title: Megalin and androgen receptor gene expression in young and old human skeletal muscle before and after three sequential exercise bouts.
abstract: Androgen signaling occurs primarily via the androgen receptor. Megalin, a low-density lipoprotein endocytic receptor located in various mammalian tissues, has been recently shown to facilitate sex hormone–binding globulin (SHBG) steroid complexes across cell membranes. The purpose of this investigation is to determine if the megalin gene is expressed in human skeletal muscle and if present to determine how megalin and androgen receptor mRNA expression change in response to sequential exercise bouts with respect to aging. Ten younger (age: 18-25 years) and 10 older (age: 60-75 years) men completed 3 workouts (M, W, F) each consisting of 9 sets of lower-body exercises with 10 repetitions per set at 80% 1 repetition maximum. Vastus lateralis muscle biopsies were extracted at baseline (T1), 48 hours after workout 1 (T2) and 2 (T3), and 24 hours after workout 3 (T4), and blood samples were collected before and 5 minutes after each workout. Muscle was analyzed for megalin and androgen receptor expression using gene-specific primers and SYBR green chemistry, and blood was analyzed for serum testosterone, SHBG, and the free androgen index. Megalin was expressed in both young and old subjects across all time points, although no between- or within-group mean differences were detected at any time point. Androgen receptor was expressed higher in young men at all time points compared to in old men (p < 0.05), and a significant correlation (p < 0.05; r = 0.506) was found between serum testosterone and androgen receptor after workout 1. Based on our data, the gene coding for megalin is expressed inside skeletal muscle, but its role, if any, in steroid cellular transport cannot be determined. This finding could lay the groundwork for more mechanistic investigations to better delineate its functional role and its potential as a therapeutic adjunct for androgen-related disorders in healthy and aged populations.
authors.affiliation: Applied Biochemistry and Molecular Physiology Laboratory, Health and Exercise Science Department, University of Oklahoma, Norman, Oklahoma, USA.
authors.forename: Chris N
authors.initials: CN
authors.lastname: Poole
authors.forename: Michael D
authors.initials: MD
authors.lastname: Roberts
authors.forename: Vincent J
authors.initials: VJ
authors.lastname: Dalbo
authors.forename: Kyle L
authors.initials: KL
authors.lastname: Sunderland
authors.forename: Chad M
authors.initials: CM
authors.lastname: Kerksick
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000375:Aging / Q000502:physiology*; D015331:Cohort Studies; D015444:Exercise / Q000502:physiology; D005260:Female; D005786:Gene Expression Regulation; D006801:Humans; D026561:Low Density Lipoprotein Receptor-Related Protein-2 / Q000235:genetics / Q000378:metabolism*; D035002:Lower Extremity; D008297:Male; D008875:Middle Aged; D018908:Muscle Weakness / Q000235:genetics / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000502:physiology; D012333:RNA, Messenger / Q000032:analysis; D011944:Receptors, Androgen / Q000235:genetics / Q000378:metabolism*; D055070:Resistance Training / Q000379:methods*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012307:Risk Factors; D018709:Statistics, Nonparametric; D055815:Young Adult
chemical_list: D026561:Low Density Lipoprotein Receptor-Related Protein-2; D012333:RNA, Messenger; D011944:Receptors, Androgen