Selective effects of phorbol 12-myristate 13-acetate on myofibrils and 10-nm filaments

Phorbol esters selectively downregulate contractile protein gene expression in terminally differentiated myotubes through transcriptional repression and message destabilization

Chronic exposure of differentiated avian skeletal muscle cells in culture to the phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (PMA), results in the selective disassembly of sarcomeric structures and loss of muscle-specific contractile proteins, leaving cytoskeletal structures and their associated proteins intact. We demonstrate here that these morphological and biochemical changes are accompanied by dramatic and selective decreases in the level of the mRNAs that encode the contractile proteins. We measured the effects of PMA on the transcriptional activity and mRNA stability of four contractile protein genes (alpha-cardiac and alpha-skeletal actin, cardiac troponin C [cTnC], and myosin light chain lf [MLClf]) and two nonmuscle genes (beta-cytoplasmic actin and the glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase [GAPDH]). The transcriptional activity of the alpha-cardiac actin and cTnC genes dramatically decreased by 8 h after the addition of PMA, while other muscle and nonmuscle genes examined showed no change. Pulse-chase experiments of in vivo labeled RNA showed significant reductions in mRNA half-lifes for all the contractile protein mRNAs examined, while the half-lifes of beta-actin and GAPDH mRNA were unchanged. All of the above effects occurred under conditions in which cellular protein kinase C (PKC) levels had been reduced by greater than 90%. The fact that many of the contractile protein genes remained transcriptionally active despite the fact that the cells were unable to accumulate their mRNAs to any significant extent indicated that the treated cells were still committed to skeletal muscle differentiation. The selective changes in the stability of the contractile protein mRNAs suggest that the control of mRNA stability may be part of the normal regulatory program of skeletal muscle differentiation and that this control may be linked to the integrity of the contractile apparatus and mediated by second messenger pathways involving PKC activation.

A protein kinase C isozyme is translocated to cytoskeletal elements on activation

Protein kinase C (PKC)1 isozymes comprise a family of related cytosolic kinases that translocate to the cell particulate fraction on stimulation. The activated enzyme is thought to be on the plasma membrane. However, phosphorylation of protein substrates occurs throughout the cell and is inconsistent with plasma membrane localization. Using an isozyme-specific monoclonal antibody we found that, on activation, this PKC isozyme translocates to myofibrils in cardiac myocytes and to microfilaments in fibroblasts. Translocation of this activated PKC isozyme to cytoskeletal elements may explain some of the effects of PKC on cell contractility and morphology. In addition, differences in the translocation site of individual isozymes--and, therefore, phosphorylation of different substrates localized at these sites--may explain the diverse biological effects of PKC.

Molecular genetics in basic myology: a rapidly evolving perspective

Myology has greatly benefited from the recent unification of concepts in molecular, cellular, and developmental biology. The interplay between intrinsic and extrinsic factors in determining the physiologic characteristics of individual myofibers has emerged as an important theme. Of special note is the manner in which the study of contractile protein gene structure and expression has contributed to our understanding of the development and ultimate plasticity of the contractile apparatus. As mechanistic models of normal myogenesis achieve increasing sophistication, the opportunities for understanding the pathogenesis of progressive muscle disfunction improve. In this article we review recent progress in basic myology which will be of interest to clinicians studying the heritable neuromuscular disorders.

Formation and alignment of Z lines in living chick myotubes microinjected with rhodamine-labeled alpha-actinin

We have used fluorescence analogue cytochemistry in conjunction with time lapse recording to study the dynamics of alpha-actinin, a major component of the Z line, during myofibrillogenesis. Rhodamine-labeled alpha-actinin microinjected into living cultured chick skeletal myotubes became localized in discrete cellular structures within 1 h and remained specifically associated with structures for up to 4 d, allowing individual identified structures to be followed during development. In the most immature cells used, alpha-actinin was found in diffuse aggregates, some of which displayed sarcomeric periodicity. Aggregates were observed to coalesce into better defined structures (Z bands) that were approximately 1.0-micron wide. Z bands condensed into narrow, more intensely fluorescent Z lines in 4-48 h. During this period, Z lines grew laterally, primarily by the addition of small beads of alpha-actinin to existing Z lines or by the merging of small Z lines. In more mature cells, alpha-actinin added to Z lines without going through a visible intermediary structure. Mean sarcomere length did not change significantly during the stages examined, although the variability of sarcomere length did decrease markedly over time for identified sets of sarcomeres. At early stages, myofibrils frequently shifted position in both the longitudinal and lateral directions. Neighboring myofibrils were frequently associated for one or more sarcomeres sporadically along their length, such that the intervening sarcomeres were often misaligned. Associations between myofibrils were often transitory. Shifts in myofibril location in conjunction with the formation, breaking, and reformation of lateral associations between myofibrils facilitated the alignment of Z lines through a trial and error process.

Role of stress fiber-like structures in assembling nascent myofibrils in myosheets recovering from exposure to ethyl methanesulfonate

When day 1 cultures of chick myogenic cells were exposed to the mutagenic alkylating agent ethyl methanesulfonate (EMS) for 3 d, 80% of the replicating cells were killed, but postmitotic myoblasts survived. The myoblasts fused to form unusual multinucleated "myosheets": extraordinarily wide, flattened structures that were devoid of myofibrils but displayed extensive, submembranous stress fiber-like structures (SFLS). Immunoblots of the myosheets indicated that the carcinogen blocked the synthesis and accumulation of the myofibrillar myosin isoforms but not that of the cytoplasmic myosin isoform. When removed from EMS, widely spaced nascent myofibrils gradually emerged in the myosheets after 3 d. Striking co-localization of fluorescent reagents that stained SFLS and those that specifically stained myofibrils was observed for the next 2 d. By both immunofluorescence and electron microscopy, individual nascent myofibrils appeared to be part of, or juxtaposed to, preexisting individual SFLS. By day 6, all SFLS had disappeared, and the definitive myofibrils were displaced from their submembranous site into the interior of the myosheet. Immunoblots from recovering myosheets demonstrated a temporal correlation between the appearance of the myofibrillar myosin isoforms and the assembly of thick filaments. The assembly of definitive myofibrils did not appear to involve desmin intermediate filaments, but a striking aggregation of sarcoplasmic reticulum elements was seen at the level of each I-Z-band. Our findings suggest that SFLS in the EMS myosheets function as early, transitory assembly sites for nascent myofibrils.

Detection of melanogenic proteins in cultured chick-embryo melanocytes

The phorbol ester, 12-0-tetradecanoylphorbol-13-acetate (TPA), was used as a reversible inhibitor of melanogenesis. Chick-melanocyte cultures of the black genotype, E/E, were grown in conditioned medium plus TPA. After growth in TPA and after its removal, the cells were pulse labeled with 3H-leucine. The membrane fraction, which included all tyrosinase activity as well as both mature and immature melanosomes, was solubilized with Triton X-100. The proteins were separated using two-dimensional electrophoresis and visualized by fluorography. One defined melanogenic protein, tyrosinase, was isolated, and its location was determined in the two-dimensional protein pattern. The protein patterns for both the TPA-inhibited cells and the cells in which the TPA effects were reversed after removal were compared. In addition to tyrosinase, at least nine TPA-sensitive proteins were found. These were designated as being putative melanogenic proteins which, along with tyrosinase, may be responsible for melanin-granule synthesis.

Stimulation and inhibition of myoblast differentiation by hormones

The growth and differentiation of L6 myoblasts are subject to control by two proteins secreted by cells of the Buffalo rat liver line. The first of these, rat insulinlike growth factor-II (formerly designated multiplication stimulating activity) is a potent stimulator of myoblast proliferation and differentiation, as well as associated processes such as amino acid uptake and incorporation into protein, RNA synthesis, and thymidine incorporation into DNA. In addition, this hormone causes a significant decrease in the rate of protein degradation. All of these actions seem to be attributable to a single molecular species, although their time courses and sensitivity to the hormone differ substantially. The second protein, the differentiation inhibitor (DI), is a nonmitogenic inhibitor of all tested aspects of myoblast differentiation, including fusion and the elevation of creatine kinase. Indirect immunofluorescence experiments demonstrated that DI also blocks accumulation of myosin heavy chain and myomesin. Upon removal of DI after 72 h incubation, all of these effects were reversed and normal myotubes containing the usual complement of muscle-specific proteins were formed. Thus, this system makes it possible to achieve specific stimulation or inhibition of muscle cell differentiation by addition of purified proteins to cloned cells in serum-free medium.

The relationship between stress fiber-like structures and nascent myofibrils in cultured cardiac myocytes

The topographical relationship between stress fiber-like structures (SFLS) and nascent myofibrils was examined in cultured chick cardiac myocytes by immunofluorescence microscopy. Antibodies against muscle-specific light meromyosin (anti-LMM) and desmin were used to distinguish cardiac myocytes from fibroblastic cells. By various combinations of staining with rhodamine-labeled phalloidin, anti-LMM, and antibodies against chick brain myosin and smooth muscle alpha-actinin, we observed the following relationships between transitory SFLS and nascent and mature myofibrils: (a) more SFLS were present in immature than mature myocytes; (b) in immature myocytes a single fluorescent fiber would stain as a SFLS distally and as a striated myofibril proximally, towards the center of the cell; (c) in regions of a myocyte not yet penetrated by the elongating myofibrils, SFLS were abundant; and (d) in regions of a myocyte with numerous mature myofibrils, SFLS had totally disappeared. Spontaneously contracting striated myofibrils with definitive Z-band regions were present long before anti-desmin localized in the I-Z-band region and long before morphologically recognizable structures periodically link Z-bands to the sarcolemma. These results suggest a transient one-on-one relationship between individual SFLS and newly emerging individual nascent myofibrils. Based on these and other relevant data, a complex, multistage molecular model is presented for myofibrillar assembly and maturation. Lastly, it is of considerable theoretical interest to note that mature cardiac myocytes, like mature skeletal myotubes, lack readily detectable stress fibers.

Tumor promoters induce a specific morphological signature in the nuclear matrix-intermediate filament scaffold of Madin-Darby canine kidney (MDCK) cell colonies

Tumor promoters such as phorbol 12-tetradecanoate 13-acetate (TPA), mezerein, teleocidin, aplysiatoxin, and benzoyl peroxide, although structurally unrelated, induce similar, profound changes in morphology in differentiated epithelial Madin-Darby canine kidney (MDCK) cell colonies. The alteration is evident in the organization of intermediate filaments in intact cells and in whole mounts of the nuclear matrix-intermediate filament (NM-IF) scaffold of the epithelial sheet. This substructure, obtained by salt extraction of the cytoskeletal framework, represents only 5% of the total cell protein but contains all of the intermediate filaments, nuclear matrix, and desmosomal core proteins arranged essentially as in the intact cell. The NM-IF is profoundly reorganized after exposure to TPA and retains the morphological changes observed in intact cells. These include bundling of the intermediate filaments, disruption of cell-cell borders, and marked deformation of the polygonal geometry of epithelia. Thus, TPA and all other complete or second-stage tumor promoters examined have a characteristic morphological signature that is not induced by mitogens, metabolic inhibitors, or agents known to disrupt microtubules or microfilaments. This signature, characteristic of tumor promoters, occurs in the absence of both protein and RNA synthesis. These results suggest that this response is prior to and independent of other biochemical markers for tumor promoters. Of the major filament systems, the cytokeratin network is implicated as an early or possibly primary site of tumor-promoter action because characteristics of the promoted cytoskeletal signature are observed in epithelial colonies after prior exposure to colchicine or cytochalasin D. Despite the massive reorganization of cytoskeletal morphology induced by TPA, the distribution of prelabeled proteins into structural fractions (i.e., cytoskeletal, chromatin, and the NM-IF) remains essentially unchanged. The sensitivity and specificity of the epithelial cell response suggest its possible use as a screen for promoting compounds.

Role of the cytoskeleton in the formation, stabilization, and removal of acetylcholine receptor clusters in cultured muscle cells

We have examined the effects of microtubule- and microfilament-disrupting drugs on the stability, formation, and removal of acetylcholine (ACh) receptors and ACh receptor clusters on the surface of aneurally cultured chick embryonic myotubes. (a) In muscle cell cultures, cytochalasin D (0.2 microgram/ml) or B (2.0 micrograms/ml) causes the dispersal of 50-60% of the existing clusters over a 24-h period (visualized with rhodamine-conjugated alpha-bungarotoxin); Colcemid (0.5 micrograms/ml) has no affect on these clusters. The total number of cell surface ACh receptors does not decline during this period (measured by [125I]alpha-bungarotoxin binding) in the presence of either drug. (b) When cells are treated with biotinylated alpha-bungarotoxin and fluorescent avidin, ACh receptors are cross-linked and rapidly internalized (Axelrod, D., 1980, Proc. Natl. Acad. Sci. USA., 77: 4823-4827). Within 6 h, I have found that 0-15% of the existing large clusters remain. Cytochalasin D or B had no effect on this removal of clusters; however, Colcemid completely prevented the removal of clusters from the cell surface. (c) Addition of chick brain extract to chick myotubes causes an increase in the synthesis and clustering of ACh receptors (Jessell et al., 1979, Proc. Natl. Acad. Sci. USA. 76: 5397-5401). Cytochalasin D caused a slight increase in the number of receptors synthesized in the presence of brain extract whereas Colcemid had no effect on the synthesis and insertion of new receptors into the plasma membrane induced by the brain extract. However, both drugs prevented the increase in the number of receptor clusters. These results are consistent with the hypothesis that receptor clusters are stabilized by actin-containing filaments, but that the movement of receptors in the plane of the membrane requires Colcemid-sensitive microtubules.

Effects of taxol and Colcemid on myofibrillogenesis

To determine the relationship between thin filaments, Z-bands, microtubules, intermediate filaments (IFs), T-tubules, and sarcoplasmic reticulum (SR) during myofibrillogenesis, myotubes were selectively depleted of their myofibrils with 12-tetradecanoylphorbol 13-acetate (TPA) and then were allowed to regenerate in (i) normal medium, (ii) taxol, and (iii) Colcemid. Myofibrils assembled in normal medium formed typical A-, I-, Z-, M-, and H-bands and associated IFs, T-tubules, and SR. Myofibrils assembled in taxol formed "A-bands" of aligned thick filaments interdigitating with long microtubules and "I-bands" consisting only of microtubules. These unprecedented sarcomeres lacked thin filaments, Z-bands, and associated IFs and SR. "Solitary A-bands," consisting exclusively of laterally aligned bipolar thick filaments 1.6 microM in length without either thin filaments or microtubules, were observed. Myofibrils assembled in Colcemid formed all myofibrillar components in the absence of microtubules but these did not achieve rigorous lateral alignment. Colcemid and taxol induced the formation of patchy Z-bands that invariably served as insertion sites for thin filaments, irrespective of the presence or absence of adjacent thick filaments. Z-bands may function as actin-organizing centers for each sarcomere.

Biological responsiveness to the phorbol esters and specific binding of [3H]phorbol 12,13-dibutyrate in the nematode Caenorhabditis elegans, a manipulable genetic system

Because of its suitability for genetic studies, the nematode Caenorhabditis elegans was examined for its responsiveness to the phorbol esters. Phorbol 12-myristate 13-acetate had three effects. It inhibited the increase in animal size during growth; it decreased the yield of progeny; and it caused uncoordinated movement of the adult. The effects on nematode size, progeny yield, and movement were quantitated. Concentrations of phorbol 12-myristate 13-acetate yielding half-maximal responses were 440, 460, and 170 nM, respectively. As was expected from the biological responsiveness of the nematodes, specific, saturable binding of phorbol ester to nematode extracts was found. [3H]phorbol 12,13-dibutyrate bound with a dissociation constant of 26.8 +/- 3.9 nM. At saturation, 5.7 +/- 1.4 pmole/mg protein was bound.

Tumor-promoting phorbol esters promote melanogenesis and prevent expression of the adrenergic phenotype in quail neural crest cells

Tumor-promoting phorbol esters were used to manipulate the in vitro development of neural crest cells. When plated at clonal density in secondary culture, quail neural crest cells from the trunk region gave rise to three types of colonies, pigmented, unpigmented, and mixed. Pigmented colonies consisted exclusively of melanocytes; up to 50% of the unpigmented mixed colonies contained adrenergic nerve cells which could be identified by a catecholamine-specific histofluorescence method. Addition of potent tumor promoters to the culture medium shortened the doubling time of neural crest cells and altered their morphologic appearance. It also delayed the onset of pigmentation, prevented the expression of the adrenergic phenotype, reduced the number of unpigmented and mixed colonies, and increased the number of pigmented colonies, most likely by directing progenitor cells preferentially to the melanogenic pathway. There was a clear correlation between the ability of phorbol esters to promote skin tumors in mice and their ability to interfere with the in vitro development of quail neural crest cells. The potent promoters 12-0-tetradecanoyl phorbol 13-acetate (TPA) and phorbol 12,13-didecanoate (PDD) were most effective, phorbol 12,13-diacetate (PDA) was considerably less effective, the nonpromoting analogues 4-0-methyl 12-0-tetradecanoyl phorbol 13-acetate (4-0-Me-TPA) and 4 alpha-phorbol 12,13-didecanoate (4alpha-PDD) and the parent alcohol phorbol (PHR) had little or no effect.

Taxol induces postmitotic myoblasts to assemble interdigitating microtubule-myosin arrays that exclude actin filaments

Taxol has the following effects on myogenic cultures: (a) it blocks cell replication of presumptive myoblasts and fibroblasts. (b) It induces the aggregation of microtubules into sheets or massive cables in presumptive myoblasts and fibroblasts, but not in postmitotic, mononucleated myoblasts. (c) It induces normally elongated postmitotic myoblasts to form stubby, star-shaped cells. (d) It reversibly blocks the fusion of the star-shaped myoblasts into multinucleated myotubes. (e) It augments the number of microtubules in postmitotic myoblasts, and these are assembled into interdigitating arrays of microtubules and myosin filaments. (f) Actin filaments are largely excluded from these interdigitating microtubule-myosin complexes. (g) The myosin filaments in the interdigitating microtubule-myosin arrays are aligned laterally, forming A-bands approximately 1.5 micrometers long.