See accompanying article on page 2431

SIKs (salt-inducible kinases) are evolutionary conserved AMPK (adenosine monophosphate-activated protein kinase) family serine-threonine kinases that were discovered in the adrenal gland of rats given a high-salt diet.¹ The SIK subfamily consists of 3 isoforms, SIK1, SIK2, and SIK3, that share a highly homologous N-terminal kinase domain, but their C-terminal regions are less conserved.² SIKs are phosphorylated by LKB1 (liver kinase B1), which is thought to be constitutively active, and, therefore, are considered to be constitutively phosphorylated. Additionally, SIKs are phosphorylated at a C-terminal serine residue in response to rises in intracellular cAMP with activation of protein kinase A, though their response to external stimuli is not fully understood.

All three SIK isoforms are broadly expressed in many tissues, including the heart and in vascular smooth muscle cells (VSMCs). The expression of SIK1 in VSMCs of the aorta increases in mice given high-salt intake. Under such conditions, SIK1 knockout mice exhibited upregulated transforming growth factor-β1 signaling with increased expression of endothelin-1, elevated systolic blood pressure, and cardiac hypertrophy.³ These observations suggest that SIK1 may be involved in the prevention of high salt–induced hypertension. Nevertheless, in a recent study, SIK1 knockout mice were protected from pressure overload–induced hypertrophy and the development of heart failure.⁴ We have shown that SIKs have a major role in the development of vascular calcifications and that pharmacological inhibition of SIKs can inhibit the process of the VSMC calcification in aortic rings and in vivo.⁵

Traumatic injury of vessel walls during angioplasty triggers a series of cellular and subcellular events that ultimately lead to VCMC proliferation, neointima formation, and restenosis, with reduction in vessel lumen diameter. Despite the reduced incidence brought by drug-eluting stents, restenosis remains a significant clinical problem. In an exciting study, Cai et al⁶ identified a novel role for SIK activity in the process of injury response and restenosis. By studying the expression of SIKs in contractile and growing VSMCs, they found that one of the three SIK isoforms, SIK3, was highly expressed in growing VSMCs, induced when stimulated by serum, and, in addition, increased in neointimal lesions after injury. The global inhibition of SIK activity in vivo, using topical application of the pan-SIK inhibitor HG-9-91-01 around femoral arteries, blocked VSMC proliferation, prevented neointimal growth, and reduced inflammation. These observations were further supported by studies in cells that showed that pan-SIK inhibitor could block proliferation, attenuate migration, and modulate actin polymerization and that specific inhibition of SIK3 using siRNAs could prevent VSMC proliferation (Figure).

Figure. SIK (salt-inducible kinase) activity promotes arterial restenosis. Balloon angioplasty or other arterial injuries induce the expression of SIK3. The inhibition of SIK activity by drugs such as HG-9-91-01 or by siRNA treatment prevent the vascular smooth muscle proliferation, the cytoplasmic retention of CRTC3 (CREB-regulated transcription coactivator 3), and the phosphorylation AKT (pAKT), that together allow the inhibition of SIK to inhibit arterial restenosis. VSMC indicates vascular smooth muscle cell.

SIKs have several known downstream targets. CRTC2 (CREB-regulated transcription coactivator 2)—a CREB (cAMP response element-binding protein) coactivator—can be phosphorylated by SIK2.⁷ CRTC2 is sequestered in the cytoplasm under basal conditions via an SIK phosphorylation-dependent association with 14-3-3 proteins. Calcium and cAMP pathways trigger its release from 14-3-3 proteins and its entry to the nucleus by activating the phosphatase calcineurin and inhibiting SIK2. It was later shown that HDAC4 (histone deacetylase 4) is phosphorylated and sequestered in the cytoplasm by SIK3 and that HDAC5 could be similarly affected.⁸ We have shown that SIK activity of promoting vascular calcifications is mediated by HDAC4 phosphorylation and cytoplasmic retention,⁵ and in the heart, it was shown that SIK1 phosphorylated and stabilized HDAC7 during pressure overload to enhance pathological remodeling.⁴ SIK2 was also shown to activate the PI3K (phosphoinositide 3-kinase)/AKT (protein kinase B) pathway through phosphorylation of p85α.⁹ Cai et al⁶ show that CRTC3 nuclear import likely contributes to SIK inhibition–mediated antiproliferative effect and show that SIK inhibition attenuated the serum-induced phosphorylation of AKT. In contrast, the SIK inhibition–mediated suppression of VSMC proliferation did not appear to be dependent on HDAC4, as HDAC4 stayed in a cytoplasmic location even with SIK inhibition or after SIK3 knockdown in cultured VCMCs.

SIKs play an important role in vascular pathology. Cai et al⁶ elegantly show their importance in restenosis formation, and we have previously shown their role in vascular calcifications. Future work will have to carefully examine the redundant or exclusive roles of each of the three SIK isoforms. Specific SIK inhibitory drugs need to be developed for systemic or local application, to examine the therapeutic potential of this axis. As SIKs play a role in several signaling pathways such as CREB, AKT, and HDACs, it will also be important to understand how SIK inhibition affects each of these pathways in different contexts and specifically in vivo.

This study was supported by the Israel Science Foundation (grant number 1385/20) for I. Kehat.

Disclosures None.

The opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.

For Sources of Funding and Disclosures, see page 2453.

请参阅第 2431 页的随附文章

SIK（盐诱导激酶）是进化保守的 AMPK（单磷酸腺苷活化蛋白激酶）家族丝氨酸-苏氨酸激酶，在给予高盐饮食的大鼠的肾上腺中发现。¹ SIK 亚家族由 3 个亚型 SIK1、SIK2 和 SIK3 组成，它们共享一个高度同源的 N 端激酶域，但它们的 C 端区域不太保守。² SIK 被 LKB1（肝激酶 B1）磷酸化，LKB1 被认为具有组成型活性，因此被认为是组成型磷酸化。此外，SIKs 在 C 末端丝氨酸残基处被磷酸化，以响应细胞内 cAMP 的升高以及蛋白激酶 A 的激活，尽管它们对外部刺激的反应尚不完全清楚。

所有三种 SIK 亚型在许多组织中广泛表达，包括心脏和血管平滑肌细胞 (VSMC)。在高盐摄入的小鼠中，SIK1 在主动脉 VSMC 中的表达增加。在这种情况下，SIK1 敲除小鼠表现出上调的转化生长因子-β1 信号转导，内皮素-1 表达增加、收缩压升高和心脏肥大。³这些观察结果表明 SIK1 可能参与预防高盐诱发的高血压。然而，在最近的一项研究中，SIK1 基因敲除小鼠免受压力超负荷引起的肥大和心力衰竭的发展。⁴我们已经表明 SIKs 在血管钙化的发展中起主要作用，并且 SIKs 的药理学抑制可以抑制主动脉环和体内 VSMC 钙化的过程。⁵

血管成形术期间血管壁的创伤性损伤引发一系列细胞和亚细胞事件，最终导致 VCMC 增殖、新内膜形成和再狭窄，血管腔直径减小。尽管药物洗脱支架降低了发病率，但再狭窄仍然是一个重要的临床问题。在一项令人兴奋的研究中，Cai 等人⁶确定了 SIK 活动在损伤反应和再狭窄过程中的新作用。通过研究 SIK 在可收缩和生长的 VSMC 中的表达，他们发现三种 SIK 同种型之一 SIK3 在生长的 VSMC 中高表达，受血清刺激时被诱导，此外，在损伤后的新生内膜病变中表达增加。使用泛 SIK 抑制剂 HG-9-91-01 在股动脉周围局部应用 SIK 活性的 体内 整体抑制，阻断了 VSMC 增殖，防止了新内膜生长，并减少了炎症。这些观察结果得到了细胞研究的进一步支持，这些研究表明 pan-SIK 抑制剂可以阻断增殖、减弱迁移和调节肌动蛋白聚合，并且使用 siRNA 特异性抑制 SIK3 可以防止 VSMC 增殖（图）。

数字。 SIK（盐诱导激酶）活性促进动脉再狭窄。球囊血管成形术或其他动脉损伤诱导 SIK3 的表达。通过药物如 HG-9-91-01 或通过 siRNA 处理抑制 SIK 活性可防止血管平滑肌增殖、CRTC3（CREB ​​调节的转录辅激活因子 3）的细胞质滞留和磷酸化 AKT（pAKT），即一起允许抑制 SIK 以抑制动脉再狭窄。VSMC 表示血管平滑肌细胞。

SIKs 有几个已知的下游目标。CRTC2（CREB ​​调节的转录辅激活因子 2）——一种 CREB（cAMP 反应元件结合蛋白）辅激活因子——可以被 SIK2 磷酸化。⁷ CRTC2 在基础条件下通过与 14-3-3 蛋白的 SIK 磷酸化依赖性结合被隔离在细胞质中。钙和 cAMP 通路通过激活磷酸酶钙调神经磷酸酶和抑制 SIK2 触发其从 14-3-3 蛋白中释放并进入细胞核。后来表明 HDAC4（组蛋白脱乙酰酶 4）被 SIK3 磷酸化并隔离在细胞质中，HDAC5 也可能受到类似的影响。⁸我们已经证明 SIK 促进血管钙化的活性是由 HDAC4 磷酸化和细胞质滞留介导的，⁵在心脏中，研究表明 SIK1 在压力超负荷期间磷酸化并稳定 HDAC7，以增强病理重塑。⁴ SIK2 还显示通过 p85α 的磷酸化激活 PI3K（磷酸肌醇 3-激酶）/AKT（蛋白激酶 B）途径。⁹ Cai 等人⁶表明 CRTC3 核输入可能有助于 SIK 抑制介导的抗增殖作用，并表明 SIK 抑制减弱了血清诱导的 AKT 磷酸化。相比之下，SIK 抑制介导的 VSMC 增殖抑制似乎并不依赖于 HDAC4，因为即使在 SIK 抑制或在培养的 VCMC 中敲除 SIK3 后，HDAC4 仍停留在细胞质位置。

SIKs 在血管病理学中发挥重要作用。Cai et al ⁶优雅地展示了它们在再狭窄形成中的重要性，我们之前已经展示了它们在血管钙化中的作用。未来的工作将不得不仔细检查三个 SIK 异构体中每一个的冗余或专有作用。需要为全身或局部应用开发特定的 SIK 抑制药物，以检查该轴的治疗潜力。由于 SIK 在多种信号通路（如 CREB、AKT 和 HDAC）中发挥作用，因此了解 SIK 抑制如何在不同环境中，特别是在体内影响这些通路中的每一种，也很重要。

这项研究得到了以色列科学基金会（资助号 1385/20）为 I. Kehat 的支持。

披露无。

本文中表达的观点不一定是编辑或美国心脏协会的观点。

有关资金来源和披露信息，请参见第 2453 页。