Linking phospholipase C isoforms with differentiation function in human vascular smooth muscle cells

The phosphoinositol-phospholipase C (PLC) family of enzymes consists of a number of isoforms, each of which has different cellular functions. PLCγ₁ is primarily linked to tyrosine kinase transduction pathways, whereas PLCδ₁ has been associated with a number of regulatory proteins, including those controlling the cell cycle. Recent studies have shown a central role of PLC in cell organisation and in regulating a wide array of cellular responses. It is of importance to define the precise role of each isoform, and how this changes the functional outcome of the cell. Here we investigated differences in PLC isoform levels and activity in relation to differentiation of human and rat vascular smooth muscle cells. Using Western blotting and PLC activity assay, we show that PLCδ₁ and PLCγ₁ are the predominant isoforms in randomly cycling human vascular smooth muscle cells (HVSMCs). Growth arrest of HVSMCs for seven days of serum deprivation was consistently associated with increases in PLCδ₁ and SM α-actin, whereas there were no changes in PLCγ₁ immuno-reactivity. Organ culture of rat mesenteric arteries in serum free media (SFM), a model of de-differentiation, led to a loss of contractility as well as a loss of contractile proteins (SM α-actin and calponin) and PLCδ₁, and no change in PLCγ₁ immuno-reactivity. Taken together, these data indicate that PLCδ₁ is the predominant PLC isoform in vascular smooth muscle, and confirm that PLCδ₁ expression is affected by conditions that affect the cell cycle, differentiation status and contractile function.