UV exposure would also lead to the activation of receptors for epidermal growth factor, IL-1, and TNF-α in keratinocytes and fibroblasts, which then activates signaling kinases throughout the skin via an unknown mechanism.  The nuclear transcription factor activator protein, AP-1, which controls the transcription of matrix metalloproteinases (MMP), is expressed and activated. MMP-1 is a major metalloproteinases for collagen degradation. This entire process is aided by the presence of reactive oxygen species that inhibits protein-tyrosine phosphatases via oxidation, thereby resulting in the up-regulation of the above-mentioned receptors. Another transcription factor NF-κB, which is also activated by UV light, also increases the expression of MMP-9.