ITPK1 mediates the lipid-independent synthesis of inositol phosphates controlled by metabolism

Abstract

Inositol phosphates (IPs) comprise a network of phosphorylated molecules that play multiple signaling roles in eukaryotes. IPs synthesis is believed to originate with IP3 generated from PIP2 by phospholipase C (PLC). Here, we report that in mammalian cells PLC-generated IPs are rapidly recycled to inositol, and uncover the enzymology behind an alternative “soluble” route to synthesis of IPs. Inositol tetrakisphosphate 1-kinase 1 (ITPK1)—found in Asgard archaea, social amoeba, plants, and animals—phosphorylates I(3)P1 originating from glucose-6-phosphate, and I(1)P1 generated from sphingolipids, to enable synthesis of IP6. We also found using PAGE mass assay that metabolic blockage by phosphate starvation surprisingly increased IP6 levels in a ITPK1-dependent manner, establishing a route to IP6 controlled by cellular metabolic status, that is not detectable by traditional [3H]-inositol labeling. The presence of ITPK1 in archaeal clades thought to define eukaryogenesis indicates that IPs had functional roles before the appearance of the eukaryote.

Can Inositol Pyrophosphates Inform Strategies for Developing Low Phytate Crops?

“…This pathway was thought be unique to these organisms, along with land plants. However, a very recent publication by Desfougères et al shows that the Lipid-Independent pathway is also present in mammals [28]. This work is the first to report evidence of the Lipid-Independent pathway in mammals and will be crucial for exploring the evolution of enzymes across organisms.…”

“…Although more work needs to be done, this suggests that animals may also utilize ITPKs in a Lipid-Independent InsP 6 pathway. In the same work, these authors also found that the plant ITPK1 could complement the yeast PLC mutant, suggesting that the plant ITPK may also have a very flexible substrate preference, and may act at several different steps in the Lipid-Independent pathway [28].…”

“…A very recent publication on non-plant organisms suggests that there might be some conserved functions of the Lipid-Independent pathway in other eukaryotes. Humans, for example, contain ITPK genes, and the expression of these enzymes appears to complement mutants defective in PLC, which cannot synthesize InsP 6 [28,46]. Although more work needs to be done, this suggests that animals may also utilize ITPKs in a Lipid-Independent InsP 6 pathway.…”

Targeting the Inositol Pyrophosphate Biosynthetic Enzymes in Metabolic Diseases

“…IP6 research is divided into two major areas: studying the role of intracellular IP6 (Int-IP6) by manipulating the enzymes that synthesize or metabolize this molecule, and assessing the impact of dietary supplementation of IP6 (Sup-IP6) in health. Besides the already established PIP2 or IP3 [8,12,14,18,20,[23][24][25][26][27], glucose-6-phosphate and sphingolipids can also be precursors of Int-IP6 [23]. Int-IP6 regulates many cellular functions including stress responses, development, phosphate homeostasis, DNA repair, RNA editing, mRNA export and post-translational modification [94,95,189].…”

“…In addition to the classic lipid-IP3 pathway (phosphatidylinositol 4,5-bisphosphate, PIP2 is cleaved to diacylglycerol and inositol 1,4,5-trisphosphate IP3. IP3 gets converted to higher IPs and IPPs), a soluble route also exists where conversion of the glucose-6-phosphate to IP1 serves as a precursor of higher IPs and IPPs [23]. Additional details of IPPs, their metabolizing enzymes, functions and mechanism of actions have been reviewed ( [8,12,14,18,20,[23][24][25][26][27][28] and references therein).…”