Gamini Siriwardana

The Children’s Medical Group, Poughkeepsie, New York, United States

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Publications (4)14.86 Total impact

  • Source
    G Siriwardana · A Bradford · D Coy · P Zeitler
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    ABSTRACT: Although GHRH has previously been shown to regulate proliferation of breast cancer cells and prevent apoptosis, the intracellular pathways mediating this effect have not been clarified. Exogenous GHRH stimulated a dose-dependent proliferative response within 24 h in MDA-231, as well as in T47D cells and in MCF-7 cells transfected with the GHRH receptor. The proliferation of MDA-MB-231 (MDA-231) cells was associated with an increase in tritiated thymidine uptake. In addition, phosphorylation of MAPK was rapidly stimulated by GHRH. The phosphorylation of MAPK by GHRH was prevented by transfection of the cells with dominant-negative Ras or Raf or by pretreatment of cells with Raf kinase 1 inhibitor. The inhibition of Ras and Raf, as well as the inhibition of MAPK phosphorylation by PD98059, also prevented GHRH-induced cell proliferation. Finally, pretreatment of cells with the somatostatin analog, BIM23014, also prevented GHRH-induced MAPK phosphorylation and cell proliferation. These results indicate that GHRH stimulates dose-dependent cell proliferation of MDA-231 breast cancer cells through a pathway that requires Ras, Raf, and MAPK phosphorylation. The results also provide support for a possible autocrine/paracrine antagonism between GHRH and somatostatin in the regulation of MDA-231 cell population maintenance. Taken together, the studies provide further insight into the possible role of GHRH as a growth factor in breast cancer.
    Preview · Article · Oct 2006 · Molecular Endocrinology
  • Philip Zeitler · Gamini Siriwardana
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    ABSTRACT: GHRH, in addition to stimulating the release of growth hormone (GH) from the pituitary, is a trophic factor for pituitary somatotrophs. Growth hormone-releasing hormone is also expressed in the gonads, gastrointestinal tract, pancreas, thymus, and lymphocytes, as well as in tumors of the pancreas, lung, central nervous system, and breast. Since GHRH has mitogenic effects, we examined the hypothesis that GHRH is an autocrine/paracrine growth factor in neoplastic breast tissue. The effect of disrupting endogenous GHRH on cell growth and apoptosis of MDA231 cells was examined through the use of a competitive GHRH antagonist, [N-acetyl-Tyr1, D-Arg2] fragment 1-29Amide (GHRHa). Cell proliferation was determined by direct cell counting and tritiated thymidine incorporation. Apoptosis was analyzed by examination of DNA laddering and nuclear condensation. GHRHa resulted in a dose-dependent, transient, and reversible decrease in cell number, proliferation rate, and tritiated thymidine uptake. Conversely, GHRHa led to a marked and dose-dependent increase in both DNA laddering and nuclear condensation. These results indicate that disruption of endogenous GHRH action in MDA231 cells results in both decreased cellular proliferation and increased apoptosis. Taken together, the findings suggest that endogenous GHRH acts as an autocrine/paracrine factor in the regulation of growth of at least some breast cancer cell types.
    No preview · Article · Jul 2002 · Endocrine
  • Philip Zeitler · Gamini Siriwardana
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    ABSTRACT: Growth hormone-releasing hormone (GHRH) is an important regulator of somatotroph development and function. However, GHRH signaling is still not completely understood. Signaling through the mitogen-activated protein kinase (MAPK) pathway has been observed in a wide variety of cell types but has not been explored as a mediator of GHRH action. In this study, we examined the phosphorylation of MAPK pathway intermediates in response to GHRH. After treatment of the GH4 rat somatotroph cell line with rGHRH (10(7) M) for 2.5 min, there was robust phosphorylation of MAPK not seen in vehicle-treated cells. Treatment of HeLa cells with GHRH resulted in no activation of MAPK, but activation was conferred by transfection with the GHRH receptor cDNA. MAPK activation by GHRH was dose dependent from 1 to 100 nM, was evident at 2.5 min, peaked at 5 min, and returned to baseline by 20 min. Pretreatment of GH4 cells with somatostatin analog BIM23014 or the MEK1 inhibitor PD98095 prevented the activation of MAPK. Finally, treatment with GHRH increased GH4 proliferation in culture, and this response was prevented by pretreatment with BIM23014 and PD98095. These results indicate that GHRH activates the MAPK pathway. Furthermore, activation of MAPK may mediate, at least in part, the effects of GHRH on somatotroph cell line proliferation. The findings support the concept that multiple pathways mediate the effects of GHRH.
    No preview · Article · Jul 2000 · Endocrine
  • Source
    P Zeitler · P Stevens · G Siriwardana
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    ABSTRACT: GHRH plays a critical role in pituitary somatotroph development and function, actions which are mediated by a G-protein coupled receptor (GHRHr) that has been recently cloned. PCR amplification of rat pituitary mRNA using primers that span the GHRHr coding region resulted in two distinct products. When sequenced, the two isoforms were identical through bp 1278 of the GHRH coding region. However, the novel variant, which we have termed GHRHrbeta, contains a 131 bp deletion (1279-1408) and resumes at bp 1409 in the 3'UTR of the previously identified transcript (GHRHralpha). The identical isoforms were present in pituitaries from dwarf (dw) rats. The predicted amino acid sequence for the alternate receptor isoform differs from the published amino acid sequence at the extreme carboxyl terminus, with the last 5 amino acids of the published sequence replaced and an additional 17 amino acids added to the sequence. When translated in vitro or expressed as an epitope-tagged construct in non-GHRHr containing cell lines, the GHRHrbeta mRNA produces a 42 kDa protein product, appropriately larger than the 40 kDa product of GHRHralpha mRNA. Furthermore, GHRHrbeta retains the ability to promote cAMP generation in response to GHRH when expressed in non-GHRHr containing cell lines. These results indicate the presence of a splice variant of rat GHRHr mRNA present in normal and dw rat pituitary that codes for a functional receptor protein with an alternate carboxyl terminal domain. These findings raise the possibilities of target cell regulation of GHRH response, modulation of response through receptor isoform interactions and the involvement of multiple intracellular signaling pathways.
    Preview · Article · Jan 1999 · Journal of Molecular Endocrinology

Publication Stats

109 Citations
14.86 Total Impact Points


  • 2002
    • The Children’s Medical Group
      Poughkeepsie, New York, United States