Evidence for the ectopic synthesis of melanin in human adipose tissue

Abstract

Melanin is a common pigment in animals. In humans, melanin is produced in melanocytes, in retinal pigment epithelium (RPE) cells, in the inner ear, and in the central nervous system. Previously, we noted that human adipose tissue expresses several melanogenesis-related genes. In the current study, we confirmed the expression of melanogenesis-related mRNAs and proteins in human adipose tissue using real-time polymerase chain reaction and immunohistochemical staining. TYR mRNA signals were also detected by in situ hybridization in visceral adipocytes. The presence of melanin in human adipose tissue was revealed both by Fontana-Masson staining and by permanganate degradation of melanin coupled with liquid chromatography/ultraviolet/mass spectrometry determination of the pyrrole-2,3,5-tricarboxylic acid (PTCA) derivative of melanin. We also compared melanogenic activities in adipose tissues and in other human tissues using the L-[U-(14)C] tyrosine assay. A marked heterogeneity in the melanogenic activities of individual adipose tissue extracts was noted. We hypothesize that the ectopic synthesis of melanin in obese adipose may serve as a compensatory mechanism that uses its anti-inflammatory and its oxidative damage-absorbing properties. In conclusion, our study demonstrates for the first time that the melanin biosynthesis pathway is functional in adipose tissue.

Pigment-producing granulomatous myopathy in Atlantic salmon: A novel inflammatory response

“…Even in mammals, there seems to be a relic of this system related to inflammatory changes in the skin [2], but mammalian leukocytes lack melanin-synthesizing abilities. It is possible that melanin production could serve as an ancient compensatory mechanism for the innate immune system in tissue with poor vascularization, as would be the case for fat tissue of morbidly obese persons as studied by Randhawa et al [45]. In humans and in some other mammals, melanin is also found in the brain.…”

Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes

“…Finally, in mammals, there is ectopic formation of melanin even in some other tissues different from skin, eye, inner ear, and brain, as in the adipose tissue [69]. Tyrosinase RNA has been detected in samples of the latter tissue, although enzyme activity is low.…”

Melanocyte stem cells as potential therapeutics in skin disorders

“…Pigment producing cells are also distributed in the eyes, ears, brain, heart, lung, and bone [1–3] . The functions of melanocytes in these other locations are not known in detail, although a role in scavenging reactive oxygen species has been reported [4,5] .…”