Erythroid membrane-bound protein kinase binds to a membrane component and is regulated by phosphatidylinositol 4,5-bisphosphate

Casein kinase 1α: biological mechanisms and theranostic potential

Casein kinase 1α (CK1α) is a multifunctional protein belonging to the CK1 protein family that is conserved in eukaryotes from yeast to humans. It regulates signaling pathways related to membrane trafficking, cell cycle progression, chromosome segregation, apoptosis, autophagy, cell metabolism, and differentiation in development, circadian rhythm, and the immune response as well as neurodegeneration and cancer. Given its involvement in diverse cellular, physiological, and pathological processes, CK1α is a promising therapeutic target. In this review, we summarize what is known of the biological functions of CK1α, and provide an overview of existing challenges and potential opportunities for advancing theranostics.

Phosphorylation regulates the Star-PAP-PIPKIα interaction and directs specificity toward mRNA targets

Star-PAP is a nuclear non-canonical poly(A) polymerase (PAP) that shows specificity toward mRNA targets. Star-PAP activity is stimulated by lipid messenger phosphatidyl inositol 4,5 bisphoshate (PI4,5P2) and is regulated by the associated Type I phosphatidylinositol-4-phosphate 5-kinase that synthesizes PI4,5P2 as well as protein kinases. These associated kinases act as coactivators of Star-PAP that regulates its activity and specificity toward mRNAs, yet the mechanism of control of these interactions are not defined. We identified a phosphorylated residue (serine 6, S6) on Star-PAP in the zinc finger region, the domain required for PIPKIα interaction. We show that S6 is phosphorylated by CKIα within the nucleus which is required for Star-PAP nuclear retention and interaction with PIPKIα. Unlike the CKIα mediated phosphorylation at the catalytic domain, Star-PAP S6 phosphorylation is insensitive to oxidative stress suggesting a signal mediated regulation of CKIα activity. S6 phosphorylation together with coactivator PIPKIα controlled select subset of Star-PAP target messages by regulating Star-PAP-mRNA association. Our results establish a novel role for phosphorylation in determining Star-PAP target mRNA specificity and regulation of 3'-end processing.

Protein kinase CK1α regulates erythrocyte survival

Protein kinase CK1 (casein kinase 1) isoforms are involved in the regulation of various physiological functions including apoptosis. The specific CK1 inhibitor D4476 may either inhibit or foster apoptosis. Similar to apoptosis of nucleated cells, eryptosis, the suicidal death of erythrocytes, is paralleled by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include increase of cytosolic Ca(2+) activity following energy depletion (removal of glucose) or oxidative stress (exposure to the oxidant tert-butyl hydroperoxide [TBOOH]). Western blotting was utilized to verify that erythrocytes express the protein kinase CK1α, and FACS analysis to determine whether the CK1 inhibitor D4476 and CK1α activator pyrvinium pamoate modify forward scatter (reflecting cell volume), annexin V binding (reflecting phosphatidylserine exposure), and Fluo3 fluorescence (reflecting cytosolic Ca(2+) activity). As a result, both, human and murine erythrocytes express CK1 isoform α. Glucose depletion (48 hours) and exposure to 0.3 mM TBOOH (30 minutes) both decreased forward scatter, increased annexin V binding and increased Fluo3 fluorescence. CK1 inhibitor D4476 (10 μM) significantly blunted the decrease in forward scatter, the increase in annexin V binding and the increase in Fluo 3 fluorescence. (R)-DRF053, another CK1 inhibitor, similarly blunted the increase in annexin V binding upon glucose depletion. The CK1α specific activator pyrvinium pamoate (10 μM) significantly enhanced the increase in annexin V binding and Fluo3 fluorescence upon glucose depletion and TBOOH exposure. In the presence of glucose, pyrvinium pamoate slightly but significantly increased Fluo3 fluorescence. In conclusion, CK1 isoform α participates in the regulation of erythrocyte programmed cell death by modulating cytosolic Ca(2+) activity.

CKI isoforms α and ε regulate Star-PAP target messages by controlling Star-PAP poly(A) polymerase activity and phosphoinositide stimulation

Star-PAP is a non-canonical, nuclear poly(A) polymerase (PAP) that is regulated by the lipid signaling molecule phosphatidylinositol 4,5 bisphosphate (PI4,5P(2)), and is required for the expression of a select set of mRNAs. It was previously reported that a PI4,5P(2) sensitive CKI isoform, CKIα associates with and phosphorylates Star-PAP in its catalytic domain. Here, we show that the oxidative stress-induced by tBHQ treatment stimulates the CKI mediated phosphorylation of Star-PAP, which is critical for both its polyadenylation activity and stimulation by PI4,5P(2). CKI activity was required for the expression and efficient 3'-end processing of its target mRNAs in vivo as well as the polyadenylation activity of Star-PAP in vitro. Specific CKI activity inhibitors (IC261 and CKI7) block in vivo Star-PAP activity, but the knockdown of CKIα did not equivalently inhibit the expression of Star-PAP targets. We show that in addition to CKIα, Star-PAP associates with another CKI isoform, CKIε in the Star-PAP complex that phosphorylates Star-PAP and complements the loss of CKIα. Knockdown of both CKI isoforms (α and ε) resulted in the loss of expression and the 3'-end processing of Star-PAP targets similar to the CKI activity inhibitors. Our results demonstrate that CKI isoforms α and ε modulate Star-PAP activity and regulates Star-PAP target messages.

CKIalpha is associated with and phosphorylates star-PAP and is also required for expression of select star-PAP target messenger RNAs

We have recently identified Star-PAP, a nuclear poly(A) polymerase that associates with phosphatidylinositol-4-phosphate 5-kinase Ialpha (PIPKIalpha) and is required for the expression of a specific subset of mRNAs. Star-PAP activity is directly modulated by the PIPKIalpha product phosphatidylinositol 4,5-bisphosphate (PI-4,5-P(2)), linking nuclear phosphoinositide signaling to gene expression. Here, we show that PI-4,5-P(2)-dependent protein kinase activity is also a part of the Star-PAP protein complex. We identify the PI-4,5-P(2)-sensitive casein kinase Ialpha (CKIalpha) as a protein kinase responsible for this activity and further show that CKIalpha is capable of directly phosphorylating Star-PAP. Both CKIalpha and PIPKIalpha are required for the synthesis of some but not all Star-PAP target mRNA, and like Star-PAP, CKIalpha is associated with these messages in vivo. Taken together, these data indicate that CKIalpha, PIPKIalpha, and Star-PAP function together to modulate the production of specific Star-PAP messages. The Star-PAP complex therefore represents a location where multiple signaling pathways converge to regulate the expression of specific mRNAs.

The alpha isoform of protein kinase CKI is responsible for hepatitis C virus NS5A hyperphosphorylation

Hepatitis C virus (HCV) has been the subject of intensive studies for nearly two decades. Nevertheless, some aspects of the virus life cycle are still a mystery. The HCV nonstructural protein 5A (NS5A) has been shown to be a modulator of cellular processes possibly required for the establishment of viral persistence. NS5A is heavily phosphorylated, and a switch between a basally phosphorylated form of NS5A (p56) and a hyperphosphorylated form of NS5A (p58) seems to play a pivotal role in regulating HCV replication. Using kinase inhibitors that specifically inhibit the formation of NS5A-p58 in cells, we identified the CKI kinase family as a target. NS5A-p58 increased upon overexpression of CKI-alpha, CKI-delta, and CKI-epsilon, whereas the RNA interference of only CKI-alpha reduced NS5A hyperphosphorylation. Rescue of inhibition of NS5A-p58 was achieved by CKI-alpha overexpression, and we demonstrated that the CKI-alpha isoform is targeted by NS5A hyperphosphorylation inhibitors in living cells. Finally, we showed that down-regulation of CKI-alpha attenuates HCV RNA replication.

The casein kinase 1 family: participation in multiple cellular processes in eukaryotes

Phosphorylation of serine, threonine and tyrosine residues by cellular protein kinases plays an important role in the regulation of various cellular processes. The serine/threonine specific casein kinase 1 and 2 protein kinase families--(CK1 and CK2)--were among the first protein kinases that had been described. In recent years our knowledge of the regulation and function of mammalian CK1 kinase family members has rapidly increased. Extracellular stimuli, the subcellular localization of CK1 isoforms, their interaction with various cellular structures and proteins, as well as autophosphorylation and proteolytic cleavage of their C-terminal regulatory domains influence CK1 kinase activity. Mammalian CK1 isoforms phosphorylate many different substrates among them key regulatory proteins involved in the control of cell differentiation, proliferation, chromosome segregation and circadian rhythms. Deregulation and/or the incidence of mutations in the coding sequence of CK1 isoforms have been linked to neurodegenerative diseases and cancer. This review will summarize our current knowledge about the function and regulation of mammalian CK1 isoforms.

Fragmentation of the Golgi apparatus. A role for beta III spectrin and synthesis of phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) synthesis has been implicated in maintaining the function of the Golgi apparatus. Here we demonstrate that the inhibition of PtdIns(4,5)P(2) synthesis in vitro in response to primary alcohol treatment and the kinetics of Golgi fragmentation in vivo were very rapid and tightly coupled. Preloading Golgi membranes with short chain phosphatidic acid abrogated the alcohol-mediated inhibition of PtdIns(4,5)P(2) synthesis in vitro. We also show that fragmentation of the Golgi apparatus in response to diminished PtdIns(4,5)P(2) synthesis correlated with both the phosphorylation of a Golgi form of beta III spectrin, a PtdIns(4,5)P(2)-interacting protein, and changes in its intracellular redistribution. The data are consistent with a model suggesting that the decreased PtdIns(4,5)P(2) synthesis and the phosphorylation state of beta III spectrin modulate the structural integrity of the Golgi apparatus.

Four casein kinase I isoforms are differentially partitioned between nucleus and cytoplasm

The casein kinase I (CKI) family consists of at least seven vertebrate genes, some of which can be alternatively spliced. Previously, we have studied the four splice variants of the chicken CKIalpha gene. The four proteins differ only by the presence or absence of two peptides, a 28-amino-acid "L" insert in the catalytic domain and a 12-amino-acid "S" insert near the extreme C-terminus. Here cells were transfected with DNA encoding all four isoforms fused to the green fluorescent protein (GFP) and the localization of each protein was examined. We noted that the L insert includes the sequence PVGKRKR, which has the characteristics of a nuclear localization signal (NLS), and we show that the CKIalphaL and CKIalphaLS isoforms which contain this sequence are targeted to the nucleus, where a fraction becomes associated with nuclear speckles. In contrast the two isoforms lacking the L insert remain predominantly cytoplasmic. Mutation of the first lysine in the putative NLS to asparagine prevented the nuclear entry of GFP-CKIalphaL. Therefore different CKIalpha isoforms are targeted to different cellular compartments in a fashion modulated by alternate transcription and in these locations presumably phosphorylate and regulate different cellular substrates.

Anti-proliferative effect of two novel palmitoyl-carnitine analogs, selective inhibitors of protein kinase C conventional isoenzymes

Previous studies have shown that palmitoyl-carnitine is an anti-proliferative agent and a protein kinase C inhibitor. Two new palmitoyl-carnitine analogs were synthesized by replacing the ester bond with a metabolically more stable ether bond. An LD50 value in the nM range was found in anti-proliferative assays using HL-60 cells and was dependent on the alkyl-chain length. The inhibitory action of these water-soluble compounds on protein kinase C in vitro was greatly increased with respect to palmitoyl-carnitine and was dependent on the length of the alkyl chain. Its effect was mediated by an increase in the enzyme's requirement for phosphatidylserine. Inhibition of the in situ phosphorylation of a physiological platelet protein kinase C substrate and of phorbol ester-induced differentiation of HL-60 cells was also observed. Finally, to test for isoenzyme selectivity, several human recombinant protein kinase C isoforms were used. Only the Ca2+-dependent classic protein kinase Cs (alpha, betaIota, betaIotaIota and gamma) were inhibited by these compounds, yet the activities of casein kinase I, Ca2+/calmodulin-dependent kinase and cAMP-dependent protein kinase were unaffected. Thus, these novel inhibitors appear to be both protein kinase C and isozyme selective. They may be useful in assessing the individual roles of protein kinase C isoforms in cell proliferation and tumor development and may be rational candidates for anti-neoplasic drug design.

Autoinhibition of casein kinase I epsilon (CKI epsilon) is relieved by protein phosphatases and limited proteolysis

Casein kinase I epsilon (CKI epsilon) is a member of the CKI gene family, members of which are involved in the control of SV40 DNA replication, DNA repair, and cell metabolism. The mechanisms that regulate CKI epsilon activity and substrate specificity are not well understood. We report that CKI epsilon, which contains a highly phosphorylated 123-amino acid carboxyl-terminal extension not present in CKI alpha, is substantially less active than CKI alpha in phosphorylating a number of substrates including SV40 large T antigen and is unable to inhibit the initiation of SV40 DNA replication. Two mechanisms for the activation of CKI epsilon have been identified. First, limited tryptic digestion of CKI epsilon produces a protease-resistant amino-terminal 39-kDa core kinase with several-fold enhanced activity. Second, phosphatase treatment of CKI epsilon activates CKI epsilon 5-20-fold toward T antigen. Similar treatment of a truncated form of CKI epsilon produced only a 2-fold activation. Notably, this activation was transient; reautophosphorylation led to a rapid down-regulation of the kinase within 5 min. Phosphatase treatment also activated CKI epsilon toward the novel substrates I kappa B alpha and Ets-1. These mechanisms may serve to regulate CKI epsilon and related forms of CKI in the cell, perhaps in response to DNA damage.

Identification of the Major Casein Kinase I Phosphorylation Sites on Erythrocyte Band 3

Human erythrocyte band 3 is a major substrate of two red blood cell protein kinases, casein kinase I and p72syk protein tyrosine kinase. Although the phosphorylation sites and physiologic consequences of p72syk phosphorylation have been characterized, little is known regarding casein kinase I phosphorylation. In this report, we identify the major phosphorylation site of casein kinase I. Using isolated components, casein kinase I was found to phosphorylate the cytoplasmic domain of band 3 (CDB3), primarily on Thr residues. Classical peptide mapping narrowed the major phosphorylation site to a peptide encompassing residues 24-91. Computer-assisted evaluation of this sequence not only showed two consensus casein kinase I phosphorylation sites, but also provided information on how to proteolytically separate and isolate the candidate sites. Following the suggested protocols, a heptapeptide containing the major phosphorylation site was isolated, subjected to amino acid sequencing, and found to be phosphorylated on Thr 42. A minor phosphorylation site was similarly identified as Ser 303. Because Thr 42 is situated near the binding sites on CDB3 of ankyrin, protein 4.1, protein 4.2, and the glycolytic enzymes, phosphorylation of CDB3 by casein kinase I could conceivably impact erythrocyte structure and/or function.

Amyloid beta-protein stimulates casein kinase I and casein kinase II activities

Amyloid beta-protein (A beta) is the major protein of cerebrovascular and plaque amyloid in Alzheimer's disease (AD). Extensive evidence has demonstrated abnormal protein phosphorylation in this disease. We investigated the effect of synthetic A beta with the amino-acid sequence corresponding to cerebrovascular A beta and plaque A beta on the activities of casein kinase I (CK I) and casein kinase II (CK II). These enzymes were purified from bovine brain and casein was used as a substrate. A beta was found to stimulate markedly CK I- and CK II-mediated phosphorylation of casein in a concentration-dependent manner. The effect of plaque A beta was considerably higher than that of cerebrovascular A beta. Heparin, which is known to be a specific inhibitor of CK II, completely inhibited A beta-stimulated CK II activity. A beta itself was not a substrate for casein kinases. These findings were confirmed using other substrates for CK I and CK II. The experiments with synthetic CK II-substrate peptide (Leu-Glu-Leu-Ser-Asp-Asp-Asp-Asp-Glu) and the phosphorylation of erythrocyte membrane proteins by intrinsic membrane-bound CK I in erythrocytes showed marked stimulation in activities of casein kinases in the presence of A beta 1-40 or blocked A beta. We propose that A beta, by stimulating casein kinases, may contribute to abnormal protein phosphorylation in AD, in particular to increased phosphorylation of microtubule-associated proteins, leading to the neurofibrillary tangles formation and neurodegeneration in this disease. Interaction of A beta with protein kinases, thus, may characterize the beginning of the disease.

[Calcium and liver]

Cells expand energy to lower the concentration of free calcium in the cytosol ([Ca2+]i) to a very low level. Extracellular Ca2+ entering via channels situated in the plasma membrane is expelled into the extracellular medium by a Ca(2+)-Mg(2+)-ATPase or by Na(+)-Ca2+ exchangers. The Ca2+ that enters the cell is sequestered, once inside the cytosol, by a Ca(2+)-Mg(2+)-ATPase, which concentrates Ca2+ in specialized domains of the endoplasmic reticulum. The nucleus and the mitochondria also concentrate Ca2+, but less efficiently. The stimulation of numerous receptors by hormones, growth factors and neurotransmitters coupled to GTP-binding proteins provokes a rapid increase in [Ca2+]i by mobilizing Ca2+ from intra- and extracellular compartments. Membrane coupling is ensured by the activation of a phospholipase C-beta, which hydrolyses a doubly phosphorylated phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). The inositol (1,4,5)-trisphosphate (InsP3) consequently formed binds to a receptor consisting in 4 homologous of 250 kDa each. The InsP3 receptor has been localized to a specialized region, rich in Ca2+, of the endoplasmic reticulum. The receptor has been purified and its sequence obtained. Reincorporated into planar bilayers, it displays the properties of a channel. In the cell, opening of the InsP3 receptor-channel provokes the release of the Ca2+ accumulated within the endoplasmic reticulum. Analyzing the kinetics of channel opening by the methods of rapid mixing, rapid filtration or flash photolysis of caged InsP3 has revealed that InsP3 opens the channel within a very short time, probably less than 30 msec. The InsP3 receptor-channel is autoregenerative. With the sustained stimulation of a Ca2+ influx the release of Ca2+ leads to an augmentation of [Ca2+]i, which is responsible for triggering cellular responses. The complexity of Ca2+ signals produced by stimulated cells has been revealed by studies in which highly effective techniques have been used to detect Ca2+ ions in the cytosol, such as bioluminescent proteins, fluorescent indicators or ionic currents sensitive to Ca2+. It appears that variations in [Ca2+]i induced by stimulation consist of oscillations of which the frequency, but not the amplitude, depends on the concentration of the hormone. Moreover, by summing the images picked up with a video recorder, it has been possible to demonstrate the changes in [Ca2+]i at the subcellular level and the waves of Ca2+ in stimulated cells.

Magnesium protects phosphatidylinositol-4,5-bisphosphate-mediated inactivation of casein kinase I in erythrocyte membrane

Recent reports suggest that membrane-bound casein kinase I (MBCK I) activity in erythrocytes is inactivated by exogenously added phosphatidylinositol 4,5-bisphosphate (PIP2) (Bazenet et al. (1990) J. Biol. Chem. 265, 7369-7376; Brockman and Anderson (1991) J. Biol. Chem. 266, 2508-2512). Here we report that PIP2-mediated inhibition of MBCK I in erythrocytes is only observed if exogenous PIP2 and the kinase are allowed to interact in the absence of Mg2+. Prior incubation of PIP2 with 1 mM Mg2+ prevents the inactivation of MBCK I by PIP2. Other divalent cations (Ni2+, Co2+, Mn2+, Cd2+, Ca2+) and trivalent metal ions (La3+, Cr3+, Al3+) did not protect MBCK I from PIP2-mediated inactivation, indicating that the protective effect is specific for Mg2+ only. We propose a role of Mg2+ in the interaction of CK I with phosphoinositides, and that PIP2-mediated inhibition of protein kinase(s) may be a non-physiological phenomenon.

Characterization of phosphoinositide kinases in normal and sickle anaemia red cells

PtdIns and PtdInsP kinases from normal erythrocyte (AA) membranes and sickle cell anaemia erythrocyte (SS) membranes have been characterized. PtdIns kinase was studied in native membranes under conditions in which PtdInsP kinase and PtdInsP phosphatase do not express any activity. Kinetic analysis of the AA and SS PtdIns kinases indicate similar Km values for PtdIns and ATP but higher Vmax values for SS PtdIns kinase. PtdInsP kinase was partially purified from erythrocyte ghosts by NaCl extraction. The kinetic parameters of PtdInsP kinase determined under these conditions were similar in AA and SS NaCl extracts. These data suggest the presence of some effector of PtdIns kinase in SS cell membranes, resulting in a greater activity of the enzyme. This leads consequently, to increase the PtdIns4P pool and to activate PtdInsP kinase, in agreement with our previous observations of a greater [32P]Pi incorporation in both polyphosphoinositides in SS cells relatively to AA cells.

The role of inositol phospholipids in the association of band 4.1 with the human erythrocyte membrane

Band 4.1 is a major protein of the erythrocyte membrane skeleton. It promotes the binding of spectrin to F-actin and may anchor the skeletal network to the plasma membrane via its association with integral membrane proteins. Here, we have investigated the involvement of inositol phospholipids in the binding of band 4.1 to erythrocyte membranes using membrane vesicles stripped of all peripheral proteins at alkaline pH. Trypsinization of these vesicles allows the discrimination of two classes of band 4.1 binding sites: trypsin-sensitive sites (60-65% of the total), largely or exclusively on band 3, and trypsin-resistant sites (35-40% of the total), composed, at least in part, of the glycophorins. ATP depletion or activation of erythrocyte phosphoinositol phospholipase C led to a reduction in membrane phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] content by 20-70% in different experiments. The resulting decrease of band 4.1 binding to vesicles by was variable, but averaged about 15-20%. The same treatments led to an average decrease in the band 4.1 binding capacity of trypsinized vesicles of 55%. Since this is equivalent to a 20% decrease in the binding capacity of non-trypsinized vesicles (consistent with the above result), it indicates that PtdIns(4,5)P2 regulates the binding of band 4.1 only to trypsin-resistant binding sites (and to only a subset of these) accounting for about 15-20% of total band 4.1 binding sites on membranes. We found that hydrolysis of > 95% of PtdIns(4,5)P2 with exogenous phospholipase C-delta (PLC delta) resulted in no further decrease in band 4.1 binding to vesicles than did hydrolysis of 65-70% of PtdIns(4,5)P2 which is accessible to erythrocyte phosphoinositol phospholipase C. This suggests that only 65-70% of total membrane PtdIns(4,5)P2 is involved in regulating band 4.1 binding. Significantly, the pool of PtdIns(4,5)P2 involved is the same pool which can be hydrolysed by erythrocyte phosphoinositol phospholipase C, and which has been shown to be metabolically labile in erythrocytes. The membrane binding capacity for band 4.1 found in this study (averaging 1000 micrograms/mg vesicle protein) is considerably higher than that found in previous studies. The results are consistent with the existence of a binding site for band 4.1 on each copy of the major transmembrane proteins (band 3 and the glycophorins). These results provide new insights into the involvement of membrane inositol phospholipids in cytoskeletal-membrane interactions.

Tyrosine-protein kinase inhibition in human erythrocytes by polyphosphoinositides (PIP and PIP2)

In human erythrocytes Ser/Thr- and Tyr-phosphorylations of cytoplasmic domain of band 3 are catalyzed by casein kinase I and Tyr-protein kinase respectively, both distributed between cytosol and membrane structures. The results reported here show that purified cytosolic Tyr-protein kinase activity, assayed on added substrates such as poly(Glu,Tyr)4:1 and isolated chymotryptic fragments of band 3 cytoplasmic domain (cdb3), is potently inhibited by PIP and even more by PIP2. Similar inhibitory effects are displayed by these polyphosphoinositides also on the endogenous Tyr-phosphorylation of band 3, when they are added to the isolated native membranes, thus suggesting their involvement in regulating in-vivo Tyr-phosphorylation of membrane proteins.

Interaction of phospholipids with proteins and peptides. New advances 1990

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

Doxorubicin interactions at the membrane: evidence for a biphasic modulation of inositol lipid metabolism

Doxorubicin, when incubated for 30 minutes with [32P]-labelled human erythrocyte membrane vesicles, produced an elevation of [32P]inositol-1,4,5-trisphosphate levels. The maximum rise was obtained with 10(-8) mol/l doxorubicin [132 (S.E. 13%) of control, n = 6, P = 0.001]. However, when the inositol lipids were examined, there was no evidence that doxorubicin stimulated the breakdown of [32P]phosphatidylinositol-4,5-bisphosphate under resting conditions, suggesting that the elevated levels of [32P]inositol 1,4,5-trisphosphate were not the result of the stimulation of phospholipase C. Instead, it was found that the dephosphorylation of inositol 1,4,5-trisphosphate by a 5'-phosphomonoesterase was partially inhibited by 10(-8) mol/l doxorubicin so that the rise in [32P]inositol 1,4,5-trisphosphate resulted from the inhibition of the breakdown of constitutively released [32P] inositol 1,4,5-trisphosphate. Similar data was also obtained with another aminoglycoside antibiotic, neomycin. The release of [32P] inositol 1,4-bisphosphate and [32P] inositol 1,4,5-trisphosphate and the breakdown of the inositol lipids in response to calcium (2.5 x 10(-4) and 10(-3) mol/l) stimulation was enhanced by doxorubicin (10(-6) to 10(-12) mol/l). These effects on resting and stimulated inositol lipid metabolism are discussed with reference to the paradoxical effects of doxorubicin to both stimulate and inhibit proliferation, according to concentration.