Identification of insulin in the tear film and insulin receptor and IGF-1 receptor on the human ocular surface.
OBJECTIVE
Insulin produces pleiotropic effects on sensitive tissues, including the ocular surface, through the tyrosine kinase insulin receptor. Cerebrospinal fluid and secreted fluids, such as milk and saliva, have been reported to contain insulin. In the present study, the presence of insulin was examined in tear film, and the expression of insulin and insulin-like growth factor (IGF)-1 receptor was examined in the human cornea and conjunctiva.


METHODS
Stimulated tear samples collected from 33 volunteers (17 men, 16 women), aged 23 to 51 years, who were fed or fasted for 12 hours, were assayed for total protein and insulin content by the biuret dye test and a radioimmunoassay, respectively. Frozen sections of human cornea (n = 4) and conjunctiva (n = 3) were incubated with anti-insulin receptor and anti-IGF-1 receptor antibodies and developed with a secondary antibody-peroxidase conjugate.


RESULTS
Insulin was detected in all tear samples analyzed, the mean concentration being 0.404 +/- 0.129 ng/mL. There were no gender-related differences. In fed subjects, tears tended toward a higher insulin content than those in fasted individuals. There was no linear correlation between insulin and total protein content (mean, 4.61 +/- 0.79 mg/mL) in the tear film. Insulin and IGF-1 receptors were detected in the plasma membrane and cytoplasm of corneal and conjunctival epithelial cells.


CONCLUSIONS
To the best of the authors' knowledge, this study represents the first demonstration of insulin in human tear film and the presence of insulin and IGF-1 receptor on the human ocular surface. These results suggest that the pancreatic hormone may play a metabolic and/or mitogenic role on the ocular surface.
Rocha
Eduardo M
EM
Laboratory of Clinical Physiopathology, Department of Clinical Medicine, Faculty of Medical Sciences, Campinas State University (UNICAMP), São Paulo, Brazil. emrocha@unicamp.br
Cunha
Daniel A
DA
Carneiro
Everardo M
EM
Boschero
Antonio C
AC
Saad
Mário J A
MJ
Velloso
Lício A
LA
D000293:Adolescent; D000328:Adult; D000368:Aged; D003228:Conjunctiva / Q000378:metabolism*; D003315:Cornea / Q000378:metabolism*; D005260:Female; D006801:Humans; D007124:Immunoenzyme Techniques; D007328:Insulin / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D011863:Radioimmunoassay; D017526:Receptor, IGF Type 1 / Q000378:metabolism*; D011972:Receptor, Insulin / Q000378:metabolism*; D013666:Tears / Q000378:metabolism*
D007328:Insulin; D017526:Receptor, IGF Type 1; D011972:Receptor, Insulin