Tuberous sclerosis complex inactivation disrupts melanogenesis via mTORC1 activation
Juxiang Cao, … , Hans R. Widlund, David J. Kwiatkowski
Juxiang Cao, … , Hans R. Widlund, David J. Kwiatkowski
Published December 5, 2016
Citation Information: J Clin Invest. 2017;127(1):349-364. https://doi.org/10.1172/JCI84262.
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Research Article Dermatology Genetics

Tuberous sclerosis complex inactivation disrupts melanogenesis via mTORC1 activation

Abstract

Tuberous sclerosis complex (TSC) is an autosomal dominant tumor-suppressor gene syndrome caused by inactivating mutations in either TSC1 or TSC2, and the TSC protein complex is an essential regulator of mTOR complex 1 (mTORC1). Patients with TSC develop hypomelanotic macules (white spots), but the molecular mechanisms underlying their formation are not fully characterized. Using human primary melanocytes and a highly pigmented melanoma cell line, we demonstrate that reduced expression of either TSC1 or TSC2 causes reduced pigmentation through mTORC1 activation, which results in hyperactivation of glycogen synthase kinase 3β (GSK3β), followed by phosphorylation of and loss of β-catenin from the nucleus, thereby reducing expression of microphthalmia-associated transcription factor (MITF), and subsequent reductions in tyrosinase and other genes required for melanogenesis. Genetic suppression or pharmacological inhibition of this signaling cascade at multiple levels restored pigmentation. Importantly, primary melanocytes isolated from hypomelanotic macules from 6 patients with TSC all exhibited reduced TSC2 protein expression, and 1 culture showed biallelic mutation in TSC2, one of which was germline and the second acquired in the melanocytes of the hypomelanotic macule. These findings indicate that the TSC/mTORC1/AKT/GSK3β/β-catenin/MITF axis plays a central role in regulating melanogenesis. Interventions that enhance or diminish mTORC1 activity or other nodes in this pathway in melanocytes could potentially modulate pigment production.

Authors

Juxiang Cao, Magdalena E. Tyburczy, Joel Moss, Thomas N. Darling, Hans R. Widlund, David J. Kwiatkowski

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Figure 8

Melanocytes isolated from TSC patients’ hypomelanotic macules accumulate early-stage melanosomes.

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Melanocytes isolated from TSC patients’ hypomelanotic macules accumulate...
(A) Late-stage melanosomes were prominent in melanocytes from normal skin and much less abundant in melanocytes from hypomelanotic macules (original magnification, ×3,000). The bottom panel demonstrates the classic stages (I–IV) of melanosome development (original magnification, ×12,000). Note that the P38 hypomelanotic macule was compared with normal skin from P16 and P30. (B) Quantification of relative amounts of stage I–IV melanosomes in control skin (N) and hypomelanotic macule melanocytes (HMs). Data represent the mean ± SD (n = 10). *P < 0.05, by Student’s t test versus normal skin control. (C) qPCR analysis of mRNA levels of TSC1, TSC2, and genes involved in pigment formation including MITF, TYR, DCT, SOX10, TRP1, and SILV in melanocytes isolated from patients with TSC, as indicated in both normal skin (N skin) and hypomelanotic macules. Note the marked difference in expression of TSC2 and pigment genes. GAPDH served as a control. *P < 0.05, by Student’s t test versus normal skin control. P16 and P38 are TSC subjects with single HM biopsies and cultures. One patient had two biopsies, LA: left leg, RA: right leg.