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Involvement of maternal embryonic leucine zipper kinase (MELK) in mammary carcinogenesis through interaction with Bcl-G, a pro-apoptotic member of the Bcl-2 family.

Lin ML, Park JH, Nishidate T, Nakamura Y, Katagiri T - Breast Cancer Res. (2007)

Bottom Line: Northern blot analyses on multiple human tissues and cancer cell lines demonstrated that MELK was overexpressed at a significantly high level in a great majority of breast cancers and cell lines, but was not expressed in normal vital organs (heart, liver, lung and kidney).We also found that MELK physically interacted with Bcl-GL through its amino-terminal region.Our findings suggest that the kinase activity of MELK is likely to affect mammary carcinogenesis through inhibition of the pro-apoptotic function of Bcl-GL.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratory of Molecular Medicine, Human Genome Center, Institute of Medical Science, The University of Tokyo, Tokyo, Japan.

ABSTRACT

Introduction: Cancer therapies directed at specific molecular targets in signaling pathways of cancer cells, such as tamoxifen, aromatase inhibitors and trastuzumab, have proven useful for treatment of advanced breast cancers. However, increased risk of endometrial cancer with long-term tamoxifen administration and of bone fracture due to osteoporosis in postmenopausal women undergoing aromatase inhibitor therapy are recognized side effects. These side effects as well as drug resistance make it necessary to search for novel molecular targets for drugs on the basis of well-characterized mechanisms of action.

Methods: Using accurate genome-wide expression profiles of breast cancers, we found maternal embryonic leucine-zipper kinase (MELK) to be significantly overexpressed in the great majority of breast cancer cells. To assess whether MELK has a role in mammary carcinogenesis, we knocked down the expression of endogenous MELK in breast cancer cell lines using mammalian vector-based RNA interference. Furthermore, we identified a long isoform of Bcl-G (Bcl-GL), a pro-apoptotic member of the Bcl-2 family, as a possible substrate for MELK by pull-down assay with recombinant wild-type and kinase-dead MELK. Finally, we performed TUNEL assays and FACS analysis, measuring proportions of apoptotic cells, to investigate whether MELK is involved in the apoptosis cascade through the Bcl-GL-related pathway.

Results: Northern blot analyses on multiple human tissues and cancer cell lines demonstrated that MELK was overexpressed at a significantly high level in a great majority of breast cancers and cell lines, but was not expressed in normal vital organs (heart, liver, lung and kidney). Suppression of MELK expression by small interfering RNA significantly inhibited growth of human breast cancer cells. We also found that MELK physically interacted with Bcl-GL through its amino-terminal region. Immunocomplex kinase assay showed that Bcl-GL was specifically phosphorylated by MELK in vitro. TUNEL assays and FACS analysis revealed that overexpression of wild-type MELK suppressed Bcl-GL-induced apoptosis, while that of D150A-MELK did not.

Conclusion: Our findings suggest that the kinase activity of MELK is likely to affect mammary carcinogenesis through inhibition of the pro-apoptotic function of Bcl-GL. The kinase activity of MELK could be a promising molecular target for development of therapy for patients with breast cancers.

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Expression and distribution of MELK in human normal tissues and breast cancer cell lines. (a) Expression of MELK in 12 breast cancer specimens (case number; 42, 102, 247, 252, 302, 473, 478, 502, 552, 646, 769 and 779) by semi-quantitative RT-PCR. GAPDH served as a quantitative internal control. (b) Multiple tissue Northern blot analysis demonstrated that an approximately 2.7 kb MELK transcript was detected in the testis, thymus and small intestine. PBL, peripheral blood leukocytes. (c) Breast cancer cell line Northern blot analysis revealed that approximately 2.4 to 2.7 kb MELK variants were specifically expressed in breast cancer cell lines, but not in normal vital organs. (d) Schematic representation of three variant transcripts identified by cDNA library screening (see Materials and methods). White boxes indicate a coding region and black boxes indicate a non-coding region. Black and grey triangles indicate initiation codons, and white triangles indicate stop codons. Exon numbers are shown above each box. (e) In vitro translation assay of each variant isolated from cDNA library screening. The number within parentheses represents the predicted molecular weight (kDa) of each variant protein. (f) Expression of MELK proteins in eight breast cancer cell lines as well as human mammary epithelial cells (HMECs) shown by western blot analysis with an anti-MELK antibody. β-Actin served as a control. (g) Schematic representation of the V1, V2 and V3 forms of MELK. The shaded boxes indicate the catalytic domain (amino acids 11 to 263 of the V1 protein). The KA1 domain is the kinase-associated domain in the carboxy-terminal region.
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Figure 1: Expression and distribution of MELK in human normal tissues and breast cancer cell lines. (a) Expression of MELK in 12 breast cancer specimens (case number; 42, 102, 247, 252, 302, 473, 478, 502, 552, 646, 769 and 779) by semi-quantitative RT-PCR. GAPDH served as a quantitative internal control. (b) Multiple tissue Northern blot analysis demonstrated that an approximately 2.7 kb MELK transcript was detected in the testis, thymus and small intestine. PBL, peripheral blood leukocytes. (c) Breast cancer cell line Northern blot analysis revealed that approximately 2.4 to 2.7 kb MELK variants were specifically expressed in breast cancer cell lines, but not in normal vital organs. (d) Schematic representation of three variant transcripts identified by cDNA library screening (see Materials and methods). White boxes indicate a coding region and black boxes indicate a non-coding region. Black and grey triangles indicate initiation codons, and white triangles indicate stop codons. Exon numbers are shown above each box. (e) In vitro translation assay of each variant isolated from cDNA library screening. The number within parentheses represents the predicted molecular weight (kDa) of each variant protein. (f) Expression of MELK proteins in eight breast cancer cell lines as well as human mammary epithelial cells (HMECs) shown by western blot analysis with an anti-MELK antibody. β-Actin served as a control. (g) Schematic representation of the V1, V2 and V3 forms of MELK. The shaded boxes indicate the catalytic domain (amino acids 11 to 263 of the V1 protein). The KA1 domain is the kinase-associated domain in the carboxy-terminal region.

Mentions: We previously reported the genome-wide expression profile analysis of 81 clinical breast cancers, representing 23,040 genes, by means of cDNA microarray analysis in combination with enrichment of cancer cells with a laser microbeam microdissection system [8]. The MELK gene (GenBank accession number NM_014791) was found to be one of the genes transactivated at a very high level in the great majority of the breast cancers examined. The subsequent semi-quantitative RT-PCR analysis of the transcript confirmed the elevated expression of MELK in 11 of 12 clinical breast cancer specimens (Figure 1a).


Involvement of maternal embryonic leucine zipper kinase (MELK) in mammary carcinogenesis through interaction with Bcl-G, a pro-apoptotic member of the Bcl-2 family.

Lin ML, Park JH, Nishidate T, Nakamura Y, Katagiri T - Breast Cancer Res. (2007)

Expression and distribution of MELK in human normal tissues and breast cancer cell lines. (a) Expression of MELK in 12 breast cancer specimens (case number; 42, 102, 247, 252, 302, 473, 478, 502, 552, 646, 769 and 779) by semi-quantitative RT-PCR. GAPDH served as a quantitative internal control. (b) Multiple tissue Northern blot analysis demonstrated that an approximately 2.7 kb MELK transcript was detected in the testis, thymus and small intestine. PBL, peripheral blood leukocytes. (c) Breast cancer cell line Northern blot analysis revealed that approximately 2.4 to 2.7 kb MELK variants were specifically expressed in breast cancer cell lines, but not in normal vital organs. (d) Schematic representation of three variant transcripts identified by cDNA library screening (see Materials and methods). White boxes indicate a coding region and black boxes indicate a non-coding region. Black and grey triangles indicate initiation codons, and white triangles indicate stop codons. Exon numbers are shown above each box. (e) In vitro translation assay of each variant isolated from cDNA library screening. The number within parentheses represents the predicted molecular weight (kDa) of each variant protein. (f) Expression of MELK proteins in eight breast cancer cell lines as well as human mammary epithelial cells (HMECs) shown by western blot analysis with an anti-MELK antibody. β-Actin served as a control. (g) Schematic representation of the V1, V2 and V3 forms of MELK. The shaded boxes indicate the catalytic domain (amino acids 11 to 263 of the V1 protein). The KA1 domain is the kinase-associated domain in the carboxy-terminal region.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Figure 1: Expression and distribution of MELK in human normal tissues and breast cancer cell lines. (a) Expression of MELK in 12 breast cancer specimens (case number; 42, 102, 247, 252, 302, 473, 478, 502, 552, 646, 769 and 779) by semi-quantitative RT-PCR. GAPDH served as a quantitative internal control. (b) Multiple tissue Northern blot analysis demonstrated that an approximately 2.7 kb MELK transcript was detected in the testis, thymus and small intestine. PBL, peripheral blood leukocytes. (c) Breast cancer cell line Northern blot analysis revealed that approximately 2.4 to 2.7 kb MELK variants were specifically expressed in breast cancer cell lines, but not in normal vital organs. (d) Schematic representation of three variant transcripts identified by cDNA library screening (see Materials and methods). White boxes indicate a coding region and black boxes indicate a non-coding region. Black and grey triangles indicate initiation codons, and white triangles indicate stop codons. Exon numbers are shown above each box. (e) In vitro translation assay of each variant isolated from cDNA library screening. The number within parentheses represents the predicted molecular weight (kDa) of each variant protein. (f) Expression of MELK proteins in eight breast cancer cell lines as well as human mammary epithelial cells (HMECs) shown by western blot analysis with an anti-MELK antibody. β-Actin served as a control. (g) Schematic representation of the V1, V2 and V3 forms of MELK. The shaded boxes indicate the catalytic domain (amino acids 11 to 263 of the V1 protein). The KA1 domain is the kinase-associated domain in the carboxy-terminal region.
Mentions: We previously reported the genome-wide expression profile analysis of 81 clinical breast cancers, representing 23,040 genes, by means of cDNA microarray analysis in combination with enrichment of cancer cells with a laser microbeam microdissection system [8]. The MELK gene (GenBank accession number NM_014791) was found to be one of the genes transactivated at a very high level in the great majority of the breast cancers examined. The subsequent semi-quantitative RT-PCR analysis of the transcript confirmed the elevated expression of MELK in 11 of 12 clinical breast cancer specimens (Figure 1a).

Bottom Line: Northern blot analyses on multiple human tissues and cancer cell lines demonstrated that MELK was overexpressed at a significantly high level in a great majority of breast cancers and cell lines, but was not expressed in normal vital organs (heart, liver, lung and kidney).We also found that MELK physically interacted with Bcl-GL through its amino-terminal region.Our findings suggest that the kinase activity of MELK is likely to affect mammary carcinogenesis through inhibition of the pro-apoptotic function of Bcl-GL.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratory of Molecular Medicine, Human Genome Center, Institute of Medical Science, The University of Tokyo, Tokyo, Japan.

ABSTRACT

Introduction: Cancer therapies directed at specific molecular targets in signaling pathways of cancer cells, such as tamoxifen, aromatase inhibitors and trastuzumab, have proven useful for treatment of advanced breast cancers. However, increased risk of endometrial cancer with long-term tamoxifen administration and of bone fracture due to osteoporosis in postmenopausal women undergoing aromatase inhibitor therapy are recognized side effects. These side effects as well as drug resistance make it necessary to search for novel molecular targets for drugs on the basis of well-characterized mechanisms of action.

Methods: Using accurate genome-wide expression profiles of breast cancers, we found maternal embryonic leucine-zipper kinase (MELK) to be significantly overexpressed in the great majority of breast cancer cells. To assess whether MELK has a role in mammary carcinogenesis, we knocked down the expression of endogenous MELK in breast cancer cell lines using mammalian vector-based RNA interference. Furthermore, we identified a long isoform of Bcl-G (Bcl-GL), a pro-apoptotic member of the Bcl-2 family, as a possible substrate for MELK by pull-down assay with recombinant wild-type and kinase-dead MELK. Finally, we performed TUNEL assays and FACS analysis, measuring proportions of apoptotic cells, to investigate whether MELK is involved in the apoptosis cascade through the Bcl-GL-related pathway.

Results: Northern blot analyses on multiple human tissues and cancer cell lines demonstrated that MELK was overexpressed at a significantly high level in a great majority of breast cancers and cell lines, but was not expressed in normal vital organs (heart, liver, lung and kidney). Suppression of MELK expression by small interfering RNA significantly inhibited growth of human breast cancer cells. We also found that MELK physically interacted with Bcl-GL through its amino-terminal region. Immunocomplex kinase assay showed that Bcl-GL was specifically phosphorylated by MELK in vitro. TUNEL assays and FACS analysis revealed that overexpression of wild-type MELK suppressed Bcl-GL-induced apoptosis, while that of D150A-MELK did not.

Conclusion: Our findings suggest that the kinase activity of MELK is likely to affect mammary carcinogenesis through inhibition of the pro-apoptotic function of Bcl-GL. The kinase activity of MELK could be a promising molecular target for development of therapy for patients with breast cancers.

Show MeSH
Related in: MedlinePlus