Regulation of actin mRNA levels and translation responds to changes in cell configuration

Abstract

The role of cell configuration in regulating cell metabolism has been studied, using a system in which cell shape and surface contact can easily be manipulated. The suspension of anchorage-dependent mouse fibroblasts in Methocel results in a coordinate decrease of DNA, RNA, and protein synthesis. These processes are restored upon reattachment of cells to a solid surface. This recovery process has two or more components: a rapid recovery of protein synthesis requiring only surface contact, and a slower restoration of nuclear events which is dependent upon extensive cell spreading (A. Ben-Ze'ev, S.R. Farmer, and S. Penman, Cell 21:365-372, 1980). In the present study, we examined 3T3 cells while in suspension culture and after attachment to a tissue culture dish surface to study cell configuration-dependent expression of specific cytoskeleton protein genes. The 3T3 line of fibroblasts used here shows these responses much more dramatically compared with 3T6 cells previously studied. We demonstrate that whereas total protein synthesis was strongly inhibited upon suspension, actin synthesis was preferentially inhibited, decreasing from 12% of total protein synthesis in control cells to 6% in suspended cells. This occurred apparently at the level of translation of actin mRNA, since the amount of actin mRNA sequences in the cytoplasm was unchanged. Reattachment initiated the rapid recovery of overall protein synthesis which was accompanied by a dramatic, preferential increase in actin synthesis reaching peak values of 20 to 25% of total protein synthesis 4 to 6 h later, but then declining to control values by 24 h. Translation in vitro and hybridization of mRNA to a cloned actin cDNA probe revealed that the induction of actin synthesis was due to increased levels of translatable mRNA sequences in the cytoplasm. These results imply a close relationship among cell cytoarchitecture, expression of a specific cytoskeletal protein gene, and growth control. The expression of the actin gene appears to be regulated at both the level of translation (during suspension) and mRNA production (during recovery).