p52 induction by cytochalasin D in rat kidney fibroblasts: Homologies between p52 and plasminogen activator inhibitor type‐1

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Abstract
Normal rat kidney (NRK) fibroblasts respond to the cell shape‐modulating chemical agent cytochalasin D (CD) with augmented synthesis of the 52‐kDa substrate‐associated protein p52. p52 is a complex glycoprotein, existing as 12 different isoforms, which include a 43‐kDa “core” protein (p43), four 50‐kDa species (p50‐0,1,2,3), and at least seven distinct pl variants of the mature 52‐kDa protein. A threshold of 2–4 μM CD was found to be necessary to augment p52 deposition into both the secreted protein‐ and saponin‐resistant cytomatrix (SAP) fractions of NRK cells. This concentration of CD was also necessary to initiate significant cell rounding. Augmented p52 production in CD‐treated NRK (NRK/CD) cells provided a means to assess the identity of this protein. p52 was found to be identical to rat plasminogen activator inhibitor type‐1 (rPAI‐1) and to PAI‐1‐like proteins of other species by comparative immunoprecipitation, 2‐D electrophoretic profile, V8 protease digest mapping, and subcellular fractionation criteria. Quantitation of rPAI‐1 cytoplasmic mRNA abundance, using the rPAI‐1 cDNA probe pSS1‐3, revealed an induction of rPAI‐1 mRNA in NRK/CD cells which paralleled the increased protein production. CD‐augmented p52(rPAI‐1) synthesis and SAP deposition was blocked by actinomycin D, implicating a need for RNA synthesis during the period of CD exposure to effect induction. Augmentation of p52 expression in NRK/CD fibroblasts, thus, appears to involve both cell shape‐associated metabolic processes and concomitant RNA synthesis.