The Ultraviolet Irradiation of Keratinocytes Induces Ectopic Expression of LINE-1 Retrotransposon Machinery and Leads to Cellular Senescence

Retrotransposons have played an important role in evolution through their transposable activity. The largest and the only currently active human group of mobile DNAs are the LINE-1 retrotransposons. The ectopic expression of LINE-1 has been correlated with genomic instability. Narrow-band ultraviolet B (NB-UVB) and broad-band ultraviolet B (BB-UVB) phototherapy is commonly used for the treatment of dermatological diseases. UVB exposure is carcinogenic and can lead, in keratinocytes, to genomic instability. We hypothesize that LINE-1 reactivation occurs at a high rate in response to UVB exposure on the skin, which significantly contributes to genomic instability and DNA damage leading to cellular senescence and photoaging. Immortalized N/TERT1 and HaCaT human keratinocyte cell lines were irradiated in vitro with either NB-UVB or BB-UVB. Using immunofluorescence and Western blotting, we confirmed UVB-induced protein expression of LINE-1. Using RT-qPCR, we measured the mRNA expression of LINE-1 and senescence markers that were upregulated after several NB-UVB exposures. Selected miRNAs that are known to bind LINE-1 mRNA were measured using RT-qPCR, and the expression of miR-16 was downregulated with UVB exposure. Our findings demonstrate that UVB irradiation induces LINE-1 reactivation and DNA damage in normal keratinocytes along with the associated upregulation of cellular senescence markers and change in miR-16 expression.

Phototherapy and DNA Damage: A Systematic Review

Phototherapy has gained popularity in the recent decades for the treatment of various immune-mediated dermatological conditions since it is more-cost effective and less toxic compared to systemic therapies. This systematic review aims to inform dermatology providers of the risks and benefits of phototherapy, especially in patients at risk for malignancies. Ionizing energy from phototherapy results in DNA photolesions, namely of cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs). Without adequate repair, these mutations increase the risk for carcinogenesis. Additionally, phototherapy can also indirectly cause DNA damage through the formation of reactive oxygen species (ROS), which damage of several structural and functional proteins and DNA. When choosing a phototherapy modality, it also important to take into consideration the side effect profiles associated with each modality. For instance, a 10-fold higher dose of NB-UVB is required to produce a similar amount of CPDs compared with BB-UVB. Patients who undergo UVA with psoralen (PUVA) can be susceptible to developing skin malignancies up to 25 years after receiving their last treatment. It would behoove providers to consider optimal radiation dosage given each patients' level of skin pigmentation and potential for photoadaptation. Additionally, there are measures have been proposed to minimize deleterious skin changes, such as a 42-degree Celsius heat treatment using a 308nm excimer laser prior to UVB phototherapy and low frequency, low intensity electromagnetic fields along with UVB. However, as performing routine skin exams, remain paramount in the prevention of phototherapy-induced neoplasia.