The Role of Retinaldehyde in Bone Metabolism

Osteoporosis is a major health problem particularly affecting the aging population. It is characterized by an imbalance in skeletal turnover and augmented marrow adiposity. This leads to a disturbance of the bone microarchitecture resulting in diminished bone stability and increased risk for fractures. Chronic inflammatory diseases, various drug therapies such as thiazolidinediones (TZDs), and lack of physical activity can affect bone mineral density. Nuclear receptors such as peroxisome proliferator-activated receptor-gamma (PPAR) are implicated in the pathophysiology of osteoporosis. The PPAR agonists TZDs are in wide clinical use for the treatment of type 2 diabetes but they have been shown to increase the bone fracture risk. PPAR plays a key role in bone turnover through modulation of marrow stromal cell differentiation and osteoclastogenesis. PPAR requires heterodimerization with the retinoid X receptor (RXR) in order to form an active transcriptional complex. RXR serves as a receptor for retinoids, the biologically active structural derivatives of vitamin A (retinol). Notably, retinoids can independently modulate the PPAR -RXR heterodimer and recapitulate important PPAR responses in inflammation and metabolic homeostasis. Recent work of Dr. Plutzky`s group identified that the retinoic acid (RA) precursor retinaldehyde (Rald) functions as a biologically active mediator in fat, where it inhibits adipogenesis by repressing PPAR-RXR transactivation. Mice lacking retinaldehyde dehydrogenase 1 (Raldh1), the main enzyme that converts Rald to RA, display higher endogenous levels of Rald, altered PPAR responses, and protection from diet-induced obesity and insulin resistance. Preliminary data presented here demonstrate a striking skeletal phenotype of Raldh1 knockout (Raldh1 -/- ) mice characterized by increased bone mineral density (BMD) and greatly enhanced trabecular bone number and volume fraction. Repression of PPAR action, blocked adipogenesis and increased bone mass in Raldh1 deficiency suggest an inverse TZD phenotype. Hence, we hypothesize that Rald and its associated enzymatic machine play an important role in bone turnover and marrow adiposity and exert important effects on gene networks that regulate skeletal homeostasis. We will pursue this hypothesis through the following concurrent, highly integrated in vitro and in vivo aims: Aim 1: Characterize the skeletal phenotype of Raldh1 -/- and WT mice. We will determine skeletal differences between female and Raldh1 -/- and wild type (WT) control mice using micro-computed tomography, bone histology and magnetic resonance imaging for marrow fat quantification, dynamic histomorphometric analysis, and bone turnover markers. Aim 2: Define the impact of Raldh1 deficiency on PPAR activation in bone. We will conduct studies with Raldh1 -/- and WT control mice treated with the potent PPAR activator rosiglitazone. This will facilitate our efforts to determine how Rald and Raldh1 modulate PPAR responses during the critical phases of bone acquisition and maintenance. Aim 3: Determine the role of Rald and Raldh1 in lineage allocation of bone marrow stromal cells (MSC) and hematopoietic progenitors. We will document whether Rald and its associated regulatory machinery are present and differentially expressed in osteoblasts and osteoclasts. We will perform comprehensive cellular studies to define the effects of Raldh1 deficiency on MSC differentiation into the osteoblast or adipocyte lineage, and to test whether the retinoid axis regulates osteoclastogenesis. These issues are of immediate translational relevance. The increasing prevalence of osteoporosis associated with the "aging" of industrialized societies demands novel therapeutic strategies. Additionally, the clinical use of PPAR agonists has been limited by unexpected bone loss and an increased risk of fractures, emphasizing a central dilemma in PPAR modulation. Possible protective effects on bone metabolism together with the reported inhibition of obesity and insulin resistance suggest a novel therapeutic potential of interfering with the retinoid axis and warrant in detail investigation.