Retinoids and retinoid-binding protein expression in rat adipocytes

Area of Science:

• Biochemistry
• Cell Biology
• Endocrinology

Background:

• Adipose tissue is known to have high levels of retinol-binding protein (RBP) mRNA.
• Previous studies indicated a potential role for adipose tissue in retinoid storage and metabolism.

Purpose of the Study:

• To investigate retinoid and RBP storage and metabolism within different adipose tissue depots and cell types in male rats.
• To quantify RBP and cellular retinol-binding protein (CRBP) mRNA levels and retinoid concentrations across six distinct adipose depots.
• To determine the cellular localization of RBP, CRBP, and other key gene expressions within adipocytes and stromal-vascular cells.

Main Methods:

• Northern blot analysis was used to measure RBP and CRBP mRNA levels in six rat adipose depots (inguinal, dorsal, mesenteric, epididymal, perinephric, brown adipose tissue) and in isolated adipocytes and stromal-vascular cells.
• Retinoid levels were measured in the adipose depots.
• Cultured adipocytes were used to assess RBP protein synthesis and secretion, and cellular retinol content.

Main Results:

• RBP mRNA levels varied significantly across adipose depots, with the highest concentrations found in epididymal and perinephric fat.
• Retinoid levels were consistent across the depots, averaging 6-7 micrograms of retinol equivalents per gram of adipose tissue.
• RBP expression was predominantly localized to adipocytes, while CRBP and apolipoprotein E (apoE) mRNA were higher in stromal-vascular cells. Adipocytes synthesized and secreted RBP protein and contained retinol.

Conclusions:

• Rat adipocytes are capable of storing retinoids and synthesizing and secreting retinol-binding protein (RBP).
• These findings highlight a dynamic role for adipocytes in retinoid storage and metabolism.
• The differential expression of RBP and CRBP in adipocytes versus stromal-vascular cells suggests specialized functions within adipose tissue.