DIFFERENCES IN THE G TOTAL ACTIN RATIO AND MICROFILAMENT STABILITY BETWEEN NORMAL AND MALIGNANT HUMAN KERATINOCYTES/

The state of polymerization of actin and the organization of actin fil aments is widely believed to be related to cellular transformation. Si nce the intracellular monomer (G) and filamentous (F) actin content re flects the state of microfilament polymerization, we measured the G/to tal actin ratio in primary cultures of normal and malignant human kera tinocytes. In normal keratinocytes the mean value of this ratio was 0. 30 +/- 0.03 (mean +/- SE, n = 15), while in basal cell carcinoma (BCC) keratinocytes it was 0.49 +/- 0.03 (n = 8) and in squamous cell carci noma keratinocytes (SCC) 0.5 +/- 0.07 (n = 4), indicating a 1.7-fold i ncrease of the G/total actin ratio in malignant cells. These results i mply that the proportion of polymerized actin is decreased markedly in malignant keratinocytes, suggesting alterations of microfilament stru ctures which probably occur during the transformation process. This wa s supported by the morphological changes of microfilament structures a s assessed by fluorescence microscopy. A different distribution of act in filaments in normal and malignant cells became evident; stress-fibr es were converging in patches at several points in SCC cells, when com pared to normal keratinocytes. Furthermore, incubation of normal and m alignant keratinocytes with cytochalasin B indicated differences in th e resistance of their microfilament networks. After 1 h exposure to 10 (-6) and 10(-5) M cytochalasin B, microfilaments in normal cells appea red to be less affected than their counterparts in neoplastic cells. E ven in a high excess of cytochalasin B (10(-4) M), normal keratinocyte s preserved their shape, while both basal cell and SCC were totally di srupted. We concluded that the G/total actin ratio was significantly i ncreased in malignant keratinocytes. This seems to be correlated with altered microfilament morphology and resistance to cytochalasin B trea tment. Our results suggest that the process of malignant transformatio n may be characterized by changes in the state of the polymerization o f actin and in the stability of the microfilament network indicating t hat both features could potentially serve as markers determine the tra nsformed state of keratinocytes.