| Summary: | The expression of phospholipase C-β1 (PLC-β1) and cyclin D3 is highly induced during skeletal
myoblast differentiation. We have previously shown that PLC-β1 activates cyclin D3 promoter during
the differentiation of myoblasts to myotubes, indicating that PLC-β1 is a crucial regulator of mouse
cyclin D3 gene. Here we report that PLC-β1 catalytic activity plays a role in the increase of cyclin D3
levels and in the induction of differentiation of C2C12 skeletal muscle cells. PLC-β1 mutational
analysis revealed the importance of His331 and His378 for the catalytic activity. We show that following
insulin administration, cyclin D3 mRNA levels are lower in cells overexpressing the PLC-β1
catalytically inactive form, as compared to wild type cells. We describe a novel signaling pathway
elicited by PLC-β1 that modulates Activator Protein-1 (AP-1) activity. Indeed, gel mobility shift assays
indicate that there is a c-jun binding site located in cyclin D3 promoter region specifically regulated by
PLC-β1 and that c-jun binding activity is significantly increased by insulin stimulation and PLC-β1
overexpression. Moreover, mutation of c-jun/AP-1 binding site decreases the basal cyclin D3 promoter
activity and eliminates its induction by insulin and PLC-β1 overexpression. Interestingly, we observed
that the ectopic expression of the Inositol Polyphosphate Multikinase (IPMK) in C2C12 myoblasts
enhances cyclin D3 gene expression and that the mutation of c-jun site in cyclin D3 promoter
determines an impairment of IPMK-dependent promoter induction. These results indicate that PLC-β1
activates a c-jun/AP-1 target gene, i.e. cyclin D3, during myogenic differentiation through IPMK
signaling.
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