Nuclear retinoic acid receptors in the lacrimal gland

Abstract

The lacrimal gland secretes and metabolizes retinoids and responds to retinoic acid in culture. Like other retinoid responsive organs it is expected to express the nuclear retinoid receptors. The goal of this study was to identify the retinoic acid receptors (RAR) in the lacrimal glands of rats, rabbits, and humans. Total RNA was prepared from whole lacrimal glands and rat lacrimal gland acinar cells grown in culture. RNA was subjected to Northern blot analysis and probed for the RARα, RARβ, and RARγ mRNAs. Nuclear extracts of rat and rabbit lacrimal glands were incubated with ³H-all-trans retinoic acid and analyzed by gel filtration chromatography. Western blots of the nuclear extracts were probed using monoclonal antibodies to RARα and RARβ. Rat lacrimal gland expresses RARα mRNA with two transcripts (3.8 and 3.0 kb), a single RARβ mRNA transcript (3.3 kb), and a single RARγ mRNA transcript (3.3 kb). Cultured rat lacrimal acinar cells also expressed the mRNA for all three RAR subtypes. Rabbit lacrimal glands express mRNAs for RARα (3.7 and 2.9 kb) and RARβ (3.2 kb) but RARγ mRNA is not detectable. Human lacrimal glands also express mRNA for RARα (3.5 and 2.3 kb), RARβ (3.4 kb) and RARγ (3.0 kb). Lacrimal gland nuclear extracts contain proteins in the 50 kDa range that specifically bind retinoic acid with K_d = 1.25 nM in rat lacrimal gland and 0.3 nM in rabbit. The monoclonal antibodies identified RARα and RARβ in both rat and rabbit lacrimal glands. The results of this study support a role for retinoids in maintaining the structure and function of the lacrimal gland. The presence of RARs suggests potential interactions of these receptors with other members of their superfamily, including androgen and thyroid receptors, which also may be involved in lacrimal function.

Key Words:

• lacrimal gland
• retinoic acid receptors
• vitamin A
• rat
• rabbit
• human