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SIRT1 promotes differentiation of normal human keratinocytes.

SIRT1 promotes differentiation of normal human keratinocytes

Blander G, Bhimavaparu A, Mammone T et al.

J Invest Dermatol, 2009, 129, 41-49

Sirtuins are associated with increasing lifespan (for a review see [1]). This is an interesting paper describing how sirtuins influence human epidermal keratinocyte differentiation and proliferation. Gene microarrays were used, which are known to provide cumbersome datasets requiring much dedicated data sorting. The authors addressed this problem by using computational systems biology to provide a causal network model generating hypotheses that were subsequently followed up by in vitro testing.

Sir2 is a gene family regulating ageing and lifespan in yeast, flies and worms. The human Sir2 gene family encodes for seven protein products, one of which, SIRT1, is a nicotinamide adenine dinucleoti-dedependent deacetylase controlling gene expression, cellular metabolism and cellular stress responses. SIRT1 has been shown to regulate cell differentiation in both muscle cells and adipocytes [2,3]. Overexpression of SIRT1 negatively influences differentiation, whereas downregulation, for example by SIRT RNA interference (RNAi), enhances differentiation. Resveratrol, which is a small molecule activator of SIRT1, increases lifespan in worms, yeast and flies, and in isolated human cells, resveratrol increases cell survival after DNA damage [4]. This study investigates the role of human SIRT1 in primary human keratinocytes either overexpressing or underexpressing SIRT1. GeneChip arrays (HG-U133a, Affymetrix, Santa Clara, CA) were used, measuring 22,278 probe sets on each microarray. A total of 109 gene expression changes were recorded in the SIRT1 overexpressing cells and 228 gene changes were measured in SIRT1 knockdown cells. These observed transcriptional changes formed the basis for the systems biology causal network modelling. Testable hypotheses were generated and were followed up in cultured human epidermal keratinocytes to elucidate the role of SIRT1 in primary keratinocytes and confirm the findings of the gene chip analysis and computational modelling.

The data generated from causal network modelling of the gene expression microarrays inferred that SIRT1 expression influenced keratinocyte proliferation rate. This hypothesis was validated by in vitro testing in the cultured keratinocytes: SIRT1 overexpression inhibited proliferation rate, whereas reducing SIRT1 increased keratinocyte proliferation.

Using the same procedure of causal network analysis of the microarray dataset, a hypothesis was generated that SIRT1 overexpression promoted cell differentiation and SIRT1 knockdown inhibited cell differentiation. Additionally, the causal network modelling identified calcium as a hypothetical candidate for mediating these effects on differentiation. Again these hypotheses were tested in in vitro culture of human keratinocytes either overexpressing or underexpressing SIRT1. This testing validated the proposed hypotheses, suggesting that in human keratinocytes SIRT1 is an important mediator of intracellular calcium-mediated signalling, although as the authors point out, both calcium and SIRT1 may also have other independent regulatory functions. Similar effects were observed experimentally using a SIRT1 antagonist (nicotinamide), which inhibited expression of keratinocyte differentiation markers; conversely, using resveratrol, a SIRT1 agonist, enhanced the expression of keratinocyte differentiation markers.

Another implication of these findings is that the effect of SIRT1 on keratinocyte differentiation is the opposite of the effects observed in muscle and fat cells, where SIRT1 overexpression actually inhibits differentiation. This may reflect fundamental differences in the biochemistry of these tissues, where sirtuins may regulate different signalling pathways.


[1.] Blander G, Guarente L. The Sir2 family of protein deacetylases. Annu Rev Biochem, 2004, 73, 417-435.

[2.] Fulco M, Schiltz RL, Lezzi S et al. Sir2 regulates skeletal muscle differentiation as a potential sensor of the redox state. Mol Cell, 2003, 12, 51-62.

[3.] Picard F, Kurtev M, Chung N et al. Sirt1 promotes fat mobilization in white adipocytes by repressing PPAR-gamma. Nature, 2004, 429, 771-776.
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Author:Knaggs, Helen
Publication:Clinical Dermatology
Date:Mar 1, 2009
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