Despite the potential of tissue engineering approaches for cartilage
repair, a major shortcoming is the low biosynthetic response of
chondrocytes. While different strategies have been investigated to
upregulate tissue formation, a novel approach may be to control nutrient
metabolism. Although known for their anaerobic metabolism of glucose,
chondrocytes are more synthetically active when cultured under
conditions that elicit mixed aerobic-anaerobic metabolism. Here, we
postulate this metabolic switch induces hypoxia inducible factor 1α
(HIF-1α) signaling leading to improved tissue growth. Transition to
different metabolic states can result in the pooling of intracellular
metabolites, several of which can stabilize HIF-1α by interfering with
proline-hydroxylase-2 (PHD2). Chondrocytes cultured under increased
media availability accelerated tissue deposition (2.2 to 3.5-fold) with
the greatest effect occurring at intermediate volumes (2 mL/106 cells).
Under higher media volumes, metabolism switched from anaerobic to mixed
aerobic-anaerobic. At and beyond this transition, maximal changes in
PHD2 activity (- 45%), HIF-1α protein expression (8-fold increase), and
HIF-1 gene target expression were observed (2.0 to 2.7-fold increase).
Loss-of-function studies using YC-1 (to degrade HIF-1α) confirmed the
involvement of HIF-1 signaling under these conditions. Lastly, targeted
metabolomic studies of glucose metabolites (14 in total) revealed that
both intracellular lactate and succinate correlated with PHD2 activity.
Although both metabolites can inhibit PHD2, this effect can most likely
be attributed to lactate as succinate was only present in trace amounts.
However, addition work (e.g., 13C flux analyses) are required to confirm
this assertion. Nevertheless, by harnessing this newly identified
metabolic switch, functional engineered cartilage implants may be
developed without the need for sophisticated methods which would allow
for improved translation into the clinical realm.