Intrinsic signals in the sexually dimorphic circulating growth hormone profiles of the rat

Abstract

Male rats secrete growth hormone (GH) in episodic bursts every 3.5–4 h. Between the peaks, GH levels are undetectable. In females, GH secretory profiles are characterized as continuous because hormone concentrations are always measurable in the circulation. These gender differences in the circulating GH profiles are responsible, to varying degrees, for observed sexual dimorphisms ranging from body growth to the expression of hepatic cytochrome P450 (P450, CYP) isoforms. Using hypophysectomized rats in which restored gender-dependent plasma GH profiles were manipulated, we have investigated the importance of the interpulse period in the masculine episodic plasma GH profile in regulating expression (mRNA, protein and/or specific catalytic activity) of male-specific CYP2A2, 2C11, 2C13 and 3A2, female-specific CYP2C12 and female-predominant CYP2A1, 2C6 and 2C7. We observed that some isoforms were induced or suppressed by discerning the length of the GH-devoid interpulse period, others responded to the pulse amplitudes, still others recognized the mean circulating concentrations of GH and some were regulated by a combination of these signals. We conclude that concealed in the gender-dependent circulating GH profiles are numerous intrinsic signals, both inductive and repressive, individually “tailored” to be recognized by each isoform of P450. There would appear to be no one signal in each gender-dependent GH profile responsible, in toto, for the characteristic sexually dimorphic expression of some dozen hepatic P450s in male and female rats.

Introduction

Circulating GH profiles in rats as well as other species have been shown to be sexually dimorphic (Shapiro et al., 1995). Male rats secrete GH in episodic bursts (∼200–300 ng/ml of plasma) every 3.5–4 h. Between the peaks, GH levels are undetectable. In females the hormone pulses are more frequent and irregular and are of lower magnitude than those in males, whereas the interpulse concentrations of GH are always measurable (Legraverend et al., 1992a, Shapiro et al., 1995). These gender differences in the circulating GH profiles, and not sexual differences in GH concentrations, per se, are responsible for observed sexual dimorphisms ranging from body growth to the expression of hepatic enzymes (Legraverend et al., 1992a, Shapiro et al., 1995). In this regard, rat, as well as murine liver each contain at least a dozen sex-dependent isoforms of P450 that are regulated by the gender-dependent profiles of circulating GH (Legraverend et al., 1992a, Waxman, 1992, Shapiro et al., 1995).²

Whereas much less is known about GH regulation of murine P450s, in the rat, P450 responses to GH regulation are almost as variable as the number of GH-dependent isoforms. That is, expression of the major female-specific CYP2C12 (as well as the non-P450 5α-reductase) is dependent upon the feminine profile of continuous GH secretion. Exposure to the masculine profile of episodic hormone release, as well as the absence of the hormone from the circulation (e.g., hypophysectomy), completely prevents expression of CYP2C12 (Ram and Waxman, 1990, Legraverend et al., 1992b). In a somewhat similar vein, female-predominant CYP2C7 expression is also dependent on the feminine GH profile and is completely suppressed in the hypophysectomized rat. However, exposure to the masculine profile allows expression of CYP2C7 at 25–40% of normal female levels (Ram and Waxman, 1990, Westin et al., 1990). Expression of the major male-specific CYP2C11 requires the episodic ‘on/off’ masculine profile of GH secretion. Although the feminine pattern of continuous hormone secretion blocks CYP2C11 expression, total GH depletion from the circulation allows CYP2C11 expression at 20–30% of intact male levels (Morgan et al., 1985, Janeczko et al., 1990, Legraverend et al., 1992b). After hypophysectomy, female-predominant CYP2A1 (male/female, ∼1:3) concentrations decline to around male levels and are restored to intact female-like levels with continuously administered GH (Waxman et al., 1990b, Yamazoe et al., 1990). Although the expression levels of CYP2C7, CYP2C11, CYP2C12, and CYP2A1 are greatest when exposed to their gender-dependent GH profiles, other isoforms are optimally expressed in the absence of GH. Male-specific CYP2A2 and CYP3A2 are maximally expressed in the hypophysectomized rat, disappear when GH is secreted constantly, but are only partially suppressed under the influence of episodic GH (Waxman et al., 1988, Waxman et al., 1995). Male-specific CYP2C13 is optimally expressed when exposed to the masculine hormone profile or under conditions of no GH, whereas the feminine GH profile completely suppresses CYP2C13 (Legraverend et al., 1992a, Legraverend et al., 1992b).

Although there are additional examples, it becomes clear that the expression or suppression of each isoform of P450 is likely to be regulated by a different “signal” or perhaps, a differential sensitivity to the ‘signal’ in the sexually dimorphic GH profiles. These signals may be recognized by the hepatocyte in the frequencies and/or durations of the pulse and interpulse periods. Alternatively, perhaps the hepatocyte can monitor the mean plasma concentration of the hormone. In the latter case, using the selective-GH deficient monosodium glutamate-treated rat, we reported that a 70–85% reduction in the feminine GH profile has little, if any, effect on the levels of gender-dependent P450 isoforms normally expressed in the female liver (Waxman et al., 1990a, Pampori and Shapiro, 1994a). More recently, using the euthyroid, hypophysectomized rat (Pampori and Shapiro, 1996), we observed that a renaturalized feminine GH secretory pattern of only 2–3% of its normal level (the lowest values we examined) remained completely suppressive of male-specific CYP2A2, CYP2C11, CYP2C13 and CYP3A2 expression. In contrast, the female-dependent isoforms exhibited a variable responsiveness to the restored hormonal profile. Female-predominant CYP2A1 and CYP2C6 were restored to pre-hypophysectomy levels when the feminine GH profile was renaturalized to just 3% of normal. While this level of GH restored female-specific CYP2C12 and 5α-reductase to ∼50% and ∼70% of normal, respectively, restoration to intact expression levels required a circulating feminine profile of 12–25% of normal. Female-predominant CYP2C7 appeared to be the least GH-sensitive isoform. Although exhibiting a commensurate increase in mRNA and protein concentrations with increasing levels of GH, the feminine GH profile had to be restored to near 100% (i.e. physiologic) to induce normal female-like expression levels of CYP2C7. Gender is an additional factor that influences the differential responsiveness of P450s to GH. That is, the renaturalized feminine GH profile is far more effective in restoring the female pattern of P450 expression when infused in females than in males (Pampori and Shapiro, 1999) and vice versa (Shapiro et al., 1993). Lastly, less well-defined, multitropic endpoints like body and organ weights also exhibit a differential sensitivity to the feminine growth hormone profile requiring replacement of 25–50% of the profile for normal maintenance (Pampori and Shapiro, 1996).

While these studies have identified ‘concentration’ as the intrinsic signal in the feminine GH profile regulating the expression of hepatic P450 isoforms observed in female rats, the present study has examined the important properties in the episodic masculine growth hormone profile, in particular, the interpulse period devoid of GH, in regulating the expression of sex-dependent hepatic P450s found in the male rat.