Page 585 - The Toxicology of Fishes

Chemical Carcinogenesis in Fishes 565

Shelton and associates (1984) fed trout diets containing the complete carcinogen DEN, with and
without Arochlors 1242 or 1254. Both Arochlors enhanced DEN liver tumor incidence. As will be shown
below, this is in contrast to earlier studies with AFB that showed an appreciable inhibition of liver
1
tumor incidence. The direction of the modulation of chemical carcinogenesis in trout by PCB depends
on the carcinogen involved.
Partial hepatectomy is a proven way to induce liver regeneration and has been used to enhance liver
tumorigenesis in rodents (Michalopoulos, 1995) and in medaka (Kyono-Hamaguchi, 1984). The liver
rapidly regenerates following this procedure, and, if the carcinogen is administered prior to partial
hepatectomy, the initiated cells preferentially respond to the growth stimulus, thereby reducing the latency
period for tumor formation (promotional effect). The cytotoxicity phase following diethylnitrosamine
exposure was studied in medaka using light microscopy, electron microscopy, and biochemical indices
(Lauren et al., 1990). Perhaps the toxicity serves as a sort of chemical hepatectomy and plays an
enhancing role in the eventual DEN-induced liver carcinogenesis in this model. Although the technique
has yet to be utilized in chemical carcinogenesis studies, Ostrander and coworkers (1993) developed a
partial hepatectomy procedure for both large (2 kg) and small (<5 gm) trout.
In recent bioassays using large numbers of larvae initiated by a brief, aqueous DEN exposure, tumor
frequency was consistently higher in females than in male medaka (Teh and Hinton, 1988). This
interesting finding was the impetus for several studies examining features of tumor growth in female
and male medaka using a two-step, initiation-promotion assay in which a subcarcinogenic dose of
initiating carcinogen was followed by continuous administration of a tumor promoter. The hypothesis
that estrogen is a promoter of hepatic neoplasia in medaka was tested, and results demonstrated that E 2
increased the prevalence of hepatocellular adenoma and carcinoma, as well as basophilic hepatic foci
of cellular alteration (FCA) (Cooke and Hinton, 1999). The effects of steroid hormones on normal liver
physiology suggest that endogenous estrogens may contribute to the increased susceptibility of female
fish to hepatic neoplasia. Interestingly a similar response was observed with a nonsteroidal estrogenic
compound, β-hexachlorocyclohexane (βHCH), which is not a strong ER agonist, suggesting that multiple
mechanisms may exist for tumor promotion putatively involving membrane initiated signaling (Cooke
and Hinton, 1999).

Enhancement of Chemical Carcinogenesis: A Focus on Medaka
One feature common to laboratory and field studies with various fish species is a higher prevalence of
hepatocellular neoplasia in females than in males. The medaka (Oryzias latipes), a small aquarium teleost
fish, is a highly valuable vertebrate model for hepatocarcinogenesis because of its small size, ease of
maintenance, rapid tumor onset, and immense potential as a bioassay subject (Hawkins et al., 1995).
Factors that appear to positively modulate tumorigenesis in medaka are sex, estradiol, and temperature.

Sex
Females have briefer latency periods and greater tumor volume. Control medaka liver morphology proved
highly correlated with sex and this in turn was correlated with ovarian maturation. Sex-specific aspects
of individual medaka and liver growth were serially investigated following repeated observations that
females were more susceptible to hepatocarcinogenesis when initiation was performed within the first
3 weeks after hatching (Teh and Hinton, 1998). Aspects of sex-specific growth in hatchling, immature,
and sexually mature control medaka were determined. For each fish, body and liver weights were
recorded, and the ratios of liver weight to body weight (hepatosomatic index [HSI]) were determined.
Body weights of control females were significantly greater than those of males at weeks 8, 20, 32, and
44. Liver weights and HSIs were significantly greater in females vs. males at all ages. Similar recordings
were made for medaka initiated with a 48-hour aqueous exposure to 250 ppm DEN. Among the DEN-
treated medaka, female body weights were significantly greater than their male counterparts at weeks
8, 16, 20, and 32. Female liver weights were greater than male values at all weeks except for 4 and 6,
and female HSIs were significantly greater than male values at all times. A higher incidence of FCA
(40%) distinguished females from males (10%) at 4 weeks, and these values reached 100% incidence
(females) and 90% (males) at week 12. Tumor latency periods for adenomas and carcinomas were