S. Adamo | Università degli Studi "La Sapienza" di Roma (original) (raw)
Papers by S. Adamo
The tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) dramatically modifies the different... more The tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) dramatically modifies the differentiative program of myotubes, developed in culture from chick embryo skeletal myogenic cells. In fact TPA selectively decreases the expression of differentiative parameters with a lag of 8-10 h from its administration to the cultures. We have tested whether the reported effect of TPA depends on the synthesis of specific products during the lag phase of TPA action. The data presented indicate that inhibition of protein synthesis by the use of cycloheximide prevents the appearance of TPA induced inhibition of the expression of differentiative products, such as creatine phosphokinase (CPK) activity and acetylcholine receptors (AChR). Following removal of cycloheximide and reinitiation of normal protein synthesis, the TPA induced inhibitory effect on CPK and AChR appears after a delay of about the same length as the time lag of TPA action. Our results indicate that inhibition of protein synthesis during the lag phase of TPA action prevents the effect of this tumor promoter on myotube differentiative parameters, and suggest that the expression of differentiative traits in cultured myotubes is affected by TPA via a regulatory step implying protein synthesis.
The tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) dramatically modifies the different... more The tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) dramatically modifies the differentiative program of myotubes, developed in culture from chick embryo skeletal myogenic cells. In fact TPA selectively decreases the expression of differentiative parameters with a lag of 8-10 h from its administration to the cultures. We have tested whether the reported effect of TPA depends on the synthesis of specific products during the lag phase of TPA action. The data presented indicate that inhibition of protein synthesis by the use of cycloheximide prevents the appearance of TPA induced inhibition of the expression of differentiative products, such as creatine phosphokinase (CPK) activity and acetylcholine receptors (AChR). Following removal of cycloheximide and reinitiation of normal protein synthesis, the TPA induced inhibitory effect on CPK and AChR appears after a delay of about the same length as the time lag of TPA action. Our results indicate that inhibition of protein synthesis during the lag phase of TPA action prevents the effect of this tumor promoter on myotube differentiative parameters, and suggest that the expression of differentiative traits in cultured myotubes is affected by TPA via a regulatory step implying protein synthesis.
Methods in enzymology, 1990
ABSTRACT
Cell Biology International Reports
Blood, 1997
The bcr1- and bcr3- promyelocytic leukemia/retinoic acid receptor alpha (PML/RAR alpha) are the t... more The bcr1- and bcr3- promyelocytic leukemia/retinoic acid receptor alpha (PML/RAR alpha) are the two major fusion proteins expressed in acute promyelocytic leukemia (APL) patients. These proteins, which are present in different lengths of PML (amino acids 1-552 and 1-394, respectively), contain most of the functional domains of PML and RAR alpha, bind all-trans-retinoic acid (t-RA), and act as t-RA-dependent transcription factors. T-RA is an effective inducer of clinical remission only in patients carrying the t(15;17) and expressing the PML/RAR alpha products. However, in APL patients achieving complete remission with t-RA therapy the bcr3-PML/RAR alpha product has been found associated with a poorer prognosis than bcr1-PML/RAR alpha. In the present study we have investigated the structural and functional properties of the bcr3-PML/RAR alpha in comparison to the previously characterized bcr1-PML/RAR alpha. In particular, we have measured the binding properties of the two endogenous ...
European journal of histochemistry : EJH, 2007
One of the most exciting aspirations of current medical science is the regeneration of damaged bo... more One of the most exciting aspirations of current medical science is the regeneration of damaged body parts. The capacity of adult tissues to regenerate in response to injury stimuli represents an important homeostatic process that until recently was thought to be limited in mammals to tissues with high turnover such as blood and skin. However, it is now generally accepted that each tissue type, even those considered post-mitotic, such as nerve or muscle, contains a reserve of undifferentiated progenitor cells, loosely termed stem cells, participating in tissue regeneration and repair. Skeletal muscle regeneration is a coordinate process in which several factors are sequentially activated to maintain and preserve muscle structure and function upon injury stimuli. In this review, we will discuss the role of stem cells in muscle regeneration and repair and the critical role of specific factors, such as IGF-1, vasopressin and TNF-alpha, in the modulation of the myogenic program and in th...
Cell growth & differentiation : the molecular biology journal of the American Association for Cancer Research, 1998
Terminal differentiation of myogenic cells has long been known to be positively regulated by insu... more Terminal differentiation of myogenic cells has long been known to be positively regulated by insulin-like growth factors (IGFs). Arg8-vasopressin (AVP) has been recently reported to potently induce myogenic differentiation. In the present study, the effects and the mechanisms of action of AVP and IGFs on myogenic cells have been investigated under conditions allowing growth and differentiation of myogenic cells in a simple serum-free medium. Under these conditions, L6 and L5 myogenic cells slowly proliferate and do not undergo differentiation (less than 1% fusion up to 7 days). AVP rapidly (2-3 days) and dose-dependently induces the formation of multinucleated myotubes. Creatine kinase activity and myosin accumulation are strongly up-regulated by AVP. Insulin or IGF-I or IGF-II, at concentrations that cause extensive differentiation in serum-containing medium, induces a modest degree of differentiation in serum-free medium. The simultaneous presence of AVP and of one of the IGFs in ...
Blood, 1996
All-trans retinoic acid (t-RA) administration leads to complete remission in acute promyelocytic ... more All-trans retinoic acid (t-RA) administration leads to complete remission in acute promyelocytic leukemia (APL) patients by inducing growth arrest and differentiation of the leukemic clone. In the present study, we show that t-RA treatment dramatically induced type II transglutaminase (type II TGase) expression in cells carrying the t(15;17) translocation and expressing the PML-RARalpha product such as the APL-derived NB4 cell line and fresh leukemic cells from APL patients. This induction correlated with t-RA-induced growth arrest, granulocytic differentiation, and upregulation of the leukocyte adherence receptor beta subunit (CD18) gene expression. The increase in type II TGase was not abolished by cycloheximide treatment, suggesting that synthesis of a protein intermediate was not required for the induction. t-RA did not significantly alter the rate of growth arrest and did not stimulate differentiation and type II TGase activity in NB4.306 cells, a t-RA-resistant subclone of the...
The American journal of physiology, 1993
Arginine vasopressin (AVP) induced concentration-dependent (10(-9) to 10(-6) M) stimulation of in... more Arginine vasopressin (AVP) induced concentration-dependent (10(-9) to 10(-6) M) stimulation of inositol phosphate production and a biphasic increment of cytosolic free Ca2+ concentration ([Ca2+]i) in skeletal myogenic cells in culture. These effects were almost completely abolished when the cells were pretreated with the AVP antagonist [deamino-Pen1,Val4,D-Arg8]-vasopressin before stimulation with AVP, thus confirming a V1 receptor-mediated effect. Inositol 1,4,5-trisphosphate production was maximally stimulated within 2-3 s of treatment with AVP, immediately followed by release of Ca2+ from intracellular deposits. Both effects were inhibited by treatment with 12-O-tetradecanoyl phorbol 13-acetate (TPA). Such effect of TPA was reversed by the protein kinase C inhibitor staurosporine. Vasopressin also regulated the intracellular pH of responsive cells with mechanisms involving both Na+ and anion transport across the plasma membrane. However, unlike in other cell types, AVP stimulated...
Cell growth & differentiation : the molecular biology journal of the American Association for Cancer Research, 1995
Human rhabdomyosarcoma RD cells express the myogenic regulatory factors MyoD and myogenin but dif... more Human rhabdomyosarcoma RD cells express the myogenic regulatory factors MyoD and myogenin but differentiate spontaneously very poorly. Prolonged treatment of RD cells with the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate (TPA) induces growth arrest and myogenic differentiation as shown by the accumulation of alpha-actin and myosin light and heavy chains, without affecting the expression of MyoD and myogenin. In this study, we show that short-term phorbol ester treatment of the cultures is sufficient to trigger myogenic differentiation but not growth arrest. Furthermore, PKC inhibitors, such as staurosporine or calphostin C, prevent TPA-induced differentiation but not cell growth arrest. These data suggest that the two events are mediated by different pathways; a possible interpretation is that the activation of one or more PKC isoforms mediates the induction of differentiation, whereas the down-regulation of the same or different isoforms mediates the growth...
Acta vitaminologica et enzymologica, 1983
Transformed cells (Balb/c, 3T12-3), induced to increase their adhesion to the substrate by treatm... more Transformed cells (Balb/c, 3T12-3), induced to increase their adhesion to the substrate by treatment with retinoic acid, display higher incorporation of (2-(3)H)-mannose into both lipids and glycoproteins than untreated controls. Stimulation of (2-(3)H)-mannose incorporation into manno-lipids is evident 8 hr after exposing the cells to retinoic acid, and stimulation of tritiated mannose incorporation into glycoproteins occurs slightly later. SDS-PAGE of (2-(3)H)-mannose labelled glycoproteins indicates that both retinoic acid and retinol treatments stimulate the incorporation of the radiolabelled sugar into a glycoprotein with subunit MW 180,000 (Gp 180) and, to a lesser extent, into other glycoproteins. 3H-leucine incorporation into a protein banding at the same position as the 3H-mannose labelled Gp 180 does not appear to be affected by retinoid treatment. A retinoic acid induced increase in the amount of Gp 180 can also be shown by lactoperoxidase catalyzed radioiodination of cul...
The Italian journal of biochemistry
The Journal of biological chemistry, Jan 10, 1979
Fusion of myoblasts is inhibited in cultures at low Ca 4+ concentration (0.44 raM); yet creatine ... more Fusion of myoblasts is inhibited in cultures at low Ca 4+ concentration (0.44 raM); yet creatine phosphokinase and myokinase activities as well as myosin synthesis and the appearance of post-mitotic myoblasts do not significantly differ from those of control cultures (grown at 1.04 mM Ca 4+) which undergo cell fusion. When Ca ~+ concentration is increased to the control value after the second day of culture, fusion occurs very rapidly and it is not inhibited by actinomycin D or cycloheximide.
Inhibition of type 4 cAMP-specific phosphodiesterase (PDE4) activity in L6-C5 and L6-E9 abolished... more Inhibition of type 4 cAMP-specific phosphodiesterase (PDE4) activity in L6-C5 and L6-E9 abolished myogenic differentiation induced by low-serum medium and IGF-I. L6-C5 cells cultured in low-serum medium displayed a PDE4 activity higher than cells cultured in serum-free medium, a condition not sufficient to induce differentiation. In the presence of serum, PDE4D3, the major isoform natively expressed in L6-C5 cells, translocated to a Triton-insoluble fraction, which increased the PDE specific activity of the fraction, and exhibited a Mr shift typical of phosphorylation of this isoform. Furthermore, serum promoted the localization of PDE4D3 to a vesicular subcellular compartment. In L6-C5 cells, IGF-I is a stronger inducer of myogenic differentiation in the presence than in absence of serum. Its ability to trigger differentiation in the absence of serum was restored by overexpressing wild-type PDE4D3, but not a phosphorylation-insensitive mutant. This finding was confirmed in single cells overexpressing a GFP-PDE4D3 fusion protein by assessing nuclear accumulation of myogenin in both L6-C5 and L6-E9. Overexpression of other PDE isoforms was less efficient, confirming that PDE4D3 is the physiologically relevant phosphodiesterase isoform in the control of myogenesis. These results show that downregulation of cAMP signaling through cAMP-phosphodiesterase stimulation is a prerequisite for induction of myogenesis.
The Journal of Cell Biology, 1989
The activity of calcium-, phospholipid-dependent protein kinase (PKc) was measured in (a) total e... more The activity of calcium-, phospholipid-dependent protein kinase (PKc) was measured in (a) total extracts, (b) crude membrane, and (c) cytosolic fractions of chick embryo myogenic cells differentiating in culture. Total PKc activity slowly declines during the course of terminal myogenesis in contrast to the activity of cAMP-dependent protein kinase, which was also measured in the same cells. Myogenic cells at day 1 of culture possess high particulate and low soluble PKc activity. A dramatic decline of particulate PKc activity occurs during myogenic cell differentiation and is accompanied, through day 4, by a striking rise of the soluble activity. The difference in the subcellular distribution of PKc between replicating myoblasts and myotubes is confirmed by phosphorylation studies conducted in intact cells. These studies demonstrate that four polypeptides whose phosphorylation is stimulated by the tumor promoter 12-O-tetradecanoyl phorbol 13-acetate in myotubes, are spontaneously pho...
The tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) dramatically modifies the different... more The tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) dramatically modifies the differentiative program of myotubes, developed in culture from chick embryo skeletal myogenic cells. In fact TPA selectively decreases the expression of differentiative parameters with a lag of 8-10 h from its administration to the cultures. We have tested whether the reported effect of TPA depends on the synthesis of specific products during the lag phase of TPA action. The data presented indicate that inhibition of protein synthesis by the use of cycloheximide prevents the appearance of TPA induced inhibition of the expression of differentiative products, such as creatine phosphokinase (CPK) activity and acetylcholine receptors (AChR). Following removal of cycloheximide and reinitiation of normal protein synthesis, the TPA induced inhibitory effect on CPK and AChR appears after a delay of about the same length as the time lag of TPA action. Our results indicate that inhibition of protein synthesis during the lag phase of TPA action prevents the effect of this tumor promoter on myotube differentiative parameters, and suggest that the expression of differentiative traits in cultured myotubes is affected by TPA via a regulatory step implying protein synthesis.
The tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) dramatically modifies the different... more The tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) dramatically modifies the differentiative program of myotubes, developed in culture from chick embryo skeletal myogenic cells. In fact TPA selectively decreases the expression of differentiative parameters with a lag of 8-10 h from its administration to the cultures. We have tested whether the reported effect of TPA depends on the synthesis of specific products during the lag phase of TPA action. The data presented indicate that inhibition of protein synthesis by the use of cycloheximide prevents the appearance of TPA induced inhibition of the expression of differentiative products, such as creatine phosphokinase (CPK) activity and acetylcholine receptors (AChR). Following removal of cycloheximide and reinitiation of normal protein synthesis, the TPA induced inhibitory effect on CPK and AChR appears after a delay of about the same length as the time lag of TPA action. Our results indicate that inhibition of protein synthesis during the lag phase of TPA action prevents the effect of this tumor promoter on myotube differentiative parameters, and suggest that the expression of differentiative traits in cultured myotubes is affected by TPA via a regulatory step implying protein synthesis.
Methods in enzymology, 1990
ABSTRACT
Cell Biology International Reports
Blood, 1997
The bcr1- and bcr3- promyelocytic leukemia/retinoic acid receptor alpha (PML/RAR alpha) are the t... more The bcr1- and bcr3- promyelocytic leukemia/retinoic acid receptor alpha (PML/RAR alpha) are the two major fusion proteins expressed in acute promyelocytic leukemia (APL) patients. These proteins, which are present in different lengths of PML (amino acids 1-552 and 1-394, respectively), contain most of the functional domains of PML and RAR alpha, bind all-trans-retinoic acid (t-RA), and act as t-RA-dependent transcription factors. T-RA is an effective inducer of clinical remission only in patients carrying the t(15;17) and expressing the PML/RAR alpha products. However, in APL patients achieving complete remission with t-RA therapy the bcr3-PML/RAR alpha product has been found associated with a poorer prognosis than bcr1-PML/RAR alpha. In the present study we have investigated the structural and functional properties of the bcr3-PML/RAR alpha in comparison to the previously characterized bcr1-PML/RAR alpha. In particular, we have measured the binding properties of the two endogenous ...
European journal of histochemistry : EJH, 2007
One of the most exciting aspirations of current medical science is the regeneration of damaged bo... more One of the most exciting aspirations of current medical science is the regeneration of damaged body parts. The capacity of adult tissues to regenerate in response to injury stimuli represents an important homeostatic process that until recently was thought to be limited in mammals to tissues with high turnover such as blood and skin. However, it is now generally accepted that each tissue type, even those considered post-mitotic, such as nerve or muscle, contains a reserve of undifferentiated progenitor cells, loosely termed stem cells, participating in tissue regeneration and repair. Skeletal muscle regeneration is a coordinate process in which several factors are sequentially activated to maintain and preserve muscle structure and function upon injury stimuli. In this review, we will discuss the role of stem cells in muscle regeneration and repair and the critical role of specific factors, such as IGF-1, vasopressin and TNF-alpha, in the modulation of the myogenic program and in th...
Cell growth & differentiation : the molecular biology journal of the American Association for Cancer Research, 1998
Terminal differentiation of myogenic cells has long been known to be positively regulated by insu... more Terminal differentiation of myogenic cells has long been known to be positively regulated by insulin-like growth factors (IGFs). Arg8-vasopressin (AVP) has been recently reported to potently induce myogenic differentiation. In the present study, the effects and the mechanisms of action of AVP and IGFs on myogenic cells have been investigated under conditions allowing growth and differentiation of myogenic cells in a simple serum-free medium. Under these conditions, L6 and L5 myogenic cells slowly proliferate and do not undergo differentiation (less than 1% fusion up to 7 days). AVP rapidly (2-3 days) and dose-dependently induces the formation of multinucleated myotubes. Creatine kinase activity and myosin accumulation are strongly up-regulated by AVP. Insulin or IGF-I or IGF-II, at concentrations that cause extensive differentiation in serum-containing medium, induces a modest degree of differentiation in serum-free medium. The simultaneous presence of AVP and of one of the IGFs in ...
Blood, 1996
All-trans retinoic acid (t-RA) administration leads to complete remission in acute promyelocytic ... more All-trans retinoic acid (t-RA) administration leads to complete remission in acute promyelocytic leukemia (APL) patients by inducing growth arrest and differentiation of the leukemic clone. In the present study, we show that t-RA treatment dramatically induced type II transglutaminase (type II TGase) expression in cells carrying the t(15;17) translocation and expressing the PML-RARalpha product such as the APL-derived NB4 cell line and fresh leukemic cells from APL patients. This induction correlated with t-RA-induced growth arrest, granulocytic differentiation, and upregulation of the leukocyte adherence receptor beta subunit (CD18) gene expression. The increase in type II TGase was not abolished by cycloheximide treatment, suggesting that synthesis of a protein intermediate was not required for the induction. t-RA did not significantly alter the rate of growth arrest and did not stimulate differentiation and type II TGase activity in NB4.306 cells, a t-RA-resistant subclone of the...
The American journal of physiology, 1993
Arginine vasopressin (AVP) induced concentration-dependent (10(-9) to 10(-6) M) stimulation of in... more Arginine vasopressin (AVP) induced concentration-dependent (10(-9) to 10(-6) M) stimulation of inositol phosphate production and a biphasic increment of cytosolic free Ca2+ concentration ([Ca2+]i) in skeletal myogenic cells in culture. These effects were almost completely abolished when the cells were pretreated with the AVP antagonist [deamino-Pen1,Val4,D-Arg8]-vasopressin before stimulation with AVP, thus confirming a V1 receptor-mediated effect. Inositol 1,4,5-trisphosphate production was maximally stimulated within 2-3 s of treatment with AVP, immediately followed by release of Ca2+ from intracellular deposits. Both effects were inhibited by treatment with 12-O-tetradecanoyl phorbol 13-acetate (TPA). Such effect of TPA was reversed by the protein kinase C inhibitor staurosporine. Vasopressin also regulated the intracellular pH of responsive cells with mechanisms involving both Na+ and anion transport across the plasma membrane. However, unlike in other cell types, AVP stimulated...
Cell growth & differentiation : the molecular biology journal of the American Association for Cancer Research, 1995
Human rhabdomyosarcoma RD cells express the myogenic regulatory factors MyoD and myogenin but dif... more Human rhabdomyosarcoma RD cells express the myogenic regulatory factors MyoD and myogenin but differentiate spontaneously very poorly. Prolonged treatment of RD cells with the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate (TPA) induces growth arrest and myogenic differentiation as shown by the accumulation of alpha-actin and myosin light and heavy chains, without affecting the expression of MyoD and myogenin. In this study, we show that short-term phorbol ester treatment of the cultures is sufficient to trigger myogenic differentiation but not growth arrest. Furthermore, PKC inhibitors, such as staurosporine or calphostin C, prevent TPA-induced differentiation but not cell growth arrest. These data suggest that the two events are mediated by different pathways; a possible interpretation is that the activation of one or more PKC isoforms mediates the induction of differentiation, whereas the down-regulation of the same or different isoforms mediates the growth...
Acta vitaminologica et enzymologica, 1983
Transformed cells (Balb/c, 3T12-3), induced to increase their adhesion to the substrate by treatm... more Transformed cells (Balb/c, 3T12-3), induced to increase their adhesion to the substrate by treatment with retinoic acid, display higher incorporation of (2-(3)H)-mannose into both lipids and glycoproteins than untreated controls. Stimulation of (2-(3)H)-mannose incorporation into manno-lipids is evident 8 hr after exposing the cells to retinoic acid, and stimulation of tritiated mannose incorporation into glycoproteins occurs slightly later. SDS-PAGE of (2-(3)H)-mannose labelled glycoproteins indicates that both retinoic acid and retinol treatments stimulate the incorporation of the radiolabelled sugar into a glycoprotein with subunit MW 180,000 (Gp 180) and, to a lesser extent, into other glycoproteins. 3H-leucine incorporation into a protein banding at the same position as the 3H-mannose labelled Gp 180 does not appear to be affected by retinoid treatment. A retinoic acid induced increase in the amount of Gp 180 can also be shown by lactoperoxidase catalyzed radioiodination of cul...
The Italian journal of biochemistry
The Journal of biological chemistry, Jan 10, 1979
Fusion of myoblasts is inhibited in cultures at low Ca 4+ concentration (0.44 raM); yet creatine ... more Fusion of myoblasts is inhibited in cultures at low Ca 4+ concentration (0.44 raM); yet creatine phosphokinase and myokinase activities as well as myosin synthesis and the appearance of post-mitotic myoblasts do not significantly differ from those of control cultures (grown at 1.04 mM Ca 4+) which undergo cell fusion. When Ca ~+ concentration is increased to the control value after the second day of culture, fusion occurs very rapidly and it is not inhibited by actinomycin D or cycloheximide.
Inhibition of type 4 cAMP-specific phosphodiesterase (PDE4) activity in L6-C5 and L6-E9 abolished... more Inhibition of type 4 cAMP-specific phosphodiesterase (PDE4) activity in L6-C5 and L6-E9 abolished myogenic differentiation induced by low-serum medium and IGF-I. L6-C5 cells cultured in low-serum medium displayed a PDE4 activity higher than cells cultured in serum-free medium, a condition not sufficient to induce differentiation. In the presence of serum, PDE4D3, the major isoform natively expressed in L6-C5 cells, translocated to a Triton-insoluble fraction, which increased the PDE specific activity of the fraction, and exhibited a Mr shift typical of phosphorylation of this isoform. Furthermore, serum promoted the localization of PDE4D3 to a vesicular subcellular compartment. In L6-C5 cells, IGF-I is a stronger inducer of myogenic differentiation in the presence than in absence of serum. Its ability to trigger differentiation in the absence of serum was restored by overexpressing wild-type PDE4D3, but not a phosphorylation-insensitive mutant. This finding was confirmed in single cells overexpressing a GFP-PDE4D3 fusion protein by assessing nuclear accumulation of myogenin in both L6-C5 and L6-E9. Overexpression of other PDE isoforms was less efficient, confirming that PDE4D3 is the physiologically relevant phosphodiesterase isoform in the control of myogenesis. These results show that downregulation of cAMP signaling through cAMP-phosphodiesterase stimulation is a prerequisite for induction of myogenesis.
The Journal of Cell Biology, 1989
The activity of calcium-, phospholipid-dependent protein kinase (PKc) was measured in (a) total e... more The activity of calcium-, phospholipid-dependent protein kinase (PKc) was measured in (a) total extracts, (b) crude membrane, and (c) cytosolic fractions of chick embryo myogenic cells differentiating in culture. Total PKc activity slowly declines during the course of terminal myogenesis in contrast to the activity of cAMP-dependent protein kinase, which was also measured in the same cells. Myogenic cells at day 1 of culture possess high particulate and low soluble PKc activity. A dramatic decline of particulate PKc activity occurs during myogenic cell differentiation and is accompanied, through day 4, by a striking rise of the soluble activity. The difference in the subcellular distribution of PKc between replicating myoblasts and myotubes is confirmed by phosphorylation studies conducted in intact cells. These studies demonstrate that four polypeptides whose phosphorylation is stimulated by the tumor promoter 12-O-tetradecanoyl phorbol 13-acetate in myotubes, are spontaneously pho...