Abstract 3002: Bone-protective curcumin circulates as a pro-drug conjugate that is activated in bone by β-glucuronidase
Authors: Kunihiro Andrew , Brickey Julia A. , Frye jennifer B. , Luis Paula B. , Schneider Claus , Funk Janet L. 
Published: August 2019

Curcumin (CURC), a turmeric-derived dietary polyphenol, prevents osteoclast formation and bone resorption in murine models of arthritis, post-menopausal osteoporosis, and bone metastatic breast cancer despite being rapidly conjugated upon ingestion in both rodents and humans to form curcumin-glucuronide (G-CURC). However, recent studies from our laboratory demonstrate that aglycone CURC is enriched in bone relative to the circulation. Studies were therefore undertaken to compare the anti-resorptive effects of aglycone CURC vs G-CURC and to elucidate mechanisms responsible for enrichment of aglycone CURC in bone, including clinically relevant influences of age, sex, and reproductive status.

CURC dose-dependently inhibited RANKL-stimulated osteoclast formation in RAW264.7 cell cultures, while G-CURC was without effect. Likewise, CURC dose-dependently inhibited TGFβ-stimulated PTHrP secretion from MDA-MB-231 breast cancer cells, which readily form PTHrP-driven osteolytic bone metastases in vivo.

In wild type (WT) C57BL/6J mice, heparanase (HPSE) and β-glucuronidase (GUSB) were the only enzymes with reported deglucuronidation activity expressed in bone (Western). However, only GUSB was able to recognize G-CURC as a substrate, deconjugating G-CURC to form CURC (LC/MS). Consistent with these findings, tissue-specific levels of CURC were highest in bone (vs. heart, muscle, or kidney) and proportional to tissue specific GUSB enzyme activity.

To verify the role of GUSB in CURC bone enrichment, CURC metabolism and GUSB enzyme expression and activity were compared among mice with normal levels of GUSB (C57BL/6J) vs. partial (C3H/HeJ) or complete (mucopolysaccharidosis VII (mps/mps]) deficiencies in GUSB expression. As expected, partial (55% of WT) or complete deficiencies in GUSB expression significantly reduced CURC enrichment in bone.

While GUSB can be positively regulated by pharmacologic doses of androgens, neither male sex nor ovariectomy (OVX) altered GUSB activity (per protein) in bone relative to intact females, although a small but statistically significant increase in free curcumin was demonstrated in males (17%) and OVX females (10%). Age did not significantly change the enrichment of CURC in bone of 4 vs 28-week mice.

These findings suggest bio-inactive G-CURC acts as pro-drug that is converted to active CURC within bone, independent of sex, age or reproductive status, where it can inhibit the formation of bone resorbing osteoclasts or secretion of osteolytic PTHrP from breast cancer bone metastases.

Citation Format: Andrew Kunihiro, Julia A. Brickey, jennifer B. Frye, Paula B. Luis, Claus Schneider, Janet L. Funk. Bone-protective curcumin circulates as a pro-drug conjugate that is activated in bone by β-glucuronidase [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3002.