Secreted proteins from adipose tissue and skeletal muscle – adipokines, myokines and adipose/muscle cross-talk

Abstract

White adipose tissue and skeletal muscle are the largest organs in the body and both are composed of distinct cell types. The signature cell of adipose tissue is the adipocyte while myocytes are the defining cell of skeletal muscle. White adipocytes are major secretory cells and this is increasingly apparent also for myocytes. Both cells secrete a range of bioactive proteins, generally termed adipokines in the case of adipocytes and myokines for muscle cells. There has, however, been some confusion over nomenclature and we suggest that the name myokine is restricted to a protein that is secreted from myocytes, while the term adipokine should be used to describe all proteins secreted from any type of adipocyte (white, brown or brite). These definitions specifically exclude proteins secreted from other cells within adipose tissue and muscle, including macrophages. There is some commonality between the myokines and adipokines in that both groups include inflammation-related proteins – for example, IL-6, Il-8 and MCP-1. Adipokines and myokines appear to be involved in local autocrine/paracrine interactions within adipose tissue and muscle, respectively. They are also involved in an endocrine cross-talk with other tissues, including between adipose tissue and skeletal muscle, and this may be bi-directional. For example, IL-6, secreted from myocytes may stimulate lipolysis in adipose tissue, while adipocyte-derived IL-6 may induce insulin resistance in muscle.

Key words::

• Adipocyte
• adipokine
• adipose tissue
• cross-talk
• myocytes
• myokines
• skeletal muscle

Acknowledgements

The authors are members of COST BM0602 and this article is based on discussions emanating from a workshop under this programme in Skagen, Denmark (7 and 8 May 2010). CAD’s studies on myokines have been funded by the University of Oslo, The Norwegian Cancer Association, Aktieselskabet Freia Chocolade Fabrik’s Medical Foundation, Throne-Holst Foundation for Nutrition Research, and the Nutrigenomics Organisation, EC-funded Network of Excellence, grant no. FOOD-2004–506360. JE’s work was supported by the Ministerium für Wissenschaft und Forschung des Landes Nordrhein-Westfalen (Ministry of Science and Research of the State of North Rhine-Westphalia), the Bundesministerium für Gesundheit (Federal Ministry of Health), and the Commission of the European Communities (Collaborative Project ADAPT, Contract No. HEALTH-F2-2008–201100). PT’s studies on adipokines have been funded by the BBRC and MRC (UK).

Declaration of interest

The authors have no conflicts of interest in relation to the contents of this article.