ABSTRACT
Adiponectin is well documented as an insulin sensitizer. However, variable and relatively mild metabolic dysfunctions in adiponectin knockout mice suggest the existence of compensatory mechanisms. Recently, a novel family of 10 secreted proteins (CTRP1 to CTRP10) sharing common structural and functional features with adiponectin was discovered. Most CTRPs are expressed by adipose tissue and circulate in plasma. Th eir circulating levels vary with the sex, genetic background and metabolic states of the animals. All CTRPs form trimers as their basic structural unit; some are further assembled into hexameric and HMW oligomeric complexes that may have distinct biological and signaling properties. Additionally, CTRPs form combinatorial associations, representing a potential mechanism to generate functionally distinct ligands with altered receptor specifi city and/or function. To date, in vitro and/or in vivo metabolic functions have been demonstrated for CTRP1, CTRP2, CTRP3, CTRP5, and CTRP9. Additionally, CTRPs have been implicated in immune response, platelet aggregation, macular degeneration, bone formation and some types of cancers. Although the receptors for CTRPs have yet to be identifi ed, liver, muscle and adipose tissues are likely targets of CTRPs. Future studies on these novel adipokines will provide new insights into physiological mechanisms that link adipose tissue to whole-body energy homeostasis.
