BioSci 145A lecture 18 page 1 © copyright Bruce Blumberg All rights reserved BioSci 145A 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

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

BioSci 145A lecture 18 page 3 © copyright Bruce Blumberg All rights reserved Oncogenes and cell growth (contd) 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

BioSci 145A lecture 18 page 4 © copyright Bruce Blumberg 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 –erbA - 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

BioSci 145A lecture 18 page 5 © copyright Bruce Blumberg 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

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

BioSci 145A lecture 18 page 7 © copyright Bruce Blumberg 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

BioSci 145A lecture 18 page 8 © copyright Bruce Blumberg 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!

BioSci 145A lecture 18 page 9 © copyright Bruce Blumberg All rights reserved Oncogenes and cell growth (contd) transcription factors and cancer –several prominent families of oncogenes are transcription factors - rel, jun, fos, erbA, 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-erbA) –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

BioSci 145A lecture 18 page 10 © copyright Bruce Blumberg 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 p53) cell-cycle control proteins (RB) DNA-repair proteins (p53) –classic examples are RB (retinoblastoma) and p53 –loss of tumor suppressor genes is implicated in several infrequent cancers of childhood retinoblastoma Wilm’s tumor

BioSci 145A lecture 18 page 11 © copyright Bruce Blumberg 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 E2F and blocking G1/S transition –phosphorylation of RB inhibits binding to E2F and releases block –some oncogenes (e.g. SV40 T-antigen, E1A) function by sequestering RB and removing block to cell growth –similar effects by loss of both alleles in human disease

BioSci 145A lecture 18 page 12 © copyright Bruce Blumberg All rights reserved Tumor suppressor genes (contd) A variety of other cell-cycle control proteins are tumor suppressor genes –p16, p21 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 –p16, p21 and p27 prevent activity of cdk-cyclin complexes cdc2, cdk2 and cdk4,6 interact with cyclins and promote cell cycle this is blocked by tumor suppressor genes

BioSci 145A lecture 18 page 13 © copyright Bruce Blumberg All rights reserved Tumor suppressor genes (contd) P53 suppresses cell growth or triggers apoptosis –more than 50% of human tumors have lost p53 protein or harbor mutations in the gene –a variety of mutations are possible recessive mutations cause loss of p53 function allowing unrestrained growth (eg. ko mice) others are dominant negative p53 mutants that interfere with normal p53 subunits in cells and allow unrestrained growth (eg rare cancers)

BioSci 145A lecture 18 page 14 © copyright Bruce Blumberg All rights reserved Tumor suppressor genes (contd) p53 has dual functions –cells normally have low levels of p53 –DNA damage induces large increase in p53 levels –increased p53 leads to growth arrest until DNA is repaired if cells are in G1 –cells in S-phase or later are triggered to become apoptotic p53 is a transcription factor that typically activates –one target is p21 -> cell cycle arrest –another is GADD45 - 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

BioSci 145A lecture 18 page 15 © copyright Bruce Blumberg All rights reserved Cancer - putting it all together