Editor's Commentary - SMAD4-dependent barrier constrains prostate cancer growth and metastatic progression

BERKELEY, CA (UroToday.com) - In a letter to Nature, Dr. Zhihu Ding and colleagues report what molecular barriers might exist that regulate prostate cancer (CaP) growth and progression.

It is known that a transgenic mouse with a prostate-specific Pten deletion (Ptenpc−/−) results in prostate intraepithelial neoplasia (PIN), which, following a long latency, can progress to high-grade adenocarcinoma, albeit with minimally invasive and metastatic features. To understand this feeble progression phenotype, they conducted transcriptome comparison of Ptenpc−/− PIN relative to wild-type prostate epithelium. In addition to the expected PI3K and p53 (also known as TRP53) pathways, knowledge-based pathway analysis revealed prominent TGFβ/BMP signaling in Ptenpc−/− PIN. IHC and western blot showed robust increase of Smad4 in PIN.

SMAD4 is a common mediator of the TGFβ signaling. The dual effects of TGFβ signaling on tumor initiation and progression are cell-specific and are yet to be determined for distinct contexts. A number of genetically manipulated mouse models with alterations in the TGFβ pathway genes, particularly the pivotal Smad4, revealed that these genes play crucial functions in maintaining tissue homeostasis and suppressing tumorigenesis. Loss of Smad4 plays a causal role in initiating squamous cell carcinomas as well as adenocarcinomas of gastrointestinal tract. However, for some cancers like pancreatic and cholangiocellular carcinomas, Smad4 deficiency does not initiate the tumorigenesis, but acts as a promoter to accelerate or synergize the development and progression of cancers that are started by other oncogenic pathways. In conditional knockout mice, they demonstrated that Ptenpc-/- mice had significant SMAD4 expression but Ptenpc-/- SMAD4pc-/- did not. In this model they found that SMAD4 deletion drives progression of Pten-deficient prostate tumors to metastasize to lymph nodes and lung. They performed ingenuity pathway analysis of primary prostate tumors from Pten pc-/-, Pten pc-/- SMAD pc-/-, and Pten pc-/- P53 pc-/- mice harvested at 15 wks. They sought to find the biological processes that underlie that metastatic phenotype in the Pten pc-/- SMAD4 pc-/- tumors. The two pathways with the most enriched genes related to cellular movement and cellular growth and proliferation. Using a Boyden chamber assay to study the 21 genes, they found 10 genes that enhanced invasion of PC3 cells. Secreted Phosphoprotein 1 (SPP1) was selected given its PCA progression-correlated expression, its prognostic potential for BCR in 79 patients, and its know link to TGFbeta signaling. Spp1 is a pro-metastasis invasion gene in human prostate cancer and in the current model. SPP1, osteopontin (OPN), and bone sialoprotein I (BP1) are important in osteoclast anchoring to bone matrix. SPP1 is upregulated 7.45 fold by PSA in SaOS-2 cells and PSA enhances TGF-β mRNA expression in SaOS-2 cells. They found the CyclinD1 promoter occupied by SMAD4 in SMAD4-induced TGFβ1-stimulated Ptenpc-/-Smad4pc-/-SMAD4-TetOn cells. Cyclin D1 is a key mediator of increased proliferation in the metastatic Pten pc-/- Smad4 pc-/- model. This resulted in SMAD4 suppression of Cyclin D1 and Spp1 mRNA upon TGF-β treatment. The loss of Smad4 resulted in markedly increased Spp1, and TGFβ treatment suppressed Spp1 expression.

Thus, inactivation of Pten and Smad4 as well as activation of Cyclin D1 and Spp1 was driving prostate cancer progression. They then hypothesized that these four key genes may carry prognostic value for metastasis risk in human CaP. They constructed numerous models using RNA or tissue microarray immunohistochemical staining to predict progression from human CaP patients. The four protein models outperformed Gleason score in predicting progression.

In summary, this complex work demonstrated that concomitant Pten and Smad4 inactivation enhances prostate tumor proliferation and drive invasion to a fully penetrant metastatic phenotype in the mouse. The human relevance of this model was confirmed by the prognostic significance of a four-marker signature in predicting BCR or lethal metastasis in human prostate cancers. This study will facilitate the development of a molecularly based prognostic assay that complements current standards and may improve evidence-based management of prostate cancer patients.

Ding Z, Wu CJ, Chu GC, Xiao Y, Ho D, Zhang J, Perry SR, Labrot ES, Wu X, Lis R, Hoshida Y, Hiller D, Hu B, Jiang S, Zheng H, Stegh AH, Scott KL, Signoretti S, Bardeesy N, Wang YA, Hill DE, Golub TR, Stampfer MJ, Wong WH, Loda M, Mucci L, Chin L, DePinho RA

 

Nature. 2011 Feb 10;470(7333):269-73
10.1038/nature09677

PubMed Abstract
PMID: 21289624

UroToday.com Prostate Cancer Section

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