Life-long caloric restriction reveals biphasic and dimorphic effects on bone metabolism in rodents

Fat targets for skeletal health

Emerging evidence points to a critical role for the skeleton in several homeostatic processes, including energy balance. The connection between fuel utilization and skeletal remodeling begins in the bone marrow with lineage allocation of mesenchymal stem cells to adipocytes or osteoblasts. Mature bone cells secrete factors that influence insulin sensitivity, and fat cells synthesize cytokines that regulate osteoblast differentiation; thus, these two pathways are closely linked. The emerging importance of the bone-fat interaction suggests that novel molecules could be used as targets to enhance bone formation and possibly prevent fractures. In this article, we discuss three pathways that could be pharmacologically targeted for the ultimate goal of enhancing bone mass and reducing osteoporotic fracture risk: the leptin, peroxisome proliferator-activated receptor gamma and osteocalcin pathways. Not surprisingly, because of the complex interactions across homeostatic networks, other pathways will probably be activated by this targeting, which could prove to be beneficial or detrimental for the organism. Hence, a more complete picture of energy utilization and skeletal remodeling will be required to bring any potential agents into the future clinical armamentarium.

Bone and fat connection in aging bone

PURPOSE OF REVIEW

The fat and bone connection plays an important role in the pathophysiology of age-related bone loss. This review will focus on the age-induced mechanisms regulating the predominant differentiation of mesenchymal stem cells into adipocytes. Additionally, bone marrow fat will be considered as a diagnostic and therapeutic approach to osteoporosis.

RECENT FINDINGS

There are two types of bone and fat connection. The 'systemic connection', usually seen in obese patients, is hormonally regulated and associated with high bone mass and strength. The 'local connection' happens inside the bone marrow. Increasing amounts of bone marrow fat affect bone turnover through the inhibition of osteoblast function and survival and the promotion of osteoclast differentiation and activation. This interaction is regulated by paracrine secretion of fatty acids and adipokines. Additionally, bone marrow fat could be quantified using noninvasive methods and could be used as a therapeutic approach due to its capacity to transdifferentiate into bone without affecting other types of fat in the body.

SUMMARY

The bone and fat connection within the bone marrow constitutes a typical example of lipotoxicity. Additionally, bone marrow fat could be used as a new diagnostic and therapeutic approach for osteoporosis in older persons.

Leptin as an endocrine signal in bone

Leptin and its actions in bone came to prominence in 2000, with the publication of two landmark articles identifying a novel interaction between energy and bone homeostasis, as well as a novel hypothalamic circuit to the skeleton. However, they also revealed the dichotomous nature of leptin's effect on the skeleton. Subsequent research has increased understanding of the factors critical to interpretation of the leptin-bone signaling. These include opposing effects in cortical and cancellous bone, central and peripheral effects, involvement of other neural and endocrine factors, and leptin receptor polymorphisms in human populations. It is clear that leptin can markedly influence the regulation of bone mass, and that study of this pathway continues to increase our knowledge of the biology of skeletal tissue and its interactions with other tissues. However, this relationship is complex and requires careful interpretation.