Abstract

Background: Adipocytes are derived from precursor cells, called preadipocytes, via a coordinated
process of differentiation that involves substantial remodelling of molecular and cellular
configuration and the extracellular matrix (ECM). Adipocytes function as energy stores and
endocrine cells, secreting several biologically active cytokine-like proteins, termed adipokines, in a
highly regulated manner to regulate essential aspects of metabolism, physiology and behaviour.
Typically, adipocyte function and adipokine production become dysregulated in obesity
contributing to the development of obesity-related diseases. Adiponectin is a key adipokine with
anti-diabetic, anti-atherogenic and anti-inflammatory effects. It is secreted into the circulation in
high concentrations in a variety of multimeric forms, the most biologically active being a high
molecular weight (HMW) multimer, a form which is decreased in obesity. The efficient
multimerisation and secretion of HMW adiponectin requires Vitamin C (VitC) to drive essential post-translational modifications (PTMs). The same PTMs are also required for collagen fibril formation,
required for ECM remodelling. Surprisingly, the mechanism(s) of VitC uptake into preadipocytes
and adipocytes is unknown, however recent papers suggest it’s transport may be mediated by the
GLUT1 glucose transporter working in tandem with stomatin: an integral plasma membrane protein
which facilitates plasma membrane ion transport and cytoskeletal anchoring. The aim of this
project was to begin to define the pathway(s) that mediate VitC transport into (pre)adipocytes and
determine whether this is compromised in obesity.
Methods: (i) Twelve transcriptomic datasets from the gene expression omnibus (GEO) detailing
changes in gene expression profiles during differentiation were analysed using GEO2R. (ii) The
volume of regulatory sites (promoter and transcription factor binding) upstream of the stomatin
gene (STOM) were analysed using genome analysis programmes ENCODE, BLAST and Ensemble. (iii)
Six GSE (Genomic Spatial Event) datasets comparing gene expression profiles of tissues or cells from
lean and obese subjects were analysed as above.
Results: (i) Analysis of differentiating gene expression profiles showed characteristic induction of
classic adipogenic markers (FABP4, C/EBP, PPARγ, PLIN1, ADIPOQ and FASN) with evidence of
differential induction of some genes in human and murine cells (eg FABP4: 1,000 vs 50-fold
induction; ADIPOQ: 50 vs 1,000-fold induction). There were no major changes in expression of the
VitC transporters responsible for transport of ascorbate (the reduced form of VitC), SVCT1/2, or the
glucose transporters, GLUT1/4, capable of transporting dehydroascorbate (DHA - the oxidised form
of VitC). However, the expression of stomatin, a protein that modulates the activity of GLUT1 such
that it transports DHA preferentially over glucose, was significantly induced in differentiating
human cells (10-fold) but not rodent cells. (ii) The number of adipogenic transcription factor
binding sites proximal to the stomatin gene was 50% higher in organisms such as humans that rely
on dietary VitC, compared with others that can synthesise VitC. (iii) There were no significant
differences between adipose tissues or cells from lean or obese subjects.
Conclusions: Collectively these results raise the intriguing hypothesis that stomatin expression is
differentially regulated during adipogenesis in those species incapable of VitC synthesis, compared
to species that can. Increased stomatin expression may facilitate uptake of DHA, via modulation of
GLUT1. Future investigations are required to test this hypothesis and, whilst stomatin gene
expression does not appear to be compromised in obesity, whether stomatin protein and/or
function is compromised remains to be investigated.