Changes in oncogene mRNA expression during human keratinocyte differentiation

Abstract

The nuclear proto-oncogenes are involved in transcriptional regulation and control many cell processes. The role of changes in proto-oncogene expression in controlling the balance between proliferation and differentiation was studied in cultured keratinocytes. Normal human keratinocytes were grown in the serum-free medium MCDB153 with an extracellular calcium concentration of 70 ΜM. After treatment with different differentiation conditions, cellular RNA was size-fractionated on agarose gels and transferred to nylon membranes which were subsequently hybridized with c-myc, c-jun, and H-ras ³²P-labelIed probes. Relative RNA loading was assessed using probes for Β-actin and ribosomal 18s RNA. Inducing differentiation by increasing the calcium concentration of the medium from 70 ΜM to 1.5 mM resulted in a marked decrease in c-myc RNA levels to 26% of control levels within 8 h. After 48 h in 1.5 mM calcium, c-myc levels had recovered to approximately 50% of control levels. There was a gradual reduction in c-jun levels to 56% of control levels by 4 days. Treatment with 10 nM TPA, which also induces keratinocyte differentiation, reduced c-myc RNA levels to 70% of control levels during the first 4 h, but thereafter c-myc levels remained approximately constant for a further 20 h. TGFΒ (2 ng/ml), which inhibits keratinocyte growth without inducing differentiation, did not alter c-myc RNA levels over a 4-day period. There were no changes in c-myc levels following the addition of retinoic acid and none of the conditions altered H-ras levels. We conclude that c-myc levels are high in proliferating keratinocytes and decrease following differentiation stimuli, but further work is required to investigate the underlying molecular mechanisms of differentiation.