Involucrin accumulation and ionophore-assisted envelope for
mation, markers of keratinocyte differentiation, were found to be
highly dependent on culture conditions in the malignant epidermal
keratinocyte line, SCC-13, derived from a human squamous cell
carcinoma. In confluent cultures, approximately one-half of the
cells were competent to form envelopes when grown in medium
without hydrocortisone or retinyl acetate supplementation. Ad
dition of hydrocortisone to the medium during growth resulted in
up to 90% competence, while addition of retinyl acetate instead
resulted in as low as 10% competence. Hydrocortisone partially
antagonized the effect of retinyl acetate when both agents were
added together. Involucrin levels, measured by radioimmunoassay,
were modulated essentially in parallel with envelope com
petence under the various conditions tested. When the cells
were grown in medium supplemented with hydrocortisone, the
levels shortly after confluence were over 50-fold higher than in
sparse cultures. Regardless of hydrocortisone or retinyl acetate
addition, less than 1% of the cells were competent in sparse
cultures of growing cells, but up to 90% exhibited this property
after growth arrest in serum-free medium containing hydrocortisone.
High levels of competence were correlated with cessation
of cell division but not with loss of colony-forming efficiency;
under optimal conditions, two-thirds of the cells were capable of
both envelope formation and colony initiation. Normal human
epidermal cells showed a 4- to 5-fold increase in envelope
competence from sparse to confluent culture but were insensitive
to the suppressive effect of retinyl acetate. The results suggest
that some potential differentiated character of malignant keratinocytes
may be suppressed in vivo by physiological agents such
as vitamin A.