br Once ingested vitamin D the most bioavailable
Once ingested, vitamin D3 – the most bioavailable form of vitamin D
– is hydroxylated in the liver into 25(OH)D3, the circulating form of vitamin D3 commonly measured as ‘serum vitamin D’ . 25(OH)D3 is then further hydroxylated in the kidneys to become the steroid hor-mones 1,25(OH)2D3 or 24,25(OH)2D3 . 1α,25(OH)2D3 has been widely studied because of its role in regulated calcium homeostasis in concert with parathyroid hormone (PTH) . 1α,25(OH)2D3 is also an essential hormone for bone health because of its role in sequestering calcium within the bones to maintain bone mineralization and strength. 24R,25(OH)2D3 is not as well studied, partially because it is often re-garded as a ‘waste product’ that cycles with 1α,25(OH)2D3 to regulate the production of the latter hormone. However, studies by our lab and others have demonstrated a role for 24R,25-(OH)2D3 in growth plate development and fracture callus healing, where 24R,25-(OH)2D3 has an anti-apoptotic eﬀect and is an essential regulator of chondrocyte sen-sitivity to 1α,25(OH)2D3.
24R,25(OH)2D3 plays a key role in fracture healing and chondrocyte diﬀerentiation via a membrane-mediated PLD dependent pathway [30,31]. Unlike 1α,25(OH)2D3, which helps regulate calcium miner-alization of the lower parts of the growth plate, 24R,25(OH)2D3 is es-sential for Lycopene regulation in earlier stages of the growth plate, particularly in the progression of pre-proliferative chondrocytes to proliferative chondrocytes . Recent studies have suggested that 24R,25(OH)2D3 or the 24-hydroxylase that produces it, CYP24A1, may modulate cell proliferation and apoptosis in cancer cells [33–35]. This eﬀect may be a direct result of the activities of either the hormone or enzyme; or may be the result of modulation of the eﬀects of 1α,25(OH)2D3. 24,25(OH)2D3 has been shown to regulate the produc-tion of the 1-hydroxylase enzyme CYP27B1, which converts 25(OH)D3 to 1,25(OH)2D3 . In a similar manner, 24R,25(OH)2D3 promotes chondrocyte sensitivity to 1α,25(OH)2D3 , encouraging a positive feedback loop where each hormone regulates the production of the other, resulting in 1α,25(OH)2D3 and 24R,25(OH)2D3 cycling within the body .
As described above, 24R,25(OH)2D3 has been shown to signal through an as-yet unidentified receptor to stimulate PLD and a sub-sequent anti-apoptosis response in chondrocytes. The identity of this receptor has been a matter of debate for decades. Larsson et al., while studying the lysosomal membranes of chick intestinal cells, reported
Fig. 1. Western blot data of (A) ERα66, (B) ERα46, and (C) ERα36 normalized to GAPDH and expressed as mean normalized signal intensity ± standard error in MCF7, HCC38, and MDA-MB-231 breast cancer cell lines.
Fig. 2. Eﬀect of 10−7 M 24R,25(OH)2D3 on proliferation, apoptosis, and metastasis markers expressed as a function of the vehicle (treatment/control) in (A) estrogen receptor negative HCC38 and (B) estrogen captoror positive MCF7 breast cancer cell lines.
that catalase-1 specifically binds 24R,25(OH)2D3 , suggesting a rapid peroxidase-involved signal transduction pathway for 24,25(OH)2D3. A number of papers from the lab of Dr. Anthony Norman have reported evidence for a membrane-bound receptor for
24R,25(OH)2D3 in chick fracture healing callus that may be involved in rapid or genomic signal transduction [39,40]. Crystallographic studies of the classical vitamin D receptor (VDR) suggest that conformational changes in the receptor may accommodate the transient binding of 24R,25(OH)2D3 to the ligand-binding pocket of VDR . Recently, another 24R,25(OH)2D3 receptor, FAM57B2, was identified by Marti-neau et al., which may be essential to the 24R,25(OH)2D3 response in fracture callus repair . Numerous studies in rat growth plate chondrocytes have characterized a specific membrane-associated signal transduction pathway that mediates both classical genomic and rapid responses of 24R,25(OH)2D3, strongly suggesting the presence of a membrane receptor or signaling complex that regulates the actions of 24R,25(OH)2D3 [43–45].