IP7K is kinetically well-equipped for cell signaling but does not regulate CDK5 activity (1013.12)
Inositol pyrophosphates (PP-InsPs) such as 1-IP7 and IP8 serve as cellular signals that interface nutrient status with cellular growth and energy homeostasis; PP-InsP turnover may impact bioenergetic diseases such as diabetes and cancer. In yeast, 1-IP7 regulates phosphate response genes through its inhibition of the Pho85 cyclin-kinase complex. Yet the relationship between cellular energy status and PP-InsP synthesis is unclear. We characterized the ATP-dependent kinase activity of human IP7K2. The IP7K2 Vmax (190 +/- 10 nmol/mg/min) is comparable to that of IP6K. Unlike IP6K, IP7K2 is not reversible in a physiologically-relevant bioenergetic environment (i.e. [ATP]>[ADP]); the equilibrium point favors 80-90% 1-IP7 and IP8 accumulation. Furthermore, IP7K2 was insensitive to physiologically-relevant changes to either [AMP], [ADP] or to [ATP] itself (since the ATP Km = 20-40 µM). We further showed that regulation of yeast cyclin-kinase by 1-IP7 is not conserved for the homologous mammalian CDK5 complex. To interrogate potential signaling roles of IP7K2, we demonstrate in L6 skeletal myoblasts that either insulin, IGF or PDGF can each raise IP8 levels 2-3 fold. Thus, we show independent of the bioenergetic context, IP7K2 is kinetically well-equipped to participate in rapid cell-signaling events and that cells place PP-InsP synthesis under receptor regulation. Precedents suggest this is a valuable observation: demonstrations of receptor-regulation of levels of IP3 - and, later, PIP3 - were critical in fermenting widespread interest in these molecules as cell-signals.
Grant Funding Source: This work was supported by the NIEHS.