Bio Sci 145 A Lecture 18 Oncogenes and

Bio. Sci 145 A Lecture 18 - Oncogenes and Cancer • Topics we will cover today – Oncogenes and cell growth – tumor suppressor genes – Important examples of transcriptional regulation • Regulation of xenobiotic metabolism • Last year’s final exam is posted • No lecture on 3/12 • We will review last year’s final exam and I will answer any and all questions on 3/14 • evaluations will be done on 3/14 as well so please come and give us your candid assessment of the course

Oncogenes and cell growth • Seven classes of proteins control cell growth – Collectively, these genes comprise the known set of genes involved in tumor formation Bio. Sci 145 A lecture 18 page 2 ©copyright Bruce Blumberg 2000. All rights reserved

Oncogenes and cell growth (contd) • Bio. Sci 145 A lecture 18 page 3 ©copyright Dominant transforming oncogenes are frequently created from proteins involved in regulating cell growth – Growth factors – Growth factor receptors – Intracellular transducers of above – Transcription factors that mediate the terminal effects of extracellular signaling Bruce Blumberg 2000. All rights reserved

Oncogenes and cell growth (contd) • • • Growth factors - proteins secreted by one cell that act on another cell (eg sis, wnt, int) – oncoprotein growth factors can only transform cells that harbor the specific receptor Growth factor receptors - transmembrane proteins that are activated by binding to extracellular ligand (protein) – very frequently protein tyrosine kinases – oncogenicity usually results from constitutive (ligand -independent) activation Intracellular transducers - several classes – protein tyrosine kinases, e. g. src – G-protein signal transduction pathways - primary effectors of activated growth factors (e. g. ras) – protein serine/threonine kinases (e. g. mos, raf) Transcription factors - these regulate gene expression directly – myc - HLH protein – fos, jun - b-ZIP proteins – erb. A - nuclear receptor common feature among these is that each type of protein can trigger general changes in cell phenotypes by: – initiating changes that lead to cell growth – respond to signals that cause cell growth – altering gene expression directly Bio. Sci 145 A lecture 18 page 4 ©copyright Bruce Blumberg 2000. All rights reserved

Oncogenes and cell growth (contd) • One example signaling pathway - MAPK (Bardwell lab) growth factor receptor tyrosine kinase ras kinase cascase (serine/threonine) • • transcription factors since the signal passes from one component to the next, inappropriate activation of one element in the cascade canl lead to widespread changes in gene expression – these pathways are not strictly linear but branch and interact with many other signaling pathways • can cause wider effects • may require mutations in parallel pathways to get oncogenesis central importance of this pathway is illustrated by the number of components that can be mutated into oncogenes – aberrant activation of mitogenic pathways can contribute to oncogenicity Bio. Sci 145 A lecture 18 page 5 ©copyright Bruce Blumberg 2000. All rights reserved

Oncogenes and cell growth (contd) • Growth factor receptors are ligand modulated dimers – EGF receptor (v-erb. B) is the prototype member • EGF binding stimulates dimerization and activates tyrosine kinase cascade • one oncogenic variant can dimerize in the absence of ligand signals constitutively • another lacks an internal regulatory domain resulting in constitutive signaling – activated kinase domain autophosphorylates and can then interact with src family proteins Bio. Sci 145 A lecture 18 page 6 ©copyright Bruce Blumberg 2000. All rights reserved

Oncogenes and cell growth (contd) • transforming activity of src-family kinases is related to kinase activity – autophosphorylation controls activity • Y 416 -> active • Y 527 -> weak, normally suppresses phosphorylation of Y 416 – some oncoproteins activate src by interfering with phosphorylation of Y 527 Bio. Sci 145 A lecture 18 page 7 ©copyright Bruce Blumberg 2000. All rights reserved

Oncogenes and cell growth (contd) • modulation of transcription factor activity is important for oncogenesis – can’t cause cancer without altering gene expression! Bio. Sci 145 A lecture 18 page 8 ©copyright Bruce Blumberg 2000. All rights reserved

Oncogenes and cell growth (contd) • transcription factors and cancer – several prominent families of oncogenes are transcription factors - rel, jun, fos, erb. A, myc, myb – actions may be quantitative or qualitative • effects may be to increase activity of the oncoprotein – increased expression could upregulate target genes and influence growth, e. g. AP-1 • alternatively, the mutations could make the oncoprotein a dominant negative inhibitor of other cellular transcription factors (e. g. v-erb. A) – many members are “immediate early” genes • transcription is immediately upregulated without the requirement for new protein synthesis when cells are treated with mitogens – likely to be involved with initiating or promoting growth • increased activity would be expected to increase oncogenesis and it does with some but not others Bio. Sci 145 A lecture 18 page 9 ©copyright Bruce Blumberg 2000. All rights reserved

Tumor suppressor genes • • oncogenesis is not typically dominant. A growing number of “tumor suppressor” genes have been identified that confer a genetic predisposition to cancers – several types of genes are involved • apoptosis proteins (eg p 53) • cell-cycle control proteins (RB) • DNA-repair proteins (p 53) – classic examples are RB (retinoblastoma) and p 53 – loss of tumor suppressor genes is implicated in several infrequent cancers of childhood • retinoblastoma • Wilm’s tumor Bio. Sci 145 A lecture 18 page 10 ©copyright Bruce Blumberg 2000. All rights reserved

Tumor suppressor genes (contd) • RB is a nuclear phosphoprotein that influences the cell cycle – unphosphorylated RB prevents cell proliferation by binding to E 2 F and blocking G 1/S transition – phosphorylation of RB inhibits binding to E 2 F and releases block – some oncogenes (e. g. SV 40 T-antigen, E 1 A) function by sequestering RB and removing block to cell growth – similar effects by loss of both alleles in human disease Bio. Sci 145 A lecture 18 page 11 ©copyright Bruce Blumberg 2000. All rights reserved

Tumor suppressor genes (contd) • • A variety of other cell-cycle control proteins are tumor suppressor genes – p 16, p 21 and D cyclins – shown by identification of inactivating mutations in a variety of human tumors in quiescent cells – RB is not phosphorylated – D cyclin levels are low or absent – p 16, p 21 and p 27 prevent activity of cdk-cyclin complexes • cdc 2, cdk 2 and cdk 4, 6 interact with cyclins and promote cell cycle • this is blocked by tumor suppressor genes Bio. Sci 145 A lecture 18 page 12 ©copyright Bruce Blumberg 2000. All rights reserved

Tumor suppressor genes (contd) • P 53 suppresses cell growth or triggers apoptosis – more than 50% of human tumors have lost p 53 protein or harbor mutations in the gene – a variety of mutations are possible • recessive mutations cause loss of p 53 function allowing unrestrained growth (eg. ko mice) • others are dominant negative p 53 mutants that interfere with normal p 53 subunits in cells and allow unrestrained growth (eg rare cancers) Bio. Sci 145 A lecture 18 page 13 ©copyright Bruce Blumberg 2000. All rights reserved

Tumor suppressor genes (contd) • • • Bio. Sci 145 A lecture 18 page 14 p 53 has dual functions – cells normally have low levels of p 53 – DNA damage induces large increase in p 53 levels – increased p 53 leads to growth arrest until DNA is repaired if cells are in G 1 – cells in S-phase or later are triggered to become apoptotic p 53 is a transcription factor that typically activates – one target is p 21 -> cell cycle arrest – another is GADD 45 - a DNA repair protein – role in inducing apoptosis is unknown at present apoptosis is an important pathway in preventing tumor formation - blocking it is a common strategy ©copyright Bruce Blumberg 2000. All rights reserved