Extracellular matrix (ECM) components display dynamic expression during obesity. The ECM is remodeled during adipose tissue growth and the expression of the ECM is generally increased in obese individuals 
. In this study, we demonstrate that the expression of biglycan is generally increased in adipose tissue of HFD fed obese mice, indicating that biglycan is associated with increased adipose tissue expansion. In addition, we show that both adipocytes and SVC cells contribute to adipose tissue biglycan. An increase in biglycan expression due to HFD was observed in earlier results by Huber, who reported elevated biglycan transcript levels in adipose tissue of high saturated fat fed db/db
. We have previously reported increased biglycan mRNA in the adipose tissue of obese humans 
. A recently published report demonstrated high expression of biglycan in various adipose depots of Psammomys obesus
. Furthermore, increased expression of biglycan was associated with impaired glucose tolerance and obesity in P. obesus,
and we recently reported improved glucose tolerance in biglycan knockout mice on HFD relative to controls 
, implicating a possible role for biglycan in glucose metabolism.
Adiponectin is an anti-diabetic hormone secreted from adipose tissue 
. Unlike many adipokines, adiponectin expression decreases with increased BMI in humans 
. Our results show an increase in circulating adiponectin in the bgn−/0
mice. We show that while bgn+/0
mice have higher body weights, their adiposity and BMI is similar to bgn−/0
mice indicating that the observed difference in adiponectin found in the bgn−/0
mice is not due to differences in adiposity. Little is known about the interaction between biglycan and adiponectin, although it is reported that biglycan can bind directly to adiponectin 
Our results show lower fasting insulin in the circulation of bgn−/0 mice and a trend towards increased insulin sensitivity as measured through HOMA-IR. As we have used whole body knockout mice, it is impossible to state at this time that the increase in adiponectin found in the bgn−/0 mice is responsible for the observed decrease in fasting insulin. However, the initial finding of a possible improvement in insulin sensitivity in bgn−/0 mice opens additional avenues for future research.
While the data from the bgn−/0 mice suggest that the absence of biglycan promotes adiponectin expression in a whole body knockout system, in vitro results indicate that biglycan absence is associated with a decline in adiponectin expression. Transient knockdown of biglycan led to decreased adiponectin production in vitro. Furthermore, medium from RAW 264.7 cells treated with biglycan increased adiponectin expression in 3T3-L1 adipocytes. Biglycan is a pro-inflammatory signal and its presence may induce increases in cytokine expression in RAW264.7 cells. We did not detect a difference in expression of TNFα, IL-6, or IL-1β due to biglycan in RAW264.7 cells that could explain the increase in adiponectin expression found in the 3T3-L1 adipocytes. However, a thorough analysis of secreted factors by the macrophage in response to biglycan is needed to understand whether any pro-inflammatory signals are produced as a mediating factor.
One possible explanation for the difference between the in vivo
and in vitro
system may be that the effect of biglycan absence stems from interactions with the extracellular matrix which are absent in a 2-D cell culture. Specifically, biglycan absence may disrupt collagen formation in adipose tissue. Collagen VI null mice have an improved metabolic phenotype which may be attributable to decreased rigidity and fibrosis in adipose tissue 
. A mechanism may exist whereby biglycan absence inhibits proper collagen VI formation as biglycan is implicated in the organization of collagen VI into hexagonal networks in vitro
Although it is unclear at this point the reason for the discordance between the in vivo
and in vitro
systems in terms of adiponectin production, it is clear that cellular and tissue level mechanisms are in place to sense the presence of biglycan and regulate adiponectin expression accordingly. Since there are multiple cell types in place in the in vivo
setting, the lack of biglycan may create a microenvironment that supports increased adiponectin expression or relieve the inhibitory function of negative mechanisms that suppress adiponectin expression. Several studies have linked increased oxidative stress and inflammation to reduced adiponectin expression 
. Since activation of PPARγ is associated with increased adiponectin expression 
, inhibition of PPARγ by nuclear factor kappa B (NFκB) during obesity 
could be a potential link between obesity and reduced adiponectin expression. We have provided evidence that biglycan knockout mice have reduced adipose tissue inflammation indicated by lower expression of inflammatory markers such as IL-6, TNFα and CD68 
. Thus the lack of biglycan in the knockout mice will prevent the inhibitory effect of inflammation on adiponectin expression, hence the higher adiponectin expression in the bgn−/0
mice. On the other hand, suppression of biglycan in vitro
in 3T3-L1 cells may send a yet unknown signal into the cell that suppresses adiponectin expression. Since biglycan and adiponectin interact leading to sequestration of adiponectin 
, the lack of biglycan might indicate that less adiponectin is needed for the same level of available adiponectin for bioactivity. Additionally, culture of cells in vitro
on plastic does not perfectly replicate the in vivo
conditions of adipocytes in adipose tissue due to the absence of other cellular and non-cellular tissue components. Furthermore, knock down of biglycan in 3T3-L1 adipocytes did not lead to an alteration in the inflammatory state of the cells, marking another major difference between biglycan absence in the in vivo
and in vitro
models. Instead, the expected disruption of collagen matrix formation in the 2-D culture condition in the absence of biglycan on plastic surface 
could affect the integrity of the extracellular matrix, and perhaps extracellular matrix characteristics that may be necessary for adiponectin expression. The induction of adiponectin in adipocytes treated with MCM from biglycan and biglycan and LPS treated macrophages may suggest that these treatments lead to production of yet unidentified factors that induce adipocytes to increase adiponectin expression. What remains constant through both the in vivo
and in vitro
results is that the absence of biglycan can impact adiponectin expression, implicating a mechanism where adipocytes can sense biglycan abundance to regulate adiponectin production.
In summary, our findings show an increase in biglycan expression in adipose tissue during obesity. We also observed a modest increase in adiponectin in bgn−/0 mice; however, transient knockdown of biglycan in 3T3-L1 cells resulted in decreased adiponectin expression. These studies indicate a complex mechanism by which adipocytes are able to sense biglycan presence in both in vivo an in vitro settings to regulate adiponectin expression. Further work will be needed to clarify the true nature of this relationship.