Telomerase activity is sufficient to bypass replicative senescence in human limbal and conjunctival but not corneal keratinocytes

The human ocular surface is covered by the conjunctival, corneal and limbal stratified epithelia. While conjunctival stem cells are distributed in bulbar and forniceal conjunctiva, corneal stem cells are segregated in the basal layer of the limbus, which is the transitional zone between the cornea and the bulbar conjunctiva. Keratinocyte stem and transient amplifying (TA) cells when isolated in culture give rise to holoclones and paraclones, respectively. Keratinocyte replicative senescence ensues when all holoclones have generated paraclones which express high levels of p16(INK4a). In the present study, we show that enforced telomerase activity induces the bypass of replicative senescence in limbal and conjunctival keratinocytes, without the inactivation of the p16(INK4a)/Rb pathway or the abrogation of p53 expression. hTERT-transduced limbal and conjunctival keratinocytes are capable to respond to both growth inhibitory and differentiation stimuli, since they undergo growth arrest in response to phorbol esters, and activate p53 upon DNA damage. Following a sustained PKC stimulation, occasional clones of p16(INK4a)-negative cells emerge and resume ability to proliferate. Telomerase activity, however, is unable to induce the bypass of senescence in corneal TA keratinocytes cultured under the same conditions. These data support the notion that telomere-dependent replicative senescence is a general property of all human somatic cells, including keratinocytes, and suggest that telomerase activity is sufficient to extend the lifespan only of keratinocytes endowed with high proliferative potentials (which include stem cells), but not of TA keratinocytes.