Insulin-Like Peptide 3 (INSL3), also known as Relaxin-Like Factor (RLF), is a 12 kDa two-chain peptide hormone belonging to the relaxin/insulin-like peptide superfamily. It is constitutively produced by testicular Leydig cells in males and ovarian theca and luteal cells in females.

Overview

INSL3 was originally discovered as a factor critical for the first transabdominal phase of testicular descent during fetal development. In the absence of functional INSL3 or its receptor RXFP2, the gubernaculum fails to develop properly, resulting in cryptorchidism (undescended testes). Unlike testosterone, which is regulated acutely by LH stimulation, INSL3 production reflects the long-term differentiation status and overall functional capacity of the Leydig cell population. This makes circulating INSL3 levels a uniquely valuable biomarker that is independent of the acute pulsatile fluctuations seen with testosterone.

In adult males, INSL3 is produced constitutively by mature Leydig cells, with serum concentrations typically ranging from 0.5 to 1.5 ng/mL. Levels are low in prepubertal boys, rise sharply during puberty as Leydig cells mature, remain relatively stable through adulthood, and decline gradually with aging. In females, INSL3 is produced by theca interna cells and luteinized granulosa cells, with much lower circulating levels (typically 20-100 pg/mL), and serves as a marker of small antral follicle differentiation and ovarian reserve.

The clinical significance of INSL3 extends beyond reproductive biology. Discovery of RXFP2 expression on osteoblasts and osteocytes has opened research into INSL3's role in bone metabolism, with evidence suggesting it functions as a circulating hormone that promotes bone formation. This positions INSL3 at the intersection of reproductive endocrinology and skeletal biology, with implications for understanding osteoporosis in hypogonadal men.

Mechanism of Action

INSL3 exerts its biological effects primarily through binding and activation of the RXFP2 receptor, a leucine-rich repeat-containing G protein-coupled receptor:

RXFP2 Receptor Activation: INSL3 binds to the large extracellular leucine-rich repeat domain of RXFP2 with high affinity (Kd ~0.5-1 nM). The B-chain of INSL3 provides the primary binding interaction with the receptor ectodomain. Receptor activation leads to coupling with inhibitory G proteins (Gi/o), resulting in a decrease in intracellular cAMP levels in most cell types studied.

Gubernacular Development Pathway: During fetal development, INSL3 produced by fetal Leydig cells acts on RXFP2 expressed in the gubernaculum to promote its growth and differentiation. This swelling reaction of the gubernaculum is essential for anchoring the testis near the inguinal canal, enabling the first (transabdominal) phase of testicular descent. Disruption of this pathway results in cryptorchidism.

Osteoblast Signaling: RXFP2 is expressed on osteoblasts and osteocytes. INSL3 binding activates signaling cascades that promote osteoblast differentiation and activity, contributing to bone formation. This involves both cAMP-dependent and MAPK-dependent pathways, with downstream effects on gene expression related to mineralization and matrix production.

Ovarian Follicular Signaling: In females, INSL3 produced by theca cells acts in a paracrine/autocrine manner within the ovarian follicle. It modulates steroidogenesis and may influence follicular development and oocyte maturation through RXFP2-mediated signaling in granulosa and theca cells.

Anti-Apoptotic Signaling: INSL3 has demonstrated anti-apoptotic effects in germ cells, particularly through activation of survival pathways. In the testis, INSL3 may support spermatogenesis by promoting germ cell survival through RXFP2-mediated signaling.

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Research

Safety Profile

INSL3 is an endogenous hormone, and safety data primarily relates to its measurement as a biomarker rather than exogenous administration. In preclinical studies, administration of recombinant INSL3 has not been associated with significant adverse effects at physiological concentrations. As a naturally circulating peptide, INSL3 is subject to normal proteolytic degradation and has a relatively short half-life. No clinical trials of exogenous INSL3 administration in humans have been conducted to date, so the safety profile of therapeutic INSL3 use remains to be established. Theoretical concerns include potential effects on reproductive tissues given the widespread expression of RXFP2 in reproductive organs. Elevated INSL3 levels in women with PCOS raise the possibility that supraphysiological INSL3 could have adverse ovarian effects, though causality has not been established.

Clinical Research Protocols

• Biomarker measurement: Serum INSL3 measured by ELISA or time-resolved fluoroimmunoassay (TRFIA). Fasting morning samples preferred for consistency.
• Reference ranges (males): Prepubertal: <0.1 ng/mL; Adult: 0.5-1.5 ng/mL; Elderly (>70 years): 0.3-0.8 ng/mL.
• Reference ranges (females): Follicular phase: 20-60 pg/mL; Luteal phase: 40-100 pg/mL; PCOS: often >100 pg/mL.
• Preclinical dosing: In animal studies, INSL3 has been administered at 0.5-5 mcg/kg intraperitoneally or subcutaneously for bone metabolism studies.
• Key trials: No interventional human clinical trials of exogenous INSL3 have been registered. Current research is observational and biomarker-focused.
• Duration: Biomarker studies range from single time-point measurements to longitudinal cohort follow-up over 4-10 years.

Subpopulation Research

• Cryptorchid males: INSL3 and RXFP2 mutations found in 1-4% of cryptorchidism cases. Low cord blood INSL3 associated with undescended testes at birth.
• Aging men: European Male Aging Study (EMAS) showed INSL3 declines ~1.3% per year after age 40, more linearly than testosterone. Low INSL3 independently predicted sexual dysfunction and reduced bone mineral density (PMID: 16757531).
• Men on TRT/AAS: Profound suppression of INSL3 due to LH suppression, often to undetectable levels. Recovery after cessation depends on duration and degree of Leydig cell atrophy.
• PCOS women: Elevated INSL3 correlates with antral follicle count and androgen levels. May serve as an additional diagnostic marker for PCOS phenotyping.
• Testicular cancer survivors: Persistently low INSL3 despite normalized testosterone (compensated Leydig cell failure), indicating subclinical gonadal damage post-chemotherapy.
• Klinefelter syndrome (47,XXY): Markedly reduced INSL3, reflecting progressive Leydig cell dysfunction. INSL3 decline may precede testosterone decline in these patients.

Pharmacokinetic Profile

Ongoing & Future Research

• Therapeutic INSL3 for osteoporosis: Preclinical work exploring whether exogenous INSL3 or RXFP2 agonists could serve as bone anabolic agents, particularly in hypogonadal men with osteoporosis.
• RXFP2 small molecule agonists: Development of non-peptide RXFP2 agonists that could overcome the pharmacokinetic limitations of peptide administration for potential bone and reproductive applications.
• INSL3 as a clinical biomarker: Efforts to standardize INSL3 immunoassays and establish consensus reference ranges for clinical use in male hypogonadism assessment.
• Reproductive toxicology screening: INSL3 proposed as a sensitive endpoint in endocrine disruptor screening, as it reflects long-term Leydig cell health more faithfully than testosterone.
• INSL3 in transgender medicine: Investigation of INSL3 as a marker of residual Leydig cell function in transgender women on estrogen therapy, and as a predictor of fertility preservation potential.
• Gubernacular biology: Continued research into the molecular mechanisms by which INSL3/RXFP2 signaling promotes gubernacular development, with implications for understanding and preventing cryptorchidism.