DNA Damage Response After Ionizing Radiation Exposure in Skin Keratinocytes Derived from Human-Induced Pluripotent Stem Cells

Purpose

Epidermal cells are positioned on the body surface and thus risk being exposed to genotoxic stress, including ionizing radiation (IR), ultraviolet rays, and chemical compounds. The biological effect of IR on the skin tissue is a significant problem for medical applications such as radiation therapy. Keratinocyte stem cells and progenitors are at risk for IR-dependent tumorigenesis during radiation therapy for cancer treatment. To elucidate the molecular mechanism of genome stability in epidermal cells, we derived skin keratinocytes from human-induced pluripotent stem cells (iPSCs) and analyzed their DNA damage response (DDR).

Methods and Materials

Skin keratinocytes were derived from iPSCs and designated as first- (P1), second- (P2), and third- (P3) passage cells to compare the differentiation states of DDR. After 2 Gy gamma-ray exposure, cells were immunostained with DNA double-strand break markers γ-H2AX/53BP1 and cell senescence markers p16/p21 for DDR analysis. DDR protein expression level, cell survival, and apoptosis were analyzed by western blotting, WST-8 assay and TUNEL assay, respectively. DDR of constructed 3D organoid modeling was also analyzed.

Results

P1, P2, and P3 keratinocytes were characterized with keratinocyte markers keratin 14 and p63 using immunofluorescence, and all cells were positive to both markers. Derived keratinocytes showed high expression of integrin α6 and CD71 (real-time (qRT)-PCR ratio: iPSCs: integrin α6: 1.12, CD71: 1.25, P1: integrin α6: 7.80, CD71: 0.43, P2: integrin α6: 5.53, CD71: 0.48), suggesting that P1 and P2 keratinocytes have potential as keratinocyte progenitors. Meanwhile, P3 keratinocytes showed low expression of integrin α6 and CD71 (qRT-PCR ratio: P3: integrin α6: 0.55, CD71: 0.10), suggesting differentiated keratinocytes. After IR exposure, the P1 and P2 keratinocytes showed an increase in DNA repair activity by a γ-H2AX/53BP1 focus assay (P1: γ-H2AX: 28.0%, 53BP1: 17.0%, P2: γ-H2AX: 37.7%, 53BP1: 28.3%) but not in P3 keratinocytes (P3: γ-H2AX: 74.7%, 53BP1: 63.7%) compared with iPSCs (γ-H2AX: 57.0%, 53BP1: 55.0%). Furthermore, in derived keratinocytes, expression of the cellular senescence markers p16 and p21 were increased compared with iPSCs (P16: non irradiated, iPSCs: 0%, P1: 12.5%, P2: 14.5%, P3: 29.7%, IR, iPSCs: 0%, P1: 19.5%, P2: 34.8%, P3: 64.5%). DDR protein expression, cellular sensitivity, and apoptosis activity decreased in derived keratinocytes compared with iPSCs.

Conclusions

We have demonstrated the derivation of keratinocytes from iPSCs and their characterization of differentiated states and DDR. Derived keratinocytes showed progenitors like character as a result of DDR. These results suggest that derived keratinocytes are useful tools for analyzing the effects of IR, such as DDR on the skin tissue from radiation therapy for cancer.