.34 [85] 1A1฀m1,m2 SCC 85170 m1m1:฀22.48.8;฀ wtwt:฀38.848.2;฀ m2m2:฀10.63.5;฀ wtwt:฀58.864.7 m1m1:฀sm+:฀6.55฀ 2.49–17.24;฀ sm++:฀8.32฀ 2.34–29.62;฀m2 m2:฀sm+:฀8.46฀ 2.48–28.85;฀ sm++:฀8.46฀ 1.68–42.73 [86] 1A1฀m1 SCC,฀AC,฀ SCLC,฀ LCLC 267151 m1m1:฀16.910.6;฀ wtwt:฀36.744.3 m1m1฀und฀m1wt:฀ 1.71฀1.07–2.69 [87] 1A1฀m1–4 LC 157314 m3:฀00;฀m4:฀2.872.87 m2:฀3.01฀1.29–7.26 [88] 1A1฀m1 AC,฀SCC 207283 m1m1:฀1.00.7;฀ m1wt:฀16.917.0;฀ wtwt:฀82.182.3 m1wt+m1m1:฀LC฀ 2.08฀1.15–3.73;฀ AC฀sm฀+:฀2.25฀ 1.13–4.48;฀m1 wt:฀LC฀1.15฀ 1.0–2.3 [89] 1A1฀m2 SCC,฀ SCLC,฀ AC 247185 m2m2:฀11.33.8;฀ m2wt:฀38.145.4;฀ wtwt:฀50.650.8 m2m2:฀LC฀3.3฀ 1.3–8.6;฀SCC฀4.9฀ 1.4–16.3;฀SCLC฀ 9.4฀2.1–42.0 [90] 1A1฀m1,฀m2 SCC,฀AC,฀ SCLC,฀ LCLC 10895 m1m1:฀22.210.5;฀ wtwt:฀36.1-;฀ m2฀m2:฀16.76.3;฀ wtwt:฀53.7– m1m1:฀LC฀2.93฀ 1.26–6.84.฀ m2m2฀LC:฀3.45฀ 1.29–9.25 [91] 1A1฀m1,฀m2 SCC,฀ SCLC,฀ AC 8563 m1m1:฀75;฀wtwt:฀4046;฀ m2m2:฀½;฀m2wt:฀ 8095;฀wtwt:฀193 m2m2฀oder฀m2wt:฀ 0.14฀0.03–0.64 [92] 2D6฀3,฀ 4,5 SCC,฀AC 106122 PM:฀0.95.7;฀HEM฀+ EM:฀6.4฀1.0–14.3 [93,฀94] EM:฀99.194.3 2E1฀DraI SCC,฀AC,฀ SCLC 4756 CC:฀010.7;฀CD:฀46.8฀ 30.4;฀DD:฀53.259.9 p฀฀0.05 [95,฀96] 2E1฀DraI SCC,฀AC,฀ SCLC,฀ LCLC 9176 CC:฀2.214.5;฀CD:฀46.2฀ 28.9;฀51.656.6 CC:฀0.13฀0.04–0.51฀ aber฀CD:฀2.1฀ 1.1–4.0 [95,฀96] 2E1฀DraI,฀ RsaI SCC,฀AC,฀ SCLC 341456 DraI:฀CC฀1.55.5;฀CD:฀ 27.526.8;฀DD฀71฀ 67.7;฀RsaI:฀c2c2฀0.6฀ 3.1.฀c1c2:฀19.622.5;฀ c1c1฀79.874.4 DraI฀allele฀LC:฀CC฀0.2฀ 0.1–0.7;฀AC:฀CC฀ 0.1฀0.0–0.5;฀ RsaI:฀c2c2:฀0.1฀ 0.0–0.5 [63] Table 5.3 ฀฀฀CYP฀enzymes฀1A1,฀2D6฀and฀2E1฀and฀the฀development฀of฀lung฀cancer risk฀for฀SCLC,฀while฀the฀development฀of฀AC฀was฀promoted฀by฀CYP2E1฀mutants฀ [63] .฀In฀ one฀further฀study,฀the฀presence฀of฀a฀p53฀mutation฀and฀a฀c2c2฀genotype฀was฀shown฀to฀cor- relate฀for฀the฀increased฀occurrence฀of฀SCC฀of฀the฀lung฀ [111] . GST฀System:฀An฀unfavourable฀combination฀of฀CYP1A1฀with฀a฀GSTM1฀00฀gene฀pos- sibly฀leads฀to฀supra-additive฀DNA฀damage฀and฀to฀an฀increased฀cancer฀risk฀because฀detoxii- cation฀may฀be฀delayed฀as฀a฀result฀of฀slower฀coupling฀of฀carcinogens฀to฀the฀GST฀system.฀ Moreover,฀damaged฀GSTM1฀activity฀is฀associated฀with฀a฀high฀inducibility฀of฀CYP1A1฀by฀ 2,3,7,8-tetrachlorodibenzo-p-dioxin฀ [95] ,฀a฀phenomenon฀relected฀in฀the฀detected฀presence฀ of฀B[a]PDE฀adducts฀in฀the฀lung฀tissue฀of฀smokers฀and฀encountered฀primarily฀in฀Caucasians.฀ High฀ B[a]PDE-DNA฀ adduct฀ levels฀ in฀ lung฀ tissue฀ in฀ Caucasians฀ were฀ associated฀ with฀ a฀ GSTM1฀phenotype฀defect฀due฀to฀high฀CYP1A1฀inducibility฀or฀with฀a฀CYP1A1฀allele฀ [112,฀ 113] .฀This฀constellation฀leads฀to฀tobacco-induced฀DNA฀damage฀and฀the฀increased฀occur- rence฀of฀lung฀cancer.฀A฀weak฀correlation฀has฀even฀been฀found฀between฀the฀number฀of฀DNA฀ adducts฀and฀the฀number฀of฀cigarettes฀smoked฀ [20] .฀A฀Swedish฀study฀has฀now฀shown฀that฀ heavy฀smokers฀with฀a฀GSTT1-positive฀genotype฀have฀a฀threefold฀increased฀risk฀for฀lung฀ cancer฀with฀23฀pack-years฀of฀smoking฀OR฀2.6฀[1.3–5.0],฀whereas฀the฀risk฀is฀increased฀ ninefold฀OR฀9.3฀[1.9–46.3฀in฀smokers฀with฀the฀GSTT1-null฀genotype฀ [114] ,฀a฀genuine฀risk฀ constellation.฀Similar฀associations฀have฀also฀been฀reported฀with฀N-acetyl฀transferase฀NAT2฀ genotype฀as฀PMs,฀particularly฀when฀combined฀with฀the฀GSTM1-null฀genotype฀ [115] . These฀studies฀indicate฀that฀the฀highest-risk฀combination฀for฀increased฀susceptibility฀to฀ lung฀cancer฀is฀the฀CYP1A1฀genotype฀plus฀a฀GSTM1฀defect฀because฀of฀the฀following: • ฀ It฀results฀in฀more฀B[a]PDE฀adducts฀ [112,฀114–116] • ฀ Approximately฀100-fold฀higher฀B[a]PDE-DNA฀levels฀occur฀than฀with฀an฀active฀form฀of฀ GSTM1฀ [112] Carriers฀of฀homozygous฀CYP1A1฀m1฀have฀higher฀BPDE-DNA฀adduct฀levels฀than฀carriers฀ of฀wild฀type฀CYP1A1฀ [114] .฀Thus,฀whatever฀their฀ethnic฀origin,฀carriers฀of฀the฀homozy- gous฀CYP1A1GSTM1฀00฀trait฀are฀at฀increased฀risk฀for฀tobacco-induced฀cancer฀of฀the฀ lungs,฀head฀and฀neck.

5.2.3.3 Peptide Receptors,

a1-Antitrypsin and Carcinogenesis Activation฀of฀gastrin-releasing฀peptide฀receptors฀GRPR฀in฀human฀airways฀has฀been฀asso- ciated฀ with฀ cigarette฀ smoking.฀ The฀ GRPR฀ gene฀ is฀ located฀ on฀ the฀ X-chromosome฀ and฀ escapes฀inactivation,฀which฀occurs฀in฀females,฀with฀the฀result฀that฀women฀are฀more฀suscep- tible฀than฀men฀to฀carcinogens.฀GRPR-mRNA฀expression฀was฀detected฀in฀more฀female฀than฀ male฀non-smokers฀55฀vs.฀0฀and฀short-term฀smokers฀1–25฀pack-years:฀75฀vs.฀20.฀ Female฀smokers฀exhibited฀GRPR-mRNA฀expression฀at฀a฀lower฀mean฀pack-year฀exposure฀ than฀male฀smokers฀37.4฀vs.฀56.3฀pack-years;฀p฀=฀−0.037,฀permitting฀the฀conclusion฀that฀ they฀have฀a฀higher฀susceptibility฀to฀tumour฀development฀ [117] . Despite฀the฀known฀association฀between฀a 1 -antitrypsin฀deiciency฀and฀COPD,฀investiga- tions฀have฀also฀been฀conducted฀to฀establish฀whether฀heterozygous฀individuals฀who฀carry฀a฀ deicient฀allele฀of฀the฀a 1 -antitrypsin฀gene฀Pi฀protease฀inhibitor฀are฀at฀an฀increased฀risk฀of฀ developing฀bronchial฀carcinoma.฀The฀Pi฀locus฀is฀polymorphic฀with฀more฀than฀70฀variants฀ reported.฀ There฀ are฀ at฀ least฀ ten฀ alleles฀ associated฀ with฀ a 1 -antitrypsin฀ deiciency฀ [118] .฀ ฀Non-smokers฀carried฀a฀deicient฀allele฀three฀times฀more฀frequently฀20.6฀than฀smokers.฀ Nevertheless,฀patients฀with฀bronchial฀carcinoma฀or฀SCC฀had฀higher฀carrier฀rates฀than฀expected฀ 15.9฀and฀23.8,฀respectively.฀It฀may,฀therefore,฀be฀concluded฀that฀patients฀with฀an฀a 1 -anti- trypsin฀deiciency฀allele฀have฀an฀increased฀risk฀for฀lung฀cancer฀speciically฀SCC฀ [118] .

5.2.3.4 Exogenous Factors and Lung Cancer

Dietary฀factors฀may฀be฀important฀for฀the฀development฀of฀lung฀cancer,฀and฀the฀consumption฀ of฀carotene-rich฀fruits฀and฀vegetables฀and฀high฀plasma฀levels฀of฀vitamin฀E฀and฀b-carotene฀ are฀reported฀to฀reduce฀the฀risk฀ [119] .฀b-carotene฀functions฀as฀an฀antioxidant฀and฀as฀a฀pre- cursor฀for฀vitamin฀A฀or฀retinol.฀Retinoids฀are฀responsible฀for฀the฀differentiation฀of฀epithe- lial฀cells฀and฀they฀may฀suppress฀the฀malignant฀transformation฀of฀epithelial฀cells฀ [120] .฀ These฀indings,฀originally฀made฀in฀retrospective฀studies,฀have฀not฀been฀conirmed฀in฀pro- spective฀studies฀involving฀almost฀30,000฀smokers฀treated฀with฀vitamin฀E฀or฀b-carotene฀ over฀a฀period฀of฀years.฀The฀incidence฀of฀cancer฀even฀increased฀+18฀with฀b-carotene.฀ The฀Beta-Carotene฀and฀Retinol฀Eficacy฀Trial฀CARET฀also฀concluded฀that฀mortality฀was฀ increased฀in฀the฀active฀treatment฀group฀ [79] . Several฀studies฀were฀also฀showing฀that฀smokers฀with฀a฀low฀body฀mass฀index฀have฀a฀ higher฀risk฀for฀developing฀lung฀malignancies฀as฀compared฀with฀smokers฀of฀average฀weight.฀ It฀is฀suggested฀that฀DNA฀adducts฀may฀play฀an฀important฀role฀in฀that฀mechanism.฀A฀current฀ study฀could฀show฀that฀overweight฀subjects฀BMI฀฀25฀with฀little฀weight฀gain฀after฀smok- ing฀cessation฀median฀weight฀gain฀of฀6฀had฀more฀persistent฀adduct฀levels฀as฀compared฀ with฀those฀with฀lower฀BMI฀and฀higher฀weight฀gain฀฀p฀=฀0.06.฀Smokers฀with฀a฀low฀body฀ mass฀index฀have฀a฀higher฀risk฀for฀developing฀lung฀malignancies฀as฀compared฀with฀smokers฀ of฀average฀weight,฀but฀there฀is฀no฀mechanistic฀explanation฀for฀this฀observation.฀Carcinogens฀ in฀cigarette฀smoke฀are฀thought฀to฀elicit฀cancer฀by฀the฀formation฀of฀DNA฀adducts,฀which฀ give฀the฀opportunity฀to฀additionally฀investigate฀the฀biological฀link฀between฀BMI฀and฀lung฀ cancer.฀Godschalk฀et฀al.฀published฀a฀study฀in฀2002฀in฀which฀DNA฀adduct฀levels฀in฀periph- eral฀blood฀lymphocytes฀of฀24฀healthy฀smoking฀volunteers฀0.76฀±฀0.41฀adducts฀per฀108฀ nucleotides฀positively฀correlated฀with฀cigarette฀consumption฀r฀=฀0.51;฀p฀=฀0.01฀and฀were฀ inversely฀related฀with฀BMI฀r฀=฀−0.48;฀p฀=฀0.02฀ [121] .฀A฀signiicant฀overall฀relationship฀ was฀observed฀when฀both฀parameters฀were฀included฀in฀multiple฀regression฀analysis฀r฀=฀ 0.63;฀p฀=฀0.007.฀Moreover,฀body฀composition฀may฀affect฀DNA฀adduct฀persistence฀because฀ lipophilic฀tobacco฀smoke-derived฀carcinogens฀accumulate฀in฀adipose฀tissue฀and฀can฀be฀ mobilized฀once฀exposure฀ceases.฀Therefore,฀DNA฀adduct฀levels฀and฀BMI฀were฀reassessed฀ in฀all฀of฀the฀subjects฀after฀a฀non-smoking฀period฀of฀22฀weeks.฀Adduct฀levels฀declined฀to฀ 0.44฀±฀0.23฀per฀108฀nucleotides฀฀p฀=฀0.002฀and฀the฀estimated฀half-life฀was฀11฀weeks฀on฀ the฀basis฀of฀exponential฀decay฀to฀background฀levels฀in฀never-smoking฀controls฀0.33฀±฀0.18฀ per฀108฀nucleotides.฀Overweight฀subjects฀BMI฀฀25฀with฀little฀weight฀gain฀after฀smok- ing฀cessation฀median฀weight฀gain฀of฀6฀had฀more฀persistent฀adduct฀levels฀as฀compared฀