To the best of our knowledge, this is the first human study to explore the relationship between obesity and the Hhip. We found that Hhip levels progressively decreased from the normal-weight and overweight groups to the obese group. In addition, the BMI was negatively associated with serum Hhip concentrations. Moreover, being overweight and obese were negatively associated with serum Hhip concentrations.
According to our previous study, the presence of prediabetes and type 2 diabetes was positively associated with serum Hhip concentrations, while the BMI was not (16). However, the average BMI in the previous study was similar among subjects with normal glucose tolerance (BMI = 22.3 kg/m2), impaired fasting glucose (BMI = 23.6 kg/m2), impaired glucose tolerance (BMI = 23.4 kg/m2), and newly diagnosed diabetes (BMI = 23.3 kg/m2), although the difference reached borderline statistical significance (p = 0.049), which may be at risk of a type 1 error, and subjects with obesity might not have been included. It is therefore unknown whether or not being overweight/obese is associated with plasma Hhip concentrations. Wei et al. reported that recombinant Hhip can increase adipocyte differentiation, which results in increased accumulation of lipid droplets in adipocytes by inhibiting the Hh signaling pathway in 3T3-L1 cells, and Hhip messenger RNA expression in adipose tissues was lower in 180-day-old than in 3-day-old pigs (13). It was suggested that serum Hhip concentrations may be negatively regulated by differentiated adipose tissues. Once one becomes obese, the production of Hhip should decrease to prevent further adipocyte differentiation. However, the mechanism as to how adipose tissues influence serum Hhip concentrations remains unclear. To address this hypothesis, further human studies are required.
Hh signaling plays an important role in inhibiting fat formation (11). A previous animal study showed the activation of Hh signaling decreased obesity induced by a high-fat diet in adult mice (10), and a deficiency of Hh signaling in myeloid cells increased the BW of mice (19). In a human study, expression of the Hh signaling transcription factor, Gli1, significantly decreased in adipose tissues of insulin-sensitive obese subjects compared to lean subjects, which may indicate that Hh signaling decreases in obese humans (20). Circulating Hh ligands and expressions of Hh ligands in adipose tissues increased in obese mice. However, serum Hh ligand levels significantly decreased in morbidly obese (BMI > 40 kg/m2) people, even in those with HbA1c > 7%, possibly due to the inhibitory effect of metformin on Hh ligand expression in adipose tissues (19). As the Hhip is a negative regulator that attenuates Hh signaling by binding to Hh ligands, further study is needed to clarify the regulatory architecture.
Cholesterol has been shown to be an endogenous Smoothened activator that being a second messenger that activating Hedgehog signaling pathway (21). Exogenously added cholesterol would activate Hh signaling pathway in vitro (22). Cholesterol is not just necessary but also sufficient to activate signaling by the Hh pathway (23). Hh signaling plays an important role in inhibiting fat formation (11) which means that elevated cholesterol level might activate Hh signaling to minimize fat formation which in the same time that Hhip should be downregulated to avoid fat formation. However, there has been no human study to discuss the relationship between cholesterol and Hhip so far. In our study, we found no difference in cholesterol level among three groups. Also, in multivariate linear regression analysis, cholesterol was not an independent factor of serum Hhip concentrations. The relationship among cholesterol level, Hh signaling pathway, and serum Hhip concentrations needs to be evaluated in human study.
There were some limitations in this study. First, this study was designed as a cross-sectional study which did not allow for causal inferences between serum Hhip concentrations and the BMI or obesity. Second, although one study revealed that the Hhip was associated with moderate to severe chronic obstructive pulmonary disease, all of our participants were apparently healthy with no airway symptoms (24). Third, we could not directly measure Hhip expression by adipose tissues. Therefore, we could not be sure whether serum Hhip concentrations were representative of those in adipose tissues. Finally, all study subjects were Taiwanese, and thus our findings might not be applicable to other ethnicities.
In summary, our results demonstrated that serum Hhip concentrations were negatively associated with the BMI, and obese subjects had lower serum Hhip concentrations than normal–weight subjects. Further research is needed to explore the pathophysiological roles and clinical implications of the Hhip in obesity.