34.10
异常增殖
在正常情况下，大多数成体细胞保持非增殖状态，除非受到内部或外部因素的刺激来替换丢失的细胞。 细胞增殖异常是指细胞生长超过正常细胞且与正常细胞不协调的情况。 在这种情况下，即使在刺激停止后，细胞分裂仍以同样的过度方式持续，导致持续性肿瘤。 肿瘤由受损细胞产生，这些细胞通过复制将损伤传递给子细胞和所有未…
细胞具有一类称为抑癌基因的特殊基因，能够中和有害遗传改变的影响，防止细胞不受控制地增殖。
例如，p53 是一种肿瘤抑制基因，在正常细胞条件下维持基础水平的表达。
Mdm2 蛋白作为 p53 活性的负调控因子，可结合 p53 蛋白的功能结构域，从而降低其转录活性。
Mdm2 还具有针对 p53 的特异性泛素连接酶活性，可催化泛素与 p53 的结合。随后，多泛素化的 p53 蛋白被蛋白酶体识别并降解，从而使其在细胞内的水平维持在极低状态。
然而，当细胞遇到细胞应激或过强的有丝分裂刺激等异常情况时，p53 基因会被诱导以发挥其肿瘤抑制功能。
例如，核仁中转录因子Myc的过表达会引发一种名为Arf的肿瘤抑制蛋白的积累。
Arf 直接与 Mdm2 结合，抑制其泛素连接酶活性，并将其隔离在核仁中，从而释放活性 p53 进入核质。
活化的p53随后结合到特定的DNA序列，并诱导其靶基因的表达，这些基因可触发加速DNA修复、阻滞细胞周期或诱导细胞凋亡，从而防止受损细胞的进一步增殖。
然而，当基因或表观遗传改变影响p53活性时，会导致细胞保护机制的崩溃。这些细胞开始剧烈增殖并形成肿瘤。
此外，脂肉瘤中也经常观察到 Mdm2 的过表达。 尽管此类肿瘤保留了正常的 p53 基因，但 Mdm2 水平的升高使 p53 处于失活状态，从而限制了其肿瘤抑制活性。
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Q1: What is abnormal proliferation in cells?
Abnormal proliferation refers to uncontrolled or excessive cell division that deviates from normal growth patterns. Unlike regulated cell division within the cell cycle, abnormal proliferation occurs when cells divide beyond physiological needs or ignore growth-limiting signals. This disruption can lead to tissue overgrowth and is a hallmark of pathological conditions including cancer and tumor formation.
Q2: How does abnormal proliferation differ from normal cell growth?
Normal cell growth is tightly regulated by internal and external signals that control when cells divide and when they stop. Abnormal proliferation bypasses these regulatory checkpoints, causing cells to divide continuously without appropriate signals. This uncontrolled growth results in excessive cell accumulation, distinguishing pathological proliferation from the orderly, purposeful growth that maintains tissue homeostasis.
Q3: What causes cells to undergo abnormal proliferation?
Abnormal proliferation results from mutations or dysfunction in genes controlling cell division, such as oncogenes and tumor suppressors. Environmental factors like radiation, chemicals, or viral infections can trigger these genetic changes. Additionally, loss of contact inhibition or failure to respond to apoptosis signals allows cells to divide indefinitely, driving uncontrolled growth and cellular pathology.
Q4: What are the consequences of abnormal cell proliferation?
Abnormal proliferation leads to excessive tissue mass, disrupting normal organ function and tissue architecture. Accumulated abnormal cells can form tumors, compress surrounding tissues, and potentially metastasize. This uncontrolled growth compromises cellular communication, nutrient distribution, and waste removal, ultimately threatening organism health and survival.
Q5: How do checkpoint mechanisms fail in abnormal proliferation?
Cell cycle checkpoints normally halt division if DNA damage or abnormalities are detected, allowing repair or triggering cell death. In abnormal proliferation, mutations disable these checkpoint proteins, permitting damaged cells to continue dividing. This failure allows accumulation of additional mutations, accelerating the progression toward malignant transformation and uncontrolled growth.
Q6: What role does contact inhibition play in preventing abnormal proliferation?
Contact inhibition is a regulatory mechanism where cells stop dividing when they touch neighboring cells, maintaining tissue organization. In abnormal proliferation, cells lose this response and continue dividing despite physical contact with other cells. This loss of contact inhibition is a critical step in cancer development, enabling cells to pile up and form tumors.