Effects of irradiance on UVA-induced skin aging

•Equivalent fluence of UVA radiation delivered at LI stimulates higher ROS and MMP-1 in dermal fibroblasts as compared to its HI counterpart.•Equivalent fluence of UVA radiation delivered at LI suppresses type I collagen in dermal fibroblasts as compared to its HI counterpart.•Pretreatment with NAC abrogates the difference in MMP-1 and collagen expressions by UVA exposure at equivalent fluence but different irradiance.•Since sunscreen use is associated with prolonging sun exposure, a sense of “false security” should be made aware to the general public. Ultraviolet A (UVA) radiation is the most relevant component of solar radiation-induced skin aging. Sunscreens were used to minimize the harmful effects of UV radiation on our skin by reducing UV irradiance. We previously found that at equivalent fluence, UVB radiation at low irradiance (LI) has higher photocarcinogenic potential as compared to its high irradiance (HI) counterpart. To examine the effects of equivalent fluence of UVA radiation administered at different irradiance on photoaging. Both the hairless mice (SKH-1) and human dermal fibroblasts were irradiated with high irradiance UVA (HIUVA) or low irradiance UVA (LIUVA; 50% irradiance of HIUVA) at equivalent fluence. Parameters related to skin photoaging were evaluated. For hairless mice receiving equivalent fluence of UVA radiation, LIUVA treated mice showed prominent skin aging as compared to its HIUVA treated counterpart. In addition, LIUVA radiation induced higher reactive oxygen species (ROS) production and c-Jun N-terminal kinases (JNK) phosphorylation as compared to their HIUVA treated counterparts. Pretreatment with N-acetylcysteine (NAC) abrogate the difference between HI and LIUVA radiation on fibroblasts in terms of intracellular ROS, JNK phosphorylation, MMP-1 expression and type I collagen expression. UVA radiation administered at LI (a scenario similar to sunscreen use) led to more severe aging process as compared to its HI counterpart. Unexpected negative effect may be imposed on the skin if sunscreen use is accompanied by longer duration spent under the sun.