Background Undercarboxylated osteocalcin (ucOCN) is released during bone resorption and acts as a hormone through GPRC6A and emerging non‑canonical pathways. While its roles in glucose metabolism, muscle, and reproduction are established, its influence on the liver—a central hub for metabolic homeostasis—remains unexplored. Age‑related decline in osteoclast activity reduces circulating ucOCN, coinciding with increased hepatic fibrosis and insulin resistance in older individuals.

Hypothesis We propose that ucOCN directly inhibits hepatic stellate cell (HSC) activation by binding GPRC6A on HSCs, which triggers a cAMP/PKA signal that phosphorylates and inhibits Smad3, thereby attenuating TGF‑β‑driven fibrotic signaling. Loss of this ucOCN‑GPRC6A axis in aging leads to unchecked HSC activation, excess collagen deposition, and impaired insulin signaling in hepatocytes.

Key Predictions

1. In vitro – Primary mouse HSCs treated with recombinant ucOCN will show decreased α‑SMA and collagen‑1α1 expression, increased phospho‑PKA, and reduced phospho‑Smad3 levels compared with vehicle. Pre‑treatment with a GPRC6A antagonist (e.g., PF‑04957325) or siRNA against GPRC6A will abolish these effects.
2. In vivo – Osteoclast‑specific Rank‑knockout mice (low bone resorption, low ucOCN) will exhibit elevated hepatic TGF‑β/Smad3 activity, higher hydroxyproline content, and impaired insulin‑stimulated AKT phosphorylation in liver versus wild‑type controls. Chronic ucOCN infusion (or vitamin K2 supplementation to increase ucOCN carboxylation shift) will normalize these parameters.
3. Human relevance – Serum ucOCN levels will inversely correlate with hepatic fibrosis scores (e.g., FIB‑4 or ELF) and directly correlate with hepatic insulin sensitivity (measured by hyperinsulinemic‑euglycemic clamp) in a cohort of older adults, independent of BMI and glycemic status.

Experimental Approach

• Cell culture: Isolate HSCs from 8‑week‑old C57BL/6 mice; treat with ucOCN (0‑100 ng/mL) ± GPRC6A inhibitor; assess α‑SMA, collagen‑1α1, p‑PKA, p‑Smad3 by Western blot and immunofluorescence.
• Animal models: Generate Oc‑Rank‑KO mice; confirm low serum ucOCN by ELISA. At 12 months, harvest liver for histology (Sirius Red), hydroxyproline assay, and signaling immunoblots. Parallel groups receive subcutaneous ucOCN (30 µg/kg/day) or vitamin K2 (45 mg/kg diet) for 8 weeks.
• Human study: Recruit 150 participants aged 60‑80 yr; collect fasting serum for ucOCN, perform liver fibrosis biomarker panel, and hyperinsulinemic‑euglycemic clamp; analyze correlations using multivariate regression adjusting for confounders.

Falsifiability If ucOCN fails to reduce HSC activation in vitro irrespective of GPRC6A status, or if osteoclast‑deficient mice show no increase in hepatic fibrosis/signaling despite low ucOCN, the hypothesis is refuted. Likewise, absence of a significant inverse relationship between serum ucOCN and hepatic fibrosis/insulin resistance in humans would invalidate the translational claim.

Implications Confirming this mechanism would position bone resorption as a regulator of liver health via a novel endocrine axis, offering therapeutic avenues—ucOCN mimetics, vitamin K2, or exercise‑induced bone turnover—to mitigate age‑related hepatic fibrosis and metabolic dysfunction.