Targeted knockout of BRG1 potentiates lung cancer development

Brahma-related gene 1 (BRG1) is a catalytic subunit of the switch in mating type/sucrose nonfermentation complex and plays an important role in cancer development. Mouse homozygous knockout experiments testing the role of BRG1 in tumorigenesis have been hampered because BRG1 inactivation is embryonic lethal. To bypass this constraint, we developed a lung-specific conditional knockout of BRG1 and examined the effect of BRG1 inactivation in an ethyl carbamate lung carcinogenesis mouse model. We found that the heterozygous loss of BRG1 resulted in increases in both the number and size of tumors when compared with controls. In contrast, when both BRG1 alleles were inactivated, neither the number nor the size of tumors increased compared with controls. In mouse lung tissue where BRG1 was homozygously inactivated, immunostaining for apoptotic markers showed significant increase in Apo-BrdUrd and cleaved caspase-3. These data indicate that a loss of cell viability underlies why biallelic inactivation of BRG1 does not increase tumorigenesis. We also examined mice when exposed to the carcinogen ethyl carbamate and then subjected to BRG1 inactivation. In these cells, loss of BRG1 after carcinogen exposure potentiated tumor development. A subset of tumors retained BRG1 expression, whereas others showed either partial or complete loss of BRG1 expression. Tumors completely devoid of BRG1 expression were significantly larger and expressed higher levels of two markers of proliferation, proliferating cell nuclear antigen and Ki67. Although biallelic inactivation of BRG1 could not initiate tumor development in untransformed cells, our results indicate that transformation and tumor progression are greatly affected by loss of BRG1.

The reversible epigenetic silencing of BRM: implications for clinical targeted therapy

The SWI/SNF chromatin-remodeling complex serves as a master switch that directs and limits the execution of specific cellular programs, such as differentiation and growth control. SWI/SNF function requires one of two paralogous ATPase subunits, Brahma (BRM) or BRM-related gene 1 (BRG1), which we previously found are lost together in cancer cell lines and primary lung cancers. Although BRG1 has been found to be mutated in cancer cell lines, the mechanisms underlying BRM silencing are not known. To address this question, we sequenced BRM in 10 BRM/BRG1-deficient cancer cell lines and found that BRM was devoid of abrogating mutations. Moreover, histone deacetylase (HDAC) inhibitors restored BRM expression in each of these BRG1/BRM-deficient cancer cell lines, indicating that epigenetic silencing is a major mechanism underlying the loss of BRM expression. Despite their ability to restore BRM expression, these HDAC inhibitors also blocked BRM function when present. However, after their removal, we observed that BRM expression remained elevated for several days, and during this period, BRM activity was detected. We also found that the suppression of BRM occurs in a broad range of human tumor types and that loss of one or both BRM alleles potentiated tumor development in mice. Thus, BRG1 and BRM are silenced by different mechanisms, and it may be possible to clinically target and reexpress BRM in a number of tumor types, potentially impacting tumor development.

The mTOR/p70S6K signal transduction pathway plays a role in cardiac hypertrophy and influences expression of myosin heavy chain genes in vivo

OBJECTIVE

Rapamycin inhibits p70 S6 kinase (p70(S6K)) activity and hypertrophy of cultured neonatal rat cardiac myocytes. The purpose of the present study was to determine whether rapamycin inhibits left ventricular (LV) hypertrophy in intact rats and whether it alters cardiac gene expression.

METHODS

300 g rats were subjected to aortic constriction (AC) or sham-operation (SH) and studied 2 and 3 days after surgery. Beginning 1 day prior to surgery, rats were injected with rapamycin (1.5 mg/kg, i.p.) or carboxymethylcellulose vehicle (V), yielding 4 groups (SH-V, SH-R, AC-V, AC-R). Total RNA was extracted for determination of mRNA levels by Northern blotting.

RESULTS

LV dry weight/body weight ratios were 0.43 +/- 0.04 (mean +/- SE) for SH-V, 0.46 +/- 0.02 for SH-R, 0.56 +/- 0.02 for AC-V, and 0.53 +/- 0.03 for AC-R. R inhibited cardiac hypertrophy induced by pressure overload (ANOVA; p < 0.05). Rapamycin had no effect on the expression of atrial natriuretic factor mRNA, but increased the levels of beta-myosin heavy chain mRNA 6-fold in hearts of SH-R and AC-R compared to SH-V. Rapamycin also increased the expression of alpha-myosin heavy chain mRNA in SH-R by 3-fold compared with SH-V, but had no effect on the AC-R group.

CONCLUSION

The data suggest that an intact mTOR signaling pathway is required for rapid hypertrophic growth of the heart in vivo. Moreover, the data suggest a novel link between the mTOR/p70(S6K) signal transduction pathway and pretranslational control of myosin gene expression in the heart.

Effects of gradual coronary artery occlusion and exercise training on gene expression in swine heart

Gradual occlusion (O) of the swine left circumflex coronary artery (LCX) with an ameroid occluder results in complete O within 3 weeks, collateral vessel development, and compensatory hypertrophy. The purpose of this investigation was to determine the independent and combined effects of O and exercise training (E) on gene expression in the swine heart. Adult Yucatan miniature swine were assigned to one of the following groups (n=6-9/group): sedentary control (S), exercise-trained (E), sedentary swine subjected to LCX occlusion (SO), and exercise-trained swine with LCX occlusion (EO). Exercise consisted of progressive treadmill running conducted 5 d/wk for 16 weeks. Gene expression was studied in myocardium isolated from the collateral-dependent left ventricle free wall (LV) and the collateral-independent septum (SEP) by RNA blotting. E and O each stimulated cardiac hypertrophy independently (p<0.001) with no interaction. O but not E increased atrial natriuretic factor expression in the LV, but not in the SEP. E decreased the expression of beta-myosin heavy chain in the LV, but not in the SEP. E retarded the expression of collagen III mRNA in SEP; but not in the LV. Exercise training and coronary artery occlusion each stimulate cardiac hypertrophy independently and induce different patterns of gene expression.

3-HMG-Coenzyme A Reductase Inhibition and Extracellular Matrix Gene Expression in the Pressure-Overloaded Rat Heart

The purpose of this study was to determine whether 3-HMG-Coenzyme A (HMG-CoA) reductase inhibition would attenuate the early pressure overload-induced activation of extracellular matrix genes in the left ventricle (LV) of the heart. Sprague-Dawley rats were randomized to 1 of 4 treatment groups: sham-operation+vehicle (SH-V), aortic constriction+vehicle (AC-V), AC+rosuvastatin (RSV, 2 mg/kg; AC-LO), and AC+RSV (10 mg/kg; AC-HI). Rats were injected with normal NaCl (V) or RSV once daily, beginning 1 day before surgery, and killed 1 or 3 days after surgery. Hemodynamic measurements were made in the open-chest anesthetized state. LV levels of transforming growth factor beta1 (TGF-beta1), procollagen 1 (C1), and fibronectin (FN) mRNA were measured by Northern blotting. AC induced a approximately 25% increase in LV weight after 3 days that was not altered by RSV treatment. LV expression of TGF-beta1, C1, and FN mRNA was approximately 2-fold, approximately 2.5-fold, and approximately 5-fold greater, respectively, in hearts of AC-V compared to SH-V rats 3 days post-operation, and was not significantly decreased by either dose of RSV. Inhibition of HMG-CoA reductase does not attenuate the pronounced aortic constriction-induced increases in the early expression of TGF-beta1, C1, and FN in this model of acute pressure overload of the rat heart.