well - established epidemiological evidence has shown that uv irradiation is a major environmental skin carcinogen , but the underlying mechanisms are still not fully elucidated .
short - wave uvc ( 200280 nm ) irradiation is filtered by atmospheric ozone and is not involved in skin damage .
the mid - wave uvb ( 280320 nm ) is partially filtered by the ozone layer , but approximately 5% reaches the earth .
uvb is biologically active and penetrates the superficial layers of skin , down to the basal layer of the epidermis , where it generates harmful reactive oxygen species ( ros ) .
chromophores , such as melanin , and the bases of the dna molecule are able to absorb the light .
such damage to the dna molecule can contribute to mutagenic lesions , and the ros produced in the course of energy release after light absorption is responsible for inflammation of the skin and sunburn .
less energetic than uvb , but present in larger amounts , uva radiation ( 320400 nm ) penetrates deeper into the skin , reaching the dermis .
several chromophores absorb uva energy , including melanin and the riboflavin - containing fad and fmn .
compared with uvb , uva causes less obvious damage and was therefore , until recently , considered rather harmless .
however , uva is more penetrating because it reaches the stem cell compartment of the skin ; and , because the generated ros alter proteins , lipids and dna , uva increases the risk of cancer development . indeed
, uva- and uvb - induced adduct formation and signature mutations have been detected in human skin squamous cell carcinomas and solar keratosis . however
, uva fingerprint mutations were mostly localized in the basal germinative layer , while uvb fingerprint mutations had a predominantly suprabasal localization , indicating that longer , more penetrating uv wavelengths are important risk factors in cancer development . in addition , uva significantly contributes to premature skin aging and wrinkle formation .
uvb and uva alter the immune response , either alone or in combination ; however , given that uva is 20-times more abundant in sunlight , the latter is generally considered the main culprit in solar - induced immunosuppression .
taken together recent data points out uva as a critical component in skin cancer development .
more detailed studies of the mechanisms behind uva - induced cell damage are , however , needed to elucidate its mechanism of action .
lysosomes are acidic cytoplasmic organelles that are responsible for the degradation and recycling of old and damaged macromolecules and organelles .
. however , research conducted over the last several decades have proven lysosomes to be advanced organelles that are crucial regulators of cell homeostasis and are involved in many cellular processes , such as plasma membrane repair , cholesterol homeostasis and cell death .
the roles of exocytosis of secretory lysosomes in the immune response , bone resorption , cell signaling and plasma membrane repair have been studied .
after damage , restoration of plasma membrane integrity is essential for the survival of the cell , and conventional lysosomes have been shown to be exocytosed during repair of the plasma membrane .
plasma membrane injury is considered a frequent event in mammalian cells , especially in those that operate under conditions of mechanical stress , such as muscle and skin cells . moreover , pore - forming toxins such as streptolysin o can induce lysosomal exocytosis within minutes .
lysosomal translocation is controlled by microtubule - mediated long - range transport and results in the formation of a lysosomal patch that eventually fuses with the plasma membrane and restores plasma membrane integrity .
damage to the plasma membrane results in calcium influx , which causes a conformational change in synaptotagmin vii ( syt vii ) at the lysosomal membrane and thus facilitates the formation of a four - helix bundle between t - and v - snares ( soluble n - ethylmaleimide - sensitive factor attachment protein receptors ) .
exocytosis can be monitored by the appearance of lysosome - associated membrane proteins ( lamp-1 and lamp-2 ) on the plasma membrane of non - permeabilized cells and by the extracellular release of lysosomal enzymes , such as -hexosaminidase , acid sphingomyelinase and n - acetyl--d - glucosaminidase .
we recently conducted a study on the damage done by uva and uvb radiation to the human keratinocyte plasma membrane , and we found that lysosomal exocytosis is involved in the repair of uva irradiation - induced plasma membrane damage .
first , we compared the plasma membrane damage induced by uva and uvb irradiation and found that uva , but not uvb , induced plasma membrane damage followed by ca - dependent lysosomal exocytosis and caspase-8-mediated apoptosis .
lysosomal exocytosis resulted in the extracellular release of the lysosomal enzymes cathepsin d and acid sphingomyelinase and the appearance of lamp-1 on the plasma membrane of non - permeabilized cells , as detected by antibodies directed to the luminal portion of lamp-1 .
we also prevented exocytosis by adding vacuolin-1 and anti - synaptotagmin vii antibodies , as shown by the significant reduction in lamp-1 translocation to the plasma membrane after uva exposure .
when lysosomal ph was increased by the addition of nh4cl , exocytosis was reduced , indicating that lysosomal acidification facilitated exocytosis ( fig .  1 ) .
following exocytosis we identified endocytosis and in the formed vacuolar structures active caspase-8 was detected and did colocalize with cathepsin d. keratinocyte cultures in monolayers lack keratinized cells from the stratum corneum in the skin ; however , it could be argued that this model is not completely relevant from a physiological perspective . to test whether the protective layer of the stratum corneum could protect keratinocytes from uva - induced plasma membrane damage
, we extended our model to reconstructed skin produced using cells derived from human foreskin .
fibroblasts were incorporated into a collagen matrix , on top of which keratinocytes were seeded and allowed to proliferate and differentiate , as described previously . after exposing reconstructed skin to uva followed by histological preparation and staining of lysosomes with lamp-2 , we found that relocation of the lysosomes from the center of the cells to the vicinity of the plasma membrane did take place in the skin model ( fig . 
in accordance with the findings from keratinocytes in mono - layer cultures , uvb - radiation did not cause relocation of lysosomes to the plasma membrane ( fig . 
thus , our findings suggest that lysosomal exocytosis might occur in keratinocytes covered by the stratum corneum following exposure of the skin to uva .
model of lysosomal exocytosis for the repair of uva - induced plasma membrane damage in human keratinocytes .
uva irradiation induces free radical - mediated plasma membrane damage , which triggers lysosomes to fuse with the plasma membrane in a synaptotagmin vii and calcium - dependent manner .
lysosomal exocytosis is detected by the appearance of lysosome - associated membrane protein ( lamp)-1 at the plasma membrane and extracellular release of the lysosomal content .
acid sphingomyelinase ( asmase ) generates ceramide , thereby inducing the formation of lipid rafts , which are important signaling platforms .
exocytosis could be reduced by the pre - treatment of keratinocytes with the antioxidant -tocopherol ( -toc ) , addition of anti - synaptotagmin vii antibodies ( syt - vii - ab ) , addition of vacuolin-1 or disruption of the lysosomal ph using nh4cl .
human reconstructed skin was exposed to uva ( 60 j / cm ) or uvb ( 500 mj / cm ) and immediately fixed and processed for immunohistochemistry .
a ) representative images of control and uva - irradiated reconstructed skin stained for lysosomal associated membrane protein-2 ( anti - lamp-2 ) and nuclei ( dapi ) and a merged image with an inset at higher magnification .
an image captured using transmitted light ( t - pmt ) shows the gross morphology , with the stratum corneum marked ( * ) .
note the redistribution of lysosomes from the perinuclear region in control keratinocytes ( arrow ) to the vicinity of the plasma membrane ( arrowhead ) in uva - exposed reconstructed skin .
b ) quantification of cells with lamp-2 located at the plasma membrane ( pm ) in 5 randomly selected areas in epidermis below stratum corneum (  500 cells ) directly after irradiation . *
these findings raise several important questions regarding the physiological function of lysosomal exocytosis in the skin .
release of lysosomal content into the extracellular space must be elucidated , and the function must be identified . as studied previously ,
the release of acid sphingomyelinase catalyzes the production of ceramide , which is important for the formation of lipid rafts and signaling platforms in the plasma membrane .
we found that cathepsin d was released into the extracellular space in keratinocyte monolayers , which has been reported to also occur in keratinocytes exposed to ionophores .
interestingly , pro - cathepsin d has been shown to have a mitogenic effect on fibroblasts when released extracellularly .
cathepsins also promote tumorigenesis by stimulating tumor growth , which enhances angiogenesis and facilitates invasion .
thus , it is possible that , following lysosomal exocytosis , the cathepsins located outside of keratinocytes might participate in uva - induced mitogen stimulation and contribute to skin cancer progression .
furthermore , uva - induced immunosuppression is mediated by several cytokines , including interleukin-4 ( il-4 ) .
il-4 was recently shown to induce cathepsin activity in macrophages , which promoted pancreatic tumor growth and invasiveness in culture , indicating that uv might promote cathepsin expression .
it will be of great interest to determine whether melanocytes exposed to uva will also present plasma membrane damage and lysosomal exocytosis .
uv - irradiation stimulates the production of melanin , but no consensus model for the delivery of melanosomes to keratinocytes has been presented .
the filopodial - phagocytosis and shedding - vesicle models have been presented , but convincing experimental proof for either model is still lacking .
therefore , it would be of interest to study melanosome transfer and compare its mechanism with those of lysosomal exocytosis .
 solar radiation is an important risk factor for skin cancer , the incidence of which is increasing , especially in the fair - skinned populations of the world . 
 while the ultraviolet ( uv)b component has direct dna damaging ability , uva - induced effects are currently mainly attributed to the production of reactive oxygen species . in our recent study , we compared the effects of uva and uvb radiation on human keratinocytes and found that uva - induced plasma membrane damage was rapidly repaired by lysosomal exocytosis , which was detected based on the expression of lysosomal membrane associated protein-1 ( lamp-1 ) on the plasma membrane of non - permeabilized cells . 
 later , the keratinocytes died through caspase-8 mediated apoptosis . in contrast , the plasma membranes of keratinocytes exposed to uvb showed no lamp-1 expression , and , although the cells died by apoptosis , no initial caspase-8 activity was detected . 
 we have also demonstrated the occurrence of uva - induced lysosomal exocytosis in reconstructed skin and shown the relocation of lysosomes from the center of cells to the vicinity of the plasma membrane . 
 thus , we suggest that lysosomal exocytosis also occurs in keratinocytes covered by the stratum corneum following exposure to uva . 
 our findings provide new insight into the mechanism of uva - induced skin damage . 
UVA-Induced Cell Damage
Exocytosis of Lysosomes
UVA-Induced Lysosomal Exocytosis in Keratinocytes
New Challenges