Modulation of cardiac PIP₂ by cardioactive hormones and other physiologically relevant interventions

Abstract

5-bisphosphate (PIP 2) affects profoundly several cardiac ion channels and transporters, and studies of PIP2-sensitive currents in excised patches suggest that PIP 2 can be synthesized and broken down within 30 s. To test when, and if, total phosphatidylinositol 4-phosphate (PIP) and PIP2 levels actually change in intact heart, we used a new, nonradioactive HPLC method to quantify anionic phospholipids. Total PIP and PIP2 levels (10-30 mol/kg wet weight) do not change, or even increase, with activation of G␣ q/phospholipase C (PLC)-dependent pathways by carbachol (50 M), phenylephrine (50 M), and endothelin-1 (0.3 M). Adenosine (0.2 mM) and phorbol 12-myristate 13-acetate (1M) both cause 30% reduction of PIP2 in ventricles, suggesting that diacylglycerol (DAG)-dependent mechanisms negatively regulate cardiac PIP2. PIP2, but not PIP, increases reversibly by 30% during electrical stimulation (2 Hz for 5 min) in guinea pig left atria; the increase is blocked by nickel (2 mM). Both PIP and PIP2 increase within 3 min in hypertonic solutions, roughly in proportion to osmolarity, and similar effects occur in multiple cell lines. Inhibitors of several volume-sensitive signaling mechanisms do not affect these responses, suggesting that PIP2 metabolism might be sensitive to membrane tension, per se. phosphatidylinositol 4,5-bisphosphate; phosphatidylinositol; diacylglycerol; phorbol ester; cardiac muscle; G protein-coupled receptors; phospholipase C; cell volume PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE (PIP 2 ) is the phospholipid precursor of three second messengers, D-myo-inositol 1,4,5-trisphosphate (IP 3 ), diacylglycerol (DAG), and phosphatidylinositol 3,4,5-trisphosphate (PIP 3 ) (66). At the same time, PIP 2 serves other cellular functions. It anchors and modulates the function of numerous cell signaling proteins and cytoskeleton at the cell membrane (11,17,42,65), including at least one transcription factor that is released by phospholipase C (PLC) activation (60). In addition, PIP 2 metabolism is coupled to membrane trafficking, including some forms of exo-and endocytosis (7, 46). Finally, PIP 2 modulates the function of phospholipases (14), receptor kinases (16, 52), and ion transporters and ion channels (25). Especially, the anchoring/recruitment functions and the modulatory functions of PIP 2 beg the question as to how, and if, PIP 2 might be used as a cell signal. For cardiac physiology, an answer to this question seems especially important at this time, because sarcolemmal mechanisms that affect both cardiac contraction (e.g., Na ϩ /Ca 2ϩ exchange) and contraction frequency [e.g., G protein-coupled inwardly rectifying K ϩ (GIRK) channels] are strongly PIP 2 dependent (27).The minimum biochemical mechanisms involved in cardiac myocyte PIP 2 metabolism (39) are summarized in Fig. 1. The dominant pathway of PIP 2 synthesis, as in other cells, is probably the sequential phosphorylation in the sarcolemma of phosphatidylinositol (PI) at the 4-and then the 5-positions of inositol (66). As in other cells, PIP 2 is hydr...

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

“…PIP 2 is a relatively minor phospholipid (∼1% of total anionic phospholipids), but it is the main polyphosphoinositide in the sarcolemma. Its concentration in myocardium is in the range of 10−30 μM or 150−450 pmol/ mg protein [4,5]. PIP 2 itself serves important signaling functions, including the regulation of ion channels and transporters and the anchoring of cytoskeletal proteins at the membrane [6,7].…”

“…This adds to the complexity of IP 3 signaling and suggests that the potential roles of IP 3 go beyond the release of Ca 2+ from internal stores. Higher inositol polyphosphates, including IP 4 , IP 5 and IP 6 , have been detected in the heart [31], as well as high affinity binding sites for IP 4 and IP 6 [32,33]. However, their physiological functions in cardiac myocytes remain poorly understood.…”

Mechanosensitivity of GIRK Channels Is Mediated by Protein Kinase C-dependent Channel-Phosphatidylinositol 4,5-Bisphosphate Interaction

“…In this case, PKC will make a positive-feedback circuit by enhancing the PIP 2 hydrolysis, and play a role of signal amplifier. PKC activation can also reduce the membrane PIP 2 content by regulation of the phosphatidylinositol transfer protein (35), and regulation of activity and location of lipid phosphatases and/or lipid kinases (21,22). The molecular identity of lipid kinases and phosphatases regulating the PIP 2 metabolism remain controversial.…”

Acute Hypertonicity Alters Aquaporin-2 Trafficking and Induces a MAPK-dependent Accumulation at the Plasma Membrane of Renal Epithelial Cells

“…Both rapid and slow responses mediate cell adaptation to increased extracellular tonicity. Rapid responses include actin and microtubule remodeling, changes in phosphoinositide metabolism, and activation of a large network of intracellular signaling pathways, including MAPK 5 and tyrosine kinase pathways (2)(3)(4)(5). The slower transcriptional response triggered by hypertonicity mediates numerous long term events, such as those involved in reducing cellular ionic strength and reinstating cell growth (6,7).…”