Epidermal
transglutaminase-K is believed to catalyze the covalent linking of
loricrin and
involucrin to form cross-linked (CE) envelopes. In normal skin,
transglutaminase-K is expressed as a band immediately below the stratum corneum, whereas in
psoriasis and healing skin its expression is considerably expanded throughout the suprabasal layers. We have investigated whether the hyperproliferative state induced by short-term application of topical
retinoic acid is similarly characterized by an increase in
transglutaminase-K enzyme activity and immunoreactivity.
Retinoic acid (0.1% cream) or vehicle were applied to human skin and occluded for 4 d. Skin biopsies were obtained for measurement of
transglutaminase-K and
transglutaminase-C activity and immunoreactivity. For comparison, cultured normal human keratinocytes were incubated for 4 d in the presence of 1 microM
retinoic acid and the subsequent
transglutaminase-K activity and immunoreactivity measured.
Transglutaminase-K activity was increased 2.8 times in
retinoic acid compared to vehicle-treated skin (p less than 0.005, n = 12) whereas there was no significant difference in
transglutaminase-C activity. However,
transglutaminase-K mRNA levels were not significantly different between
retinoic acid- and vehicle-treated skin. In vehicle-treated skin,
transglutaminase-K immunoreactivity was limited to a narrow, substratum corneal band, but was considerably expanded in a diffuse suprabasal pattern in
retinoic acid-treated epidermis. In contrast,
transglutaminase-K immunostaining was decreased and its enzymatic activity reduced sixfold in
retinoic acid-treated keratinocytes (p less than 0.01, n = 4). These results demonstrate that
retinoic acid treatment in vivo, in contrast to in vitro, leads to not only increased
transglutaminase-K protein expression but also increased enzymatic activity in the absence of detectable increases in
mRNA levels. These data, taken with the previously reported lack of in vivo modulation of the
differentiation markers keratins 1 and 10 by
retinoic acid, indicate that certain aspects of keratinocyte terminal differentiation that are altered in vitro by
retinoic acid do not occur in vivo in human skin.