Partial purification and differential androgen sensitivity of protein phosphokinases from nuclei of rat ventral prostate

CK2 signaling in androgen-dependent and -independent prostate cancer

Protein serine/threonine kinase casein kinase 2 (CK2) is a key player in cell growth and proliferation but is also a potent suppressor of apoptosis. CK2 has been found to be dysregulated in all the cancers that have been examined, including prostate cancer. Investigations of CK2 signaling in the prostate were originally initiated in this laboratory, and these studies have identified significant functional activities of CK2 in relation to normal prostate growth and to the pathobiology of androgen-dependent and -independent prostate cancer. We present a brief overview of these developments in the context of prostate biology. An important outcome of these studies is the emerging concept that CK2 can be effectively targeted for cancer therapy.

Androgenic regulation of phosphorylation and stability of nucleolar protein nucleolin in rat ventral prostate

Nucleolin is an abundant nucleolar phosphoprotein which has been implicated as a factor in various stages of ribosome synthesis, including transcription. Since androgens exert a profound effect on the rRNA synthesis in the target organ prostate, we have examined the nature of androgenic regulation of the amount and phosphorylation of nucleolin in this tissue. Phosphorylation of prostatic nucleolin is catalyzed in part by heparin-sensitive casein kinase 2 (CK-2) and by another (heparin-insensitive) protein kinase. Both the amount and phosphorylation of prostatic nucleolin are profoundly sensitive to androgens. Rapid reduction in the level and phosphorylation of nucleolin occurs following androgen deprivation, which corresponds to the ensuing cessation of prostatic growth leading to involution. Further, the loss of nucleolin phosphorylation and its degradation appear to be concordant. Administration of a single injection of 5 alpha-dihydrotestosterone to castrated animals causes an early increase in the amount and phosphorylation of nucleolin, starting in the prereplicative phase in the prostatic cell nucleus. These data suggest that early androgenic regulation of nucleolin expression and phosphorylation may play a role in nucleolar control mechanisms relevant to prostatic cell growth.

Differential regulation of prostatic protein kinase C isozymes by androgens

Multiple isozymes of Ca2+/phospholipid-dependent protein kinase (PKC) were isolated from the rat ventral prostate. The enzyme exists mainly as type II (beta), and type III (alpha) forms, and it is possible that type II isozyme may comprise the subspecies beta 1 and beta 2. The total and specific activities of prostatic PKC isoforms were reduced in castrated animals; this decrease was specific since administration of androgens to castrated animals reversed such a decline. Also, there was a differential response to androgen deprivation so that type III isozyme declined at a faster rate than that of type II. Thus, our studies show for the first time that PKC of the rat ventral prostate comprises multiple isozymes, and that the activity of these various forms are differentially regulated by androgens.

Tissue-specific decline in cytosolic casein kinase II of the ventral prostate in aging rats

An age-associated decline in the activity of cytosolic casein kinase II in the rat ventral prostate was observed. The decrease in specific and total activity of casein kinase II of prostatic cytosol obtained from 12-month old rats compared with that from 3-month old rats was 50% and 70%, respectively. This decrease was tissue specific since no such alteration in enzymic activity was found in liver, lung, or heart. The decline in activity was not due to an increase in the concentration of a casein kinase inhibitor or to altered androgenic status with aging. Rather, the reduction in the activity of the enzyme was commensurate with the decrease in the total concentration of the enzyme protein as determined by an ELISA using anti-casein kinase II antibodies. The activity of casein kinase II in the nuclear fraction of the rat prostate was not altered in aging animals. It may be concluded that the cytosolic form of casein kinase II in the prostate of older animals is regulated at the transcriptional level (but not via the androgen-receptor complex mechanism). An alternative explanation for this observation may relate to altered stability of the enzyme in the prostate of older animals.

Purification of nuclear cAMP-independent protein kinases from rat ventral prostate

Two nuclear cAMP-independent protein kinases (designated PK-N1 and PK-N2) were purified from rat ventral-prostate and liver. The yield of enzyme units was 4-5% and 7-9% for each enzyme from the prostatic nuclei and liver nuclei, respectively. The average fold purification for prostatic nuclear protein kinase N1 and N2 was 1360 and 1833, respectively. The respective average specific activity of the two enzymes towards casein was 81,585 and 110,000 nmol 32P incorporated/hr/mg of enzyme. Protein kinase N1 comprised one polypeptide of Mr 35,000 which underwent phosphorylation in the presence of Mg2+ + ATP. Protein kinase N2 comprised two polypeptides Mr 40,000 and 30,000 of which only the Mr 30,000 polypeptide was autophosphorylated. Both enzymes were active towards casein, phosvitin, dephosphophosvitin, spermine-binding protein, and non-histone proteins in vitro. Little activity was detected towards histones. Both enzymes were stimulated by 150-200 mM NaCl. MgCl2 requirement varied with the protein substrate but was between 2-4 mM for both enzymes. With dephosphophosvitin as substrate, the apparent Km for ATP for N1 protein kinase was 0.01 mM. GTP did not replace ATP in this reaction. Protein kinase N2 was active in the presence of ATP or GTP. The apparent Km was 0.01 mM for ATP, but 0.1 mM for GTP.

Differential localization and androgen sensitivity of prostatic nuclear protein kinases in euchromatin and heterochromatin fractions

We have examined the distribution and androgenic regulation of protein kinases and phosphoproteins in euchromatin and heterochromatin fractions of rat ventral prostate chromatin. Available procedures to prepare euchromatin and heterochromatin fractions were found to result in the loss of various chromatin-associated protein kinases even though there was no gross change in the gel electrophoretic profile of proteins in these fractions. This loss was prevented by the addition of 0.5 mM phenylmethylsulfonyl fluoride throughout the preparative procedures, which indicates that the protein kinases associated with the chromatin may be particularly susceptible to proteolytic degradation during further subfractionation. By utilizing an improved method for fractionation of chromatin, we have demonstrated a marked enrichment of protein kinase activity (towards phosvitin and endogenous chromosomal proteins) in the euchromatin fraction as compared with heterochromatin. Both of these fractions were also examined for the incorporation of 32P into two main classes of nonhistone proteins (namely, H2SO4-soluble and -insoluble nonhistones). The amount of 32P incorporated into heterochromatin-associated proteins of both classes was markedly less than that in the euchromatin-associated proteins. Protein kinase activities (especially those active towards phosvitin and nonhistone proteins) in the euchromatin fraction as compared with the heterochromatin were significantly reduced within 24 h after androgenic deprivation in the animal. The decreased phosphorylation of nonhistone proteins could be attributed to the loss of endogenous protein kinase activity. The results indicate that not only are chromatin-associated protein phosphokinases preferentially localized in euchromatin fractions but also that these euchromatin-associated protein kinases display the greatest sensitivity to androgenic status of the animal.

Mechanisms and significance of polyamine stimulation of various protein kinase reactions

An overview of the work on polyamine effects on certain protein kinase reactions is presented. In general, the reactions catalyzed by the messenger-independent protein kinases but not by cyclic nucleotide-, Ca2+-, Ca2+-calmodulin-, and Ca2+-anionic lipid-dependent protein kinases, are markedly enhanced by polyamines. The extent of this stimulation depends critically on the nature of the protein substrate and several other factors. A variety of other polycationic compounds including Co3+(NH3)6, polybrene, and certain aminoglycoside antibiotics exert polyamine-like effects in the same reactions. These observations suggest that the charge properties rather than any strict chemical structure play a role in the action of polyamines. Available data do not support a specific "cofactor" function of these amines for the protein kinases involved in the polyamine-stimulable reactions. It appears that the action of polyamines is mediated via their influence on the conformational status of the protein substrates thereby altering the availability of the phosphorylatable sites to the active sites on the protein kinases. Although this notion is supported by several lines of evidence, at present a role of the influence of polyamines on both the substrate and enzyme cannot be ruled out. Possible physiological relevance of the polyamine-stimulable protein kinase reactions observed in the in vitro experiments remains problematic in the absence of precise knowledge on the "effective" or free concentrations of intracellular polyamines.

Purification of cytosolic cAMP-independent protein kinases from rat ventral prostate

Two cAMP-independent protein kinases were purified from rat ventral-prostate and liver cytosol, and were designated PK-C1 and PK-C2 to distinguish them from the nuclear protein kinases described in the preceding paper. The yield of the prostate enzymes was about 5% each, and about 10% each for the liver enzymes. The average fold purification of the prostatic enzymes was 1892 and 3176 for protein kinase C1 and C2, respectively. Their average respective specific activity towards casein was 40,111 and 67,340 nmol 32P incorporated/hr per mg of enzyme protein. protein kinase C1 comprised one polypeptide of Mr 39,000 which underwent phosphorylation in the presence of Mg2+ + ATP. Protein kinase C2 comprised three polypeptides of Mr 41,000; 38,000; 26,000. Of these only the Mr 26,000 polypeptide was autophosphorylated. The Mg2+ requirement for protein kinase C1 and C2 was between 1 and 4 mM depending on the nature of the protein substrate. Both enzymes were stimulated by 100-200 mM NaCl. Km for ATP for C1 and C2 kinases was 0.01 mM; GTP could be used only by protein kinase C2 but with a markedly lower affinity. The enzymes were active towards casein, phosvitin, dephosphophosvitin, and spermine-binding protein in vitro, but demonstrated little activity towards histones. Despite several similarities in these general properties of cytosolic protein kinases C1 and C2 with those of nuclear protein kinases N1 and N2, a number of differences are also noted.

Polyamine-stimulated phosphorylation of prostatic spermine-binding protein is mediated only by cyclic AMP-independent protein kinases

Spermine-binding protein (a rat ventral prostatic protein with high affinity for spermine) was phosphorylated in situ through the action of intrinsic cellular protein kinase(s), suggesting it to be a phosphoprotein in vivo. The purified protein served as a substrate in a number of cyclic AMP-independent protein kinase reactions in vitro, but not for cyclic AMP-dependent, Ca2+ + calmodulin-dependent or Ca2+ + phospholipid-dependent protein kinases. Available data indicate that at least one of the cyclic AMP-independent protein kinases (cytosolic protein kinase C2) may be physiologically relevant in mediating the phosphorylation of this protein. The phosphorylation reaction was stimulated several-fold in the presence of spermine. Spermidine was somewhat less effective, whereas putrescine, cadaverine and 1,6-hexanediamine were minimally active. Phospho amino acid analysis of 32P-labelled spermine-binding protein indicated that phosphoserine was the only labelled phospho amino acid. Spermine-binding protein did not undergo autophosphorylation, or modify the stimulative effect of spermine on the phosphorylation of other substrates such as non-histone proteins. In situ the phosphorylation of spermine-binding protein in tissue from castrated rats was markedly diminished as compared with the normal. Since the phosphorylation of spermine-binding protein appears to be mediated by cyclic AMP-independent protein kinase(s) whose activity in the prostate is under androgenic control, it is suggested that androgen-dependent modulation of the protein kinase(s) exerts a regulatory control (via phosphorylation-dephosphorylation) on the spermine-binding activity and stability of this protein in vivo. Further, since this protein is a substrate for only the cyclic AMP-independent protein kinases, it could serve as a tool for the investigation of such kinases.

Phosphorylation of prostatic nuclear matrix proteins is under androgenic control

Nuclear matrix fraction was isolated from rat ventral prostatic nuclei previously incubated with [gamma-32P]ATP to label nuclear phosphoproteins with 32P. A significant portion of the radioactivity was recovered in the phosphoproteins intrinsic to the nuclear matrix fraction. At 12 h after androgen deprivation (i.e., when a significant portion of the nuclear androgen receptor was known to be depleted), the rate, but not the extent, of phosphorylation of nuclear proteins (predominantly nonhistone proteins) was markedly reduced. Nuclear matrix fraction isolated from such preparations demonstrated a profound reduction in the rate of incorporation of 32P into the matrix-associated proteins without any apparent change in the gel electrophoretic profile of these proteins. The results indicate that the cAMP-independent protein kinase activity which catalyzes the phosphorylation of nuclear matrix proteins is under androgenic control. This may be germane to nuclear matrix-associated initial events in androgen action.

Phosphorylation of the androgen receptor by a nuclear cAMP-independent protein kinase

The androgen receptor was purified from rat ventral prostate. The purified receptor migrated as a single band of mol. wt. 87000 on SDS-polyacrylamide gels, had a kd for R-1881 (17 beta-hydroxy-17 alpha-methyl-estra-4,9,11-trien-3-one) binding as 6 nM, and sedimentation coefficient of 4.5 S. Phosphorylation of the purified receptor was studied by incubating it with [gamma-32P]ATP in the presence of several purified protein kinases including cAMP-dependent protein kinase, and four cAMP-independent protein kinases (which were active towards substrates such as phosvitin and casein). Phosphorylation of the 87000 mol. wt. androgen receptor protein occurred only in the presence of a nuclear cAMP-independent protein kinase (of the N2 type). No auto-phosphorylation of the receptor was detected. The results indicate that the androgen receptor is a phosphoprotein. Further, phosphorylation of the androgen receptor by only a specific nuclear cAMP-independent protein kinase may be important in determining the dynamics of its function.

Differential androgenic control of prostatic cytosolic cAMP-dependent and -independent protein kinases

Whether or not various cytosolic protein kinases (and especially the type I cAMP-dependent protein kinase) of rat ventral prostate are specifically regulated with respect to total activity or specific activity by androgen has been investigated. Following androgen deprivation, the total activity per prostate of cAMP-dependent protein kinase (with histone as substrate) changed little at 24 h, declining by about 20% at 96 h. Under these conditions, its specific activity remained unaltered at 24 h, but was markedly enhanced at 96 h postorchiectomy. Type II cAMP-dependent protein kinase in rat ventral prostate cytosol was the only form of cAMP-dependent protein kinases present as determined by measurement of catalytic activity as well as [32P]-8-N3-cAMP binding to the regulatory subunits. There was no alteration in the distribution of the isoenzymes of cAMP-dependent protein kinases or the response of these kinase activities to cAMP owing to castration of animals. The prostatic cytosol also contains free regulatory subunit (with molecular weight similar to that of regulatory subunit R1) which coelutes with type II cAMP-dependent protein kinase. This finding was confirmed by using [32P]-8-N3-cAMP photoaffinity labeling of cAMP-binding proteins. With respect to cAMP-independent protein kinase (measured with dephosphophosvitin as substrate), a decline of 31% in its specific activity was observed in cytosol of prostates from rats castrated for a period of 24 h without significant further change at later periods following castration. However, there was a marked progressive reduction in total activity of this enzyme per prostate (loss of 72% at 96 h postorchiectomy). The increase in specific activity of cAMP-dependent, but not cAMP-independent, protein kinase in the face of decreasing total activity in the cytosol at later periods of castration (e.g., at 96 h) may reflect a slower loss of the former enzyme protein than the bulk of the cytosolic proteins. Administration of testosterone to castrated animals prevented these changes. These data do not indicate a specific regulation by steroid of the type I cAMP-dependent protein kinase in the prostate. Rather, the cAMP-independent protein kinase (with dephosphophosvitin as substrate) appears to be modulated by the androgenic status of the animal.

Studies of prostatic cytosol protein kinases of the aging ACI rat

Changes in prostatic cytosolic cyclic AMP (cAMP)-dependent and independent protein kinases associated with aging in ACI rats were determined. A decrease in prostatic weight and tissue protein content (but not DNA) with age was noted. The cytosolic cAMP-dependent protein kinase was present predominantly as the type II isozyme. Total content of this enzyme per prostate declined by 37% in 12-month-old rats but remained relatively stable thereafter. However, the activity ratio (+cAMP/-cAMP) increased with aging, indicating an elevation in the amount of the holoenzyme or a decrease in the amount of the free catalytic subunit. Concomitantly the specific activity (activity/unit protein in the cytosol) of this enzyme was found to be increased; this is most likely due to the loss of bulk proteins in the cytosol, and may indicate a slower loss of this enzyme protein in relation to other cytosolic proteins. By comparison, the cAMP-independent protein kinase demonstrated a marked and progressive decline in amount as well as in specific activity with aging, e.g. 80-90% decline at 12 months of age. The above changes in these enzymes did not appear to be due to low circulating androgen in the animals, since they were not reversed on administration of 3.0 mg of testosterone propionate per rat daily for four consecutive days. It is therefore suggested that the above-described changes in prostatic cytosolic protein kinases are primarily associated with cellular senescence in this organ.

Differential effects of polyamines on the phosphorylation of chromatin-associated proteins

Studies are presented on the nature of chromatin-associated phosphoproteins whose phosphorylation is influenced by polyamines. After labelling with 32P, chromatin-associated proteins were separated into four fractions. Fraction I comprised neutral and basic non-histone phosphoproteins, including high-mobility-group non-histones; fraction II consisted mostly of histones; fraction III consisted of a class of (salt-soluble) acidic non-histone phosphoproteins; and fraction IV consisted of residual (salt-insoluble) acidic non-histone phosphoproteins. The average relative distribution of protein in the four fractions (I-IV) was about 1:4:2:1 for both liver and prostate. However, tissue-dependent differences were observed in the incorporation of 32P in various protein fractions. In the presence of polyamines (e.g. 1 mM-spermine or 2 mM-spermidine) maximal stimulation of phosphorylation was observed in non-histone proteins of fraction I (160-180%), followed by that in non-histone proteins of fraction III (80-110%). The phosphorylation of residual non-histone proteins in fraction IV, and the small extent of phosphorylation of histones in fraction II, remained unaltered in the presence of polyamines. Thus polyamines do not stimulate the phosphorylation of all non-histone proteins; their stimulative effect is most prominent in the phosphorylation of neutral and basic non-histone proteins and a class of salt-soluble acidic non-histone proteins. In accord with our hypothesis, these differential effects of polyamines on phosphorylation of endogenous non-histone proteins may relate to the conformation of these substrates rather than to endogenous kinases.

Fractionation and partial purification of rat liver nuclear protein kinases

We have fractional and partially purified several rat liver nuclear protein kinases by utilizing endogenous (nonhistone proteins) and exogenous acidic (dephosphophosvitin) and basic (lysine-rich histone) protein substrates. Three enzymes were active towards endogenous substrates, two towards dephosphophosvitin and two towards lysine-rich histone. Of the latter only one was cAMP-dependent, however, only minimal cAMP binding activity was detected. Several features of these enzyme reactions are described revealing distinct differences in the characteristics of each of these enzymes.

3',5'-cyclic-nucleotide-dependent protein kinase of squamous cell carcinoma of the prostate

An adenosine 3'5'-cyclic-monophosphate (Cyclic AMP)-dependent protein kinase has been identified and partially purified from the rat prostate tumor induced by 20-methylcholanthrene. This enzyme is stimulated 2- to 3-fold by the nucleotide. Equilibrium studies at pH 5.0 suggest the presence of a major class of binding site for cyclic AMP with an association constant of approximately 10(8) M-1. The concentration of binding site is about 1 pmol/mg of protein of the enzyme preparation. The enzyme is stimulated by other cyclic nucleotides as well, but only by higher concentrations. In comparing the ability of different histone subfractions, casein and protamine, to serve as substrate for this particular protein kinase, maximal cyclic-AMP-dependent enzyme activity was observed with histones. The results suggest that factors contributing to the malignant growth of the prostatic tissue do not directly involve changes in the characteristics of a cyclic-AMP-dependent protein kinase.

Phosphoproteins and protein kinase activities of ventral prostate chromatin during sexual maturation in the rat

1. In the ventral prostates of rats undergoing normal sexual development, chromatin-associated phosphokinase activities toward dephosphophosvitin and lysine-rich histones were maximal at 48 days of age, whereas phosphorylation of endogenous chromosomal proteins was greatest in the sexually mature adult. 2. Polyacrylamide gel electrophoretic analysis of chromatin proteins from these animals revealed age-dependent changes in the amount of phosphorylation of specific proteins. 3. These were most evident in highly phosphorylated H2SO4-soluble nonhistone proteins and in those nonhistones (13-16 K and 20-35 K mol. wt) which are insoluble in H2SO4.