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Expression of Platelet-derived Growth Factor-independent Phenotypes in BALB/c 3T3 Cell Variant with High Susceptibility to Chemically or Physically Induced Neoplastic Cell Transformation: Dissociation from Activation of Protein Kinase C¹

Department of Molecular Bioregulation, Institute of Molecular and Cellular Biology for Pharmaceutical Sciences, Kyoto Pharmaceutical University, Kyoto 607, Japan

A31-I-13, a clonal cell variant of nontransformed BALB/c 3T3 that is highly susceptible to chemically or physically induced malignant cell transformation but is not sensitive to cell killing or susceptible to induced somatic cell mutation compared with another less transformation-susceptible A31-I-1 cell variant, was previously found to be constitutively competent [platelet-derived growth factor (PDGF)-independent] to synthesize DNA (M. Tatsuka et al., J. Cell. Physiol., 139: 18–23, 1989). The present study has demonstrated that density-arrested, quiescent A31-I-13 cells autonomously exhibit disruption of actin filamentous bundles and perturbations of dynamic morphology. PDGF induced these cytoskeletal modulations in quiescent A31-I-1 cells, which require PDGF for the induction of DNA synthesis. Furthermore, the cytoskeletal modulations of quiescent A31-I-13 cells were not accompanied by an increased production of plasminogen activators, activation of protein kinase C, or phosphorylation of a Triton X-100-soluble protein (molecular weight, 90,000) known as 80K, a major substrate for protein kinase C. However, these modulations were accompanied by the tyrosine phosphorylation of Triton X-100-insoluble (cytoskeletal) proteins with molecular weights of 24,000, 32,000–33,000, and 36,000. These Triton X-100-insoluble proteins, as well as the 80K protein, were phosphorylated by the exposure of quiescent A31-I-1 cells to PDGF. Thus the pathway for producing the transformation-susceptible phenotype in A31-I-13 appears to coincide with the PDGF signaling pathway but does not involve the protein kinase C pathway.

¹ Supported in part by Grants 02857353 and 03857337 to M. T. from the Ministry of Education, Science, and Culture of Japan.

² To whom requests for reprints should be addressed, at John Sealy Center for Molecular Science, The University of Texas Medical Branch at Galveston, 301 University Boulevard, J-51, Galveston, TX 77555-1051.

³ Deceased February 12, 1992.

Received 1/13/92. Accepted 5/26/92.