Mechanisms of male reproductive toxicity

1. Mechanisms of male reproductive toxicity Markku Sallmén Finnish Institute of Occupational Health

2. Time trends in male reproductive health • Declining semen quality • Increasing incidence of hypospadias and cryptorchidism • Increasing incidence in testicular cancer in Northern European countries including Estonia and Finland => are there common causal environmental factors behind?

3. Timing of male reproductive hazards • The adult life: most studies on male reproductive health have focused on this period • Prenatally (examples): ionizing radiation, cadmium, oestrogens (diethylstilbestrol, DES)

4. Sex-dependent differences in germ cell kinetics • Male: germ cells undergo expensive mitosis during fetal development, but do not enter meiosis before puberty • Female: germ cells initiate their first meiotic division before birth

5. Prenatal events Postnatal manifestations O E S T R O G E N S Synthesis of Müllerian Inhibiting Substance in fetal Sertoli cells Proliferation of Sertoli cells Impaired spermatiogenesis Normal germ cell division Testicular cancer Regression of Müllerian ducts Cryptorchidism Synthesis of Testosterone in fetal Leydic cells Poor virilization Adapted from Bonde and Giwercman (1995)

6. Site of action of male reproductive toxicants Affected site Exposure Epididymis Epicholorohydrin, Chlorometahane Spermatid Chlorometahane Spermatocyte Heat Spermatogonium DBCP Sertoli cell Phtalate esters, dinitrobenzene Leydig cells Ethanol Capillary Cadmium Adapted from Bonde and Giwercman (1995)

7. Severity of the damage and site of action Affected site Effect Spermatogonium azoospermia without recovery Spermatocyte and decreased capacity to reproduce Spermatids transient (stem cells unaffected) Epididymial or testicular transient impairment of sperm spermatozoa motility, decreased viability Adapted from Bonde and Giwercman (1995)

8. Severity of the damage and site of action Affected site Effect Sertoli cells number and morphology of sperm cells, may be irreversible Leydig cells reduced testosterone and thus disturbance of Sertoli cell function Hypothalamus - Pituitary axis disturbed endocrine homeostasis, reduced semen quality Adapted from Bonde and Giwercman (1995)

9. Male-mediated developmental toxicity • gene mutation • chromosomal abnormalities • spontaneous abortion • congenital malformations • cancer

10. Male-mediated developmental toxicity: mechanisms • Direct germ-cell effects by either genetic or epigenetic mechanisms • Indirect effects by transmission of agents to the mother via seminal fluid and maternal exposure to toxicants brought home by the father

11. Male-mediated developmental toxicity: mechanisms • Seminal fluid transfer: methadone, cyclophosphamide • Household contamination: lead, beryllium, polychlorinated biphenyls

12. Epigenetic mechanisms • Change in gene activity during development (without gene or chromosomal mutations!) • genomic imprinting for example through change in DNA methylation or removal of proteins that control imprinting

13. Epigenetic mechanisms • 5-azazytidine, a non-mutagenic chemical • exposure in vitro has caused 10-30% of previously inactive genes to become reactivated corresponds about 1 million-fold increase over spontaneous reversion rates

14. Lead: proposed mechanisms of developmental toxicity • Seminal fluid transfer • Household contamination • Direct toxic effects on sperm • Mutations • Epigenetic effects • Effects on chromatin stability

15. Lead: epigenetic effects Gandley et al. Environ Research 1999 • Fertility was reduced in male rats with PbB 27-60 µg/dl • Changes in 2-cell gene expression with PbB 15-23 µg/dl • Lead may affect fetal development in the absence of decreased fertility

16. Lead: effects on chromatin stability • Chromatin structure altered at rather low exposure • Lead and other cations (mercury, copper) may replace zinc in chromatin structure => Failure or delay in sperm chromatin decondensation in fertilitzation process => reduced fertility or DNA damage possible