Modulation of the signal recognition particle 54-kDa subunit (SRP54) in rat preosteoblasts by the extracellular matrix.

Rat preosteoblastic cells, UMR201, develop a more mature phenotype when subcultured onto a type I collagen gel when compared with their growth on plastic. Basal osteopontin mRNA expression is up-regulated, whereas retinoic acid-induced alkaline phosphatase expression is reduced in cells on collagen when compared with cells plated onto plastic. We have used differential display polymerase chain reaction (PCR) of mRNA to identify other mRNA species that are regulated by collagen and/or retinoic acid in UMR201 cells. A number of differentially expressed PCR products were isolated, whose sequences did not correspond to known sequences in the data bank. However, one species which was up-regulated by growth on collagen showed 95 and 94% homology to the murine and canine 54-kDa subunit of the signal recognition particle (SRP54), respectively. In time course experiments, using reverse transcription PCR, it was found that SRP54 mRNA was up-regulated in UMR201 cells as early as 1 h after subculture onto collagen, when compared with cells subcultured onto plastic, and levels remained elevated after 48 h. The increased expression of SRP54 paralleled the increased expression of a known secreted protein, osteopontin. SRP54 recognizes signal sequences of proteins destined for secretion and retards them for further elongation in the endoplasmic reticulum. The increased expression may correlate with the synthesis of specific extracellular matrix molecules in differentiating osteoblasts.