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Mediator kinases CDK19 and CDK8, pleiotropic regulators of transcriptional reprogramming, are differentially regulated by androgen signaling, but both kinases are upregulated in castration-resistant prostate cancer (CRPC). Genetic or pharmacological inhibition of CDK8 and CDK19 reverses the castration-resistant phenotype and restores the sensitivity of CRPC xenografts to androgen deprivation in vivo. Prolonged CDK8/19 inhibitor treatment combined with castration not only suppressed the growth of CRPC xenografts but also induced tumor regression and cures. Transcriptomic analysis revealed that Mediator kinase inhibition amplified and modulated the effects of castration on gene expression, disrupting CRPC adaptation to androgen deprivation. Mediator kinase inactivation in tumor cells also affected stromal gene expression, indicating that Mediator kinase activity in CRPC molded the tumor microenvironment. The combination of castration and Mediator kinase inhibition downregulated the MYC pathway, and Mediator kinase inhibition suppressed a MYC-driven CRPC tumor model even without castration. CDK8/19 inhibitors showed efficacy in patient-derived xenograft models of CRPC, and a gene signature of Mediator kinase activity correlated with tumor progression and overall survival in clinical samples of metastatic CRPC. These results indicate that Mediator kinases mediated androgen-independent in vivo growth of CRPC, supporting the development of CDK8/19 inhibitors for the treatment of this presently incurable disease.

Conventional chemotherapy not only kills tumor cells but also changes gene expression in treatment-damaged tissues, inducing production of multiple tumor-supporting secreted factors. This secretory phenotype was found here to be mediated in part by a damage-inducible cell-cycle inhibitor p21 (CDKN1A). We developed small-molecule compounds that inhibit damage-induced transcription downstream of p21. These compounds were identified as selective inhibitors of a transcription-regulating kinase CDK8 and its isoform CDK19. Remarkably, p21 was found to bind to CDK8 and stimulate its kinase activity. p21 and CDK8 also cooperate in the formation of internucleolar bodies, where both proteins accumulate. A CDK8 inhibitor suppresses damage-induced tumor-promoting paracrine activities of tumor cells and normal fibroblasts and reverses the increase in tumor engraftment and serum mitogenic activity in mice pretreated with a chemotherapeutic drug. The inhibitor also increases the efficacy of chemotherapy against xenografts formed by tumor cell/fibroblast mixtures. Microarray data analysis revealed striking correlations between CDK8 expression and poor survival in breast and ovarian cancers. CDK8 inhibition offers a promising approach to increasing the efficacy of cancer chemotherapy.

The nuclear factor-κB (NFκB) family of transcription factors has been implicated in inflammatory disorders, viral infections, and cancer. Most of the drugs that inhibit NFκB show significant side effects, possibly due to sustained NFκB suppression. Drugs affecting induced, but not basal, NFκB activity may have the potential to provide therapeutic benefit without associated toxicity. NFκB activation by stress-inducible cell cycle inhibitor p21 was shown to bemediated by a p21-stimulated transcription-regulating kinase CDK8. CDK8 and its paralog CDK19, associated with the transcriptional Mediator complex, act as coregulators of several transcription factors implicated in cancer; CDK8/19 inhibitors are entering clinical development. Here we show that CDK8/19 inhibition by different small-molecule kinase inhibitors or shRNAs suppresses the elongation of NFκB-induced transcription when such transcription is activated by p21-independent canonical inducers, such as TNFα. On NFκB activation, CDK8/19 are corecruited with NFκB to the promoters of the responsive genes. Inhibition of CDK8/19 kinase activity suppresses the RNA polymerase II C-terminal domain phosphorylation required for transcriptional elongation, in a gene-specific manner. Genes coregulated by CDK8/19 and NFκB include IL8, CXCL1, and CXCL2, which encode tumor-promoting proinflammatory cytokines. Although it suppressed newly induced NFκB-driven transcription, CDK8/19 inhibition in most cases had no effect on the basal expression of NFκB-regulated genes or promoters; the same selective regulation of newly induced transcription was observed with other transcription signals potentiated by CDK8/19. This selective role of CDK8/19 identifies these kinases as mediators of transcriptional reprogramming, a key aspect of development and differentiation as well as pathological processes.

Breast cancers (BrCas) that overexpress oncogenic tyrosine kinase receptor HER2 are treated with HER2-targeting antibodies (such as trastuzumab) or small-molecule kinase inhibitors (such as lapatinib). However, most patients with metastatic HER2⁺ BrCa have intrinsic resistance and nearly all eventually become resistant to HER2-targeting therapy. Resistance to HER2-targeting drugs frequently involves transcriptional reprogramming associated with constitutive activation of different signaling pathways. We have investigated the role of CDK8/19 Mediator kinase, a regulator of transcriptional reprogramming, in the response of HER2⁺ BrCa to HER2-targeting drugs. CDK8 was in the top 1% of all genes ranked by correlation with shorter relapse-free survival among treated HER2⁺ BrCa patients. Selective CDK8/19 inhibitors (senexin B and SNX631) showed synergistic interactions with lapatinib and trastuzumab in a panel of HER2⁺ BrCa cell lines, overcoming and preventing resistance to HER2-targeting drugs. The synergistic effects were mediated in part through the PI3K/AKT/mTOR pathway and reduced by PI3K inhibition. Combination of HER2- and CDK8/19-targeting agents inhibited STAT1 and STAT3 phosphorylation at S727 and upregulated tumor suppressor BTG2. The growth of xenograft tumors formed by lapatinib-sensitive or -resistant HER2⁺ breast cancer cells was partially inhibited by SNX631 alone and strongly suppressed by the combination of SNX631 and lapatinib, overcoming lapatinib resistance. These effects were associated with decreased tumor cell proliferation and altered recruitment of stromal components to the xenograft tumors. These results suggest potential clinical benefit of combining HER2- and CDK8/19-targeting drugs in the treatment of metastatic HER2⁺ BrCa.

Background/Objective: Ovarian clear cell carcinomas (OCCCs) are a rare histological subtype of epithelial ovarian cancer characterized by resistance to platinum-based therapy. CDK8/19, a component of the regulatory CDK module associated with Mediator complex, has been implicated in transcriptional reprogramming and drug resistance in various solid tumors. Our study aimed to investigate the therapeutic potential of CDK8/19 kinase inhibition using selective inhibitors SNX631 and SNX631-6 in OCCC treatment, both as monotherapy and in combination with standard chemotherapeutics. Methods: CDK8 and Ki67 levels were evaluated via immunohistochemistry in benign, primary, and metastatic ovarian cancer tissues. The efficacy of SNX631 alone and in combination with cisplatin or paclitaxel was assessed in OCCC cell lines (ES-2, TOV-21-G, RMG-1). In vivo evaluation utilized xenograft models with subcutaneous and intraperitoneal delivery of the OCCC ES2 cells and oral delivery of SNX631-6, with the monitoring of tumor growth, metastatic spread, and survival. Results: CDK8 protein levels were elevated in OC tissues, particularly in OCCC primary and metastatic lesions compared to benign tissue. While CDK8/19 inhibition showed limited effects on in vitro cell proliferation, SNX631-6 demonstrated significant antitumor and anti-metastatic activity in vivo. Notably, SNX631-6 enhanced the efficacy of cisplatin, substantially inhibiting tumor growth and extending overall survival. Conclusions: Therapeutically achievable doses of CDK8/19 inhibitors may provide clinical benefit for OCCC patients by inhibiting tumor growth and reversing platinum resistance, potentially addressing a critical treatment challenge in this rare ovarian cancer subtype.

TWIK-1 two-pore domain K(+) channels are expressed abundantly in astrocytes. In the present study, we examined the extent to which TWIK-1 contributes to the linear current-voltage (I-V) relationship (passive) K(+) membrane conductance, a dominant electrophysiological feature of mature hippocampal astrocytes. Astrocytes from TWIK-1 knockout mice have a more negative resting potential than those from wild type animals and a reduction in both inward rectification and Cs(+) permeability. Nevertheless, the overall whole-cell passive conductance is not altered significantly in TWIK-1 knockout astrocytes. The expression of Kir4.1 and TREK-1, two other major astrocytic K(+) channels, or of other two-pore K(+) channels is not altered in TWIK-1 knockout mice, suggesting that the mild effect of TWIK-1 knockout does not result from compensation by these channels. Fractionation experiments showed that TWIK-1 is primarily localized in intracellular cytoplasmic fractions (55%) and mildly hydrophobic internal compartment fractions (41%), with only 5% in fractions containing plasma membranes. Our study revealed that TWIK-1 proteins are mainly located in the intracellular compartments of hippocampal astrocyte under physiological condition, therefore a minimal contribution of TWIK-1 channels to whole-cell currents is likely attributable to a relatively low level presence of channels in the plasma membrane.

CDK8 and CDK19 paralogs are regulatory kinases associated with the transcriptional Mediator complex. We have generated mice with the systemic inducible Cdk8 knockout on the background of Cdk19 constitutive knockout. Cdk8/19 double knockout (iDKO) males, but not single Cdk8 or Cdk19 KO, had an atrophic reproductive system and were infertile. The iDKO males lacked postmeiotic spermatids and spermatocytes after meiosis I pachytene. Testosterone levels were decreased whereas the amounts of the luteinizing hormone were unchanged. Single-cell RNA sequencing showed marked differences in the expression of steroidogenic genes (such as Cyp17a1, Star, and Fads ) in Leydig cells concomitant with alterations in Sertoli cells and spermatocytes, and were likely associated with an impaired synthesis of steroids. Star and Fads were also downregulated in cultured Leydig cells after iDKO. The treatment of primary Leydig cell culture with a CDK8/19 inhibitor did not induce the same changes in gene expression as iDKO, and a prolonged treatment of mice with a CDK8/19 inhibitor did not affect the size of testes. iDKO, in contrast to the single knockouts or treatment with a CDK8/19 kinase inhibitor, led to depletion of cyclin C (CCNC), the binding partner of CDK8/19 that has been implicated in CDK8/19-independent functions. This suggests that the observed phenotype was likely mediated through kinase-independent activities of CDK8/19, such as CCNC stabilization.

CDK8 and CDK19 paralogs are regulatory kinases associated with the transcriptional Mediator complex. We have generated mice with the systemic inducible Cdk8 knockout on the background of Cdk19 constitutive knockout. Cdk8/19 double knockout (iDKO) males, but not single Cdk8 or Cdk19 KO, had an atrophic reproductive system and were infertile. The iDKO males lacked postmeiotic spermatids and spermatocytes after meiosis I pachytene. Testosterone levels were decreased whereas the amounts of the luteinizing hormone were unchanged. Single-cell RNA sequencing showed marked differences in the expression of steroidogenic genes (such as Cyp17a1, Star, and Fads ) in Leydig cells concomitant with alterations in Sertoli cells and spermatocytes, and were likely associated with an impaired synthesis of steroids. Star and Fads were also downregulated in cultured Leydig cells after iDKO. The treatment of primary Leydig cell culture with a CDK8/19 inhibitor did not induce the same changes in gene expression as iDKO, and a prolonged treatment of mice with a CDK8/19 inhibitor did not affect the size of testes. iDKO, in contrast to the single knockouts or treatment with a CDK8/19 kinase inhibitor, led to depletion of cyclin C (CCNC), the binding partner of CDK8/19 that has been implicated in CDK8/19-independent functions. This suggests that the observed phenotype was likely mediated through kinase-independent activities of CDK8/19, such as CCNC stabilization.

Paralogs CDK8 and CDK19 are regulatory kinases associated with the transcriptional Mediator complex. We have e generated mice with the systemic inducible Cdk8 knockout on the background of Cdk19 constitutive knockout. Cdk8/19 double knockout (iDKO) males, but not single Cdk8 and Cdk19 KO, had an atrophic reproductive system and were infertile. The iDKO males lacked postmeiotic spermatids and spermatocytes after meiosis I pachytene. Testosterone levels were decreased whereas the amounts of the luteinizing hormone were unchanged. Single cell RNA sequencing showed marked differences in the expression of steroidogenic genes (such as Cyp17a1, Star and Fads) in Leydig cells concomitant with alterations in Sertoli cells and spermatocytes likely associated with impaired synthesis of steroids. Star and Fads were also downregulated in cultivated Leydig cells after iDKO. The treatment of primary Leydig cells culture with a CDK8/19 inhibitor did not induce the same changes in gene expression as iDKO, and prolonged treatment of mice with a CDK8/19 inhibitor did not affect the size of testes. iDKO, in contrast to single knockouts or treatment with a CDK8/19 kinase inhibitor, led to depletion of cyclin C (CcnC), the binding partner of CDK8/19 that has been implicated in CDK8/19-independent functions. This suggests that the observed phenotype was likely mediated through kinase-independent activities of CDK8/19, such as CcnC stabilization.Competing Interest StatementI.B.R is Founder and President and M.C. is Director of Research of Senex Biotechnology, Inc. Other authors declare no conflict of interest.Footnotes* Figure 2 revised; Supplemental files updated.

Castration-resistant prostate cancer (CRPC) remains incurable due to its high plasticity. We found that Mediator kinases CDK8 and CDK19, pleiotropic regulators of transcriptional reprogramming, are differentially affected by androgen, which downregulates CDK8 and upregulates CDK19. Accordingly, expression of CDK8 decreases while CDK19 increases during prostate carcinogenesis, but both CDK19 and CDK8 are upregulated in metastatic CRPC. Genetic inactivation of CDK8 and CDK19 suppresses CRPC tumor growth in castrated male mice and renders CRPC responsive to androgen deprivation. Restoration of active CDK19 or CDK8 kinases reverses this phenotype, indicating that CRPC becomes dependent on Mediator kinase activity for in vivo growth under the conditions of androgen deprivation. Selective CDK8/19 inhibitors suppress androgen-independent growth of cell line-based and patient-derived CRPC xenografts, whereas prolonged inhibitor treatment induces tumor regression and even leads to cures. Mediator kinase activity was found to affect tumor and stromal gene expression preferentially in castrated mice, orchestrating castration-induced transcriptional reprogramming. These results warrant the exploration of Mediator kinase inhibitors for CRPC therapy.