Zonation of cytochrome P450 enzyme expression in rat liver. Isozyme-specific regulation by pituitary dependent hormones.

The effect of hypophysectomy and subsequent infusion of growth hormone (GH) or injections of triiodothyronine (T3) on the acinar expression pattern of four homonally regulated P450 isozymes was studied to elucidate the involvement of pituitary dependent hormones in regulating the characteristic centrilobular expression pattern of most members of the cytochrome P450 (CYP) gene family in rat liver. Hypophysectomy was previously observed to allow high expression of CYP2B1/2 and 3A1/2 in the normally silent periportal region. In the present study, it had much less effect on the zonation of the ethanol-inducible P450 2E1 form: only a moderate shift of 2E1 staining towards the periportal region was observed by immunohistochemistry. Subsequent injections with T3 moderately decreased CYP2E1 expression in the periportal region and no significant countereffect of GH was discerned. T3 treatment, previously observed to block only the periportal expression of CYP3A1/2, counteracted the increased CYP2B1/2 expression caused by hypophysectomy equally in the periportal and perivenous region. This was true both at the protein and mRNA level, as analysed from cell lysates obtained by in situ perfusion of livers by zone-restricted digitonin treatment. Thus, although hypophysectomy and subsequent GH and T3 treatment affect the total expression of CYP2B1/2, 2E1, and 3A1/2 similarly, the zonal effects were isozyme-specific. In contrast, the perivenous zonation normally seen for the dioxin-inducible P450 1A2 form was steepened rather than diminished by hypophysectomy, both in male and female rats. Administration of GH by the female-type continuous infusion had no effect in male rats, but partially counteracted the effect of hypophysectomy in females, suggesting an involvement of GH. In contrast to other CYP genes investigated, the female-characteristic expression of CYP2C12 was found to be completely non-zonated. Hypophysectomy and continuous GH administration dramatically affected the amount of mRNA of both P450 2C12 and the male-specific 2C11 form, but analysis of periportal and perivenous cell lysates indicated that these effects were not zone-specific. The distribution of the GH receptor was investigated to explain the zonal effects of GH. Immunohistochemically, a moderate perivenous dominance was observed, whereas the mRNA abundance of both GH receptor and GH binding protein was slightly higher in the periportal region. Thus, zonal regulation by GH does not appear to result from a GH receptor zonation; rather, a sinusoidal GH gradient may be involved. These data, combined with our previous results, indicate that pituitary-dependent hormones regulate the zone-specific expression of some P450 forms strongly (i.e. 2B1/2 and 3A1/2), and other forms are moderately regulated (i.e. 1A2 and 2E1), or are affected across the whole acinus (i.e. 2C11, 2C12).