FDG/PET Imaging for the Assessment of Brown Adipose Tissue in HIV Lipodystrophy

Among individuals infected with HIV, highly active antiretroviral therapy has reduced the incidence of morbidity and mortality however, despite recent improvements in newer antiretrovirals patients continue to exhibit secondary effects related to body composition such as lipoatrophy of the periphery, increased adiposity of the trunk and lipomatosis, especially of the dorsocervical spine. Changes in body composition have been reported in 40-50% of HIV-infected patients. Several studies have shown that antiretroviral therapy contributes to changes in body composition and is coupled with increased dyslipidemia, insulin resistance and diabetes.

Accumulation of fat over the dorsocervical spine, or "buffalo" has been reported in 2% to 13% of HIV-infected patients. Enlargement of adipose tissue in the dorsocervical region involves subcutaneous fat and is therefore unique to fat accumulation of the abdominal area. Guallar et al. examined dorsocervical adipose tissue after surgical removal and found that adipose tissue in this area showed substantial levels of the marker gene of brown fat, uncoupling protein 1 (UCP-1) suggesting there may be brown adipose tissue (BAT) in HIV infected individuals with lipomatosis of the dorsocervical spine. Until recently, BAT was known to be present in rodents throughout their lifetime and was thought to be present in humans only during infancy and early childhood. However, recent studies using 18F-FDG PET-CT have confirmed the presence of BAT in adults. Brown adipose tissue is known to affect whole-body metabolism and may be related to insulin sensitivity as well as susceptibility to weight gain.

Using 18F-FDG PET techniques, our group has evaluated HIV-infected subjects with lipoatrophy and noted there was significantly increased glucose uptake into subcutaneous tissue which may suggest presence of BAT in HIV-infected patients. However our previous study did not specifically examine areas of BAT in the subjects. Therefore, using 18F-FDG PET-CT in addition to fat biopsies we propose to explore the presence of BAT in fat depots among HIV-infected patients with fat redistribution, focusing specifically in the cervical area.

Also, as recent studies suggest down regulation of Dicer, a major component of miRNA has an important role in the differentiation and function of brown and white adipose tissue and may contribute to lipodystrophy. We will perform subcutaneous fat biopsies of the dorsocervical and abdominal fat in a subset of HIV-infected and non-HIV-infected men in order to explore further the question of down regulation of Dicer and its implication on metabolic abnormalities in this population.