1. 1 Endocrine properties of skeletal muscle Josef Brandauer Joslin Diabetes Center josef.brandauer@joslin.harvard.edu

2. 2 Lecture overview Endo-/para-/juxta-/autocrine signaling ‘Classic’ and ‘novel’ hormone-secreting organs Protein secretion from skeletal muscle Interleukin-6 (IL-6) Retinol-binding protein 4 (RBP4) – a novel myokine?

3. 3 Endocrine signaling Endocrine vs. exocrine: Endocrine: ductless, vascular, intracellular vacuoles/granules storing hormones Exocrine: ducts or hollow lumen; much less vascularized (salivary gland, sweat glands)

4. 4 Other ‘-crine’ signaling Paracrine: Target cell is close to signal-releasing cell E.g.: Allergic response, scar formation, blot clotting, etc. Juxtacrine: requires physical contact between two cells. E.g.: Notch signalling Autocrine: secreting cell = target cell E.g.: Monocytes (IL-1)

5. 5 Hormones Hormao: Gr., ‘to set in motion’ First use of ‘hormone’: Description of secretin, Bayliss and Starling, 1902

6. 6 “Classic” endocrine glands Hypothalamus: Thyrotropin-releasing hormone  ACTH (pituitary) Pituitary: TSH, GH Thyroid: Triiodothyronine: BMR, catecholamine sensitivity, protein synthesis Adrenal medulla: Norepinephrine, epinephrine Pancreas: insulin, glucagon Liver: IGF-1

7. 7 “Novel” endocrine glands Heart: ANP, BNP: blood pressure regulation Adipose tissue: leptin, adiponectin, etc.: satiety, insulin sensitivity, etc. Skeletal muscle: Interleukin-6, others?

8. 8 Protein secretion from skeletal muscle What purpose could this serve? -- Communicate a change in skeletal muscle environment to other tissue(s) -- Which change(s)? -- Which tissue(s)?

9. 9 Interleukin-6 (IL-6) Febbraio, 2002

10. 10 Interleukin-6 (IL-6) Steensberg, 2000

11. 11 Interleukin-6 (IL-6) Febbraio, 2002

12. 12 IL-6 before after hindlimb contraction

13. 13 Retinol-binding protein 4 (RBP4) Adipose-tissue G4KO animals are IR in vivo – in vitro glucose transport is not affected

14. 14 (s)RBP4 RBP4 occurs in serum as a tetramer with retinol and transthyretin, the carrier protein for thyroxine AT mRNA and serum protein levels are increased in adipo-G4KO mice sRBP4 is elevated in other obesity/diabetes models (ob/ob, HFD, etc.) responsive to TZD treatment (AT mRNA, serum) RBP4 -/- animals are more insulin sensitive than WT control. RBP4 injection impairs IRS-1 associated PI3k activity in skeletal muscle

15. 15 Connection between exercise and sRBP4 Vitamin A (retinol) levels increase acutely with endurance exercise … known since the 1950s.  does this imply that sRBP4 is also increased?  can acute rodent treadmill exercise, or in vitro contraction, function as a model?

16. 16 Acute exercise increases RBP4 in SD rats AU, arbitrary units; Sed, sedentary, Exer., exercise; 60 post, 60 min after exercise. (N = indicated in bars).

17. 17 Where does the exercise-induced increase in sRBP4 originate? 1.Liver 2.Adipose tissue 3.Other tissues?

18. 18 Acute exercise does not transiently induce RBP4 mRNA in liver…

19. 19 … or visceral fat

20. 20 … but … … acute exercise transiently increases RBP4 mRNA in gastrocnemius muscle Additionally, muscle-specific hRBP4 transgenic animals show an interesting sRBP4 pattern:

21. 21 Soooo….. … is RBP4 a “myokine”?

22. 22 What does an individual muscle fiber show?

23. 23 Retinol-binding protein 4 (RBP4) non-contracted contracted

24. 24 Conclusion I Acute exercise-induced increases in sRBP4 concentration are likely a result of increased skeletal muscle gene expression and RBP4 secretion Liver and adipose tissue, 2 major sources of sRBP4, are likely not involved in this process Evidence for a myokine role of RBP4  If RBP4 is a myokine, what would be its physiological role during exercise?

25. 25 Conclusion I Acute exercise-induced increases in sRBP4 concentration are likely a result of increased skeletal muscle gene expression and RBP4 secretion Liver and adipose tissue, 2 major sources of sRBP4, are likely not involved in this process Evidence for a myokine role of RBP4  If RBP4 is a myokine, what would be its physiological role during exercise?

26. 26 What are the effects of RBP4 on exercise capacity? fvb mice H4IIe H4IIe Levels of the gluconeogenic enzyme hepatic PEPCK are regulated, either directly or indirectly, by (circulating?) RBP4.  Implications on hepatic glucose production in vivo?  functional connection to acute exercise? First, some background…

27. 27 Hypothesis Due to the involvement of RBP4 in regulating hepatic glucose output, RBP4 deficient animals have reduced exercise capacity, and elevated post-exercise liver glycogen levels.

28. 28 Descriptive data – RBP4 KO/WT mice

29. 29 Exercise – graded exhaustive treadmill protocol

30. 30 Sedentary and exercise lactate levels Decreased gluconeogenesis? Why?

31. 31 Liver glycogen levels

32. 32 Conclusion II RBP4 KO mice have reduced exercise capacity compared to WT controls Some evidence that BG levels during exercise will decrease at an accelerated rate in KO animals Despite no apparent difference in sedentary glycogen, post- exercise glycogen levels are approximately 3-fold greater in KO compared to WT animals Liver glycogen metabolism during acute exercise may be impaired in RBP4 KO animals Post-exercise blood lactate levels are increased in RBP4 KO vs. WT animals  impaired gluconeogenesis?