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포유류 생식 내분비 기능 조절에서 Ghrelin의 역할

Role of Ghrelin in the Control of Reproductive Endocrine Function

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참고문헌

주제분류생물학

초록

시상하부-뇌하수체-생식소(hypothalamus-pituitary-gonad, HPG) 호르몬 축의 활성에 영향을 미치는 수많은 인자들은 생식 기능을 조절하고 사춘기 개시와 폐경기 진입과 같은 뚜렷한 생식 능력의 단계 전이를 초래한다. 지방세포로부터 분비되는 다기능적 호르몬인 leptin의 발견 이후, 곧 이어 생식과 신체의 에너지 균형 사이의 긴밀한 관계에 대한 증거들이 밝혀졌다. 위장관으로부터 분비되는 또 다른 다기능 호르몬인 ghrelin은 이미 알려져 있던 growth hormone secretagogue receptor(GHSR)의 내인성 리간드이며, 에너지 항상성의 조절에서 leptin에 상응하는 물질로 알려졌다. 예상대로, ghrelin 또한 HPG 축의 활성의 조절을 통해 생식 능력을 조절함이 증명되었다. 이 논문은 ghrelin의 발견과 유전자 구조, 조직 내의 분포, 그리고 역할과 HPG 축에서의 생식 호르몬 분비 조절에 대한 포유동물의 생식에서의 ghrelin-GHSR 신호에 관한 최신의 정보를 요약한 것이다. 뇌하수체에서의 POMC 유전자 발현과 유사하게, preproghrelin 유전자는 alternative splicing과 번역 후 변형(posttranslational modification)을 거치는 복잡한 레퍼토리의 전사체들과 펩티드 산물을 만들어 낸다. 에너지 항상성을 제외한 신체 생리 기능의 조절에서의 preproghrelin 유전자 산물의 역할에 관한 정보는 제한적이지만, 신진 대사와 생식 사이에서의 ghrelin의 상호작용에 관해서는 충분한 증거들이 있다. 흰쥐와 인간에서, ghrelin 수용체인 GHSRs(GHSR1a와 GHSR1b)의 분포는 본래 ghrelin의 표적으로 여겨진 시상하부와 뇌하수체 뿐만 아니라 정소와 난소에서도 확인되었다. 뇌와 생식소에서도 preproghrelin 유전자 발현이 확인되었는데, 이것은 HPG 축에서 ghrelin이 국부적인 역할을 담당할 가능성을 시사한다. 비록 뇌하수체에서의 기능은 아직 확실치 않지만, ghrelin은 시상하부의 GnRH, 뇌하수체의 생식소자극호르몬과 생식소의 성 스테로이드 호르몬 분비에 대한 음성적인 조절자로서의 역할을 수행하는 것으로 보인다. 최근의 연구들은 사춘기 개시, 그리고 아마도 폐경기 진입의 조절에서 ghrelin의 관여를 시사한다. 이제 ghrelin이 ‘뇌-위장관’ 축의 필수적인 호르몬 요인이며, 신진 대사와 생식 사이를 연결하는 조절 물질이라는 가능성은 매우 높다. ‘배부름’을 반영하는 leptin 신호와는 정반대인 ghrelin 신호는 신체 에너지 균형 상태로 볼 때 ‘배고픔’을 표현하는 것으로 생각되며, 항상성의 유지에서 최우선 사항으로 고려되지 않는 생식으로의 에너지 투자가 이루어지지 않도록 하는데 필수적일 것으로 사료된다. 생식능력 조절에 있어서 ghrelin의 보다 명확한 작용 메커니즘과 역할에 대한 깊은 통찰력을 얻고 성공적인 생의학적 적용을 위해서는 향후 더 많은 연구들이 필요하다.

Abstract

Numerous factors can affect the activities of hypothalamus-pituitary-gonad (HPG) hormonal axis, resulting in alteration of reproductive capacity or status such as onset of puberty and menopause. Soon after the finding of leptin, a multifunctional hormone secreted from adipocytes, a close relationship between reproduction and body energy balance have been manifested. Ghrelin, another multifunctional hormone from gastrointestinal tract, is an endogenous ligand of growth hormone secretagogue receptor (GHSR), and is thought to be a counterpart of leptin in the regulation of energy homeostasis. As expected, ghrelin can also modulate the reproductive capacity through the modulation of activities of HPG axis. This paper summarizes the current knowledge on the discovery, gene structures, tissue distribution and roles of ghrelin and GHSRs in mammalian reproduction in particular modulation of reproductive hormone secretion in HPG axis. Like POMC gene expression in pituitary gland, preproghrelin gene can generate a complex repertoire of transcripts which further undergo alternative splicing and posttranslational modifications. Concerning the roles of preproghrelin gene products in the control of body physiology except energy homeostasis, limited knowledge is available so far. Several lines of evidence, however, show the interplay of ghrelin between metabolism and reproduction. In rat and human, the distribution of ghrelin receptor GHSRs (GHSR1a and GHSR1b) has been confirmed not only in the hypothalamus and pituitary which were originally postulated as target of ghrelin but also in the testis and ovary. Expression of the preproghrelin gene in the brain and gonads was also verified, suggesting the local role (s) of ghrelin in HPG axis. Ghrelin might play a negative modulator in the secretions of hypothalamic GnRH, pituitary gonadotropins and gonadal steroids though the action on pituitary is still questionable. Recent studies suggest the involvement of ghrelin in regulation of puberty onset and possibly of menopause entry. It is now evident that ghrelin is a crucial hormomal component in 'brain-gut' axis, and is a strong candidate links between metabolism and reproduction. Opposite to that for leptin, ghrelin signaling is likely representing the 'hunger' state of body energy balance and is necessary to avoid the energy investment into reproduction which has not a top priority in maintaining homeostasis. Further researches are needed to gain a deep insight into the more precise action mechanism and role of ghrelin in reproduction, and to guarantee the successful biomedical applications.

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