Not So Giants: Mice Lacking Both Somatostatin and Cortistatin Have High GH Levels but Show No Changes in Growth Rate or IGF-1 Levels

Abstract

Somatostatin (SST) and cortistatin (CORT) are two highly related neuropeptides involved in the regulation of various endocrine secretions. In particular, SST and CORT are two primary negative regulators of GH secretion. Consequently, single SST or CORT knockout mice exhibit elevated GH levels; however, this does not lead to increased IGF-1 levels or somatic growth. This apparent lack of correspondence has been suggested to result from compensatory mechanisms between both peptides. To test this hypothesis, in this study we explored, for the first time, the consequences of simultaneously deleting endogenous SST and CORT by generating a double SST/CORT knockout mouse model and exploring its endocrine and metabolic phenotype. Our results demonstrate that simultaneous deletion of SST and CORT induced a drastic elevation of endogenous GH levels, which, surprisingly, did not lead to changes in growth rate or IGF-1 levels, suggesting the existence of additional factors/systems that, in the absence of endogenous SST and CORT, could counteract GH actions. Notably, elevation in circulating GH levels were not accompanied by changes in pituitary GH expression or by alterations in the expression of its main regulators (GHRH and ghrelin) or their receptors (GHRH receptor, GHS receptor, or SST/CORT receptors) at the hypothalamic or pituitary level. However, although double-SST/CORT knockout male mice exhibited normal glucose and insulin levels, they had improved insulin sensitivity compared with the control mice. Therefore, these results suggest the existence of an intricate interplay among the known (SST/CORT), and likely unknown, inhibitory components of the GH/IGF-1 axis to regulate somatic growth and glucose/insulin homeostasis.

Obesity- and gender-dependent role of endogenous somatostatin and cortistatin in the regulation of endocrine and metabolic homeostasis in mice

“…In contrast, despite clear alterations in pituitary axes, lack of SST or CORT did not drastically impacted mice growth, body composition or glucose/insulin metabolism, which is in agreement with previous data reported in single SST-KO or CORT-KO mice or even in double SST/CORT-KO mice under standard dietary conditions13252930. Indeed, although HF-feeding demonstrated a significant effect on all the experimental groups in terms of increased BW gaining, reduced food intake, elevated adipose tissue weight, altered body composition and impaired glucose/insulin metabolism, lack of SST or CORT showed minor effects.…”

“…Remarkably, these changes in plasma GH levels were not accompanied by parallel alterations in the expression of pituitary GH mRNA, which is consistent with previous reports2527293839 and reinforces the idea that SST and CORT could be exerting regulatory roles at different levels of the regulatory axis of the GH secretion, including the hypothalamic level, the control of secretory pulses or the regulation of other hormonal systems (ghrelin, glucocorticoids, etc.) as previously suggested132527293033. Nevertheless, it is important to highlight that, despite the difficulty of assessing GH levels due to its pulsatile secretion pattern, the results presented herein serve to confirm previous results on the crucial role of endogenous SST and CORT on the control of GH secretion and to expand them to different dietary patterns.…”

“…The fact that both peptides exhibit a marked inhibitory action on GH release and other endocrine secretions suggests that lack of SST or CORT would cause profound alterations in relevant endocrine/metabolic processes. However, genetically engineered animal models have revealed that, under normal conditions, the single or combined deletion of SST and/or CORT does not drastically influence the metabolic phenotypic of these animals132930. Indeed, despite the fact that SST and CORT are two primary inhibitors of GH secretion by acting through their canonical receptors (sst1–5)13263132 and, that SST-KO or CORT-KO mice display markedly elevated plasma GH levels1325, SST-KO and CORT-KO mice do not exhibit a correspondingly enhanced somatic growth1325 or major changes in body mass, IGF plasma levels or glucose/insulin metabolism132425293032.…”

“…C57Bl6/J SST-KO (kindly provided by Dr. Ute Hochgeschwender) and CORT-KO mice were bred in-house and maintained under standard conditions (12 h light/dark cycle; lights on at 07:00 h; 22–24C), with free access to tap water and food [standard rodent chow (SAFE-diets, Barcelona, Spain)]. The development and validation of Sst−/− and Cort−/− mice as well as the methods to determine the genotypes by conventional PCR of tail-snip DNA have been previously reported1327303346. Specifically, male and female C57Bl/6 J wild-type (Sst +/+ and Cort +/+; Controls, WT), SST-KO (Sst −/−) and CORT-KO (Cort −/−) littermate mice were generated from heterozygous breeding pairs (Sst+/− or Cort +/− mice, respectively).…”

“…Total RNA from mouse tissues (hypothalamus, pituitary and liver) was extracted, reverse transcribed, and amplified by quantitative real-time PCR using specific sets of primers, as previously described13304748. To control for variations in the amount of RNA used and the efficiency of RT reaction, mRNA copy number of each transcript was adjusted by a normalization factor (NF) obtained from the expression of three housekeeping genes [Beta-actin (ACTB), cyclophilin A and/or Hypoxanthine-guanine phosphoribosyltransferase (HPRT)], using the Genorm 3.3 application49, where the expression of these housekeeping genes was not significantly altered between the experimental groups.…”