Zinc protoporphyrin IX enhances chemotherapeutic response of hepatoma cells to cisplatin

AIM: To investigate the effect of zinc protoporphyrin IX on the response of hepatoma cells to cisplatin and the possible mechanism involved.

METHODS: Cytotoxicity was determined using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Apoptosis was determined by a flow cytometric assay. Western blotting was used to measure protein expression. Heme oxygenase (HO)-1 activity was measured by determining the level of bilirubin generated in isolated microsomes. Reactive oxygen species (ROS) production was monitored by flow cytometry. Caspase-3 activity was measured with a colorimetric assay kit. Mice were inoculated with 1 × 10⁷ tumor cells subcutaneously into the right flanks. All mice were sacrificed 6 wk after the first treatment and tumors were weighed and measured.

RESULTS: Overexpression of HO-1 in HepG2 cell line was associated with increased chemoresistance to cis-diaminedichloroplatinum (cisplatin; CDDP) compared to other cell lines in vitro. Inhibition of HO-1 expression or activity by zinc protoporphyrin IX (ZnPP IX) markedly augmented CDDP-mediated cytotoxicity towards all liver cancer cell lines in vitro and in vivo. In contrast, induction of HO-1 with hemin increased resistance of tumor cells to CDDP-mediated cytotoxicity in vitro and in vivo. Furthermore, cells treated with ZnPP IX plus CDDP exhibited marked production of intracellular ROS and caspase-3 activity, which paralleled the incidence of cell apoptosis, whereas hemin decreased cellular ROS and caspase-3 activity induced by CDDP.

CONCLUSION: ZnPP IX increases cellular sensitivity and susceptibility of liver cancer cell lines to CDDP and this may represent a mechanism of increasing ROS.

S-100 modulates Ca2+-independent phosphorylation of an endogenous protein (Mr = 19K) in brain

A new brain enzyme (tentatively named protein kinase X), which catalyzes protamine phosphorylation modulated by S-100, was reported recently. An endogenous substrate protein (Mr = 19K) for protein kinase X was isolated from brain by means of S-100-Sepharose 4B affinity chromatography. S-100, but not calmodulin, promoted phosphorylation of the 19K Mr protein in a Ca2+-independent manner, and this reaction was inhibited by gossypol. The substrate protein, localized in the particulate fraction, was present at a much higher level in brain from adult than neonatal rats (2-day-old), a developmental change similar to that seen for protein kinase X. It is suggested that a protein phosphorylation system modulated by S-100 exists in brain, and that this process may be involved in regulation of certain neural functions.

S-100 and other acidic proteins promote Ca2+-independent phosphorylation of protamine catalyzed by a new protein kinase from brain

A new protein kinase modulated by S-100 (tentatively referred to as protein kinase X) was partially purified from pig brain extracts. The activity of protein kinase X, which was independent of Ca2+, was demonstrated when protamine (free base), but not protamine sulfate and other proteins (including histone), was used as substrate. The enzyme activity, found to distribute in both soluble and particulate fractions and to occur at the highest level in brain compared with other tissues (heart, kidney, liver, skeletal muscle, spleen, and testis) of rats, was also modulated by other acidic proteins (calmodulin, troponin C, and stimulatory modulator) in a Ca2+ -independent manner. S-100 and other acidic proteins appeared to function as "substrate modifiers" by interacting with protamine (a highly basic protein), but not with the enzyme, thus rendering protamine in the complex a superior phosphate acceptor. The two isoforms of S-100 (i.e., a and b) were equally effective. Although the enzyme was not inhibited by many agents (trifluoperazine, melittin, cytotoxin I, polymyxin B, and spermine) shown to inhibit markedly phospholipid/Ca2+- or calmodulin/Ca2+ -stimulated protein kinase, gossypol was found to inhibit specifically protein kinase X. The present findings suggest that S-100, a major acidic protein specific to nervous system, may promote phosphorylation by protein kinase X of certain neural proteins resembling protamine or containing protamine-like domains, in addition to its presumed role of a low-affinity Ca2+ -binding protein.

Polyamines inhibit phospholipid-sensitive and calmodulin-sensitive Ca2+-dependent protein kinases

Effects of polyamines on various protein kinases were investigated. It was found that both phospholipid-sensitive Ca2+-dependent protein kinase and myosin light-chain kinase (a calmodulin-sensitive species of Ca2+-dependent protein kinase) were inhibited to different degrees by polyamines, with an approximate order of inhibitory potency of spermine = 1, 12-diaminododecane greater than spermidine = 1, 10-diaminodecane much greater than cadaverine = putrescine. Kinetic analysis revealed that spermine inhibited the phospholipid-sensitive enzyme non-competitively with respect to Ca2+ (Ki = 0.84 mM) and phosphatidylserine (Ki = 0.90 mM); it also inhibited myosin light-chain kinase non-competitively with respect to Ca2+ (Ki = 1.82 mM) and calmodulin (Ki = 2.73 mM). 1, 12-Diaminododecane, in comparison, inhibited the phospholipid-sensitive enzyme competitively with respect to Ca2+ (Ki = 0.45 mM) and phosphatidylserine (Ki = 0.50 mM); it also inhibited myosin light-chain kinase competitively with respect to calmodulin (Ki = 0.63 mM) but non-competitively with respect to Ca2+ (Ki = 1.49 mM). Moreover, spermine (0.5 mM) was found to inhibit markedly phosphatidylserine/Ca2+- and calmodulin/Ca2+-stimulated phosphorylation of endogenous proteins in rat brain particulate fraction. All the polyamines tested were practically without effect on cyclic AMP-dependent and cyclic GMP-dependent protein kinases. Polyarginine, like spermine, was found to be a more selective inhibitor of Ca2+-dependent protein kinases, whereas polyglutamate preferentially inhibited the cyclic nucleotide-dependent enzymes. The present results indicated that, in addition to certain lipophilic compounds (such as trifluoperazine, palmitoylcarnitine, adriamycin and naphthalenesulphonamide) and polypeptides with hydrophobic regions (such as melittin and polymyxin B) previously reported, polycationic compounds (exemplified by polyamines) could also inhibit the two classes of Ca2+-dependent protein kinases requiring either phospholipid or calmodulin as a cofactor. Because of the high cellular concentration (up to 10 mM) and the differential effects of polyamines, it is suggested that spermine, and to smaller extents spermidine and putrescine, may be involved in the regulation of certain Ca2+-dependent protein-phosphorylation systems in vivo.

Comparative abilities of lanthanide ions La3+ and Tb3+ to substitute for Ca2+ in regulating phospholipid-sensitive Ca2+-dependent protein kinase and myosin light chain kinase

Although lanthanide ions La3+ and Tb3+ were only slightly able to substitute for Ca2+ to activate phospholipid-sensitive Ca2+-dependent protein kinase (PL-Ca-PK), they potentiated the ability of a suboptimal concentration of Ca2+ to stimulate the enzyme. In comparison, the lanthanides were more effective Ca2+ substitutes for myosin light chain kinase, a calmodulin-sensitive Ca2+-dependent protein kinase. Both enzymes, however, were inhibited by high concentrations of lanthanides either in the presence or absence of Ca2+. Similar effects of the lanthanides were also noted on phosphorylation of endogenous substrates in the particulate fraction of rat brain stimulated by either phosphatidylserine/Ca2+ or calmodulin/Ca2+. The La3+- or Tb3+-stimulated activity of PL-Ca-PK, as the Ca2+-stimulated activity, was inhibited by various agents, such as trifluoperazine, polymyxin B, cobra cytotoxin I, melittin, and spermine.