['deregulated', 'c-myc', 'is', 'capable', 'of', 'uncoupling', 'p2/', 'induction', 'from', 'growth', 'arrest', 'normally', 'associated', 'with', 'differentiation,', 'Finally,', 'it', 'is', 'important', 'to', 'point', 'out', 'that', 'Mlcells,', 'like', 'their', 'normal', 'counterparts', 'in', 'vivo,', 'when', 'treated', 'with', 'hematopoietic', 'cytokines', 'undergo', 'differentiation', 'and', 'growth', 'arrest,', 'and,', 'after', '5-7', 'days,', 'apoptosis', '(Selvakumaran', 'ef', 'al.,', '1994a),', 'thereby', 'providing', 'a', 'good', 'model', 'system', 'to', 'molecularly', 'dissect,', 'not', 'only', 'growth', 'arrest,', 'but', 'also', 'apoptosis', 'coupled', 'to', 'differentiation,', 'Following', 'cytokine', 'induced', 'differentiation', 'of', 'Micells', 'in', 'addition', 'to', 'rapid', 'induction', 'of', 'p2/,', 'AfyD1/8', 'and', 'MyDJIJ6', ',', 'implicated', 'in', 'growth', 'arrest', 'and', 'apoptosis', '(Hoffman', '&', 'Liebermann,', '1994),', 'are', 'induced', 'as', 'primary', 'response', 'genes', '(Lord', 'et', 'al.,', '1990a;', 'Abdollahi', 'ef', 'al,', '1991);', 'MyDII8', 'expression', 'declines', 'after', '1', 'day,', 'whereas', 'AfyD!16', 'expression', 'is', 'cyclic', '(Abdollahi', 'et', 'ai,', '1991a;', 'Zhan', 'et', 'al.,', '1994),', 'Expression', 'of', 'Gadd-+5,', 'a', 'member', 'in', 'the', 'MyD118', 'gene', 'family', 'and', 'another', 'p53', 'target', 'gene', 'implicated', 'in', 'growth', 'arrest,', 'was', 'detected', 'only', 'at', 'fate', 'times', '(Zhan', 'ef', 'al,', '1994).', 'In', 'sharp', 'contrast,', 'expression', 'af', 'CR6,', 'the', 'third', 'known', 'member', 'in', 'the', 'MyD118', 'gene', 'family,', 'was', 'high', 'in', 'untreated', 'proliferating', 'Mland', 'M1p53,', 'and', 'to', 'rapidly', 'decline', 'following', 'induction', 'of', 'differentiation', '(unpublished).', 'It', 'was', 'also', 'observed,', 'that', 'expression', 'of', 'Bel-2', 'and', 'bax,', 'was', 'rapidly', 'suppressed', 'upon', 'induction', 'of', 'M1', 'differentiation.', '(Selvakumaran', 'et', 'al.,', '1993).', 'The', 'roles', 'any', 'one,', 'or', 'combinations,', 'of', 'these', 'gene', 'products', 'play', 'in', 'growth', 'arrest', 'and', 'apoptosis', 'which', 'are', 'coupled', 'to', 'differentiation,', 'is', 'being', 'investigated', 'using', 'M1', 'cells', 'as', 'a', 'model', 'system.', 'Growth-arrest', 'and', 'apoptosis', 'uncoupled', 'from', 'terminal', 'differentiation.', 'The', 'ability', 'to', 'induce', 'growth', 'arrest', 'and', 'apoptosis', 'by', 'either', 'wt', 'p53', 'or', 'TGF-B1', 'in', 'M1', 'cells,', 'and', 'the', 'availability', 'of', 'genetically', 'engineered', 'M1', 'variants,', '(Selvakumaran', 'et', 'al.,', '1994;', 'Selvakumaran', 'et', 'ai,', '1994b).', 'made', 'it', 'possible', 'to', 'compare', 'and', 'contrast,', 'within', 'the', 'same', 'cell', 'type,', 'p53-', 'dependent', 'and', 'independent', 'growth-arrest', 'and', 'apoptotic', 'pathways', 'uncoupled', 'from', 'differentiation.', 'As', 'shown', 'by', 'Selvakumaran', 'et', 'ai,', '(1994b),', 'the', 'kinetics', 'of', 'apoptosis', 'of', 'M1', 'cells', 'induced', 'by', 'p53', 'were', 'more', 'rapid', 'than', 'apoptosis', 'induced', 'by', 'TGFB1.', 'Elevated', 'expression', 'of', 'bcl-2', 'blocks', 'the', 'TGFBl-induced', 'apoptotic', 'pathway;', 'however,', 'ectopic', 'expression', 'of', 'bel-2', 'at', 'levels', 'which', 'block', 'TGFB', 'l-induced', 'apoptosis', 'of', 'M1', 'cells,', 'delayed,', 'but', 'did', 'not', 'block,', 'p53-', 'induced', 'apoptosis.', 'Both', 'p53', 'and', 'TGFB1', 'down-regulate', 'endogenous', 'bel-2', 'expression,', 'but', 'only', 'p53', 'up-regulates', 'bax', 'expression,', 'bax', 'being', 'identified', 'as', 'a', 'p53-immediate', 'early', 'response', 'gene,', 'Thus,', 'the', 'p53-mediated', 'up-regulation', 'of', 'bax', 'provides', 'at', 'least', 'a', 'partial', 'explanation', 'for', 'the', 'more', 'rapid', 'rate', 'of', 'apoptosis', 'induced', 'by', 'p53', 'compared', 'to', 'by', 'TGFB1,', 'as', 'well', 'as', 'for', 'the', 'ineffectiveness', 'of', 'ectopic', 'bel-2', 'to', 'abrogate', 'p53-mediated', 'apoptosis.', 'Following', 'stimulation', 'of', 'M1', 'cells', 'with', 'TGFB1', 'has', 'revealed', 'that', 'AfyD/J76', 'andgadd45', 'were', 'not', 'expressed;', 'however,', 'MfyDIJ8', 'was', 'rapidly', 'induced.', 'MyDJ18', 'also', 'was', 'expressed', 'when', 'MI', 'cells', 'were', 'concomitantly', 'treated', 'with', 'TGF', 'l', 'and', 'cycloheximide,', 'indicating', 'that', 'MyDJJ&', 'is', 'a', 'TGFBI', 'primary', 'response', 'gene.', 'It', 'is', 'of', 'interest', 'to', 'point', 'out', 'that', 'AfyD218', 'was', 'observed', 'to', 'be', 'a', 'TGFBI', 'primary', 'response', 'gene', 'in', 'other', 'cell', 'types', 'as', 'well,', 'including', 'the', 'lymphoid', 'cell', 'line', 'EL4', 'and', 'the', 'mink', 'lung', 'epithelial', 'cell', 'line', 'MvtLu', '(Selvakumaran', 'ef', 'ai,', '1994),', 'Using', 'M1', 'cell', ' ', 'Growth', 'arrest/apoptosis:', 'p53-dependent/independent', 'DA', 'Liebermann', 'et', 'al', ' ', ' ', 'lines', 'which', 'express', 'antisense', '(AS)', 'MyDJJ8', 'transcripts', 'at', 'elevated', 'levels,', 'thereby', 'blocking', 'the', 'early', 'expression', 'of', 'the', 'MyDII8', '.', 'gene', 'product,', 'it', 'was', 'demonstrated', 'that', 'TGFB', '1', 'induced', 'apoptosis', 'was', 'delayed,', 'indicating', 'that', 'MyD2J8', 'is', 'a', 'positive', 'modulator', 'of', 'TGFBimediated', 'cell', 'death,', 'Interestingly,', 'failure', 'to', 'exprtss', 'MyD/18', 'at', 'early', 'times', 'was', 'observed', 'to', 'have’', 'no', 'effect', 'on', 'TGFBi-induced', 'growth', 'arrest,', 'suggesting', 'that', 'TGFB1-induced', 'growth', 'arrest', 'and', 'apoptosis', 'are', 'distinctly', 'regulated.', 'In', 'further', 'support', 'of', 'a', 'tole', 'for', 'MyD1]8', 'in', 'TGF', 'l-mediated', 'apoptosis', 'is', 'the', 'observation', 'that', 'there', 'is', 'eleyated', 'and', 'prolonged', 'expression', 'of', 'MyD/18', 'both', 'in', 'the', 'Mimyc', 'and', 'Mlmyb', 'cells', 'following', 'treatment', 'with', 'TGFB1,', 'where', 'these', 'cell', 'variants', 'undergo', 'accelerated', 'apoptosis', 'relative', 'to', 'parental', 'M1', 'cells,', 'with', 'highest', 'expression', 'in', 'the', 'Mimyb', 'cells', '(Selvakumaran', 'et', 'al.,', '1994),', 'Interestingly,', 'MyD//8,', 'shown', 'to', 'play', 'a', 'role', 'in', 'TGF', '1-induced', 'apoptosis,', 'was', 'found', '‘not', 'to', 'be', 'induced', 'upon', 'p53', 'mediated', 'apoptosis.', 'Recently,', 'the', 'Cdk', 'inhibitors', 'piSiXK4#', 'and', 'p27', 'have', 'been', 'implicated', 'in', 'the', 'TGF-B-', 'induced', 'growth', 'arrest', 'pathway', '(Polyak', 'etal.,', '1994;', 'Toyoshima', '&', 'Hunter,', '1994;', 'Hannon', '&', 'Beach,', '1994).', 'Whether', 'they', 'may', 'play', 'arole', 'in', 'TGFB', '1', 'induced', 'apoptosis,', 'remains', 'to', 'be', 'determined.', 'M1', 'cells', 'treated', 'with', 'hematopoietic', 'cytokines', 'undergo', 'differentiation', 'and', 'growth', 'arrest,', 'and,', 'only', 'after', '5-7', 'days,', 'apoptosis', '(Selvakumaran', 'et', 'al.,', '1994a),', 'It', 'was', 'of', 'interest,', 'therefore,', 'to', 'investigate', 'the', 'effects', 'of', 'concomitantly', 'treating', 'cells', 'with', 'IL-6', 'and', 'TGF-B1', 'to', 'ascertain', 'which', 'stimulus', 'is', 'dominant,', 'and', 'what', 'happens', 'to', 'the', 'players', 'regulating', 'differentiation,', 'growth', 'arrest', 'and', 'apoptosis.', 'It', 'was', 'shown', 'that', 'IL-6', 'overrides', 'the', 'rapid', 'loss', 'of', 'viability', 'and', 'apoptosis', 'associated', 'with', 'TGF-B1', 'treatment', 'of', 'M1', 'cells,', 'undergoing', 'terminal', 'differentiation', 'and', 'growth', 'arrest,', 'comparable', 'to', 'IL-6', 'treatment', 'alone.', 'The', 'expression', 'of', 'MyDII8', 'is', 'elevated', 'in', 'M1', 'cells', 'concomitantly', 'treated', 'with', 'IL-6', 'and', 'TGF-Bicompared', 'to', 'cells', 'treated', 'with', 'either', 'inducer', 'alone.', 'Since', 'treatment', 'with', 'both', 'IL-6', 'and', 'TGF-B1', 'results', 'in', 'a', 'delay', 'in', 'apoptosis', 'until', 'after', 'the', 'cells', 'become', 'differentiated,', 'in', 'spite', 'of', 'the', 'elevated', 'levels', 'of', 'MyDI18,', 'other', 'modulators', 'of', 'apoptosis,', 'either', 'positive', 'or', 'negative,', 'must', 'be', 'present', 'to', 'determine', 'the', 'final', 'outcome.', 'One', 'obvious', 'modulator', 'whose', 'overexpression', 'was', 'shown', 'to', 'block', 'TGF-8', '1-induced', 'apoptosis', 'is', 'bcl-2.', 'Interestingly,', 'Bel-2', 'protein', 'was', 'found', 'to', 'be', 'down', 'regulated', 'more', 'rapidly', 'and', 'completely', 'by', 'IL-6', 'than', 'by', 'TGP-81,', 'and', 'combined', 'treatment', 'results', 'in', 'a', 'response', 'similar', 'to', 'IL-6.', 'Thus,', 'modulators', 'other', 'than', 'dci-2', 'must', 'work', 'in', 'conjunction', 'with', 'MyD118.', 'MyD116,', 'a', 'differentiation', 'primary', 'response', 'gene', 'is', 'a', 'potential', 'candidate,', 'both', 'by', 'preliminary', 'antisense', 'experiments', 'and', 'expression', 'kinetics.', 'That', 'MyDJJ6', 'can', 'participate', 'in', 'promoting', 'céll', 'survival', 'is', 'supported', 'by', 'the', 'recently', 'reported', 'striking', 'homology', 'between', 'part', 'of', 'the', 'protein', 'sequence', 'encoded', 'by', 'the', 'MyD/I6', 'cDNA', 'and', 'the', 'sequence', 'of', 'a', 'herpes', 'simplex', 'virus', '(HSV)', 'protein', '134.5,', 'shown', 'to', 'be', 'a', 'determinant', 'of', 'viral', 'virulence', '(McGeoh', 'et', '..', 'al,,', '1991', '&', 'ref.', 'therein),', 'where', 'virulence', 'is', 'believed', 'to', 'be', 'mediated', 'via', 'increased', 'cell', 'survival', '(Chou', '&', 'Roizmann,', '1992),', 'The', 'function', 'of', 'y)34.5,', 'which', 'blocks', 'the', 'apoptotic', 'NO', 'response', 'of', 'the', 'host', 'to', 'infection,', 'was', 'observed', 'to', 'map', 'in', '&', 'the', 'homologous', 'domain', 'of', 'MyD116/Gadd34', '(Chou', '&', '=', 'Roizman,', '1994).', 'IL-6', 'was', 'observed', 'also', 'to', 'override', 'p53', 'induced', 'apoptosis', 'of', 'M1', 'cells', '(Selvakumaran', 'er', 'al.,', '1994a;', 'OO', 'Yonish-Rouach', 'etal.,', '1991}.', 'Thus', 'using', 'M1', 'cells', 'as', 'model', 'OI', 'system,', 'it', 'is', 'now', 'possible', 'to', 'investigate,', 'at', 'the', 'molecular', '&', 'level,', 'how', 'the', 'induction', 'of', 'differentiation', 'may', 'override', ' ', ' ']