Is Visfatin an Adipokine or Myokine? Evidence for Greater Visfatin Expression in Skeletal Muscle than Visceral Fat in Chickens

Abstract

Visfatin, an adipokine hormone produced primarily by visceral adipose tissue in mammals, has been implicated in the immune system, cellular aging, and glucose metabolism. Increased visceral adiposity and hyperglycemia have been correlated with elevated plasma visfatin levels in humans. The present study investigated visfatin cDNA and protein expression as well as plasma visfatin levels in chickens that are selected for rapid growth and are naturally hyperglycemic relative to mammals. By RT-PCR, we detected visfatin cDNA in multiple tissues in the chicken. The deduced amino acid sequence of full-length chicken visfatin was 92-93% homologous to mammalian visfatin. Using real-time quantitative PCR and Western blotting, chicken skeletal muscle was found to contain 5- and 3-fold greater quantities of visfatin mRNA and protein than abdominal fat pad, respectively. Visfatin mRNA and protein quantities were not significantly different among sc and visceral adipose tissue depots. Skeletal muscle visfatin mRNA and protein quantities as well as plasma visfatin levels determined by enzyme immunoassay were significantly higher in 8-wk-old compared with 4-wk-old chickens, possibly due to rapid skeletal muscle growth and visceral fat accretion occurring in broiler chickens during this period. However, fasting and refeeding did not affect plasma visfatin levels in the chicken. Collectively, our results provide novel evidence that skeletal muscle, not the visceral adipose tissue, is the primary source of visfatin in chickens, thereby raising the possibility that visfatin may be acting as a myokine affecting skeletal muscle growth and metabolism.

NAMPT (visfatin) in the chicken testis: influence of sexual maturation on cellular localization, plasma levels and gene and protein expression

“…The same reference standard yielded an intra-assay CV of 6.46% in a previous study (Krzysik-Walker et al 2008). However, the greater intra-assay CV observed in the present study should not affect our interpretation as the plasma visfatin levels were higher by 28-fold in adult chickens compared with that of prepubertal chickens.…”

“…In addition, we observed a lower molecular weight (w28 kDa) NAMPT protein in both sperm protein extract and in seminal plasma. Chicken adipose and liver protein extracts contained a similar 28-kDa NAMPT protein (Krzysik-Walker et al 2008), which may have resulted from proteolytic degradation (Samal et al 1994) or due to alternative splicing, as tissuespecific visfatin splice variants have been previously reported in the pig ).…”

“…5A). The molecular weight of testicular NAMPT protein was identical to that expressed in chicken skeletal muscle, adipose tissue, and liver (Krzysik-Walker et al 2008). NAMPT protein quantity relative to ACTB quantity was not significantly different between prepubertal and adult chicken testes ( Fig.…”

“…Our previous study had established the specificity of an anti-human NAMPT antibody to detect chicken NAMPT (Krzysik-Walker et al 2008). A 52-kDa NAMPT protein was detected in the protein extracts obtained from both prepubertal (14-weekold) and adult (29-week-old) chicken testes (Fig.…”

“…Recently, we have shown that chicken NAMPT cDNA and protein are highly homologous to mammalian NAMPT (Krzysik-Walker et al 2008). In addition, NAMPT protein levels were elevated with age in the skeletal muscle and plasma of male chickens.…”