Tissue context-activated telomerase in human epidermis correlates with little age-dependent telomere loss

Abstract

Telomerase- and telomere length regulation in normal human tissues is still poorly understood. We show here that telomerase is expressed in the epidermis in situ independent of age but was repressed upon the passaging of keratinocytes in monolayer culture. However, when keratinocytes were grown in organotypic cultures (OTCs), telomerase was re-established, indicating that telomerase activity is not merely proliferation-associated but is regulated in a tissue context-dependent manner in human keratinocytes. While not inducible by growth factors, treatment with the histone deacetylation inhibitor FK228 restored telomerase activity in keratinocytes grown in monolayer cultures. Accordingly, CHIP analyses demonstrated an acetylated, active hTERT promoter in the epidermis in situ and in the epidermis of OTCs but a deacetylated, silenced hTERT promoter with subsequent propagation in monolayer culture suggesting that histone acetylation is part of the regulatory program to guarantee hTERT expression/telomerase activity in the epidermis. In agreement with the loss of telomerase activity, telomeres shortened during continuous propagation in monolayer culture by an average of approximately 70 base pairs (bp) per population doubling (pd). However, telomere erosion varied strongly between different keratinocyte strains and even between individual cells within the same culture, thereby arguing against a defined rate of telomere loss per replication cycle. In the epidermis in situ, as determined from early-passage keratinocytes and tissue sections from different age donors, we calculated a telomere loss of only approximately 25 bp per year. Since we determined the same rate for the non-regenerating melanocytes and dermal fibroblasts, our data suggest that in human epidermis telomerase is a protective mechanism against excessive telomere loss during the life-long regeneration.

The impact of age on the physical and cellular properties of the human limbal stem cell niche

“…In other systems, ageing has been linked with reduced telomerase activity and consequent shortening of telomere length in some stem cell populations, for example, in hematopoietic stem cells (Rossi et al 2007;Cawthon et al 2003). However, findings in stem cells from muscle (O'Connor et al 2009) and skin Krunic et al 2009), a tissue with many functional and phenotypic similarities to the cornea, agree with the results of our study by showing that age had no significant effect in telomerase and telomere lengths.…”

Are the Effects of the Cholera Toxin and Isoproterenol on Human Keratinocytes’ Proliferative Potential Dependent on Whether They Are Co-Cultured with Human or Murine Fibroblast Feeder Layers?

“…For this study, we restricted our experiments to the first 3 passages after primoculture (P1, P2, and P3) because we were mainly interested in the translational value of our findings on our culture protocols. It is known that as passages advance, keratinocytes can suffer chromosomal rearrangements [24] and a reduction of telomerase activity which leads to telomere shortening [25]. Therefore, it is generally preferred to use P1 to P3 keratinocytes when producing bio-engineered skin equivalents for clinical purposes [5,6,26].…”

Damage at the root of cell renewal—UV sensitivity of human epidermal stem cells

“…The high proliferative capacity of somatic stem cells in actively regenerating tissues is assured by the activity of telomerase that prevents inappropriate telomer shortening and subsequent premature initiation of senescence [50]. Although the expression of telomerase in epidermal stem cells could only been shown in mice so far [51], is more than presumable that the telomerase expression that could be detected in human epidermis [52,53] also derives from the epidermal stem cell compartment. This assumption is further supported by reports that describe telomerase enzyme activity in rapidly adhering human keratinocytes [54].…”