Interactions between oral contraceptive status and GSTM1 and GSTT1 deletions on insulin-like growth factor-1 (IGF-1) plasma levels in young healthy women

Objective: Insulin-like growth factor-1 (IGF-1) is essential for the pubertal growth spurt and for normal mammary gland development. IGF-1 increases premenopausal breast cancer risk. Oral contraceptives (OCs) decrease IGF-1 in most women. The endogenous estrogens and their metabolites also influence IGF-1 levels. Glutathione S-transferases (GSTs) are involved in estrogen metabolism. We aimed to study IGF-1 levels and body size in relation to GSTM1 and GSTT1 deletions, and GSTP1*1B and current oral contraceptive (OC) status.

Design: Questionnaires on reproductive factors and OC use were completed and blood samples were obtained during menstrual cycle day 18-23 in healthy women (≤40 years) from breast cancer high-risk families. IGF-1 was analyzed with radioimmunoassay. Genetic analyses were done with PCR based methods. Initially 258 women were included. After exclusion 229 women were finally included in the analyses of IGF-1 in relation to GSTM1 and GSTT1.

Results: Among the 142 non-OC users, GSTM1*0/*0 or GSTT1*0/*0 alone were associated with lower IGF-1 levels while homozygous GSTM1*0/*0/GSTT1*0/*0 carriers had higher IGF-1 levels (P(interaction)=0.024). In the 87 OC users, GSTM1*0/*0 or GSTT1*0/*0 alone were associated with higher IGF-1 levels while homozygous GSTM1*0/*0/GSTT1*0/*0 carriers had lower IGF-1 levels (P(interaction)=0.010). Among all 229 women, a three-way interaction model showed an interaction between the GSTM1*0/*0/GSTT1*0/*0 genotype and OC use on IGF-1 levels (P(interaction)=0.003). GSTP1*1B was not associated with IGF-1. The GSTM1*1/GSTT1*0/*0 genotype was associated with high body weight (P=0.003) and GSTM1*0/*0/GSTT1*0/*0 was associated with early growth (P=0.003).

Conclusion: Both OC use and GSTT1 and GSTM1 genotypes may influence IGF-1 levels. Further studies are warranted to confirm our finding and elucidate the clinical importance.